Language：English   Publishing type：Research paper (scientific journal)  

Space weather manifests in power networks as quasi-DC currents flowing in and out of the power system through the grounded neutrals of high-voltage transformers, referred to as geomagnetically induced currents. This paper presents a comparison of modeled geomagnetically induced currents, determined using geoelectric fields derived from four different impedance models employing different conductivity structures, with geomagnetically induced current measurements from within the power system of the eastern states of Australia. The four different impedance models are a uniform conductivity model (UC), one-dimensional n-layered conductivity models (NU and NW), and a three-dimensional conductivity model of the Australian region (3DM) from which magnetotelluric impedance tensors are calculated. The modeled 3DM tensors show good agreement with measured magnetotelluric tensors obtained from recently released data from the Australian Lithospheric Architecture Magnetotelluric Project. The four different impedance models are applied to a network model for four geomagnetic storms of solar cycle 24 and compared with observations from up to eight different locations within the network. The models are assessed using several statistical performance parameters. For correlation values greater than 0.8 and amplitude scale factors less than 2, the 3DM model performs better than the simpler conductivity models. When considering the model performance parameter, P, the highest individual P value was for the 3DM model. The implications of the results are discussed in terms of the underlying geological structures and the power network electrical parameters.

DOI： 10.1029/2018SW002047

Language：English   Publishing type：Research paper (international conference proceedings)  

Intense X-ray fluxes during solar flares are known to cause enhanced ionization in the Earth’s ionospheric D, E and F region. This sudden change of ionospheric electron density profile is serious problem to radio wave communication and navigation system. The ground magnetograms often record the sudden change in the sunlit hemisphere during the enhanced X-ray flux, due to the sudden increase in the global ionospheric current system caused by the flare-induced enhanced ionospheric conductivity. These geomagnetic field disturbances are known as ‘‘solar flare effects’’ (SFEs) or geomagnetic crochets [Campbell, 2003]. The typical SFE is increase variation on the equatorial magnetic data. On Ionosonde observation during solar flare event, the High-Frequency (HF) radio wave blackout is often detected in ionogram due to the sudden disturbance in ionosphere. Two intense X-class solar flares occurred on 6 and 10 September 2017. We investigated the magnetic field and Ionosonde responses to the intense solar flare events. Dayside magnetic field variations sudden increased due to the ionospheric disturbance resulting from solar flare. There is no response in night side magnetometer data. The magnitude of SFE (magnetic field) is independent of solar flare x-ray magnitude. We found HF radio wave blackout in ionogram at dayside Ionosonde stations. The duration of blackout is dependent of latitude and local time of Ionosonde stations. There is the different feature of ionogram at night side.

DOI： 10.1051/e3sconf/20186201007

Language：English   Publishing type：Research paper (international conference proceedings)  

We examined the Schumann resonance (SR) at low-latitude station KUJ by comparing with solar X-ray flux and solar proton flux at a geostationary orbit. For intense solar activity in October-November 2003, the reaction of the SR frequency to X-ray enhancement and SPEs was different. The SR frequency in H component increased at the time of the Xray enhancement. The response of SR seems to be caused by the increase of the electron density in the ionospheric D region which ionized by the enhanced solar X-ray flux. In the case of the SPEs, the SR frequency in D component decreased with enhancement of solar proton flux. We suggest that the SPEs caused the decrease of altitude on the ionopheric D region at high-latitude region, and the SR frequency decreased.

DOI： 10.1051/e3sconf/20186201012

Language：English   Publishing type：Research paper (international conference proceedings)  

For study of coupling processes in the Solar-Terrestrial System, International Center for Space Weather Science and Education (ICSWSE), Kyushu University has developed a real time magnetic data acquisition system (the MAGDAS project) around the world. The number of observational sites is increasing every year with the collaboration of host countries. Now at this time, the MAGDAS Project has installed 78 real time magnetometers-so it is the largest magnetometer array in the world. The history of global observation at Kyushu University is over 30 years and number of developed observational sites is over 140. Especially, Collaboration between IKIR is extended back to 1990's. Now a time, we are operating Flux-gate magnetometer and FM-CW Radar. It is one of most important collaboration for space weather monitoring. By using MAGDAS data, ICSWSE produces many types of space weather index, such as EE-index (for monitoring long tern and shot term variation of equatorial electrojet), Pc5 index (for monitoring solar-wind velocity and high energy electron flux), Sq-index (for monitoring global change of ionospheric low and middle latitudinal current system), and Pc3 index (for monitoring of plasma density variation at low latitudes). In this report, we will introduce recent development of MAGDAS/ICSWSE Indexes project and topics for new open policy for MAGDAS data will be also discussed.

DOI： 10.1051/e3sconf/20172001013

Language：English   Publishing type：Research paper (scientific journal)  

Pi2 magnetic oscillations on the dayside are considered to be produced by the ionospheric current that is driven by Pi2-associated electric fields from the high-latitude region, but this idea has not been quantitatively tested. The present study numerically tested the magnetospheric-ionospheric current system for Pi2 consisting of field-aligned currents (FACs) localized in the nightside auroral region, the perpendicular magnetospheric current flowing in the azimuthal direction, and horizontal ionospheric currents driven by the FACs. We calculated the spatial distribution of the ground magnetic field produced by these currents using the Biot-Savart law in a stationary state. The calculated magnetic field reproduced the observational features reported by previous studies: (1) the sense of the H component does not change a wide range of local time sectors at low latitudes, (2) the amplitude of the H component on the dayside is enhanced at the equator, (3) the D component reverses its phase near the dawn and dusk terminators, (4) the meridian of the D component phase reversal near the dusk terminator is shifted more sunward than that near the dawn terminator, and (5) the amplitude of the D component in the morning is larger than that in the early evening. We also derived the global distributions of observed equivalent currents for two Pi2 events. The spatial patterns of dayside equivalent currents were similar to the spatial pattern of numerically derived equivalent currents. The results indicate that the oscillation of the magnetospheric-ionospheric current system is a plausible explanation of Pi2s on the dayside and near the terminator.

DOI： 10.1002/2017JA024246

Language：English   Publishing type：Research paper (scientific journal)  

The solar energy can mainly be divided into two categories: the solar radiation and the solar wind. The former maximizes at the equator, generating various disturbances over a wide height range and causing vertical coupling processes of the atmosphere between the troposphere and middle and upper atmospheres by upward propagating atmospheric waves. The energy and material flows that occur in all height regions of the equatorial atmosphere are named as “Equatorial Fountain.” These processes from the bottom also cause various space weather effects, such as satellite communication and Global Navigation Satellite System positioning. While, the electromagnetic energy and high-energy plasma particles in the solar wind converge into the polar region through geomagnetic fields. These energy/particle inflow results in auroral Joule heating and ion drag of the atmosphere particularly during geomagnetic storms and substorms. The ion outflow from the polar ionosphere controls ambient plasma constituents in the magnetosphere and may cause long-term variation of the atmosphere. We propose to clarify these overall coupling processes in the solar-terrestrial system from the bottom and from above through high-resolution observations at key latitudes in the equator and in the polar region. We will establish a large radar with active phased array antenna, called the Equatorial Middle and Upper atmosphere radar, in west Sumatra, Indonesia. We will participate in construction of the EISCAT_3D radar in northern Scandinavia. These radars will enhance the existing international radar network. We will also develop a global observation network of compact radio and optical remote sensing equipment from the equator to polar region.

DOI： 10.1002/2016RS006035

Language：English   Publishing type：Research paper (scientific journal)  

A physical process for the latitudinal motion of an auroral arc based on the four-side bound Cowling channel model is proposed. Assuming that an upward field-aligned current (FAC) is associated with the auroral arc that forms a Cowling channel with finite lengths not only latitudinally but also longitudinally and that the upward FAC region is primarily embedded in a purely northward electric field, the primary Hall current driven by the northward electric field accumulates positive excess charges at the eastern edge of the channel and negative charges at the western edge for a perfect or partial Cowling channel with a nonzero Cowling efficiency. The charges produce a westward secondary electric field, indicating that a westward electric field can thus be produced by a purely northward primary electric field. This secondary electric field moves the arc with its magnetospheric source drifting together with the magnetospheric plasmas equatorward and simultaneously produces the electric field outside the channel that moves the downward FAC equatorward of the upward FAC region equatorward together with the upward FAC. Thus, the whole 3-D current system is expected to move equatorward as often observed in the afternoon auroral zone.

DOI： 10.1029/2012JA017953

Language：English   Publishing type：Research paper (scientific journal)  

Energy flow is an important aspect of magnetosphere-ionosphere coupling. Electromagnetic energy is transported as Poynting flux from the magnetosphere to the ionosphere, where it is dissipated as Joule heating. Recently Richmond derived an "Equipotential Boundary Poynting Flux (EBPF) theorem", that the Poynting flux within a flux tube whose boundary is an equipotential curve is dissipated inside the ionospheric foot point of the flux tube. In this article we study Richmond's EBPF theorem more closely by considering the curl-free and divergence-free parts as well as the Hall and Pedersen parts of the ionospheric current system separately. Our main findings are that i) divergence-free currents are on average dissipationless, ii) the curl-free Pedersen current is responsible for the whole ionospheric Joule heating and iii) pointwise match between vertical Poynting flux and ionospheric Joule heating is broken by gradients of Hall and Pedersen conductances. Results i) and ii) hold when integrated over the whole ionosphere or any area bounded by an equipotential curve. The present study is limited to quasi-static phenomena. The more general topic of electrodynamic Joule heating and Poynting flux, including inductive effects, will be addressed in a future study.

DOI： 10.1029/2012JA017841

Language：English   Publishing type：Research paper (scientific journal)  

In order to correctly describe the dynamical behavior of the magnetosphere-ionosphere (MI) coupling system and the scale-dependent auroral process, we develop a synthesis formulation that combines the process of (1) the inverse Walen separation of MHD disturbance into parallel- and antiparallel-propagating shear Alfvén wave to the ambient magnetic field, (2) the shear Alfvén wave reflection process including (3) the scale-dependent electrostatic coupling process through the linearized Knight relation, (4) two-layer ionosphere model, and (5) dynamic conductance variations. A novel procedure that applies the inverse Walen relation to the incompressional MHD disturbances at the inner boundary of the MHD region enables to extract the component of the shear Alfvén wave incident to the ionosphere. The extracted incident electric field supplies an electromotive force for the generation of the MI coupling system, and the reflected electric field is generated such that it totally satisfies the synthesis MI-coupling equation. A three-dimensional ionospheric current system is represented by a two-layer model in which the Pedersen and the Hall current are confined in the separated layers, which are connected by field-aligned currents driven by the linear current-voltage relation between two layers. Hence, our scheme possibly reproduces two types of the scale-dependent MI-decoupling process of the perpendicular potential structure: due to the parallel potential drop at the auroral acceleration region and the other due to the parallel potential differences inside the ionosphere. Our newly formulation may be well suited for description of scale-dependent auroral process and mesoscale ionospheric electrodynamics interlocked with the dynamical development of magnetospheric processes.

DOI： 10.1029/2011JA016460

Language：English   Publishing type：Research paper (scientific journal)  

We formulate the evolution of ionospheric conductivity in the framework of 3-D M-I coupling. Two important physical processes are taken into account. One is the ionization process by precipitating mono-energetic particles, which are accelerated by parallel-potential drops in the auroral acceleration region. The other process reflects the fact that part of field-aligned current (FAC) carried by electrons is closed with a perpendicular ionic current. Here, whereas the electric current is divergence-free, the divergence of electron current is finite. Therefore, the ionospheric electron density changes, and so does the conductivity. If the energy of electron precipitation is below ∼10 eV, this second process plays an important role in plasma transportation, production, and evacuation processes. In this case the density variation does not extend in space at the perpendicular electron velocity, but it rather moves at the ion perpendicular velocity. If the energy of electron precipitation is above ∼1 keV, in contrast, the precipitation has a nonlinear effect on plasma evolution. That is, the propagation speed of the density variation increases with increasing upward-FAC density, and the propagation takes place in the direction of the converging current into the upward FAC region. The Cowling effect on the plasma evolution process is crucially important. Our formulation is more general than the previous studies and is not limited to certain geometries, current component or interaction modes between the ionosphere and magnetosphere. It is therefore better-suited for describing the self-organized M-I coupling system, which evolves with current systems, conductivity, and magnetospheric processes interacting with each other.

DOI： 10.1029/2011JA016449

Language：English   Publishing type：Research paper (scientific journal)  

The question to which extent the divergence of the Hall current can be connected to the Pedersen current or to the closure current in the magnetosphere through field-aligned currents (FACs), that is, the Cowling channel process in the polar region, has long been debated but not fully understood. The present study reformulates the Cowling channel by introducing a two-layer model consisting of Hall and Pedersen conductivity layers with channel boundaries not only in the direction perpendicular to the channel but also in the direction along it. This new model enables us to better and more physically understand the connection between the Hall current, Pedersen current, and FAC. In particular, the finiteness of the channel along its direction enables us to understand that the primary nonzero electric field along the channel and FACs at the channel boundaries that faced each other in the channel direction carries the necessary energy for the Hall current to set up the secondary electric field from the magnetosphere. A case for a possible connection between the Pedersen and Hall currents is shown based on a polar current system derived from the Kamide-Richmond-Matsushita method. A more comprehensive analysis based on data is presented in the companion paper.

DOI： 10.1029/2010JA015989

Language：English   Publishing type：Research paper (scientific journal)  

An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp ≤ 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity.

DOI： 10.1029/2011JA016487

Language：English   Publishing type：Research paper (scientific journal)  

The Cowling channel mechanism describes the creation of a secondary polarization electric field at sharp conductance boundaries in the ionosphere due to excess charges for the case in which the release of these charges to the magnetosphere is fully or partially impeded. The secondary currents generated by the polarization electric field effectively modify the effective ionospheric conductivity inside the Cowling channel. While the Cowling mechanism is generally accepted for the equatorial electrojet, there is a long-standing discussion about the importance of this mechanism and its efficiency in the auroral electrojet. We present a statistical investigation that enables us to identify the most probable geospace conditions and MLT locations for a high Cowling efficiency. This investigation is based on more than 1600 meridional profiles of data from the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE) network in Scandinavia, in particular, ground magnetic field data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network and electric field data from the Scandinavian Twin Auroral Radar Experiment (STARE) radar, supported with pointwise ionospheric conductance measurements from the European Incoherent Scatter (EISCAT) radar. We analyze the data in the framework of a 3-D ionospheric model, but our data set is filtered so that only electrojet-type situations are included so that the gradients of all measured quantities in longitudinal direction can be neglected. The analysis results in a steep peak of high Cowling channel efficiency probability in the early morning sector (0245-0645 MLT), with the largest probability around 0500 MLT and for medium and high geomagnetic activity. In agreement with an earlier single-event study by Amm and Fujii (2008), this indicates that the Cowling mechanism may be most effective in the early morning part of the central substorm bulge. Further, our analysis results in an almost monotonic increase of the probability of high Cowling channel efficiency with increasing geomagnetic activity.

DOI： 10.1029/2010JA015988

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2009JA014676

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2009JA014924

Language：English   Publishing type：Research paper (bulletin of university, research institution)  

Language：English   Publishing type：Research paper (scientific journal)  

A new index, EE-index (EDst, EU, and EL), is proposed to monitor temporal and long-term variations of the equatorial electrojet by using the MAGDAS/CPMN real-time data. The mean value of the H component magnetic variations observed at the nightside (LT = 18-06) MAGDAS/CPMN stations along the magnetic equatorial region is found to show variations similar to those of Dst; we defined this quantity as EDst. The EDst can be used as a proxy of Dst for the real-time and long-term geospace monitoring. By subtracting EDst from the H component data of each equatorial station, ir is possible to extract the Equatorial Electrojet and Counter Electrojetcomponents, which are defined as EU and EL, respectively.

DOI： 10.1186/BF03352828

Language：English   Publishing type：Research paper (scientific journal)  

Ground Pi 2 pulsations are mixtures of several components reflecting (1) propagations of fast and shear Alfvén wave, (2) resonances of plasmaspheric/magnetospheric cavity and magnetic field lines, and (3) tansformations to ionospheric current systems. However, it has been unclear how they coupled with each other and how their signals are distributed at different latitudes. The present work is intended to pilot the future possibilities whether we can identify the global system of Pi 2 pulsations by Independent Component Analysis (ICA). We have successfully decomposed an isolated Pi 2 event on a quiet day observed at the CPMN stations into two components. One was the global oscillation that occurs from nightside high to equatorial latitudes with the common waveform and has an amplitude maximum at nightside high latitude. Another component was localized at nightside high latitudes. Its amplitudes were quite weak at low latitudes, but were enhanced near dayside dip equator.

DOI： 10.1029/2007GL030174

Language：English  

Language：English  

Language：English   Publishing type：Research paper (other academic)  

Language：English  

Language：English  

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Under a model of altitude distribution of the Alfvén speed VA, one-dimensional transient response of the inner magnetosphere at the magnetic equator to earthward propagating impulse- and step-like MHD disturbances is considered. The waveforms of transient compressional oscillations due to these disturbances at some L shells are directly simulated by a numerical inversion of the Laplace transform with orthonormal Laguerre functions. The present paper concentrates on the analysis of waveforms. Then, it is verified that the compressional oscillations are due to the poles of the system under consideration. The oscillation arising from the cavity resonance all over the inner magnetosphere is most dominant. However, its amplitude becomes smaller as the characteristic time scale T of an incident disturbance grows large, and it is negligibly small for T greater than several times of eigenperiod of the resonance. On the other hand, when T is relatively small (e.g., T £10 s), the oscillations due to the cavity resonances trapped around the trough in VA are outstanding. It is also found that the relative phase between the cavity-mode oscillations all over the inner magnetosphere at the earth's surface and another L shell increases monotonically with L when the inner magnetosphere has no strong gradient or a strong positive gradient of VA at its outer boundary. However, the relative phase is nearly zero and nearly 180 inside and outside a specific L shell, respectively, when the inner magnetosphere has a strong negative gradient at its outer boundary. The one-dimensional cavity-mode type resonance of the inner magnetosphere is certainly a cause of equatorial Pi2 pulsations. However, some constituents of the Pi2's may be not cavity-mode oscillations but quasi-steady-state oscillations forced by some damped sinusoidal waves incident on the outer boundary of the inner magnetosphere.

Language：English   Publishing type：Research paper (scientific journal)  

One-dimensional transient response of the inner magnetosphere at the magnetic equator is investigated using two models of altitude distribution of the Alfvén speed VA- The present paper concentrates on the transfer function of the system under consideration and its poles, which govern the transient response of the system. The poles, which are mathematical counterparts of the cavity resonances, appear owing to the inhomogeneity of VA and their locations depend on the altitude distribution of VA as well as the position of external source (or outer boundary of the inner magnetosphere). Even if there exists no strong Alfvén velocity gradient at the outer boundary, an observable cavity-mode oscillation in the Pi2 range can be excited because of the existence of a strong gradient of the plasmapause within the inner magnetosphere. However, the existence of a strong gradient at the outer boundary brings about a long-lived nature of the cavity-mode oscillation as well as calls some new poles into existence. While the surface of the solid earth forms the inner boundary at which the almost perfect reflection of wave takes place, the ionosphere is of secondary importance as a reflector of wave. The existence of the solid earth plays an essential role in the observability of the compressional oscillation arising from the cavity resonance all over the inner magnetosphere. The real part of each pole has a negative value, meaning that the cavity-mode oscillation decays with a damping factor of absolute value of the real part of the pole. Such a damping is primarily due to the leakage of energy through the outer boundary of the inner magnetosphere.

DOI： 10.5636/jgg.49.21

Language：English   Publishing type：Research paper (scientific journal)  

Two basic but novel equations directly describing the generation of shear Alfvén and compressional waves in the inner magnetosphere filled with a cold magnetized plasma are derived. The shear Alfvén wave is characterized by the field-aligned current and the compressional wave by the compressional component of the magnetic field. Such a generation arises from the effects of inhomogeneous Alfvén speed and curvilinear field line. Around the magnetic equator, if the Alfvén speed is inversely proportional to a power of the geocentric distance, these effects have magnitudes of the same order and their signs are identical. Considered in the present study is a situation that the earthward propagating compressional wave is launched from a large scale oscillating current wedge centered at midnight and symmetric about the magnetic equator. Then, it is found that the field-aligned current excited around the equator by the compressional wave has opposite senses in direction in the northern and southern hemispheres, in the pro- and post-midnight sectors as well as just inside the plasmapause and in its surrounding regions. As a result of the excitation of shear Alfvén wave, two types of oscillations appear on a field line: One is a forced oscillation and the other is an eigenoscillation. Although a modulation of the compressional wave may be caused locally (or microscopically) around the equator by the eigenoscillation of field line, the modulation can be globally (or macroscopically) neglected. So far as the propagation along the source longitude (source-earth line) around the equator is concerned, the coupling between compressional and shear Alfvén waves can be almost neglected and so one-dimensional response of the inner magnetosphere around the equator plays a significant role in the compressional oscillation.

DOI： 10.5636/jgg.48.1451

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Publisher：Space Weather  

The ionosphere over Iran is located on the upper edge of equatorial ionization anomaly (EIA). The Equatorial Electrojet (EEJ) as the proxy of temporal evolution of the EIA is explored using the MAGDAS stations near the magnetic equator during quiet and active geomagnetic conditions. The possible impact of EIA on Total Electron Content (TEC) over Iran is investigated. Analyzing data from 140 stations over 7 months, it is found that EEJ influences TEC and its latitudinal penetration. During geomagnetically active days, EEJ's behavior and TEC's latitudinal variations are investigated. Diurnal EEJ patterns and counter electrojets (CEJ) are analyzed, alongside solar wind timing effects on EEJ and TEC using data from stations in Brazil, Peru, the Philippines, and Sri Lanka. Statistical correlations between maximum TEC and parameters extracted from the daily EEJ profile, including EEJ (Formula presented.), integrated EEJ (IEEJ), and IEEJ (Formula presented.) —highlight Iran's position at the EIA's upper edge, indicating high susceptibility to equatorial dynamics. In addition, the Rate of Total Electron Content Index (ROTI) has been analyzed under geomagnetic active conditions. The results demonstrate that ROTI exhibits good agreement with TEC in terms of latitude penetration and the detected gradient in TEC. Employing Artificial Neural Networks (ANN) for local TEC prediction, Iran is segmented into eight regions based on GNSS receiver distribution. Each region's ANN model, trained during the quiet geomagnetic condition, assesses predictability and accuracy. The ANN model demonstrates reliable local TEC forecasting and confirms the direct impact of equatorial dynamics on Iran's ionosphere.

DOI： 10.1029/2024SW004032

Scopus

Publisher：Earthquake Science  

Among electromagnetic methods of short-term earthquake prediction, an approach is being actively developed based on the phenomenon of magnetic ultra-low-frequency (ULF) power depression occurring a few days before an earthquake. In particular, a nighttime geomagnetic power depression in the band 0.03–0.05 Hz was observed approximately 5 days before the catastrophic Tohoku 2011 earthquake. To verify the reliability of this method, we performed an extended analysis using data from magnetometer arrays JMA, MAGDAS, PWING, and INTERMAGNET. The selected stations included sites close to the epicenter (<300 km) and remote points (∼10000 km). The band-integrated spectral power of nighttime magnetic noise decreased significantly from March 6–9, several days before the earthquake. However, such variations occur simultaneously not only at nearby stations but also at distant stations. During this event, the ULF power depression was caused by low global geomagnetic activity, as evidenced by the planetary index SME. Thus, the depression of geomagnetic ULF noise cannot be considered a reliable short-term precursor.

DOI： 10.1016/j.eqs.2024.06.003

Web of Science

Scopus

Publisher：Atmosphere  

Ionospheric scintillation is a pressing issue in space weather studies due to its diverse effects on positioning, navigation, and timing (PNT) systems. Developing an accurate and timely prediction model for this event is crucial. In this work, we developed two machine learning models for the prediction of ionospheric scintillation events at the equatorial anomaly during the maximum and minimum phases of solar cycle 24. The models developed in this study are the Random Forest (RF) algorithm and the eXtreme Gradient Boosting (XGBoost) algorithm. The models take inputs based on the solar wind parameters obtained from the OMNI Web database from the years 2010–2017 and Pc5 wave power obtained from the Bear Island (BJN) magnetometer station. We retrieved data from the Scintillation Network and Decision Aid (SCINDA) receiver in Egypt from which the S4 index was computed to quantify amplitude scintillations that were utilized as the target in the model development. Out-of-sample model testing was performed to evaluate the prediction accuracy of the models on unseen data after training. The similarity between the observed and predicted scintillation events, quantified by the R2 score, was 0.66 and 0.74 for the RF and XGBoost models, respectively. The corresponding Root Mean Square Errors (RMSEs) associated with the models were 0.01 and 0.01 for the RF and XGBoost models, respectively. The similarity in error shows that the XGBoost model is a good and preferred choice for the prediction of ionospheric scintillation events at the equatorial anomaly. With these results, we recommend the use of ensemble learning techniques for the study of the ionospheric scintillation phenomenon.

DOI： 10.3390/atmos15101213

Web of Science

Scopus

Publisher：Research in Astronomy and Astrophysics  

A geomagnetic storm is a global disturbance of Earth's magnetosphere, occurring as a result of the interaction with magnetic plasma ejected from the Sun. Despite considerable research, a comprehensive classification of storms for a complete solar cycle has not yet been fully developed, as most previous studies have been limited to specific storm types. This study, therefore, attempted to present complete statistics for solar cycle 24, detailing the occurrence of geomagnetic storm events and classifying them by type of intensity (moderate, intense, and severe), type of complete interval (normal or complex), duration of the recovery phase (rapid or long), and the number of steps in the storm's development. The analysis was applied to data from ground-based magnetometers, which measured the Dst index as provided by the World Data Center for Geomagnetism, Kyoto, Japan. This study identified 211 storm events, comprising moderate (177 events), intense (33 events), and severe (1 event) types. About 36% of ICMEs and 23% of CIRs are found to be geoeffective, which caused geomagnetic storms. Up to four-step development of geomagnetic storms was exhibited during the main phase for this solar cycle. Analysis showed the geomagnetic storms developed one or more steps in the main phase, which were probably related to the driver that triggered the geomagnetic storms. A case study was additionally conducted to observe the variations of the ionospheric disturbance dynamo (Ddyn) phenomenon that resulted from the geomagnetic storm event of 2015 July 13. The attenuation of the Ddyn in the equatorial region was analyzed using the H component of geomagnetic field data from stations in the Asian sector (Malaysia and India). The variations in the Ddyn signatures were observed at both stations, with the TIR station (India) showing higher intensity than the LKW station (Malaysia).

DOI： 10.1088/1674-4527/ad5b34

Web of Science

Scopus

DOI： 10.35848/1347-4065/ad6659

Web of Science

Language：English  

DOI： 10.4049/jimmunol.2300450

Web of Science

PubMed

Publisher：Journal of Atmospheric and Solar-Terrestrial Physics  

Pc5 geomagnetic pulsations can accelerate electrons in the radiation belts, which can pose adverse threats to both astronauts and satellites in space. The estimation of Pc5 waves in space is crucial to radiation belt dynamics studies and will help mitigate these challenges. Here, we explore the advantages of the Feed-forward Neural Network (FFNN) and Random Forest (RF) algorithm for effective estimation of Pc5 geomagnetic pulsations observed in space at geostationary orbit during solar cycle 23. The dataset used in this study is the vector magnetic field measurements retrieved from the Geostationary Operational Environmental Satellite-10 (GOES-10) and the solar wind parameters: Bz and Vx component of the solar wind in the Geocentric Solar Ecliptic (GSE) coordinate system, proton density, flow pressure, and plasma beta obtained from the OMNI Web database during part of solar cycle 23. Pc5 geomagnetic pulsations were extracted from the toroidal component of the magnetic field time series using a bandpass Butterworth filter. The continuous wavelet transform (CWT) was utilized to study the characteristics of the extracted wave in the time-frequency domain for its validation. The validated Pc5 events were used as the target in the model's development, with the solar wind parameters as the inputs. In addition to the solar wind parameters, we included an attribute of the magnetic field time series as an input variable in the model. The dataset is carefully divided to ensure effective training and testing of the models. Finally, we trained both models using the same inputs and targets and explored their estimation abilities. The model was tested during the maximum, descending, and minimum phases of solar cycle 23. Both the FFNN and RF models have a similar estimation, with average cross-correlation score (R) values of 0.74 and 0.73 and corresponding average root mean squared error (RMSEs) of 0.16 nT and 0.67 nT, respectively. The model was deployed to investigate the response of Pc5 waves during three storm days in each testing year. The machine learning (ML) model outputs showed good coherence with the observed Pc5 waves. To validate the models, we studied the correlation between the estimated Pc5 events with the Kp index, and a good correlation was seen to exist between both events. This validates the good performance of the developed models. This work will aid in the study of radiation belt dynamics and the construction of electron depletion regions in the radiation belt.

DOI： 10.1016/j.jastp.2024.106258

Scopus

Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

Starting from 29 January 2022, a series of solar eruptions triggered a moderate geomagnetic storm on 3 February 2022, followed subsequently by another. Despite the typically minimal impact of unintense storms on space technology, 38 out of the 49 Starlink satellites underwent orbital decay, re‐entering Earth's atmosphere. These satellite losses were attributed to enhanced atmospheric drag conditions. This study employs numerical simulations, utilizing our test particle simulation code, to investigate the dynamics of the inner radiation belt during the two magnetic storms. Our analysis reveals an increase in proton density and fluxes during the transition from the recovery phase of the first storm to the initial phase of the second, primarily driven by intense solar wind dynamic pressure. Additionally, we assess Single Event Upset (SEU) rates, which exhibit a 50% increase in comparison to initial quiet conditions.

DOI： 10.1029/2023SW003789

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Language：English  

DOI： 10.1038/s41598-024-65420-7

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DOI： 10.1016/j.jcrysgro.2023.127553

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

This paper investigates the ionospheric responses to the February and April 2023 geomagnetic storms using Total Electron Content (TEC) derived from the 10 Global Navigation Satellite System (GNSS) stations at the transition of low to mid-latitudes (Approx. 20 ∼ 30°N) across different longitudes in the American and Asian sectors. Significant variations in both sectors were recorded during the recovery phase and were mainly attributed to the Prompt Penetration Electric Field (PPEF), Disturbance Dynamo Electric Field (DDEF) and the thermospheric neutral composition changes as O/N2 depletion. Similarly, the American and Asian sectors showed TEC enhancements, primarily attributed to the PPEF, during the main phase of the April storm. The negative TEC variations during the recovery phase were observed due to the DDEF and changes in O/N2. In the Asian sector, positive and negative variations were recorded over Pakistan and China, respectively, during the main phase of the April storm. These variations resulted in high and low concentrations of the O/N2 ratio due to Magnetosphere-Ionosphere-Thermosphere coupling.

DOI： 10.1016/j.asr.2023.12.039

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Language：English   Publisher：ALife Robotics Corporation Ltd.  

Measuring the altitude distribution of electron density in the upper atmosphere, known as the ionosphere, using HighFrequency radio wave reflections often causes the low signal-to-noise ratio of ionospheric echoes due to radio frequency interference. We propose a model for converting low-signal-to-noise-ratio ionospheric echo video images (Ionogram) into noise-reduced images using image processing techniques, for tracing the ionospheric echoes from Ionogram. The proposed method consists of three processing parts: noise removal optimized for individual Ionogram images, extraction of ionospheric echoes by penalized background subtraction technique, and fine-tuning of ionospheric echo signals using a minimum spanning tree algorithm. The proposed model successfully reproduces fine Ionograms with 98% recall and 99% precision.

DOI： 10.5954/icarob.2024.gs5-3

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DOI： 10.1016/j.jcrysgro.2023.127550

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

The physical destruction and fatalities caused by earthquake events has compelled scientists to develop a method for predicting Earthquakes. It is almost impossible to detect earthquake events due to limited seismometer sensitivity; therefore, a non-seismological predictor was established by using the Ultra-Low Frequency (ULF) magnetic field as a potential earthquake precursor. ULF waves respond to magnetic activity that are transferred to Earth by solar winds, and become ultimately linked to earthquake events. The statistical correlation analysis is used between ULF Pc4 and Pc5 pulsation, solar wind parameters; IMF- component (Bz), solar wind speed (Vsw), solar wind pressure (Psw), solar wind input energy (IEsw), and near-equator geomagnetic storm (SYM/H ). Through a multiple regression modelled approach, the relationship between solar wind (SW) parameters (IMF-Bz, Vsw, Psw, IEsw) and near-equator geomagnetic index (SYM/H ) with ULF wave pulsations (Pc4 - Pc5) prior to an earthquake (EQ) event (magnitude, M = 3.0–6.0, depth, d < 100 km, epicentre distance from magnetometer station, r < 100 km) was determined. The solar wind and geomagnetic index parameters were obtained from OMNIWeb, where ULF pulsations (Pc4 - Pc5) were computed from the magnetometer Davao station (Philippines) (7.00oN, 125.40oE) located in a low latitude region. The ULF Pc5 band demonstrated the best fit for the ULF earthquake precursor model in relation to solar wind parameters (Vsw, Psw) and geomagnetic index (SYM/H ) with R2 and R2-adjusted values of (R2 =0.5011, R2-adj = 0.4940). The Vsw, Psw and SYM/H are major factors that contribute to the emission of ULF Pc5 prior to an earthquake, as determined by the stepwise regression method. This ULF earthquake precursor model is intended to address the problem seismologists face when predicting seismic events by assisting non-seismologists in locating the most effective EM-ULF wave bands in solar-terrestrial activities for detecting seismic events.

DOI： 10.1016/j.asr.2023.10.036

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Publishing type：Research paper (international conference proceedings)   Publisher：Springer Proceedings in Physics  

This study elucidates the development of the global three-dimensional current system from polar to equatorial regions during substorms. We extracted 50 cases from 1934 isolated substorms between 2006 and 2013. These 50 cases met two criteria: the electric field was calm for 1 h before the substorm, and clear electric field fluctuations were observed during substorm onset. We compared the 210 MM and mid-latitude magnetic field data to capture each event's distribution and temporal evolution of the substorm current wedge. Additionally, we captured the electric field response using the mid-latitude Paratunka station. The results revealed that the electric field amplitude was proportional to the SuperMAG aurora electrojet index magnitude (correlation coefficient = 0.67). In the central region of the substorm current wedge (SCW), predominantly westward electric fields were observed between the upward and downward field-aligned currents (FACs) (i.e., among 12 cases, wherein the observation points were at the center of the SCW, 10 showed westward electric fields). Moreover, a tendency for larger amplitudes of these westward electric fields was observed at the SCW center. This may reflect the projection of the primary potential formed by the charges injected into the upward and downward FAC regions on the ionosphere. Contrarily, a tendency for eastward electric fields was observed near the upward FAC of the SCW (i.e., among 24 cases, wherein the observation points were near or slightly inside the upward FAC, 14 showed eastward electric fields), which might be due to the twisting of the potential distribution associated with Hall polarization.

DOI： 10.1007/978-981-97-0142-1_7

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Publishing type：Research paper (international conference proceedings)   Publisher：Springer Proceedings in Physics  

It is well known that the geomagnetic field exhibits consistent diurnal, seasonal, and solar cycle fluctuations. In this paper, the seasonal variations of the quiet day (Sq) of the geomagnetic field in the recent solar cycle were obtained to characterise the profiling behaviour of the geomagnetic field using the horizontal component (H-component) at Langkawi. Long-term data analysis was limited to Magnetic Data Acquisition System (MAGDAS) data from 2011, 2012, 2013, 2016, and 2018. It was discovered that the seasonal variations of the geomagnetic field component during Sq demonstrated Langkawi as an equatorial station where the horizontal component of the geomagnetic field, MSq(H), peaked during the equinox for most of the year. The value is comparable to the predicted value by the model of the International Geomagnetic Reference Field (IGRF) at 41,794 nT. This information can be used to improve the accuracy of geomagnetic models and predictions, as well as gain a better understanding of the regular variations of Sq in Langkawi, one of equatorial observatories in Southeast Asia.

DOI： 10.1007/978-981-97-0142-1_12

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Publishing type：Research paper (international conference proceedings)   Publisher：Springer Proceedings in Physics  

During solar storms, conductivities in Earth’s ionospheric regions become more enhanced. This phenomenon produces effects on Earth’s current systems to which emissions of solar radiation largely contribute. Such radiations are analyzed as short timescale variations of the X-ray and EUV fluxes. This was observed for the two X- and three M-class solar flares on 17, 20, and 21 April 2022. All of which are geomagnetically quiet days with nominal Kp, Hp, and Dst indices. Magnetic responses were observed from magnetometers in the Philippines, located in the low-latitude (MUT) and near-equatorial (DAV) stations. These coexisting variations observed with Earth’s magnetic field are referred to as magnetic crochets. Measurements also showed that solar flare effects (Sfe) are more pronounced near the equator, where high conductivity generates more variations in Earth’s magnetic fields. Yet, the largest variations were not consistently observed for the strongest flares. At MUT, manifest variations in Hsfe (horizontal) and Zsfe (downward) were aligned with the X1.1 and M1.89 flares on day 17. Variations are generally homogeneous on days 20 and 21. Meanwhile, the largest variations at DAV were found during the M9.7 flare on day 21. It is inferred that increases in Hsfe remain greater than in Zsfe at both stations. Fluctuations in Hsfe are nonetheless more prolonged and heightened at DAV station.

DOI： 10.1007/978-981-97-0142-1_9

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Publishing type：Research paper (scientific journal)   Publisher：Instituto Nazionale di Geofisica e Vulcanologia, INGV  

Fluctuations in Solar Wind (SW) and Interplanetary Magnetic Field (IMF) significantly affect the Geomagnetic Field (GF) measurements particularly during extreme space weather events. The current study investigates the variations of horizontal (H), vertical (Z) geomagnetic components under quiet and disturbed IMF and solar conditions as well as the effect of underground conductivity on the stationary geomagnetic measurements in Japan. Results of data analysis show that the response of the GF to IMF and solar parameters fluctuations is variable. The H- and Z-components were in good harmony with high visual correlation along a latitudinal profile across Japan during quiet times. On the other hand, during the disturbed times related to a Coronal Mass Ejection (CME) launched on 13 May 2005, the GF components varied with the disturbance of IMF and solar parameters. The H-components showed highly correlated variations with a significant reduction along the examined profile due to intensified ionospheric currents, while the Z-components recorded in the northern part of Japan showed abnormal daily variations pattern (positive daily variations) with an enhanced amplitude which is opposite to the normal behavior of daily variations recorded in the central and southern parts of Japan (negative daily variations). The observed enhanced and abnormal daily variation of Z-components in north Japan, which we consider a remarkable observation here, is possibly linked with underground conductivity anomaly in this region.

DOI： 10.4401/ag-9021

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Publisher：IEEE Symposium on Wireless Technology and Applications, ISWTA  

Geomagnetic storms impact communication technology by disturbing the ionosphere, affecting GPS signal propagation, and causing navigation errors. These storms also induce voltages in long conductors such as communication cables, power lines, and pipelines through geomagnetically induced currents (GICs), leading to potential disruptions. This study addresses the gap in understanding geomagnetic field variations in low-latitudes, where infrastructure is less equipped for GIC disruptions. Furthermore, limitations in the availability of magnetic field components often necessitate the use of dB/dt as a GIC proxy for this region. This study aims to assess the relationship between the rate of change of the total magnetic field (dB/dt) and the horizontal magnetic field component (dH/dt) across different latitudes during the geomagnetic storm of 22-23 June 2015. Utilizing data from high (Narsarsuaq, Greenland), mid (Boulder, USA), and low (Alibag, India) latitude stations through correlation, cross-correlation, and linear regression analyses. The results reveal significant latitude dependent differences with strong correlations at mid and low latitudes. A strong linear correlation between dB/dt and dH/dt was observed, particularly at mid and low-latitudes. Cross-correlation analysis reveals synchronous responses at zero lag, indicating highly predictable relationships at lower latitude regions. Regression analysis demonstrates greater predictability of dB/dt and dH/dt at lower latitudes than at higher latitudes. These findings indicate that dB/dt is an effective proxy for GIC studies in low-latitude regions and enhance our understanding of geomagnetic impacts on infrastructure and highlight the importance of developing strategies tailored to different latitudes. This research contributes to the field by providing new insights into the use of dB/dt as a reliable measure for assessing GIC risks in low-latitude regions.

DOI： 10.1109/ISWTA62130.2024.10651826

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Publishing type：Research paper (international conference proceedings)  

Measuring the altitude distribution of electron density in the upper atmosphere, known as the ionosphere, using High- Frequency radio wave reflections often causes the low signal-to-noise ratio of ionospheric echoes due to radio frequency interference. We propose a model for converting low-signal-to-noise-ratio ionospheric echo video images (Ionogram) into noise-reduced images using image processing techniques, for tracing the ionospheric echoes from Ionogram. The proposed method consists of three processing parts: noise removal optimized for individual Ionogram images, extraction of ionospheric echoes by penalized background subtraction technique, and fine-tuning of ionospheric echo signals using a minimum spanning tree algorithm. The proposed model successfully reproduces fine Ionograms with 98% recall and 99% precision.

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Publisher：Proceedings of International Conference on Artificial Life and Robotics  

Measuring the altitude distribution of electron density in the upper atmosphere, known as the ionosphere, using High- Frequency radio wave reflections often causes the low signal-to-noise ratio of ionospheric echoes due to radio frequency interference. We propose a model for converting low-signal-to-noise-ratio ionospheric echo video images (Ionogram) into noise-reduced images using image processing techniques, for tracing the ionospheric echoes from Ionogram. The proposed method consists of three processing parts: noise removal optimized for individual Ionogram images, extraction of ionospheric echoes by penalized background subtraction technique, and fine-tuning of ionospheric echo signals using a minimum spanning tree algorithm. The proposed model successfully reproduces fine Ionograms with 98% recall and 99% precision.

Scopus

Publishing type：Research paper (international conference proceedings)   Publisher：Springer Proceedings in Physics  

Coronal mass ejection (CME) occurs in the Sun’s corona that travels and reaches Earth’s magnetosphere, and it can trigger a geomagnetic storm. In this work, we presented a case study of a geomagnetic storm triggered by CME and observed the presence of ionospheric disturbance dynamo (Ddyn) at the equatorial station. Our analysis utilized CME data from the SOHO/LASCO CME Catalog, which contains information such as CME speed, solar source location, digital data, and the CME LASCO/C2 movies. The Disturbance storm time (Dst) index calculated from ground-based magnetometer data describes the geomagnetic storm phases. On the other hand, the variation of the Ddyn at the equatorial region was derived from the H component of Earth’s magnetic field. Four stations at the dip equator cover the longitude sectors of Asia (DAV and TIR) and Africa (AAB and ILR). This study shows that halo CME on 4 August 2011 triggered intense geomagnetic storms (Dst = − 115 nT) on 6 August 2011. During this geomagnetic storm, after a few hours of Dst peak, the signature of Ddyn can be clearly observed in two stations of the African sector (ILR and AAB), while no presence of Ddyn in two other stations (TIR and DAV) in the Asian sector.

DOI： 10.1007/978-981-97-0142-1_23

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Language：Others   Publishing type：Research paper (scientific journal)  

The field of space weather research has witnessed growing interest in the use of machine learning techniques. This could be attributed to the increasing accessibility of data, which has created a high demand for investigating scientific phenomena using data-driven methods. The dataset, which is based on bibliographic records from the Web of Science (WoS) and Scopus, was compiled over the last several decades and discusses multidisciplinary trends in this topic while revealing significant advances in current knowledge. It provides a comprehensive examination of trends in publication characteristics, with a focus on publications, document sources, authors, affiliations, and frequent word analysis as bibliometric indicators, all of which were analysed using the Biblioshiny application on the web. This dataset serves as the document profile metrics for emphasising the breadth and progress of current and previous studies, providing useful insights into hotspots for projection research subjects and influential entities that can be identified for future research.

DOI： 10.1016/j.dib.2023.109667

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

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During a few solar energetic particle (SEP) events, solar protons were trapped within the geomagnetic field and reached the outer edge of the inner radiation belt. We reproduced this phenomenon by modeling the proton flux distribution at the Low‐Earth Orbit (LEO) for different geomagnetic conditions during solar particle events. We developed a three‐dimensional relativistic test particle simulation code to compute the 70–180 MeV solar proton Lorentz trajectories in low L‐shell range from 1 to 3. The Tsyganenko model (T01) generated the background static magnetic field with the IGRF (v12) model. We have selected three Dst index values: −7, −150, and −210 nT, to define quiet time, strong, and severe geomagnetic storms and to generate the corresponding inner magnetic field configurations. Our results showed that the simulated solar proton flux was more enhanced in the high‐latitude regions and more expanded toward the lower latitude range as long as the geomagnetic storm was intensified. Satellite observations and geomagnetic cutoff rigidities confirmed the numerical results. Furthermore, the LEO proton flux distribution was deformed, so the structure of the proton flux inside the South Atlantic Anomaly (SAA) became longitudinally extended as the Dst index decreased. Moreover, we have assessed the corresponding radiation environment of the LEO mission. We realized that, for a higher inclined LEO mission during an intense geomagnetic storm (Dst = −210 nT), the probability of the occurrence of the Single Event Upset (SEU) rates increased by 19% and the estimated accumulated absorbed radiation doses increased by 17% in comparison with quiet conditions.

DOI： 10.1029/2023SW003664

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DOI： 10.22541/essoar.170000392.22424750/v1

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Indian Journal of Physics  

The solar surface activity produces space weather. Solar flares, coronal mass ejections (CMEs), and coronal holes are examples of solar activity phenomena that can compress the magnetosphere and cause a geomagnetic storm. Previous studies have indicated that these solar events disturb the near-Earth region. This study examines the behavior of the terrestrial geomagnetic field (H-component) and focuses on how the geomagnetic index (SYM/H) responds to solar wind parameters related to solar activity. Results showed that on September 9, 2017, a powerful geomagnetic storm (SYM/H = 200 nT) was observed that was brought on by powerful solar flares, CMEs, and coronal holes that happened on September 7 and 8, 2017, which generated significant alterations in the magnetosphere’s solar wind. On October 3 and 18, 2017, a moderate geomagnetic storm occurred, which resulted in a strong southbound situation with a high solar wind pressure that blasted high-density plasma due to CME associated with a coronal hole event into the magnetosphere. A moderate geomagnetic storm (SYM/H = 80 nT) on November 14, 2017, was triggered, according to findings, by a wider direction and high speed of CME with CIR event occurrence implies the accelerated large-scale solar wind that exacerbated the reconnection of the flowing interplanetary magnetic field into the inner magnetosphere. There were fewer injections of ions and energetic solar particles into the magnetosphere on December 12, 2017, which were thought to be a source of a weak geomagnetic storm(SYM/H = 49 nT). Additionally, the response of solar wind parameters and geomagnetic storm due to solar flare, CMEs and coronal holes to the modest drop in the ground geomagnetic field resulted in H-component reading at the RAN and DLT station. This study has contributed to the study of earthquake precursors in ultra-low frequency (ULF) emission with the effect of solar wind.

DOI： 10.1007/s12648-023-02721-z

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Annals of Geophysics  

Studying Pc3-4 geomagnetic pulsations at equatorial and very low latitude regions is an important issue to understand their generation and propagation mechanisms. Pc3-4 amplitudes and their latitudinal dependency across the dip equator up to low latitudes (± 25°) are investigated using geomagnetic data simultaneously obtained by the MAGDAS/CPMN stations along the 210° Magnetic Meridian (MM) chain. Forty-five Pc3 events and thirty-two Pc4 events were selected for this study. Our results show a clear dependence of Pc3-4 amplitudes on geomagnetic latitudes. At the dip equator, most of the selected Pc3 events (~75%) showed an enhancement in amplitudes, while the rest (~25%) showed an attenuation. After that, the amplitudes decreased gradually by increasing latitudes. These results suggest mixed generation and propagation mechanisms for the equatorial and very low latitudes Pc3s. For better understanding, the Interplanetary Magnetic Field (IMF) and solar conditions are examined during the selected events. Results indicate that Pc3 events with enhanced amplitudes at dip equator are mainly occurred in daytime with no preference to IMF (magnitude and direction) and solar parameters, which suggests the ionospheric currents model as a generation and propagation mechanism for these events. While the attenuation observed in the other Pc3 events was associated with intense and abrupt fluctuations in the IMF and solar parameters, which in turn suggests the compressional wave model for generating these Pc3 events. On the other hand, these two models can explain the observed enhancement in the Pc4 amplitudes at the dip equator. Therefore, our obtained results clarified the origin of equatorial Pc3-4 pulsations.

DOI： 10.4401/ag-8979

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

The east–west electric field and the characterized horizontal configuration of the geomagnetic fields around the equatorial region cause electron redistribution in the ionospheric bottomside F- region. This electric field E in turn causes the flow of current in a narrow band around this region called Equatorial Electrojet (EEJ) current. The paper discusses the robust least square modelling approach for modelling some selected ionospheric profile parameters below the maximum height of the F2 layer from the computed EEJ values at Jicamarca station (12°S, 76.9 °W) in the Southern American sector and Ilorin (8.50°N, 4.68°E) in the West African sector. The modelled parameters are the maximum electron density, NmF2, the maximum height, hmF2 and the thickness parameter, B0 of the F2- layer which were then compared with the recent ionospheric international standard model (IRI-2016). The proposed new model for ionospheric parameters based on ground-based magnetic field data at specific locations improved the model fit for hmF2 with RMSE value ∼ 30 km, an improvement to the IRI hmF2 model options. In addition, the derived NmF2 model performs poorly as compared to the IRI URSI and CCIR model options for NmF2. The model was however able to reproduce the temporal changes in the NmF2, for example, the semi-annual variation in the observational NmF2 value. The specific local features that were not fully reproduced in the IRI B0 model options can be seen in the derived model.

DOI： 10.1016/j.asr.2023.04.040

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Sains Malaysiana  

This study investigated the simultaneous effects of solar flares (SFs) on both the D and E layers of Earth’s ionosphere. The analysis focused on the M and X-class SFs that occurred during the 24th solar cycle as these two classes of SFs are known to produce significant effects on Earth’s environment, particularly during the daytime period. The data utilized to detect the SF events in this study were ground-based magnetometer data from the equatorial regions. Effects of the selected SF events on the E layer were investigated based on the EUEL index constructed using the geomagnetic data. Meanwhile, the changes in the strength of radio VLF signals in the D ionospheric layer during the selected SF events were monitored using Sudden Ionospheric Disturbance (SID) data. Two case studies were performed which consisted of four SF events from a total of 23 events that were detected by geomagnetic data during the period of study. Further analysis on the selected SF events showed the common effects of SFs on the D layer, which is the increment on the VLF signal measured from the SID stations although a different response was detected in the EUEL index variations. This indicates that the VLF signal always shows an increment even though a decrement in the ionization of the E layer occurs as a result of the SF events. The difference in responses could be attributed to the distinct changes in electron density of both layers during the SF occurrence. Further studies are needed to elucidate the underlying mechanism responsible for this unique response, utilizing appropriate parameters such as total electron content, as well as the electron density data to thoroughly analyze the ionospheric response during SF events.

DOI： 10.17576/jsm-2023-5208-11

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

Geomagnetic micropulsations are ultra-low frequency (ULF) signals observed in the magnetosphere as well as on the ground. These signals offer us an effective means to study the coupling of the magnetosphere-ionosphere processes in the space field. The most prominent observed type of such ULF waves is the Pc5 pulsations (with a frequency range of 1–7 mHz), known to have their maximum amplitude in the auroral oval. The low magnitude of Pc5 signals withstands against distinguishing them from the background noise. This study presents a machine learning approach for the automatic detection of geomagnetic Pc5 pulsations in the auroral zone using artificial neural networks (ANN) guided by discrete wavelet transform. Our ANN algorithm is validated and tested using a huge amount of datasets of auroral ground-based geomagnetic records from the Svalbard network during the two solar cycles 23 and 24. The wavelet-based coherence was used to determine the signal's consistency detected by the magnetometer stations of the Svalbard network; since they are sensitive to all sorts of space wave-related phenomena that left their footprint on the magnetic field time series. The Daubechies wavelet transform was utilized to classify and extract Pc5 signals from the artificial noise and the results are correlated with the geomagnetic pulsation records as detected by our ANN-based model. The ANN-based model showed a good correlation of an average of 98% for the different phases of the two studied solar cycles. The statistical regression analysis of the post-processed results yielded a high coefficient of determination of R2 = 0.9 and norms of residuals of 8–21 nT. The Pc5 events detected by the ANN-based algorithm during the two solar cycles showed a good correlation with the Kp index, which enables our model for space weather forecasting.

DOI： 10.1016/j.asr.2022.06.063

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

This study presents the first results of a newly installed ground-based station Alex Magnetometer and Telluric Station (AMTS) located in Alexandria governorate in Egypt (geographic coordinate: 30° 51.68′ N, 29° 33.67′ E). The station consists of a fluxgate digital magnetometer and a two-channel digital Long Period Telluric Recorder (LPTR). The station commenced operation on October 12th, 2021, by providing real-time data of the geoelectric and geomagnetic field components. Despite the low solar activity, the AMTS detected several signatures of geomagnetic storms. An illustrative case study was conducted on the strong geomagnetic storm of November 4th, 2021. The magnetic field components recorded by the station demonstrated a prompt increase synchronized with the Sudden Storm Commencement (SSC) captured by the ACE satellite arriving at 19:42 UT on November 3rd, 2021. The three geomagnetic field components recorded by the AMTS during the storm were consistent with the nearby INTERMAGNET stations. The digital fluxgate magnetometer's high sampling frequency (400 Hz) and the LPTR with a sampling frequency (1 Hz), allowed us to capture all ULF pulsations. The structural pearl-shaped Pc1 pulsation was obtained in all phases of the geomagnetic storm with repetitive oscillations in the main storm phase. The wavelet analysis of micropulsations recorded at AMTS showed a high coherence with the nearby-four INTERMAGNET stations demonstrating high data integrity and reliability. The magnetometer and telluric measurements at the AMTS will fill the data gap in the northern section of Africa and allow us to study geoelectric and geomagnetic micropulsations in the region.

DOI： 10.1016/j.asr.2022.09.014

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A novel quantum algorithm for solving the Boltzmann-Maxwell equations of the
6D collisionless plasma is proposed. The equation describes the kinetic
behavior of plasma particles in electromagnetic fields and is known for the
classical first-principles equations in various domains, from space to
laboratory plasmas. We have constructed a quantum algorithm for a future
large-scale quantum computer to accelerate its costly computation. This
algorithm consists mainly of two routines: the Boltzmann solver and the Maxwell
solver. Quantum algorithms undertake these dual procedures, while classical
algorithms facilitate their interplay. Each solver has a similar structure
consisting of three steps: Encoding, Propagation, and Integration. We conducted
a preliminary implementation of the quantum algorithm and performed a parallel
validation against a comparable classical approach. IBM Qiskit was used to
implement all quantum circuits.

DOI： 10.22541/essoar.168748458.80915597/v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

The presence of the dawn-to-dusk electric field of about 4 mV/m in the Jovian inner magnetosphere and its response to the enhancement of the solar wind dynamic pressure are still a mystery of the rotation-dominated Jovian magnetosphere. Previous studies have suggested that magnetosphere-ionosphere (M-I) coupling via Region 2-like (R2-like) field-aligned current (FAC) could be the origin of the Jovian dawn-to-dusk electric field. This study investigates whether the dawn-to-dusk electric field is formed from this scenario by using a Jovian ionosphere model and a two-dimensional ionospheric potential solver. Our results show that the dawn-dusk asymmetry in the ionospheric potential form even at middle latitudes and that the dawn-to-dusk electric field is induced in the inner magnetosphere if the electric potential is mapped to the magnetospheric equatorial plane. Around the Io orbit, the calculated electric field strength for the ionosphere without meteoroid influx is too large, 200 mV/m at dawn and 88 mV/m at dusk. One of the solutions is to consider long-lived meteoric ions in the Jovian ionosphere, which reduce the electric field strength to 15 mV/m at dawn and 12 mV/m at dusk. The model also shows that the electric field strength increases with the intensity of R2-like FAC, consistent with its response to the solar wind dynamic pressure observed by the Hisaki satellite.

DOI： 10.1029/2022ja031248

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Language：English   Publisher：The Japanese Association of Rehabilitation Medicine  

<b>Objectives: </b>Increased long-term impairment is common among intensive care unit (ICU) survivors. However, predictors of activities of daily living (ADL) in ICU survivors are poorly understood. We aimed to focus on the trajectory of physical function and explore the clinical variables that affect ADL at hospital discharge.

<b>Methods: </b>We enrolled 411 patients admitted to the ICU from April 2018 to October 2020. Physical function was evaluated at ICU admission, ICU discharge, and hospital discharge. We assessed physical function (grip strength, arm and calf circumference, quadriceps thickness, and Barthel index). Patients were assigned to the high or low ADL group based on their Barthel index at discharge. Propensity score matching analysis was performed to minimize selection biases and differences in clinical characteristics.

<b>Results: </b>After matching propensity scores, 114 of the 411 patients (aged 65±15 years) were evaluated. The high ADL group showed better physical function at ICU discharge and hospital discharge than the low ADL group. An overall decreasing trend in muscle mass was observed over time; the rates of decline were lower in the high ADL group than in the low ADL group. The cutoff values for relative changes in calf circumference and quadriceps thickness to predict high ADL were −7.89% (sensitivity: 77.8%, specificity: 55.6%) and −28.1% (sensitivity: 81.0%, specificity: 58.8%), respectively.

<b>Conclusions: </b>The relative decreases in calf circumference and quadriceps thickness during hospitalization were lower in patients who maintained their ADL. Assessment of the trajectory of physical function can predict ADL status at hospital discharge among ICU survivors.

DOI： 10.2490/prm.20230010

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-20K19447/

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Publishing type：Research paper (scientific journal)   Publisher：ASM Science Journal  

The physical destruction and fatalities caused by earthquake events compelled scientists to develop Earthquake predictions. Due to limited seismometer sensitivity, it was impossible to detect earthquake events; therefore, non-seismological is established, which in Ultra Low Frequency (ULF). The study of electromagnetic waves (EM) in the Ultra-Low Frequency (ULF) range is a promising tool for investigating seismomagnetic effects that act as an earthquake precursor. This study analysed the ULF frequency range as a short-term earthquake precursor at a depth < 100 km and an epicentral distance < 100 km (distance from Cebu magnetometer station) measured using a ground magnetometer installed in Cebu (10.36°N, 123.91°E), in the Philippines. This study also intended to determine the emission of magnetic pulsation (Pc4 and Pc5), solar wind parameters and near equatorial geomagnetic storms (SYM/H) in low latitude regions prior to an earthquake event. Findings show that the most evident ULF that acts as a potential earthquake precursor was at a frequency range of Pc5 (1.7-6.7 mHz) compared to Pc4 (6.7-22 mHz). It also shows that high solar wind changes and geomagnetic storms respond to the emission of ULF magnetic pulsations (Pc4 and Pc5) prior to earthquake events at low latitudes. Thus, it can be concluded that magnetic pulsations are signatures that indicate the probability of short-term earthquakes precursor.

DOI： 10.32802/ASMSCJ.2023.1189

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Publishing type：Research paper (scientific journal)   Publisher：Advances in Space Research  

Pc5 geomagnetic pulsations are ultra-low frequency (ULF) waves whose period lies within 150–600 s. Their detection is crucial for accurate space weather modelling, monitoring, and analysis. Identification of these pulsations using manual approaches is difficult and time-consuming because of the smaller magnitudes and the limited durations within which they occur. To overcome this challenge, we propose a robust Cascade Forward Neural Network (CFNN) model combined with the wavelet technique for automatically detecting Pc5 events from satellite datasets observed at geostationary orbits. The dataset used in this study is the magnetic field vector measurements retrieved from the Geostationary Operational Environmental Satellite-10 (GOES-10) from 2000 to 2009. Pc5 geomagnetic pulsations were extracted from the toroidal component of the field perturbation using a bandpass Butterworth filter. Continuous wavelet transform (CWT) analysis using the mother Morlet wavelet was utilized to validate the integrity of the extracted signal in the time–frequency domain. The extracted Pc5 signal was decomposed into details and approximations using Daubechies wavelet transform to separate the intelligible signal from the incoherent noise that left their trace on the magnetic field time series. The detail signal was subjected to denoising using the heuristic Stein Unbiased Risk Estimate (SURE) approach with soft thresholding to obtain the denoised Pc5 events. The denoised Pc5 events were utilized as the target in the machine learning whereas the inputs were obtained from 5-dimensional space transformation of the toroidal component of the time series. The developed algorithm performed well and demonstrated a detection accuracy of 80 % and a Root Mean Square Error (RMSE) of 0.13 nT indicating a high performance in detecting Pc5 events. For effective validation of the model, Pc5 events detected by our model were correlated with the Kp index and the amplitude of the Pc5 events observed at geostationary orbit. A good correlation was obtained in both cases, making our model a practical and preferred choice for Pc5 pulsation detection in contrast to conventional frequency analysis tools which take much time for signal processing on large data sets.

DOI： 10.1016/j.asr.2023.11.001

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CiNii Research

DOI： 10.2113/RGG20214362

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Language：English  

DOI： 10.3389/fcimb.2022.925444

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PubMed

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The intensity of the equatorial electrojet (EEJ) shows temporal and spatial variability that is not yet fully understood nor accurately modeled. Atmospheric solar tides are among the main drivers of this variability but determining different tidal components and their respective time series is challenging. It requires good temporal and spatial coverage with observations, which, previously could only be achieved by accumulating data over many years. Here, we propose a new technique for modeling the EEJ based on principal component analysis (PCA) of a hybrid ground-satellite geomagnetic data set. The proposed PCA-based model (PCEEJ) represents the observed EEJ better than the climatological EEJM-2 model, especially when there is good local time separation among the satellites involved. The amplitudes of various solar tidal modes are determined from PCEEJ based tidal equation fitting. This allows to evaluate interannual and intraannual changes of solar tidal signatures in the EEJ. On average, the obtained time series of migrating and nonmigrating tides agree with the average climatology available from earlier work. A comparison of tidal signatures in the EEJ with tides derived from neutral atmosphere temperature observations show a remarkable correlation for nonmigrating tides such as DE3, DE2, DE4, and SW4. The results indicate that it is possible to obtain a meaningful EEJ spectrum related to solar tides for a relatively short time interval of 70 days.

DOI： 10.1029/2022JA030691

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

Studies on counter-electrojet currents (CEJ) using ground data revealed that this current could occur simultaneously among locations that are less than 30 degrees longitude apart. In our work, the symmetricity of CEJ variation between the west and east of Southeast Asia, separated by similar to 25 degrees, was preliminarily examined according to its types: morning (MCEJ) and afternoon (ACED. Since most of the past studies had overlooked the occurrence after dusk, the monitoring period was also extended from 18:00 to 21:00 LT, namely, the post-sunset depletion (PSD). The magnetometer station in Davao, Philippines (DAV) and Langkawi, Malaysia (LKW) were chosen to represent the east and west parts. The EEJ index (i.e., EUEL) over the periods of the solar cycle 24 (2008-2018) was utilized specifically during magnetically quiet days (Kp < 3). As the result, both parts symmetrically showed that MCEJ and ACEJ were positively and negatively correlated with the F10.7 index. Contrarily, MCEJ and ACEJ were asymmetrically prominent in the east and west. CEJ types also varied symmetrically with the season, especially for MCEJ and ACEJ (at high level), prominent during Equinox and J-solstice. Post-sunset depletion (PSD) in both parts was symmetrically solar activity independent, as no correlation with the F10.7 index was observed in the extended observation. PSD that varied symmetrically with season was also solar activity independent, except in the east during Equinox, where it was negatively correlated with the F10.7 index. Our finding also revealed that PSD was prominent during Equinox, except for the high level in the west part.

DOI： 10.3390/app12147138

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

DOI： 10.1029/2022gl098105

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022GL098105

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Journal of Geophysical Research: Space Physics  

The equatorial ionosphere endured plasma density irregularities during a geomagnetic storm on 21–22 December 2014. To understand the underlying mechanism, we analyzed the rate of the total electron content change (ROTI) data obtained from a global navigation satellite system, along with solar wind, interplanetary magnetic field (IMF), geomagnetic indices, Jicamarca incoherent scatter radar, and magnetometer data. The results indicate that the ROTI enhancement related to plasma density irregularities (plasma bubbles) occurred three times in the equatorial and low latitude regions of the American sector during the geomagnetic storm. The first, second, and third enhancements which have a longitudinal extent of ∼20° appeared in the post-sunset, pre-midnight, and post-midnight sectors, respectively. The second enhancement occurred during the recovery phase of the storm-time substorm even though the IMF remained southward. During this period, the direction of the dayside equatorial electrojet (EEJ) changed from eastward to westward, while the nightside upward plasma velocity at Jicamarca increased to 28.8 m/s. The response of the EEJ and upward ion drift implies that the westward and eastward electric fields were intensified on the dayside and nightside, respectively. Therefore, these results suggest that an over-shielding electric field penetrates the dayside/nightside equator simultaneously in association with a substorm recovery phase, and that the electric field generates plasma bubbles by the Rayleigh-Taylor instability mechanism. Plasma bubbles induced by the penetration of an over-shielding electric field due to substorm activity have not previously been reported.

DOI： 10.1029/2021JA030240

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Effect of Meteoric Ions on Ionospheric Conductance at Jupiter
Ionospheric Pedersen and Hall conductances play significant roles in electromagnetic coupling between the planetary ionosphere and magnetosphere. Several observations and models have suggested the existence of meteoric ions with interplanetary origins in the lower part of Jupiter’s ionosphere; however, no models have considered the contributions of meteoric ions to ionospheric conductance. This study is designed to evaluate the contribution of meteoric ions to ionospheric conductance by developing an ionospheric model combining a meteoroid ablation model and a photochemical model. We find that the largest contribution to Pedersen and Hall conductivities occurs in the meteoric ion layer at altitudes of 350–600 km due to the large concentration of meteoric ions resulting from their long lifetimes of more than 100 Jovian days. Pedersen and Hall conductances are enhanced by factors of 3 and 10, respectively, in the middle- and low-latitude and auroral regions when meteoric ions are included. The distribution of Pedersen and Hall conductances becomes axisymmetric in the middle- and low-latitude regions. Enhanced axisymmetric ionospheric conductance should impact magnetospheric plasma convection. The contribution of meteoric ions to the ionospheric conductance is expected to be important only on Jupiter in our solar system because of Jupiter’s intense magnetic and gravitational fields.

DOI： 10.1029/2022JA030312

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2022JA030312

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We use the in-situ observations of DMSP and SWARM satellites to report the changes of the topside ionospheric electron temperature during the October 2016 storm. Electron temperature in the afternoon sector dramatically increases in low latitudes in the recovery phase of the storm. Furthermore, the temperature enhancements have an obvious dependence on longitude and are mainly centralized around 100°–150°E in different satellite observations. The temperature enhancements attain more than 2,000 K at 840 km and 1,500 K at 450 km around the magnetic equator. The decrease in the electron-ion collision cooling rate, resulting from the lessened topside electron density, could not fully explain the temperature enhancement. At the same time, the electron densities in crests of the equatorial ionization anomaly are suppressed drastically at 100°–150°E, which cause a less heat conduction effect from the equatorial topside ionosphere to low altitudes via magnetic field lines and heat the topside ionospheric electron temperature. Further analysis indicates that dayside westward disturbance dynamo electric field presents a significant longitude structure and is a primary driver for the topside ionospheric temperature enhancement during the storm.

DOI： 10.1029/2022JA030278

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The solar quiet (Sq) ionospheric current variations exhibit spatial and temporal patterns that can be identified by the prevailing eigenmodes based on the empirical orthogonal function (EOF) analysis. In this study, the Sq current function over the American and European/African sectors are derived using ground magnetometer data from 2006 to 2019 based on the spherical harmonic analysis technique. Subsequently, we decomposed the Sq current function into eigenmodes by applying the EOF analysis, where the first three eigenmodes capture 96% of the overall Sq current variance. Additionally, these eigenmodes are utilized to model the Sq current function and compare its properties between the two longitudinal sectors. We observed that the EOF model could reconstruct the observed Sq current function with the first three eigenmodes in both longitudinal sectors. Moreover, the EOF model unveils a clear association of the Sq current function with several driven features, such as magnetic latitude, local time, season, and solar activity. Both longitudinal sectors had comparable Sq current patterns under varying solar activity, while their amplitudes varied. Besides, the newly developed model could reproduce a refined Sq current variability over the two longitudinal sectors as long as the observed Sq variations have sufficient duration. Our EOF model shows that the variations of the Sq current function with solar activity can be explained by the first three eigenmodes, which could be used as a basis for further numerical modeling of the Sq current variations.

DOI： 10.1029/2021JA029903

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

Data imputation studies include reconstruction or estimation of imperfect data gaps caused by system sensing failure, and non-responsive data transmission remains an open issue. In space weather applications, imputation of ground electromagnetism is significant in capturing the complex interaction of sun–earth prior to the subsequent analysis of the space weather effects. Key contributions to the demonstration of supervised machine learning (ML) imputation approach with artificial neural network, K-nearest neighbour, support vector regression (SVR), and General Regression Neural Network (GRNN) for MAGDAS-9 ground electromagnetism dataset have not yet been established. A total of 1,585,950 data points were analysed with supervised ML models which included performance benchmark with statistical analysis namely zero value substitution, listwise deletion, mean substitution, and hot deck imputation. To achieve low reconstruction errors, different imputation models with hyperparameter tuned settings are varied, and computational time execution has been shown to contribute to imputation performance. Performance metrics measured by mean square error (MSE), mean absolute error (MAE),mean absolute percentage error (MAPE), and execution time respectively demonstrate the capability of SVR to perfectly impute missing data for all ground electromagnetism components at an average of 0.314 MSE, 0.738 MAPE, closeness to 0.510 MAE and 0.91-second at various percentage level of data missingness. A comparison with traditional imputation shows that the supervised ML with SVR model has improved imputation performance by up to 80% of data gap. The outcome of the proposed imputation will benefit space weather applications for event characterisation, which will cover a large number of missing data in the MAGDAS-9 dataset.

DOI： 10.1016/j.aej.2021.04.096

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The thermospheric wind response to a sudden westward turning of the ion velocity at a high latitude was studied by analyzing data obtained with a Fabry-Perot interferometer (FPI; 630 nm), Dynasonde, and Swarm A & C satellites during a conjunction event. The event occurred during a geomagnetically quiet period (Kp= 0+) through the night, but some auroral activity occurred in the north. The collocated FPI and Dynasonde measured the thermospheric wind (U) and ionospheric plasma velocity (V), respectively, in the F region at the equatorward trough edge. A notable scientific message from this study is the possible role of thermospheric wind in the energy dissipation process at F-region altitude. The FPI thermospheric wind did not instantly follow a sudden V change due to thermospheric inertia in the F region. At a pseudo-breakup during the event, V suddenly changed direction from eastward to westward within 10 min. U was concurrently accelerated westward, but its development was more gradual than that of V, with U remaining eastward for a while after the pseudo-breakup. The delay of U is attributed to the thermospheric inertia. During this transition interval, U.V was negative, which would result in more efficient generation of frictional heating than the positive U.V case. The sign of U.V, which is related to the relative directions of the neutral wind and plasma drift, is important because of its direct impact on ion-neutral energy exchange during collisions. This becomes especially important during substorm events, where rapid plasma velocity changes are common. The sign of U.V may be used as an indicator to find the times and locations where thermospheric inertia plays a role in the energy dissipation process.

DOI： 10.1186/s40623-022-01710-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have studied the radiation environment of LEO spacecraft when passing through the South Atlantic Anomaly (SAA) during the geomagnetic storm event of 15 May 2005. The 70-180 MeV proton flux information were numerically obtained from a simulation model of the inner radiation belt. The radiation analysis was consisted of two parts: the estimation of the Single Event Upset (SEU) rates and the absorbed radiation doses. One of the characteristics of the SAA at 800 km altitude during the recovery phase of the geomagnetic storm was the enhancement of its southern proton flux cell. It was found that the excess SEU rates and absorbed radiation doses could reach at least 200 % and 70 % respectively during the recovery phase in comparison to the initial phase of the geomagnetic storm. The estimated radiation results were in a good agreement with observations.

DOI： 10.1109/JRFID.2022.3163441

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Publisher：Journal of Mechanical Engineering  

This paper discusses geomagnetic field attempt modelling using an Artificial Neural Network (ANN). The local horizontal component of geomagnetic field data was collected on April 2011 (equinox) during a solar quiet day at recent solar cycle inclination-24 using the Magnetic Data Acquisition System (MAGDAS) in Langkawi, Malaysia, in the low latitude region. The calculated average values (mean) of the H component geomagnetic field variation during Equinox 2011 characterised the dominant geomagnetic field during that particular solar cycle. The difference in amplitude of maximum and minimum values shows a regular diurnal variation of the geomagnetic field during Sq in the low latitude region. The output training utilised these calculated mean values during the modelling attempt. Meanwhile, the input training utilised proton density, solar wind plasma speed, plasma flow pressure, and Interplanetary Magnetic Field (IMF) space data using Non-Linear Auto Regressive Input (NARX). The best modelling outputs were 3, 2, and 3 for input delay (nu), output delay (ny), and hidden layer size (h), respectively. Residual test and model fit analysis show an unbiased and high overlap between predicted and actual geomagnetic field average values, suggesting the model can potentially anticipate the geomagnetic field average during solar quiet in November (month of Equinox) on solar cycle inclination.

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Publishing type：Research paper (scientific journal)  

This paper discusses geomagnetic field attempt modelling using an Artificial Neural Network (ANN). The local horizontal component of geomagnetic field data was collected on April 2011 (equinox) during a solar quiet day at recent solar cycle inclination-24 using the Magnetic Data Acquisition System (MAGDAS) in Langkawi, Malaysia, in the low latitude region. The calculated average values (mean) of the H component geomagnetic field variation during Equinox 2011 characterised the dominant geomagnetic field during that particular solar cycle. The difference in amplitude of maximum and minimum values shows a regular diurnal variation of the geomagnetic field during Sq in the low latitude region. The output training utilised these calculated mean values during the modelling attempt. Meanwhile, the input training utilised proton density, solar wind plasma speed, plasma flow pressure, and Interplanetary Magnetic Field (IMF) space data using Non-Linear Auto Regressive Input (NARX). The best modelling outputs were 3, 2, and 3 for input delay (nu), output delay (ny), and hidden layer size (h), respectively. Residual test and model fit analysis show an unbiased and high overlap between predicted and actual geomagnetic field average values, suggesting the model can potentially anticipate the geomagnetic field average during solar quiet in November (month of Equinox) on solar cycle inclination.

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PSTEP: project for solar–terrestrial environment prediction
<title>Abstract</title>Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

DOI： 10.1186/s40623-021-01486-1

Language：English  

The east–west component of magnetic field variation (∆D-component) at Davao station (Philippines, geomagnetic latitude: – 2.22˚N) are used to investigate the characteristics of the long-term Inter-Hemispheric Field-Aligned Currents (IHFACs) based on the time-series analysis from August 1998 to July 2020. Recent in situ satellite and ground-based observations have reported that dusk-side current polarity of IHFAC is often opposite to that of the noon IHFAC, being inconsistent with Fukushima's IHFACs model. We investigated the consistency of the dusk-side IHFAC polarity derived from the observations with the polarity expected from Fukushima’s IHFACs model and examined the solar cycle dependence of IHFACs. It was confirmed that the dusk-side IHFACs during June and December solstices flow in the same direction of the noontime IHFACs, which was consistent with the IHFAC polarities suggested by the Fukushima model. The dusk-side IHFACs around March and September–November months disagreed with the Fukushima model. The ∆D variations clearly showed seasonal asymmetry in the dawn and noon sectors, whereas the ∆D variations in the dusk sector demonstrated seasonal symmetry. Solar cycle dependence of IHFACs was exhibited in the dusk sector. For the dawn and noon sectors, the yearly peak-to-peak ∆D amplitude in the later solar cycle SC24 decreased by about 35&#37; in comparison with the earlier solar cycle SC23. In contrast, the dusk-side yearly peak-to-peak ∆D amplitude increased by about 200&#37;. The dusk-side IHFAC yearly amplitude tended to be in inverse proportion to solar activity. [Figure not available: see fulltext.]

DOI： 10.1186/s40623-021-01481-6

Language：Others   Publishing type：Research paper (other academic)  

Short-term earthquake forecasting is impossible due to the seismometer's limited sensitivity in detecting the generation of micro-fractures prior to an earthquake. Therefore, there is a strong desire for a non-seismological approach, and one of the most established methods is geomagnetic disturbance observation. Previous research shows that disturbances in the ground geomagnetic field serves as a potential precursor for earthquake studies. It was discovered that electromagnetic waves (EM) in the Ultra-Low Frequency (ULF) range are a promising tool for studying the seismomagnetic effect of earthquake precursors. This study used a multiple regression approach to analyse the preliminary study on the relationship between Pc4 (6.7-22 mHz) and Pc5 (1.7-6.7 mHz) ULF magnetic pulsations, solar wind parameters and geomagnetic indices for predicting earthquake precursor signatures in low latitude regions. The ground geomagnetic field was collected from Davao station (7.00 N, 125.40 E), in the Philippines, which experiences nearby earthquake events (Magnitude <5.0, depth <100 km and epicentre distance from magnetometer station <100 km). The Pc5 ULF waves show the highest variance with four solar wind parameters, namely SWS, SWP, IMF-Bz, SIE and geomagnetic indices (SYM/H) prior to an earthquake event based on the regression model value of R2 = 0.1510. Furthermore, the IMF-Bz, SWS, SWP, SWE, and SYM/H were found to be significantly correlated with Pc5 ULF geomagnetic pulsation. This Pc5 ULF magnetic pulsation behaviour in solar winds and geomagnetic storms establishes the possibility of using Pc5 to predict earthquakes.

DOI： 10.1088/1755-1315/880/1/012010

Language：English   Publishing type：Research paper (scientific journal)  

Empirical Modeling of Ionospheric Current Using Empirical Orthogonal Function Analysis and Artificial Neural Network
Given the potential importance of solar quiet (Sq) ionospheric current in geomagnetic field modeling, it is vital to obtain accurate parameters characterizing its variations, particularly the spatial and temporal variations. In this paper, we derived the Sq current function based on the spherical harmonic analysis (SHA) technique using a 14-year (2006–2019) quiet geomagnetic field record over the American sector. The empirical orthogonal function (EOF) analysis was then applied to deduce temporal and spatial variations of the Sq current. It is observed that the first EOF mode of the Sq current function is dominated by solar activity, while the second and third EOF modes exhibit annual and semiannual variations, respectively. Also, the artificial neural network (ANN) model of Sq current function was constructed to validate the EOF model predictions. While the Sq current intensity predicted by the ANN model is underestimated by 2.83&#37;, the EOF model underpredicted the Sq current intensity by 1.92&#37; relative to the observation. The root mean square error (RMSE) of the EOF model is 0.64 kA. This RMSE is about 79&#37; smaller than that of the ANN model. In addition, both the EOF and ANN models capture the variation of the total Sq current (Jtotal) intensity with respect to solar activity. In principle, the EOF model had an optimal performance at nearly 98&#37; accuracy, with the ANN model exhibiting almost the same degree of accuracy, which appears to be a reference point for ionospheric conditions when looking for space weather applications.

DOI： 10.1029/2021SW002831

Language：Others   Publishing type：Research paper (other academic)  

The Sun's magnetic activity influences disturbances that perturb interplanetary space by producing large fluxes of energetic protons, triggering geomagnetic storms and affecting the ground geomagnetic field. The effect of two solar events, namely Coronal Mass Ejection (CME) and Coronal Holes, on geomagnetic indices (SYM/H), solar wind parameters and ground geomagnetic fields has provided magnetic ground data, which were extracted from the Terengganu (TRE, -4.21 N, 175.91 E) Magnetometer (MAGDAS) station, and investigated in this study. Results show that the physical dynamic mechanism in the Earth's magnetosphere is triggered by various solar wind parameters associated with CMEs and Coronal hole events during the minimum solar cycle of 24 at low latitudes. It is important to study solar wind-magnetosphere coupling because it has an impact on ground-based technological systems and human activities.

DOI： 10.1088/1755-1315/880/1/012009

Language：Others   Publishing type：Research paper (scientific journal)  

Previous studies have reported that coronal mass ejections (CMEs) and solar flares lead to the development of huge storms and high-speed streams. Our aim in this paper is to investigate the response of the geomagnetic index SYM/H to the solar wind parameters, such as solar wind speed V, dynamic pressure P, input energy IE and the interplanetary magnetic field (IMF) Bz component, associated with solar flares and CME events. The response of the ground geomagnetic field (H-component) to the solar wind parameters and the IMF Bz component at three low-latitude stations has also been analysed. Our findings show that the delay of the solar wind changes in the Earth's magnetosphere in response to the weak geomagnetic storm (SYM/H = -30 nT) at the beginning of 2014 December 21. A weak storm of SYM/H = -30 nT in the middle of 2014 November 5 is suggested by a low magnetic reconnection process in the magnetosphere. In addition, the strong southward IMF Bz component and high solar wind changes in the magnetosphere system, which were a result of the X2.0 solar flare event and the CMEs on 2014 October 27, responded to the moderate storm (SYM/H = -60 nT) at the beginning of 2014 October 28. This dynamic physical process in the magnetosphere caused by solar wind variation is seen to excite the Earth's H-component through ultra low frequency at the ground-based magnetometers at the BCL (Vietnam), TIR (India) and SCN (Indonesia) stations during the geomagnetic storm. This study relates to seismic investigations and geomagnetic-induced current on the ground.

DOI： 10.1093/mnras/stab1161

Language：English   Publishing type：Research paper (scientific journal)  

Identification of fractal properties in geomagnetic data of southeast asian region during various solar activity levels
The fractal properties of geomagnetic northward component data (H-component) in the equatorial region during various phases of solar activity over Southeast Asia were investigated and then quantified using the parameter of the Hurst exponent (H). This study began with the identification of existence of spectral peaks and scaling properties in international quiet day H-component data which were measured during three levels of solar activity: low, intermediate, and high. Then, various cases of quiet and disturbed days during different solar activity levels were analyzed using the method that performed the best in the preceding part. In all the years analyzed, multifractal scaling and spectral peaks exist, signifying that the data have fractal properties and that there are external factors driving the fluctuations of geomagnetic activity other than solar activity. The analysis of various cases of quiet and disturbed days generally showed that quiet days had anti-persistence tendencies (H < 0.5) while disturbed days had persistence tendencies (H > 0.5)—generally a higher level of Hurst exponent compared to quiet days. As for long-term quiet day H-component data, it had a Hurst exponent value that was near H ≃0.50, while the long-term disturbed day H-component data showed higher values than that of the quiet day.

DOI： 10.3390/universe7070248

Language：English   Publishing type：Research paper (scientific journal)  

The Ionosphere at Middle and Low Latitudes Under Geomagnetic Quiet Time of December 2019
The ionospheric electron density shows remarkable day-to-day variability due to solar radiance, geomagnetic activity and lower atmosphere forcing. In this report, we investigated the ionospheric variations at middle and low latitudes during a period under geomagnetic quiet time (Kpmax = 1.7) from November 30 to December 8, 2019. During the quiescent period, the ionosphere is not undisturbed as expected in the Asian-Australian and the American sectors. Total electron content (TEC) has multiple prominent enhancements at middle and low latitudes in the two sectors, and TEC depletions also occur repeatedly in the Asian-Australian sector. The low-latitude electric fields vary significantly, which is likely to be modulated by the notably changing tides in the mesosphere and lower thermosphere region. It is worth noting that the variations of TEC and the electric fields are not consistent in the two sectors, particularly on December 4–6. Further investigation reveals that the increase in TEC depends on altitude. The TEC enhancements are mainly contributed by the altitude below 500 km in both two sectors, which indirectly reflects that the driving sources may come from the lower atmosphere. Especially, a mid-latitude band structure continuously appears at all local times in the North American sector on December 6–8, which is also mainly contributed by the altitude below 500 km.

DOI： 10.1029/2020JA028964

Language：Others   Publishing type：Research paper (scientific journal)  

MAGDAS PEN was established on 19th September 2019 as one of the MAGDAS observatory arrays located at Universiti Sains Malaysia (USM) (5.15°, 100.50°). The main objective of the MAGDAS project is to monitor global electromagnetic and the ambient plasma density in the geospace environment. This installation has contributed to a better understanding of the Sun-Earth coupling system. This paper presents the installation process of one of the MAGDAS magnetometers named YU-8 T magnetic sensor and provides a preliminary analysis of geomagnetic HDZ components amplitude-time that was observed at PEN station. A one-month HDZ-geomagnetic field data was processed from 1st November to 30th November 2019. The daily variations with a consistent pattern in delta H geomagnetic field components are observed throughout the day with eastward electric field effects that are observed during solar peak hours. The delta H-component gradually increases around 0700LT and reaches the maximum reading at 1300LT with a range of value ~ 40-70nT. The value slowly decreases that started from 1400LT until the night time. The reading during the night time shows a constant variation with magnitude varies in between −10nT to + 10nT. The average H-component value of the night time is used as the baseline for the observation system. Overall, the observed trends portray a good sign of solar quiet field and Sq with no solar-terrestrial disturbances.

DOI： 10.1016/j.asr.2021.01.009

Language：English   Publishing type：Research paper (scientific journal)  

The equatorial ionospheric current system is composed of equatorial electrojet (EEJ) and solar quiet (Sq) that flow in the eastward direction. That being mentioned, this study looked into the changes in ionospheric currents stemming from geomagnetic storm disturbances at different levels of solar activity during solar cycle 24. This analysis was performed by employing an EEJ index known as EUEL, which is calculated from northward geomagnetic component data measured using ground-based magnetometer at five different latitudes across the Southeast Asia region. Based on the outcomes, the impacts of geomagnetic storm on EEJ currents were observed at 1200 LT and 1100 LT, respectively, during high and low solar activity levels. Time delay was associated with changes in EEJ peak time at varied activity levels. This case study reveals that EEJ value at peak time of high solar activity decreased significantly during the main phase of geomagnetic storm.

DOI： 10.1007/s12648-020-01734-2

Language：English   Publishing type：Research paper (scientific journal)  

Comparison of eej longitudinal variation from satellite and ground measurements over different solar activity levels
The longitudinal variability and local time of equatorial electrojet (EEJ) current using simultaneous data recorded by ground and satellite magnetometers at different levels of solar activity were investigated. In this study, we used data from the CHAMP and Swarm satellites to obtain EEJ current measurements around the globe. The ground data were provided by the MAGDAS, INTERMAGNET, and IIG networks. The ground observation was carried out by analyzing magnetometer data in four different sectors: the South American, Indian, African, and Southeast Asian sectors. These ground data were normalized to the dip equator to overcome the latitudinal variation of each station. The analysis for both measurements was performed using quiet day data. Both the ground and satellite data were categorized according to solar activity level; low, moderate, and high. The results revealed that, during the low solar activity, there was a good agreement between the longitudinal profiles of the EEJ measured using the satellite and the ground data. In general, strong correlations were obtained in most of the sectors where ground data were available between 11 and 13 local time (LT). Besides that, our analysis revealed that the different times of maximum EEJ appearances were seasonally dependent only at certain longitude sectors.

DOI： 10.3390/universe7020023

Language：Others   Publishing type：Research paper (international conference proceedings)  

Geomagnetic data has been demonstrated to exhibit fractal properties, which are analysed using various fractal methods. These methods allow the characterization of geomagnetic activity during certain periods using the Hurst exponent. In this study, the geomagnetic activity during the quiet period of the month of December 2011 is analysed using the r-DFA method, of which viability to identify fractal properties of geomagnetic data has not been tested yet, and also using its established predecessor; the detrended fluctuation analysis (DFA). The results show that the r-DFA method is indeed viable to be used upon geomagnetic time series, with comparable if not better performance compared to its predecessor.

DOI： 10.1088/1742-6596/1768/1/012004

Language：Others   Publishing type：Research paper (international conference proceedings)  

A negative decrement in geomagnetic data in equatorial region is not necessarily cause by the well-known counter electrojet, CEJ but it is also can be affected by solar activity such as a solar flare. Relying on this matter, we did a comparison between CEJ and unusual solar flare effect called as SFE∗ on geomagnetic data before introducing several types of CEJ that can be extracted from EUEL index. This study used ground data from MAGDAS network located in Langkawi (LKW) with 1-minutes resolution during solar cycle 24 and applied EUEL index. From the results, it is crucial to ensure the minimum duration of EUEL depletion is within an hour because less than an hour depletion could be possibly cause by solar activity. Four interval of time that has been introduced in our study composed of several types of CEJ which are morning CEJ, afternoon CEJ and evening CEJ, with additional the controversial CEJ occurrence which is night CEJ. We also successfully demonstrated a rare CEJ occurrence that associated with solar flare effect using EUEL index.

DOI： 10.1088/1742-6596/1768/1/012005

Language：English   Publishing type：Research paper (scientific journal)  

Correlations between earthquake properties and characteristics of possible ulf geomagnetic precursor over multiple earthquakes
In this study, we improved and adapted existing signal processing methods on vast geomagnetic field data to investigate the correlations between various earthquake properties and characteristics of possible geomagnetic precursors. The data from 10 magnetometer stations were utilized to detect precursory ultra-low frequency emission and estimate the source direction for 34 earthquakes occurring between the year 2007–2016 in Southeast Asia, East Asia, and South America regions. As a result, possible precursors of 20 earthquakes were identified (58.82&#37; detection rate). Weak correlations were obtained when all precursors were considered. However, statistically significant and strong linear correlations (r ≥ 0.60, p < 0.05) were found when the precursors from two closely located stations in Japan (Onagawa (ONW) and Tohno (TNO)) were exclusively investigated. For these stations, it was found that the lead time of the precursor is strongly (or very strongly) correlated with the earthquake magnitude, the local seismicity index, and the hypocentral depth. In addition, the error percentage of the estimated direction showed a strong correlation with the hypocentral depth. It is concluded that, when the study area is restricted to a specific location, the earthquake properties are more likely to have correlations with several characteristics of the possible precursors.

DOI： 10.3390/universe7010020

Language：English   Publishing type：Research paper (scientific journal)  

The Multi-Optimized Parameter Technique for Near Online Automatic Determination of Geomagnetic Sudden Commencement Arrival Time
This paper introduces a new technique in the automatic detection of storm sudden commencement (SC) using the discrete wavelet transform (DWT). A geomagnetic storm is a global simultaneous phenomenon affecting the whole Earth, which means that all ground magnetometers running online will record this event. An algorithm using different characteristic features of the SC is proposed. The selection of an optimal threshold for feature parameters is critical for the success of SC automatic detection. Therefore, this paper uses particle swarm optimization (PSO) to determine the optimal feature threshold values. The developed algorithm is based on data records from a network of ground magnetometers. This algorithm is implemented via multi-resolution analysis (MRA) of the DWT using the Haar wavelet filter. Four-year data sampled at one sample/s from six ground stations from low to high latitudes were analyzed to develop and test this technique. Data representing 450 days from five stations operating simultaneously are available. The confusion matrix of all possible outcomes shows that the accuracy of the proposed algorithm is 97.33&#37;.

DOI： 10.1007/s13369-020-04773-3

Language：English   Publishing type：Research paper (scientific journal)  

Integrating Pre-Earthquake Signatures From Different Precursor Tools.
Potential earthquake precursors include, among others, electromagnetic fields, gas emissions, Land Surface Temperature (LST), Sea Surface Temperature (SST), and Surface Air Temperature (SAT) anomalies. These observables have been individually studied, before earthquakes, by many researchers. The ionospheric studies concerning earthquakes (EQs) using magnetic data from Low Earth Orbit (LEO) satellites are increasingly being used to detect ionospheric anomalies before large EQs. Also, LST, SST, and SAT values retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua satellites and Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) are considered as physical precursors before EQs. In this work, we jointly analyze magnetic, MODIS, and MERRA-2 data in space and time around the epicenters before the selected EQs in Mexico, Japan, Chile, and Indonesia. Our analyses present interesting findings where anomalies in temperature and magnetic field, preceding the considered EQs, are confirmed through different methods. Particularly, we utilize the Fast Fourier Transform (FFT) and the Discrete Cosine Transform (DCT) for analyzing magnetic data over the designated EQs regions. We use the magnetic data acquired by Swarm satellites in the top side ionosphere along with MODIS and MERRA-2. Five case studies are described to prove the effectiveness of our analyses. Precursory anomalies were observed using these methods in different anomalous days from the considered four regions of interest around the epicenter. It is concluded that these methods could be effective and reliable in detecting anomalies preceding the upcoming EQs.

DOI： 10.1109/ACCESS.2021.3060348

Language：English   Publishing type：Research paper (scientific journal)  

Evolution of field-aligned current in the meridional plane during substorm: multipoint observations from satellites and ground stations
We report the propagation of substorm-associated magnetic fluctuations by multipoint magnetic observations from ground and space in the same meridional plane. The first Quasi-Zenith Satellite (QZS-1), which has a unique orbit of quasi-zenith orbit with an inclination of 41 ∘, an apogee of 7.1 RE radial distance, and an orbital period of 24 h, can stay for a long time in the near-earth magnetotail away from the magnetic equator. We examined a substorm event during 15:00–16:00 UT on July 09, 2013 when QZS-1 was located at 31 ∘ dipole latitude and 23.5 h dipole magnetic local time. The Engineering Test Satellite VIII (ETS-VIII), Time History of Events and Macroscale Interactions during Substorms D (THEMIS-D) at a radial distance of ∼ 10 RE, and THEMIS-E at a radial distance of ∼ 7 RE were located near the equator in the similar magnetic meridian. The dipolarization was first observed at THEMIS-D at 15:14:30 UT. Then, ∼ 1 min later, magnetic fluctuations were observed by ETS-VIII and THEMIS-E. At the same time, the magnetic bay and Pi2 pulsation were observed at low-latitude magnetic observatories and the Radiation Belt Storm Probes B satellite in the inner magnetosphere. We found that QZS-1 away from the equator observed a strong azimuthal magnetic field fluctuation with a long delay of 15 min from the first dipolarization at THEMIS-D near the equator. The speed of the poleward propagating magnetic fluctuation between the ionospheric footprints is calculated to be 310 [m / s] , which is consistent with a typical speed of auroral poleward expansion. A similar time delay of the onset of the negative bay was observed between the Tixie (AACGM MLAT = 66. 8 ∘) and Kotelny (AACGM MLAT = 71. 0 ∘) observatories near the ionospheric footprint of satellites. We suggest that the long delay time of the magnetic fluctuation at QZS-1 was associated with the crossing of field-aligned current during the poleward expansion of the substorm current system. The distribution of azimuthal magnetic field variations in the magnetosphere indicates that the east side downward current extended more west side in the higher latitude part of the current wedge.

DOI： 10.1186/s40623-020-01182-6

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the relations between the monthly average values of the critical frequency (f0F2) and the physical properties of the coronal mass ejections (CMEs), then we examined the seasonal variation of f0F2 values as an impact of the several CMEs properties. Given that, f0F2 were detected by PRJ18 (Puerto Rico) ionosonde station during the period 1996–2013. We found that the monthly average values of f0F2 are varying coherently with the sunspot number (SSN). A similar trend was found for f0F2 with the CMEs parameters such as the CME energy (linear correlation coefficient R = 0.73), width (R = 0.6) and the speed (R = 0.6). The arrived CMEs cause a plasma injection into the ionosphere, in turn, increasing the electron density, and consequently, f0F2 values. This happens in the high latitudes followed by the middle and lower latitudes. By examining the seasonal variation of f0F2, we found that the higher correlation between f0F2 and CMEs parameters occurs in the summer, then the equinoxes (spring and autumn), followed by the winter. However, the faster CMEs affect the ionosphere more efficiently in the spring more than in the summer, then the winter and the autumn seasons.

DOI： 10.3390/universe6110200

Language：English   Publishing type：Research paper (scientific journal)  

Prominent Daytime TEC Enhancements Under the Quiescent Condition of January 2017
It is well known that the ionospheric electron densities in the F-region usually undergo day-to-day variability with the magnitude of ~20–35&#37;, associated with the solar as well as geomagnetic activities and the meteorological forcing from the lower atmosphere. In this study, we reported remarkable enhancements in the total electron content (TEC) from the Beidou geostationary (GEO) satellites during the quiescent geophysical condition of January 2017 prior to the arrival of the sudden stratosphere warming (SSW). The daytime TEC around the equatorial ionization anomaly region increased by 75–160&#37; and lasted for several days. The equatorial electrojet and Sq current in the ionospheric E-region at low and middle latitudes showed the corresponding increase during the TEC enhancement interval. The possible contributions from the variations of solar EUV flux and geomagnetic activity are ruled out. Our further analysis showed that the diurnal tides in the temperature from Sounding of the Atmosphere using Broadband Emission Radiometry and the solar and lunar semidiurnal tides of neutral winds from two radars over China became strong during this period of interest. These results suggest that the lower atmospheric tidal forcing could contribute to the great and long-duration TEC enhancement in the ionosphere even though the SSW had not happened yet.

DOI： 10.1029/2020GL088398

Language：Others   Publishing type：Research paper (scientific journal)  

This paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ∼3 days, also known as ultrafast Kelvin waves (UFKWs). The whole atmosphere model Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30-km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the Gravity field and steady-state Ocean Circulation Explorer satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward-propagating ∼3-day variations with zonal wavenumber 1 in the equatorial zonal electric field and F region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ∼3-day variations due to other sources. This issue can be overcome by combining ground-based measurements from different longitudes. As a demonstration, we analyze ground-based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ∼3-day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.

DOI： 10.1029/2020JA027939

Language：English   Publishing type：Research paper (scientific journal)  

On 2 February 2018, the China Seismo-Electromagnetic Satellite (CSES) ZhangHeng 01 (ZH-01) was successfully launched, carrying on board, in addition to a suite of plasma and particle physics instruments, a high precision magnetometer package (HPM), able to observe the ultra-low frequency (ULF) waves. In this paper, a night time Pi2 pulsation observed by CSES is reported for the first time. This Pi2 event occurred on 3 September 2018, and began at 14:30 UT (02:37 magnetic local time), when the satellite was in the southern hemisphere between -49 and -13 magnetic latitude (MLAT). Kakioka (KAK) ground station in Japan detected the same Pi2 between 14:30-14:42 UT (23:30-23:42 local time). The Pi2 oscillations in the compressional, toroidal, and poloidal components at the CSES satellite and the H-component at the KAK station are investigated by estimating coherence, amplitude, and cross-phase. We noticed a high degree of similarity between the Pi2 event in the horizontal component at KAK and the ionospheric fluctuations in the compressional component at CSES. This high correlation indicated the magnetospheric source of the Pi2 event. In addition, Pi2 is exhibited clearly in the ffiBy component at CSES, which is highly correlated with the ground H component, so the Pi2 event could be explained by the Substorm Current Wedge (SCW). This interpretation is further confirmed by checking the compressional component of Van Allen Probe (VAP) B satellite inside the plasmasphere, which, for the first time, gives observational support for an earlier model. This ULF wave observation shows the consistency and reliability of the high precision magnetometer (HPM) equipped by two fluxgate magnetometers (FGM1 and FGM2) onboard CSES.

DOI： 10.3390/rs12142300

Language：English   Publishing type：Research paper (scientific journal)  

Normalized Polarization Ratio Analysis for ULF Precursor Detection of the 2009 M 7.6 Sumatra and 2015 M 6.8 Honshu Earthquakes
The utilization of geomagnetic field data especially in the ultra-low frequency range has been shown by prior studies to have the potential to detect earthquake precursors. The most widely used signal processing method for this purpose is arguably the polarization ratio analysis. In this paper, the method was improved by introducing a new normalization process with dissimilar ranges for the vertical and horizontal components. The normalized method was applied to geomagnetic field data that were recorded at locations near the earthquakes which had occurred in Sumatra, Indonesia on 30th September 2009 (M7.6) and Honshu, Japan on 12th May 2015 (M6.8) to evaluate the method's reliability and effectiveness in two different regions, i.e., the equatorial and mid-latitudinal regions. The results showed that the precursors obtained from the normalized method were less disturbed by random fluctuations and had more distinguishable amplitudes compared to the non-normalized classic method in both studied regions.

DOI： 10.17576/jkukm-2020-si3(1)-06

Language：English   Publishing type：Research paper (scientific journal)  

Persistence of the Long-Duration Daytime TEC Enhancements at Different Longitudinal Sectors During the August 2018 Geomagnetic Storm
In this study, the ionospheric responses in the Asian-Australian, American, and African sectors during the August 2018 geomagnetic storm were investigated based on the Beidou geostationary orbit (GEO) satellite and Massachusetts Institute of Technology (MIT) Madrigal total electron contents (TECs), combined with measurements from ionosondes, magnetometers, and Global Ultraviolet Imager (GUVI). The middle- and low-latitude TECs were dominated by positive responses over the three longitudinal sectors during the storm on 26–29 August. It is unique that daytime TECs at the Asian-Australian, American, and African sectors displayed large enhancements larger than 10 TEC units (TECu) on 27–29 August, during the recovery phase, when the ionosphere is usually dominated by plasma depletions due to the ionospheric disturbance dynamo and/or disturbed thermospheric compositions. The combination and competition of the disturbed vertical plasma drifts through the solar wind-magnetosphere-ionosphere (SW-M-I) coupling and disturbed neutral compositions contribute significantly to the daytime TEC responses at different longitudinal sectors on 26 August during the main and early recovery phases. The eastward equatorial electrojet and O/N2 during the recovery phase on 27–29 August are larger than the quiet reference, which suggest that the enhanced upward vertical plasma drifts combining with higher O/N2 make an important contribution on the daytime positive ionospheric storm during the recovery phase. The enhanced vertical plasma drifts could not be driven by the SW-M-I coupling or ionospheric disturbance dynamo associated with the geomagnetic storm. Further studies should be untaken to explore the dominant sources for the enhanced upward vertical drifts during the recovery phase.

DOI： 10.1029/2020JA028238

Language：English   Publishing type：Research paper (scientific journal)  

Ionospheric Current Variations Induced by the Solar Flares of 6 and 10 September 2017
We examine the global ionospheric current in relation to X9.33 disk and X8.28 limb flares, which had significant differences in their solar X-ray and extreme ultraviolet (EUV) fluxes using the ground-based magnetometer data. At the peak of X9.33 flare, when X-ray and EUV radiations were significantly enhanced, the northern current vortex was situated at (40°N, 12 LT), while the southern current vortex was found at (30°S, 13LT). In comparison to the X8.28 flare, the northern current vortex was seen at (16°N, 12LT), while the southern current vortex was situated at (35°S, 14LT), which was 2 hr earlier in local time compared to those observed in the X9.33 flare. The changes in the total current intensity of the X9.33 flare is about 16&#37; less than that of the X8.28 flare, thus revealing that the current variations relative to both flares are due to solar flux and universal time variations. The daytime X9.33 flare northern current vortex is stronger, while the southern vortex is less intense than the corresponding vortex of X8.28 flare. Even though both flares happened in equinox, the current vortices are nearly symmetric. There were significant hemispheric changes in the focus position leading to the hemispheric asymmetry. Our results indicated that both the enhanced X-ray and EUV fluxes during flares could have impacts on the ionospheric electric field and current, but their relative contributions and the underlying physics need further investigations.

DOI： 10.1029/2020SW002608

Language：English   Publishing type：Research paper (scientific journal)  

Multi-Instrument Observations of the Atmospheric and Ionospheric Response to the 2013 Sudden Stratospheric Warming Over Eastern Asia Region.
We investigate the atmospheric and ionospheric response to the 2013 sudden stratospheric warming (SSW) by using multiple instruments located in Eastern Asia. Three meteor radars and five ionosondes are used to investigate the mesospheric zonal wind fields and ionospheric parameters of F -layer virtual height ( h 'F), F2-layer peak height ( h-{m} ext{F}-{2} ), and critical frequency ( f-{o} ext{F}-{2} ) at mid- and low-latitudes (10.7°N to 40.3°N). The vertical total electron content (TEC) data derived from the ground-based global positioning system receiver network are analyzed to study the ionospheric perturbations in the equatorial ionization anomaly (EIA) region. The changes in equatorial electrojet (EEJ) are observed by using the magnetometer data from stations on and off the magnetic equator. The variations of the h-{m} ext{F}-{2} at Sanya and EEJ strength presented the semidiurnal pattern with increase/decrease and eastward/westward currents in the morning/afternoon hours. In addition, the EIA crest moved poleward/equatorward in the morning/afternoon. The f-{o} ext{F}-{2} showed the most significant enhancements during daytime at Wuhan and Shaoyang but the f-{o} ext{F}-{2} at Sanya and Chumphon reduced mildly. Most importantly, based on the time-period wavelet analysis, the diurnal tidal components in the f-{o} ext{F}-{2} over Beijing, Wuhan, and Sanya seemed similar those in zonal winds and the semidiurnal tides in the low-latitude h-{m} ext{F}-{2} showed the similar temporal variations as those in EEJ strength during the later phase of SSW. Therefore, apart from the local tides propagating from lower atmosphere having influence on the mid- and low-latitude ionosphere directly during the early phase, the equatorial fountain effect modulated by the enhanced tides also disturbed the EIA region.

DOI： 10.1109/TGRS.2019.2944677

Language：English   Publishing type：Research paper (scientific journal)  

Investigation on the Variability of the Geomagnetic Daily Current During Sudden Stratospheric Warmings
The magnetic field records of the magnetometer networks in the American, East Asian-Australian, and European-African sectors were employed in this present work. We used them to investigate equatorial electrojet (EEJ), counter electrojet (CEJ), tidal variability in EEJ strength and ionospheric current during the 2005/2006 and 2008/2009 sudden stratospheric warming (SSW) events. In addition to the well-investigated tidal variability in EEJ strength over the American and East Asian sectors, we investigated that of the African sector for the first time. Interestingly, the tidal components in EEJ strength during both SSW events clearly exhibit marked longitudinal differences with high, moderate, and low amplitudes in the American, East Asian, and African sectors, respectively. An exception found around day 71 in the African sector after the 2008/2009 SSW event had higher solar diurnal tidal component as compared to that of the Asian sector. Over the American sector, solar and lunar semidiurnal tides were strongly associated with CEJ current during both SSW events, whereas at the African and East Asian sectors such variabilities are not evident. A solar diurnal tidal component was strongly related to a reduction in the EEJ strength over the East Asian sector. In addition, a prolonged period of CEJ occurrence that begins during the SSW precondition and ends when the SSW was evolving characterized the African sector during both SSW events. There is a steady shift in phase at later hours when both SSW events are evolving.

DOI： 10.1029/2019JA026667

Language：English   Publishing type：Research paper (scientific journal)  

Deformation of Ionospheric Potential Pattern by Ionospheric Hall Polarization
The present study shows that the ionospheric Hall polarization can deform the high-latitude ionospheric convection field, which is widely considered to be a manifestation of the convection field in the magnetosphere. We perform the Hall polarization field separation with a potential solver by changing the conductance distribution step by step from a uniform one to a more realistic one. We adopt dawn-dusk and north-south symmetric distributions of conductance and region 1 (R1) field-aligned current (FAC). The pair of the primary field of the R1 system and each gradient of off-diagonal component of conductance tensor (Hall conductance) generates the Hall polarization field and consequently causes potential deformations as follows. (a) The equatorward gradient causes clockwise rotation. (b) The gradient across the terminator, together with the effect of the equatorward gradient, causes the dawn-dusk asymmetry. (c) The high conductance band in the auroral region causes kink-type deformations. In particular, a nested structure at the equatorward edge of the band in the midnight sector well resembles the Harang Reversal. Result (a) can explain the clockwise bias inexplicable by the IMF-By effect alone, the combination of (a) and (b) can explain the clearness and unclearness in the round or crescent shapes of the dawn-dusk cells depending on the IMF-By polarity, and (c) suggests that the ionosphere may not need the upward-FAC for the formation of the Harang Reversal. We suggest that the final structure of the ionospheric potential is established by the combined effects of the magnetospheric requirements (external causes) and ionospheric polarization (internal effect).

DOI： 10.1029/2018JA026013

Language：English   Publishing type：Research paper (scientific journal)  

Three different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high-latitude field-aligned currents and reconfiguration of high-latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high-frequency radar observations.

DOI： 10.1029/2019JA027069

Language：English  

Language：English  

Seasonal dependence of semidiurnal equatorial magnetic variation during quiet and disturbed periods
The analysis of 20-year long-term semidiurnal lunar tidal variations gave the evidence that the semidiurnal variations are completely different between the magnetic quiet and disturbed periods. This is the first time that the seasonal dependence of disturbance-time semidiurnal variation has been provided from the analysis of the EE-index. We found the Kp dependence of semidiurnal variation: For full and new moon phase, counter troughs are amplified during disturbance time, possibly related to disturbance dynamo. For all moon phase, there are positive enhancements in dawn and strong depressions after sunset, resulting from the penetration of polar electric filed. For Seasonal dependence, semidiurnal variations are divided to three seasonal groups, and characterized as deep trough, enhanced crest and weak structure for D-solstice, Equinoxes and J-solstice, respectively. There is no significant longitudinal difference between Ancon and Davao, except for the amplitude of semidiurnal variations. The deep troughs occur during D-solstice and the enhanced crests during Equinoxes, at both Ancon and Davao.

DOI： 10.1051/e3sconf/201912702025

Language：English   Publishing type：Research paper (scientific journal)  

The mantle electrical conductivity-depth structure of Malaysia was determined for the first time using solar quiet day ionospheric current (Sq) variations. Spherical harmonic analysis (SHA) was employed to separate the external and internal field contribution to the Sq variations. A transfer function was applied in estimating the conductivity-depth profile for the paired of external and internal coefficients of the SHA. We observed a downward increase in electrical conductivity with initial magnitude of 0.0065 S/m at a depth of ∼ 56 km which gradually rose to 0.0106 and 0.0140 S/m at 118 and 180 km. Subsequently, the conductivity profile rose to about 0.0228 S/m at 380 km (near the base of the upper mantle) and reached 0.0260 S/m at 435 km, after which a sharp steep increase was observed at 450 km with conductivity profile of 0.0278 S/m. Consequently, the conductivity profile increases significantly to about 0.1367 S/m at a depth of 973 km and reached its peak value 0.1975 S/m at the depth of 1097 km in the lower mantle with no indication of leveling off. An evidence of discontinuity was observed near 390–460 km and 675–746 km. A slight increase in conductivity values at depth between 150 and 300 km corresponds to the region of unusual global low velocity zone with high electrical conductivity. The conductivity profile showed a less steep increase above 450 km below which a steep increase was observed. The present profile showed the deepest penetration depth which may be attributed to the influence of equatorial electrojet current (EEJ) that is actively supported by the conductive properties of the Earth's interior within the study region.

DOI： 10.1016/j.jastp.2018.01.019

Language：English   Publishing type：Research paper (scientific journal)  

This study utilized magnetic field records of the horizontal component obtained from magnetic data acquisition system at northern island of Malaysia to delineate the diurnal variations of SqH and their monthly mean MSqH for a period of 4 years. The results show that for the entire period of study, the daytime SqH amplitudes increase from ∼ 20 to ∼ 160 nT and their MSqH amplitudes increased from ∼ 40 to about 135 nT in 2008 to 2013, respectively. These variabilities of SqH and MSqH reached peak amplitudes between 10:00 and 12:00 LT hrs. The standard deviation fairly responds to the daily variability of SqH with a phase shift in January 2008 and 2013 which may likely have connection to the sudden stratospheric warming event that occurred in these months. We observed the extension of eastward field that continued beyond the sunset (18:00 LT) hours is more prevalent with higher magnitudes during the deep minimum solar activity and the probable causes are discussed. The observed counter equatorial electrojet associated with late reversal of night-time westward electric field (WEF) shows inverse relationship with increasing solar activity and those that are not associated to late reversal of night-time WEF seems to indicate linear relationship exception of deep minimum solar activity year (2008). We also observed a gradual shift of ECEJ to the morning sector with increasing solar activity. Throughout the years, days with stronger morning counter equatorial electrojet magnitudes generally weaken the expected strong eastward current and shifted its peak to earlier local time. The annual and seasonal variations of SqH show semiannual variation with equinoctial maxima and solstitial minima. About 40&#37; of the SqH shows positive night-time to pre-sunrise variations which suggest the existence of weak electric currents not necessarily of ionospheric origin with appreciable influence at this longitude sector. The extension of eastward field beyond the sunset hours suggests likely modification of the evening side ionosphere over this region.

DOI： 10.1007/s12648-019-01388-9

Language：English   Publishing type：Research paper (scientific journal)  

Assessment of signal processing methods for geomagnetic precursor of the 2012 M6.9 Visayas, Philippines earthquake
Abstract: This study assessed two signal processing methods on geomagnetic data to detect precursory signals appearing before the M6.9 Visayas, Philippines earthquake on 6 February 2012. It aimed to compare the polarization ratio analysis method with the diurnal variation ratio method in terms of reliability and effectiveness. The geomagnetic data were obtained from the MAGDAS magnetometer network for Cebu (CEB) and Legazpi (LGZ) stations which served as the primary and remote stations, respectively. The polarization ratio analysis method was performed on the primary station data to obtain power spectral density in an ultra-low-frequency range before finding the ratio of vertical to total horizontal component. Meanwhile, the diurnal variation ratio method was used to calculate the difference between the daily maximum and minimum values. Then, the ratio of the daily differences of the primary station to the remote station for each individual component was calculated. The disturbance storm time index which describes global geomagnetic activity originating from the Sun was utilized to verify that any observed geomagnetic fluctuations were not caused by solar-terrestrial effect. A precursory anomaly was found using the polarization ratio analysis method which appeared 2 weeks before the earthquake. It is concluded that this method might be effective and reliable in detecting geomagnetic anomalies preceding upcoming earthquakes. In contrast, although the diurnal variation ratio method did show perceivable fluctuations, the running averages were not statistically significant to be considered a precursor. The discrepancy between the analytical results of the two methods may be attributed to the detectability of the earthquake being studied which had a relatively low magnitude. Hence, future studies which utilize more earthquake events need to be conducted to reach a definitive conclusion. Graphic abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s11600-019-00319-w

Language：English   Publishing type：Research paper (scientific journal)  

Magnetic records of the declination (D) component for the solar quiet year 2011–2013 obtained from Magnetic Data Acquisition System (MAGDAS) at Langkawi (Geog. Lon. 99.68∘ E, Geog. Lat. 6.30∘N), Malaysia were utilized in this study. The minutes averages were used to delineate the diurnal (Sq(D)) variation. The monthly mean (MSq(D)) and their seasonal variabilities (SVq(D)) were also analysed. The Sq(D) and their MSq(D) exhibit smooth regular occurring pattern in the month of April–September and became highly perturbed in October–March across the years. The highest positive (∼3.5 arc-min) and the negative (∼−3.0 arc-min) values were observed in August 2011 during the dawn and noon sectors. These maxima shifted to July and September in 2012 with peaks ∼3.2 and −3.0 arc-min. In 2013, the positive maximum (∼3.0 arc-min) and its negative (∼−2.5 arc-min) were again seen in August. This implies that the dawn and noon sectors of August 2011 and 2013 are strongly influenced by IHFACs and this effect shifted to July and September in 2012. IHFACs through the years flow from the winter to summer hemisphere during the noon and dusk sectors and flow in opposite direction during the dawn sector. The day-to-day magnitudes of Sq(D) and MSq(D) seems to suggest the inter-hemispheric imbalance of the ionospheric Sq current earlier established by Van Sabben as the cause of IHFACs is not strongly affected by the changes in annual solar variation. Dusk-side IHFACs were observed to be northbound in all the seasons with the exception of June solstice. The direction of IHFACs does not change except in April and November. The current intensity is not large in solstices except in August 2011 and 2013 but it shifted to July in 2012. The result further showed that the magnitude of the duskside IHFACS is determined to some extent by the strength of the noontime IHFACs. IHFACs were generally observed to be greater during the daytime than night-time hours.

DOI： 10.1016/j.jastp.2018.01.012

Language：English   Publishing type：Research paper (other academic)  

The intense current that flowing eastward in the dayside in the E layer of ionosphere at dip equator region is known as equatorial electrojet (EEJ). In this work, we attempt to compare the longitudinal profile of EEJ current estimated from the Challenging Minisatellite Payload (CHAMP) and European Space Agency satellite (SWARM) with the ground-based magnetometer network from six equatorial observatory pairs across the globe. Ground magnetometer were obtained from the network such as MAGDAS, INTERMAGNET and IIG. We process our data using the EUEL index during quiet days for ground measurement. Then, correlation coefficient were obtained after applying appropriate normalization techniques for both measurements. Analysis showed the longitudinal variations of EEJ current from ground indicates that this current was strongest at South American (77 °W) and Southeast Asian (99.78°E). Meanwhile, satellite measurement shows four peaks of EEJ current at -100°W, 0°E, 100°E and 180° E which are at North America, Atlantic ocean, Southeast Asian and Pacific ocean respectively.

DOI： 10.1063/1.5111232

Language：English   Publishing type：Research paper (other academic)  

This study present the statistical investigation of equatorial geomagnetic crochets due to strong solar flare of class M to X. Our analysis utilized data from eight magnetometer stations provided by MAGnetic Data Acquisition System network (MAGDAS) at different longitudes sector during period from January 2008 to March 2015. We have considered 52 cases of solar flare events. Based on our analysis, 65% of the studied solar flares show no effect on geomagnetic variation with most of it from class M8, M9 and X1. The absence of solar flare effect might due to the influence of coronal mass ejection (CME) on the observation data or due to the station's location is at nighttime. The other 35% shows geomagnetic crochets to the magnetic data variation where majority are mix effect dominated by positive crochets. Apart from that, we also found an interesting solar flare effect where negative geomagnetic crochets were detected from all equatorial magnetometer data. The unique effect is due to X-class solar flare on 9th August 2011. Further investigation using extended latitudinal and longitudinal magnetometer is recommended to observe the whole picture of the solar flare effect to global ionospheric current.

DOI： 10.1063/1.5111233

Language：English   Publishing type：Research paper (scientific journal)  

Magnetic Data Acquisition System (MAGDAS) is a magnetometer initiated by the International Center for Space Weather Science and Education in Kyushu University, Japan, to study space weather. The latest of real-time Magnetic Data Acquisition System/Circum-pan Pacific Magnetometer Network was successfully installed at the East Coast Environmental Research Institute in Universiti Sultan Zainal Abidin, Terengganu, Malaysia, by Kyushu University. This is the fifth magnetometer under the MAGDAS network (geographic latitude and longitude: 5.23°, 103.04° and geomagnetic latitude and longitude: − 4.21°, 175.91°). In this study, the results of data plot obtained at Terengganu (TRE) station were shown to have reliable patterns of geomagnetic elements. The amplitude variations for each component were also proximate with other stations and a standard model. This study compared MAGDAS-II data for the H component with solar wind data (input energy, IMF, dynamic pressure and speed).

DOI： 10.1007/s12648-018-1318-x

Language：Others   Publishing type：Research paper (scientific journal)  

Ion transport from the plasma sheet to the ring current is the main cause of the development of the ring current. Energetic (>150 keV) ring current ions are known to be transported diffusively in several days. A recent study suggested that energetic oxygen ions are transported closer to the Earth than protons due to the diffusive transport caused by a combination of the drift and drift-bounce resonances with Pc 3–5 ultralow frequency waves during the 24 April 2013 magnetic storm. To understand the occurrence conditions of such selective oxygen increase (SOI), we investigate the phase space densities (PSDs) between protons and oxygen ions with the first adiabatic invariants (μ) of 0.1–2.0 keV/nT measured by the Radiation Belt Storm Probes Ion Composition Experiment instrument on the Van Allen Probes at L ~ 3–6 during 90 magnetic storms in 2013–2017. We identified the SOI events in which oxygen PSDs increase while proton PSDs do not increase during a period of ~9 hr (one orbital period). Among the 90 magnetic storms, 33&#37; were accompanied by the SOI events. Global enhancements of Pc 4 and Pc 5 waves observed by ground magnetometers during the SOI events suggest that radial transport due to combination of the drift-bounce resonance with Pc 4 oscillations and the drift resonance with Pc 5 oscillations can be the cause of SOIs. The contribution of the SOI events to the magnetic storm intensity is roughly estimated to be ~9&#37; on average.

DOI： 10.1029/2018JA026168

Language：English   Publishing type：Research paper (scientific journal)  

Response of the Ionosphere-Plasmasphere Coupling to the September 2017 Storm: What Erodes the Plasmasphere so Severely?
We report an extreme erosion of the plasmasphere arising from the September 2017 storm. The cold electron density is identified from the upper limit frequency of upper hybrid resonance waves observed by the Plasma Wave Experiment instrument onboard the Exploration of energization and Radiation in Geospace/Arase satellite. The electron density profiles reveal that the plasmasphere was severely eroded during the recovery phase of the storm and the plasmapause was located at L = 1.6–1.7 at 23 UT 8 September 2017. This is the first report of deep erosion of the plasmasphere (LPP < 2) with the in situ observation of the electron density. The degree of the severity is much more than what is expected from the relatively moderate value of the SYM-H minimum (−146 nT). We attempt to find a possible explanation for the observed severe depletion by using both observational evidence and numerical simulations. Our results suggest that the middle latitude electric field had penetrated from the high-latitude storm time convection for several hours. Such an unusually long-lasting penetration event can cause this observed degree of severity.

DOI： 10.1029/2019SW002168

Language：English   Publishing type：Research paper (other academic)  

Statistical study of equatorial geomagnetic crochets
This study present the statistical investigation of equatorial geomagnetic crochets due to strong solar flare of class M to X. Our analysis utilized data from eight magnetometer stations provided by MAGnetic Data Acquisition System network (MAGDAS) at different longitudes sector during period from January 2008 to March 2015. We have considered 52 cases of solar flare events. Based on our analysis, 65&#37; of the studied solar flares show no effect on geomagnetic variation with most of it from class M8, M9 and X1. The absence of solar flare effect might due to the influence of coronal mass ejection (CME) on the observation data or due to the station's location is at nighttime. The other 35&#37; shows geomagnetic crochets to the magnetic data variation where majority are mix effect dominated by positive crochets. Apart from that, we also found an interesting solar flare effect where negative geomagnetic crochets were detected from all equatorial magnetometer data. The unique effect is due to X-class solar flare on 9th August 2011. Further investigation using extended latitudinal and longitudinal magnetometer is recommended to observe the whole picture of the solar flare effect to global ionospheric current.

DOI： 10.1063/1.5111233

Language：English   Publishing type：Research paper (other academic)  

Longitudinal profile of equatorial electrojet derived from ground and satellite based magnetometers
The intense current that flowing eastward in the dayside in the E layer of ionosphere at dip equator region is known as equatorial electrojet (EEJ). In this work, we attempt to compare the longitudinal profile of EEJ current estimated from the Challenging Minisatellite Payload (CHAMP) and European Space Agency satellite (SWARM) with the ground-based magnetometer network from six equatorial observatory pairs across the globe. Ground magnetometer were obtained from the network such as MAGDAS, INTERMAGNET and IIG. We process our data using the EUEL index during quiet days for ground measurement. Then, correlation coefficient were obtained after applying appropriate normalization techniques for both measurements. Analysis showed the longitudinal variations of EEJ current from ground indicates that this current was strongest at South American (77 °W) and Southeast Asian (99.78°E). Meanwhile, satellite measurement shows four peaks of EEJ current at -100°W, 0°E, 100°E and 180° E which are at North America, Atlantic ocean, Southeast Asian and Pacific ocean respectively.

DOI： 10.1063/1.5111232

Language：English   Publishing type：Research paper (scientific journal)  

Equatorial electrojet (EEJ) current is an intense eastward electric current flowing at about 100- 120km altitude within ±3° latitude in E region of the equatorial ionosphere. This study reported dependence of EEJ longitudinal profile on the three phases of solar cycle 24 (SC-24). The analysis was carried out using EUEL index, calculated from the northward H component of geomagnetic field. This work utilised data from various ground-based magnetometer networks such as MAGDAS, INTERMAGNET and IIG. The results of this study showed that the highest and lowest value of EEJ longitudinal profiles varies with solar phases. The nature of the longitudinal disparity in the EEJ strength indicates that it is strongest in American sector and lowest in African sector during solar minimum in 2008. However, during solar maximum in 2013 the highest EEJ intensity was recorded in Southeast Asian sector. On the other hand, the lowest value of EEJ current was recorded in African sector in 2008 which was found to be shifting to Indian sector started in 2011. In addition, we discovered the transition point of the highest value of EEJ started in 2012, while the transition point of minimum value EEJ started in 2010.

Language：English   Publishing type：Research paper (scientific journal)  

The response of the ionospheric bottomside electron density profile parameters of the F2 layer, namely, the maximum electron density (NmF2), the maximum height of F2 layer (hmF2), and bottomside thickness (B0) parameter to the equatorial electrojet (EEJ) current is examined for a Peruvian location at the Jicamarca station (12 °S, 76.9 °W) in the South American sector. The results of the analysis show that both hmF2 and B0 increase for ~2 h before sunrise and exhibit a postsunset peak during the equinoctial and summer months. The increase in the peak height, hmF2, is observed to terminate before midday, while B0 continued to increase throughout the daytime. The apparent midday and postnoon peaks in NmF2 occur in all the seasons under study. It was demonstrated that a relationship exists between EEJ and the profile parameters hmF2 and B0 during low and moderate solar conditions. Conversely, the correlation coefficient between EEJ and NmF2 is statistically significant only during solar minimum conditions but correlates poorly, if at all, with EEJ during moderate solar activity.

DOI： 10.1029/2018EA000537

Language：English   Publishing type：Research paper (scientific journal)  

Longitudinal variability of the equatorial counter electrojet during the solar cycle 24
Ground and space-based geomagnetic data were used in the investigation of the longitudinal, seasonal and lunar phase dependence of the equatorial counter electrojet (CEJ) occurrence in the Peruvian, Brazilian, African, Indian and Philippine sectors during geomagnetically quiet days from the solar cycle 24 (2008 to 2018). We found that CEJ events occur more frequently during the morning (MCEJ) than in the afternoon (ACEJ). The highest MCEJ and ACEJ occurrence rates were observed for the Brazilian sector. Distinct seasonal dependence was found for each longitudinal sector under investigation. The lunar phase dependence was determined for the first time for the Philippine sector (longitude 125°E), and it was shown to be less pronounced than in the Peruvian, Brazilian and African sectors. We demonstrate that differences in CEJ rates derived from ground-based and satellite data can arise from the longitudinal separation between low-latitude and equatorial stations that are used to determine the signal and its consequent time delay in their sunrise/sunset times at ionospheric heights.

DOI： 10.1007/s11200-018-0286-0

Language：English   Publishing type：Research paper (other academic)  

In recent years, it has been clarified from previous studies that plasma bubbles and equatorial electrojets (EEJs) are related. In general, EEJs are calculated by subtracting the magnetic field H component of the magnetic equator from that at low latitude. However, in this study, EE-index data at Langkawi (magnetic equator), which includes all local current systems, were used for the analysis during the period from January 1, 2011, to November 8, 2014. By using the EE-index, it was found that plasma bubbles tend to occur for larger EEJ strengths. This result differs from the previous studies. In addition, if an equatorial counter electrojet (CEJ) occurs, it is understood that plasma bubbles will rarely occur due to the westward current; however, we found that when the lunar tidal effect is strong, plasma bubbles can occur even in conjunction with CEJs. Finally, we want to find the relationship between plasma bubbles and ionospheric current to predict them.

DOI： 10.1088/1742-6596/1152/1/012022

Language：English  

The Magnetic Data Acquisition System (MAGDAS-9) was successfully installed at Universiti Teknologi MARA Johor, Pasir Gudang Campus, Malaysia (JOH station), under the joint collaboration between International Center for Space Weather Science and Education (ICSWSE), National Space Agency (ANGKASA) and UiTM Johor. This is the first solar-powered magnetometer under the Magnetic Data Acquisition System of Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN). In this paper, construction process, installation process and analysis of first geomagnetic observations at MAGDAS JOH station are presented. In addition, analysis on time derivative of horizontal component of geomagnetic field (dH/dt) to indicate the possibility of Geomagnetically Induced Currents (GIC) occurrences is also presented in this paper. The obtained geomagnetic components: H, D, Z and Total F are approximately similar to World Magnetic Model 2015 (WMM2015) while the diurnal variation from this geomagnetic observation showing a good variation pattern. The analysis of dH/dt has indicated 2 significant events of GIC.

DOI： 10.1088/1742-6596/1152/1/012030

Language：English   Publishing type：Research paper (scientific journal)  

In recent years, it has been clarified from previous studies that plasma bubbles and equatorial electrojets (EEJs) are related. In general, EEJs are calculated by subtracting the magnetic field H component of the magnetic equator from that at low latitude. However, in this study, EE-index data at Langkawi (magnetic equator), which includes all local current systems, were used for the analysis during the period from January 1, 2011, to November 8, 2014. By using the EE-index, it was found that plasma bubbles tend to occur for larger EEJ strengths. This result differs from the previous studies. In addition, if an equatorial counter electrojet (CEJ) occurs, it is understood that plasma bubbles will rarely occur due to the westward current; however, we found that when the lunar tidal effect is strong, plasma bubbles can occur even in conjunction with CEJs. Finally, we want to find the relationship between plasma bubbles and ionospheric current to predict them.

DOI： 10.1088/1742-6596/1152/1/012022

Language：English   Publishing type：Research paper (scientific journal)  

The geomagnetically induced current (GIC) is one of the most widely recognized phenomena caused by geomagnetic disturbances. Realistic predictions of magnetic field fluctuations may be used to evaluate the induction of electric fields to ground surfaces, and thus to estimate the occurrence of GICs. Although many GICs occur at high latitudes, they are now being studied at low and mid-latitudes as well. The purpose of this research was to understand the dynamics, observation, and prediction in Japan for GICs occurring at the low and mid-latitudes. In this study, the influence of geomagnetic field variations on Earth's electric field was examined. The magnetic field and the electric field components of 3 observation points for 1 year in 2015 are visually examined, and the characteristics of the fluctuations of the magnetic field and the surface electrical field were also analysed.

DOI： 10.1088/1742-6596/1152/1/012035

Language：English   Publishing type：Research paper (other academic)  

The behavior of the ionospheric current during disturbed days using 10 geomagnetic observatories along 180-degree magnetic meridian was analysed. Separation of external and internal current during geomagnetic storm on 12 April until 15 April 2012 data was carried out using spherical harmonic analysis method. The contour of day to day variation of total current, external current and internal current were distinguished based on geomagnetic storm phase. These current then were correlate with the solar wind parameter, IMF Bz, and geomagnetic indices in order to identify their relationship. The results reveal that these current intensities were found higher during the initial phase. This due to the enhancement of the magnetic field during the initial phase caused by the increment of solar wind dynamic pressure and southward IMF Bz. Hence, the induced current identified in the internal current. This study demonstrated that the electric current induction can be obtained from the weak geomagnetic storm.

DOI： 10.1088/1742-6596/1152/1/012029

Language：English   Publishing type：Research paper (other academic)  

The geomagnetically induced current (GICs) impact is no longer concentrated at high latitude region strictly. The objective of this study was to extend the GICs observation conducted at equatorial region using geomagnetic data extracted from magnetometer installed at Langkawi National Observatory (LNO), Malaysia for the long-term analysis involving the solar cycle 24. The observation is now continued with the GICs comparison between LKW stations with five low latitude station which is BCL, PTN, MND, DAW, and TWV obtained from the magnetic data acquisition system (MAGDAS). The assessment of the GICs behavior revealed the possibility of LKW station experiences GICs is high, based on the time derivative of the horizontal component (dH/dt) value. Extend from that, the GICs estimation was successful done by applying the plane wave method as a model to calculate the North-South and the East-West component, Ex and Ey respectively. The equation of the GICs was provided by excluding the topology constant parameters, which is a and b constant and the value of the underground conductivity for low latitude station. Subsequently, the comparison was limited in years of 2008 - 2014 due to insufficient geomagnetic data. Indeed, the GICs calculation is able to derive the GICs estimation and showed a good agreement with the corresponding value of the time derivative of the horizontal component. However, the accurate current value (A) is able to gain by conducting a direct measurement of the high voltage power transmission system.

DOI： 10.1088/1742-6596/1152/1/012028

Language：English   Publishing type：Research paper (international conference proceedings)  

The first solar-powered Magdas-9 installation and possible geomagnetically induced currents study at Johor, Malaysia
The Magnetic Data Acquisition System (MAGDAS-9) was successfully installed at Universiti Teknologi MARA Johor, Pasir Gudang Campus, Malaysia (JOH station), under the joint collaboration between International Center for Space Weather Science and Education (ICSWSE), National Space Agency (ANGKASA) and UiTM Johor. This is the first solar-powered magnetometer under the Magnetic Data Acquisition System of Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN). In this paper, construction process, installation process and analysis of first geomagnetic observations at MAGDAS JOH station are presented. In addition, analysis on time derivative of horizontal component of geomagnetic field (dH/dt) to indicate the possibility of Geomagnetically Induced Currents (GIC) occurrences is also presented in this paper. The obtained geomagnetic components: H, D, Z and Total F are approximately similar to World Magnetic Model 2015 (WMM2015) while the diurnal variation from this geomagnetic observation showing a good variation pattern. The analysis of dH/dt has indicated 2 significant events of GIC.

DOI： 10.1088/1742-6596/1152/1/012030

Language：English   Publishing type：Research paper (other academic)  

The geomagnetically induced current (GIC) is one of the most widely recognized phenomena caused by geomagnetic disturbances. Realistic predictions of magnetic field fluctuations may be used to evaluate the induction of electric fields to ground surfaces, and thus to estimate the occurrence of GICs. Although many GICs occur at high latitudes, they are now being studied at low and mid-latitudes as well. The purpose of this research was to understand the dynamics, observation, and prediction in Japan for GICs occurring at the low and mid-latitudes. In this study, the influence of geomagnetic field variations on Earth's electric field was examined. The magnetic field and the electric field components of 3 observation points for 1 year in 2015 are visually examined, and the characteristics of the fluctuations of the magnetic field and the surface electrical field were also analysed.

DOI： 10.1088/1742-6596/1152/1/012035

Language：English  

The behavior of the ionospheric current during disturbed days using 10 geomagnetic observatories along 180-degree magnetic meridian was analysed. Separation of external and internal current during geomagnetic storm on 12 April until 15 April 2012 data was carried out using spherical harmonic analysis method. The contour of day to day variation of total current, external current and internal current were distinguished based on geomagnetic storm phase. These current then were correlate with the solar wind parameter, IMF Bz, and geomagnetic indices in order to identify their relationship. The results reveal that these current intensities were found higher during the initial phase. This due to the enhancement of the magnetic field during the initial phase caused by the increment of solar wind dynamic pressure and southward IMF Bz. Hence, the induced current identified in the internal current. This study demonstrated that the electric current induction can be obtained from the weak geomagnetic storm.

DOI： 10.1088/1742-6596/1152/1/012028

Language：English  

Space weather manifests in power networks as quasi-DC currents flowing in and out of the power system through the grounded neutrals of high-voltage transformers, referred to as geomagnetically induced currents. This paper presents a comparison of modeled geomagnetically induced currents, determined using geoelectric fields derived from four different impedance models employing different conductivity structures, with geomagnetically induced current measurements from within the power system of the eastern states of Australia. The four different impedance models are a uniform conductivity model (UC), one-dimensional n-layered conductivity models (NU and NW), and a three-dimensional conductivity model of the Australian region (3DM) from which magnetotelluric impedance tensors are calculated. The modeled 3DM tensors show good agreement with measured magnetotelluric tensors obtained from recently released data from the Australian Lithospheric Architecture Magnetotelluric Project. The four different impedance models are applied to a network model for four geomagnetic storms of solar cycle 24 and compared with observations from up to eight different locations within the network. The models are assessed using several statistical performance parameters. For correlation values greater than 0.8 and amplitude scale factors less than 2, the 3DM model performs better than the simpler conductivity models. When considering the model performance parameter, P, the highest individual P value was for the 3DM model. The implications of the results are discussed in terms of the underlying geological structures and the power network electrical parameters.

DOI： 10.1029/2018SW002047

Language：English   Publishing type：Research paper (scientific journal)  

The magnetic field records of the magnetometer networks in the American, East Asian-Australian, and European-African sectors were employed in this present work. We used them to investigate equatorial electrojet (EEJ), counter electrojet (CEJ), tidal variability in EEJ strength and ionospheric current during the 2005/2006 and 2008/2009 sudden stratospheric warming (SSW) events. In addition to the well-investigated tidal variability in EEJ strength over the American and East Asian sectors, we investigated that of the African sector for the first time. Interestingly, the tidal components in EEJ strength during both SSW events clearly exhibit marked longitudinal differences with high, moderate, and low amplitudes in the American, East Asian, and African sectors, respectively. An exception found around day 71 in the African sector after the 2008/2009 SSW event had higher solar diurnal tidal component as compared to that of the Asian sector. Over the American sector, solar and lunar semidiurnal tides were strongly associated with CEJ current during both SSW events, whereas at the African and East Asian sectors such variabilities are not evident. A solar diurnal tidal component was strongly related to a reduction in the EEJ strength over the East Asian sector. In addition, a prolonged period of CEJ occurrence that begins during the SSW precondition and ends when the SSW was evolving characterized the African sector during both SSW events. There is a steady shift in phase at later hours when both SSW events are evolving.

DOI： 10.1029/2019JA026667

Language：English   Publishing type：Research paper (other academic)  

This paper presents the preliminary observation on the possible Geomagnetic Induced Current (GIC) activities at Malaysia region during the quiet period of solar cycle 24 (year 2017). In fact, the damage of power system operation might lead to the malfunction of the transformer, which is the main component of the power system. Previously, the cause of the damage has been identified in the long-term investigation, where it drives from the space weather effect so-called (GIC). In this work, the quiet period has been selected by respecting the value of K-Planetary index (Kp) must be less than 3 extracted from OMNIWeb data source. The main objective of the current work is to investigate the behavior of the GIC amplitude at Malaysia region as well as to identify the parameter (ionospheric current and magnetospheric current) that contribute significantly towards the GIC activity at Malaysia region. The type of analysis was carried out using the data of geomagnetic field by extracting the time derivative of the horizontal component (dH/dt) as the indicator for GIC activity. The geomagnetic data was obtained from the MAGDAS observatories at Langkawi (LKW), Perak (PER), and Johor (JOH) stations. Subsequently, the dH/dt value is less than 30 nT/min represent no GIC activity occurred during that particular date for all three stations. However, the results revealed that the ionospheric current, DP2 corresponds well to the dH/dt amplitude. Plus, the latitudinal correlation gave the best representation of the control factor of GIC activity.

DOI： 10.1109/ICSEngT.2018.8606390

Language：English  

Equatorial electrojet (EEJ) current is an intense eastward electric current flowing at about 100- 120km altitude within ±3° latitude in E region of the equatorial ionosphere. This study reported dependence of EEJ longitudinal profile on the three phases of solar cycle 24 (SC-24). The analysis was carried out using EUEL index, calculated from the northward H component of geomagnetic field. This work utilised data from various ground-based magnetometer networks such as MAGDAS, INTERMAGNET and IIG. The results of this study showed that the highest and lowest value of EEJ longitudinal profiles varies with solar phases. The nature of the longitudinal disparity in the EEJ strength indicates that it is strongest in American sector and lowest in African sector during solar minimum in 2008. However, during solar maximum in 2013 the highest EEJ intensity was recorded in Southeast Asian sector. On the other hand, the lowest value of EEJ current was recorded in African sector in 2008 which was found to be shifting to Indian sector started in 2011. In addition, we discovered the transition point of the highest value of EEJ started in 2012, while the transition point of minimum value EEJ started in 2010.

Language：English  

The geomagnetically induced current (GICs) impact is no longer concentrated at high latitude region strictly. The objective of this study was to extend the GICs observation conducted at equatorial region using geomagnetic data extracted from magnetometer installed at Langkawi National Observatory (LNO), Malaysia for the long-term analysis involving the solar cycle 24. The observation is now continued with the GICs comparison between LKW stations with five low latitude station which is BCL, PTN, MND, DAW, and TWV obtained from the magnetic data acquisition system (MAGDAS). The assessment of the GICs behavior revealed the possibility of LKW station experiences GICs is high, based on the time derivative of the horizontal component (dH/dt) value. Extend from that, the GICs estimation was successful done by applying the plane wave method as a model to calculate the North-South and the East-West component, Ex and Ey respectively. The equation of the GICs was provided by excluding the topology constant parameters, which is a and bconstant and the value of the underground conductivity for low latitude station. Subsequently, the comparison was limited in years of 2008 – 2014 due to insufficient geomagnetic data. Indeed, the GICs calculation is able to derive the GICs estimation and showed a good agreement with the corresponding value of the time derivative of the horizontal component. However, the accurate current value (A) is able to gain by conducting a direct measurement of the high voltage power transmission system.

DOI： 10.1088/1742-6596/1152/1/012029

Language：English   Publishing type：Research paper (scientific journal)  

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-018-0785-9

Language：English   Publishing type：Research paper (scientific journal)  

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.[Figure not available: see fulltext.].

DOI： 10.1186/s40623-018-0785-9

Language：English   Publishing type：Research paper (scientific journal)  

We have investigated the longitudinal propagation and extension of the poleward boundary intensifications (PBIs) of auroral emission by examining four events, Events 1–4, which show different spatiotemporal structures. In Event 1 an auroral form extended both eastward and westward immediately following the arrival of a fast polar cap flow and became dynamic as the polar cap flow enhancement continued. The PBI extended 3 hr in local time in a few minutes, which questions the conventional idea that the PBIs are an ionospheric manifestation of distant reconnection. In Events 2 and 3, an auroral form was already dynamic and was collocated with an upward field-aligned current (FAC), which, along with an adjacent downward FAC, formed a longitudinal flow channel confined near the poleward boundary of the auroral oval. Auroral structures propagated in the direction of this longitudinal convection flow. In Event 4, as a transient westward convection flow arrived, a new auroral form developed and extended also westward but noticeably faster than the convection flow, and it faded as it extended. These results suggest that the longitudinal ionospheric convection plays a critical role in the formation and development of the PBIs. They are consistent with a recently proposed idea that the PBIs are an effect of the ionospheric electrostatic polarization, which deflects the enhanced polar cap flow from equatorward to along the auroral oval at its poleward boundary. The contrast between Events 1 and 4 suggests that morphological differences of the PBIs reflect different durations and intensities of the polar cap flow enhancement.

DOI： 10.1029/2017JA024375

Language：English   Publishing type：Research paper (scientific journal)  

In this paper, we present our investigation of geomagnetic crochets observed following the X-class solar flare events on 9 August 2011 (X6.9) and 24 September 2011 (X1.4). Both events occurred during geomagnetically quiet times (Kp ≤ 3). The analysis of the geomagnetic crochets near the magnetic equator revealed very different responses in the ionospheric currents between these two events. During the 9 August 2011 event, a reduction of the eastward equatorial electrojet was observed in all daytime stations studied, while the daytime eastward equatorial electrojet was observed to enhance during the 24 September 2011 event. A spherical harmonic analysis was applied to the observations from nearly 160 ground stations at middle and low latitudes to derive the equivalent ionospheric current system for the geomagnetic crochets as well as background solar-quiet (Sq) variations for both events. The equivalent current system of the geomagnetic crochets for the 24 September 2011 event showed a similar pattern with the background Sq current system and thus can be attributed to the enhancement of Sq currents. However, the crochet current system for the 9 August 2011 event revealed a different pattern from the background Sq current system in both Northern and Southern Hemispheres. It is suggested that unusual geomagnetic crochets during the 9 August 2011 event may be dominated by the ionospheric currents at D-region altitudes.

DOI： 10.1029/2018JA025601

Language：Others   Publishing type：Research paper (scientific journal)  

Changes in ionospheric currents system at southeast Asia region during geomagnetic storm in solar's minimum phase
Equatorial ionospheric current system consists of equatorial electrojet (EEJ) and solar quiet (Sq). EEJ current is a ribbon of current flowing eastwards along the dip equator region. On the other hand, Sq current is a loop of current that flows at the Earth's northern and southern hemispheres in different directions. This study is focusing on analysing the effects of a solar event on the latitudinal profile of the current system, specifically in Southeast Asian region. Data used in this study are geomagnetic data from five magnetometer stations in MAGDAS network which are Muntinlupa (MUT), Cebu (CEB), Davao (DAV), Manado (MND) and Pare Pare (PRP). The EEJ current intensity is known to be highest around noon time between 1000 and 1100 LT during solar minimum and thus this study analyses the current system from 1000 to 1400 LT. The analysis showed that the geomagnetic storm occurred on 23 April 2008, which is caused by coronal mass ejection (CME), gives rises to the currents on that day. This increment is clearly observed on the currents at the southern hemisphere stations, which are MND and PRP. Our analysis also found that this geomagnetic storm gives increment to the current at the non-peak time. Apart from that, the current profile on this day is compared with the current profile from a quiet day on 21 April 2008.

DOI： 10.17576/jsm-2018-4708-34

Language：English   Publishing type：Research paper (scientific journal)  

Global Distribution of ULF Waves During Magnetic Storms: Comparison of Arase, Ground Observations, and BATSRUS + CRCM Simulation
During 26–29 March 2017 magnetic storm, the Arase satellite observed typical ultra low frequency (ULF) waves and acceleration of relativistic electrons. We simulate the global distribution of these ULF waves using CRCM with BATSRUS global magnetospheric magnetohydrodynamic model. The simulation can qualitatively reproduce the ULF waves observed by Arase at frequencies of 2–3 mHz. However, the simulated ULF wave power is 1–2 orders of magnitude smaller than the observation. The simulated ULF wave activity has a good correlation with the solar-wind dynamic pressure variation, while the wave activity on the ground is enhanced even during the recovery phase, possibly due to the Kelvin-Helmholtz instability and/or substorms. We also study the 3–6 April 2017 magnetic storm, in which low ULF wave activity and weak acceleration of relativistic electrons are seen. We suggest that the existence of ULF waves plays an important role in accelerating electrons up to relativistic energies.

DOI： 10.1029/2018GL078857

Language：English   Publishing type：Research paper (scientific journal)  

Generation of Electron Acoustic Waves in the Topside Ionosphere From Coupling With Kinetic Alfven Waves: A New Electron Energization Mechanism
Results from a new drift kinetic model in the topside ionosphere capture the mode conversion from kinetic Alfven waves to electron acoustic waves. When the kinetic Alfven waves propagate into the transition region, where the electron density of ionospheric origin becomes comparable to that of magnetospheric origin, the steep temperature gradient leads to the mode conversion. The electron acoustic waves are short-lived by dissipating their energy into the electron energization, thus revealing a new type of electron acceleration in the topside ionosphere.

DOI： 10.1029/2018GL077898

Language：English  

Study of the Solar Wind Influence on the Jovian Inner Magnetosphere Using an Ionospheric Potential Solver

Language：English  

Anomalous ultra low frequency signals possibly linked with seismic activities in Sumatra, Indonesia

Language：English  

Earth's ionosphere: Theory and phenomenology of cowling channels
The Cowling channel is a generic name of a current system forming inside a high conductivity band in which a secondary polarization electric field modifies the current flow. The polarization field is excited when a divergent part of Hall current driven by the primary electric field is prevented from flowing out to the magnetosphere as the field-aligned current. The purpose of this chapter is to review the recent development of the Cowling channel model. Recent work provides an extension of the theoretical description of the classical Cowling channel with respect to the following aspects: (1) taking into account the 3D nature of the ionosphere by introducing two current layers at different altitudes, and (2) considering finite length of the Cowling channel by introducing a conductance boundary not only at the meridional borders of the Cowling channel, but also at its zonal boundaries. Using this improved model, we discuss current closure and the energy principle for evolution of the Cowling channel. Energy flow inside the Cowling channel and impact of the polarization effect on Joule dissipation in the more general M-I coupling scheme are also provided. We also clarify how shear Alfvén waves interact to the Cowling channel and their application to the global magnetosphere- ionosphere coupling simulations.

DOI： 10.1002/9781119324522.ch25

Language：English  

The Time Derivative of the Horizontal Geomagnetic Field for the Low Latitude MAGDAS Langkawi Station for the Estimation of Geomagnetically Induced Current

DOI： 10.1007/978-981-10-6574-3_6

Language：English   Publishing type：Research paper (other academic)  

Estimation of Time Derivative of Horizontal Geomagnetic Component for GIC Assesment in Malaysia during Quiet Period
This paper presents the preliminary observation on the possible Geomagnetic Induced Current (GIC) activities at Malaysia region during the quiet period of solar cycle 24 (year 2017). In fact, the damage of power system operation might lead to the malfunction of the transformer, which is the main component of the power system. Previously, the cause of the damage has been identified in the long-term investigation, where it drives from the space weather effect so-called (GIC). In this work, the quiet period has been selected by respecting the value of K-Planetary index (Kp) must be less than 3 extracted from OMNIWeb data source. The main objective of the current work is to investigate the behavior of the GIC amplitude at Malaysia region as well as to identify the parameter (ionospheric current and magnetospheric current) that contribute significantly towards the GIC activity at Malaysia region. The type of analysis was carried out using the data of geomagnetic field by extracting the time derivative of the horizontal component (dH/dt) as the indicator for GIC activity. The geomagnetic data was obtained from the MAGDAS observatories at Langkawi (LKW), Perak (PER), and Johor (JOH) stations. Subsequently, the dH/dt value is less than 30 nT/min represent no GIC activity occurred during that particular date for all three stations. However, the results revealed that the ionospheric current, DP2 corresponds well to the dH/dt amplitude. Plus, the latitudinal correlation gave the best representation of the control factor of GIC activity.

DOI： 10.1109/ICSEngT.2018.8606390

Language：Others   Publishing type：Research paper (scientific journal)  

Solar flare is referring to sun's activity define as sudden intense bright light coming from its surface. It can immediately affect earth's ionosphere system thus perturb any currents flowing in the layer. The activity can be monitored using ground based magnetometer data. Regularly, the event will enhance the ionospheric conductivity thus increase the magnitude of the currents however the opposite effect has been reported recently. There were studies reported that a solar flare is capable to increase current intensity at some location and reduces it at other location. Previous study were focused on solar flare effect at some particular area sector or locally. Therefore, our approach is to analyze solar flare feature and its effect using extensive magnetometer data distribution which involve stations from five sectors. Data are obtained from Magnetic Data Acquisition System/Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN) and solar flare is identified using Geostationary Operational Environmental Satellite 15 (GOES 15) X-ray flux data. Our study discovered a new effect of solar flare which is reduction of current intensity at all magnetic equator data and the proposed factor is existence of counter electrojet current. Apart from that, we also found that this solar flare occurred on geomagnetic storm recovery phase during June solstice in inclining phase of solar cycle.

DOI： 10.17576/jsm-2018-4703-21

Language：English   Publishing type：Research paper (scientific journal)  

DOI： DOI10.4283/ JMAG.2017.22.2.333.

Language：English   Publishing type：Research paper (scientific journal)  

Nighttime Pc3 pulsations: MM100 and MAGDAS observations
In this paper, we present a statistical and case analysis of nighttime Pc3 pulsations observed from middle to equatorial latitudes during the year 2003. We found two groups of nighttime Pc3 pulsations. Pc3s of the first group are in fact the nightside counterpart of morning Pc3 pulsations with large azimuthal scales slowly attenuating toward midnight. Such night signatures of morning Pc3 waves are observed during the periods of fast solar wind (V > 500 km/s). The second type is the locally generated night Pc3 pulsations. They can be observed under moderate solar wind velocities. Maximal occurrence rates and amplitudes for these pulsations are recorded at middle geomagnetic latitudes near the local magnetic midnight. Probably, they are associated with auroral activations or local non-substorm bursty processes.

DOI： 10.1186/s40623-017-0647-x

Language：English   Publishing type：Research paper (other academic)  

Characteristics of Schumann Resonance Parameters at Kuju Station
The ground magnetic field variation in the extremely low frequency (ELF) range has been measured by an induction magnetometer at Kuju, Japan (KUJ; M.Lat. = 23.4 degrees, M. Lon. = 201.0 degrees) since 2003. The first mode of the Schumann resonance (SR) around 8 Hz can be seen at KUJ. The SR in H (horizontal northward component) shows maximum peaks around 08 UT and 15 UT. In the case of D (horizontal eastward component), the SR shows its maximum peak around 08 UT. These peaks are coincident with the enhancement of lightning activity in Africa and Asia. Thus, we found the influence of the lightning activity on the observed SR at KUJ.

DOI： 10.1051/e3sconf/20172001004

Language：English   Publishing type：Research paper (scientific journal)  

Geomagnetic induced currents (GICs) have been considered an issue for high-latitude power networks for some decades. More recently, GICs have been observed and studied in power networks located in lower latitude regions. This paper presents the results of a model aimed at predicting and understanding the impact of geomagnetic storms on power networks in Australia, with particular focus on the Queensland and Tasmanian networks. The model incorporates a “geoelectric field” determined using a plane wave magnetic field incident on a uniform conducting Earth, and the network model developed by Lehtinen and Pirjola (1985). Model results for two intense geomagnetic storms of solar cycle 24 are compared with transformer neutral monitors at three locations within the Queensland network and one location within the Tasmanian network. The model is then used to assess the impacts of the superintense geomagnetic storm of 29–31 October 2003 on the flow of GICs within these networks. The model results show good correlation with the observations with coefficients ranging from 0.73 to 0.96 across the observing sites. For Queensland, modeled GIC magnitudes during the superstorm of 29–31 October 2003 exceed 40 A with the larger GICs occurring in the south-east section of the network. Modeled GICs in Tasmania for the same storm do not exceed 30 A. The larger distance spans and general east-west alignment of the southern section of the Queensland network, in conjunction with some relatively low branch resistance values, result in larger modeled GICs despite Queensland being a lower latitude network than Tasmania.

DOI： 10.1002/2017SW001613

Language：English   Publishing type：Research paper (scientific journal)  

The ionospheric thickness (B0) and shape (B1) are bottomside profile parameters introduced by the International Reference Ionosphere (IRI) model. We have validated these parameters with the latest version of the IRI-2012 model and compared them with the solar quiet of geomagnetic H-component (SqH). The B0, B1 and SqH are calculated from the measurements obtained from digisonde DPS-4 sounder and the Magnetic Data Acquisition System (MAGDAS) magnetometer, respectively at Ilorin (geo latitude 8.50°N, geo longitude 4.68°E, and Magnetic dip 4.1°S) an equatorial station in the African sector. The study was for the year 2010, a year of low solar activity (with 27-day averaged solar index, F10.7 = 80 sfu). The results show that B0 for the entire months was higher during the daytime than during the night time. On the other hand, the magnitude of B1during the daytime period is lower than nighttime values and exhibit oscillatory pattern. By comparing the experimental observations of the profile parameters with the IRI-2012 model prediction, we found that B0 was fairly represented by the IRI model options during the nighttime period while discrepancies exist between the model estimates and the experimental values during the morning till midday. A close agreement exists between the observed B1 values and IRI model options. We observed a positive and significant correlation coefficient between B0 and SqH indicating a plausible relationship between these parameters while a weak and negative correlation coefficient between B1 and SqH was observed. We concluded that the difference in the relationship of SqH and the profile parameters B0 and B1 observed can be attributed to their sensitivity to the electric field which is responsible for the E × B drift which in turn modulate the height of the F2.

DOI： 10.1016/j.asr.2017.02.003

Language：English   Publishing type：Research paper (other academic)  

The equatorial electrojet (EEJ) is an eastward current flow around the dip equator in E-region of the ionosphere within the range of 90 to 120 km altitude. The longitudinal variation of EEJ was studied using the ground-based measurement from six different longitudinal sectors. The ground magnetometer data were provided by magnetometer networks such as those from Magnetic Data Acquisition System (MAGDAS) /Circum-pan Pacific Magnetometer Network (CPMN), Indian Institute of Geomagnetism (IIG) and International Real-time Magnetic Observatory Network (INTERMAGNET). The EUEL index used in this study was calculated from geomagnetic northward H component. The H-component was normalized to the dip equator using the CM4 model. This study present results of the longitudinal variation of EEJ during the three phases of solar cycle 24: solar minimum (2008), inclination phase (2011) and solar maximum (2014). Results show that EEJ current is higher in the American sector and lowest between African and Indian sector in 2008 and 2011. On the other hand, during the year of solar maximum, this current component is comparable in American and Southeast Asian sector.

DOI： 10.1088/1742-6596/852/1/012019

Language：English   Publishing type：Research paper (other academic)  

Equatorial electrojet is an eastward flowing current at about ±3° dip equator. This current intensity is always higher during noontime as it is greatly influenced by the ionization of Earth ionosphere. Apart from that, previous study had shown that EEJ current varies with longitude and latitude as well as solar cycle. The aim of this study is to investigate the peak time of EEJ current at different longitude sectors using simultaneous data in 2009. By eliminating Sq current contribution and normalizing ground-based data from MAGDAS/CPMN, IIG and WDC network, we managed to reach our purpose and gain reliable output. We found out that EEJ is strongest in South American sector. Our results show that the peak EEJ during fall solar minimum is observed at 11 LT for South American, Indian and Southeast Asian sector but it is 1 hr earlier in African sector.

DOI： 10.1088/1742-6596/852/1/012015

Language：English   Publishing type：Research paper (scientific journal)  

This study tries to bring out the fact that storm time substorms can compete and at times significantly contribute to the geomagnetically disturbed time prompt penetration electric field effects on low and equatorial latitudes. Observations of unusual equatorial plasma drift data from Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere during two space weather events show that substorms can induce both eastward and westward penetration electric fields under steady southward interplanetary magnetic field (IMF Bz) conditions. During the first event on 2 January 2005, the enhancement of the daytime eastward electric field over Jicamarca due to substorm is found to be comparable with the Sq and interplanetary electric field (IEFy) generated electric fields combined. During the second event on 19 August 2006, the substorm is seen to weaken the daytime eastward field thereby inducing a westward field in spite of the absence of northward turning of IMF Bz (overshielding). The westward electric field perturbation in the absence of any overshielding events is observationally sparse and contrary to the earlier results. Further, the substorm-induced field is found to be strong enough to compete or almost nullify the effects of storm time IEFy fields. This study also shows quantitatively that at times substorm contribution to the disturbed time prompt electric fields can be significant and thus should be taken into consideration in evaluating penetration events over low latitudes.

DOI： 10.1002/2016JA023754

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Longitudinal variation of equatorial electrojet and the occurrence of its counter electrojet
We examined the longitudinal variability of the equatorial electrojet (EEJ) and the occurrence of its counter electrojet (CEJ) using the available records of the horizontal component H of the geomagnetic field simultaneously recorded in the year 2009 (mean annual sunspot number Rz Combining double low line 3.1) along the magnetic equator in the South American, African, and Philippine sectors. Our results indicate that the EEJ undergoes variability from one longitudinal representative station to another, with the strongest EEJ of about 192.5nT at the South American axis at Huancayo and a minimum peak of 40.7nT at Ilorin in western Africa. Obtained longitudinal inequality in the EEJ was explicable in terms of the effects of local winds, dynamics of migratory tides, propagating diurnal tide, and meridional winds. The African stations of Ilorin and Addis Ababa registered the greatest &#37; of CEJ occurrence. Huancayo in South America, with the strongest electrojet strength, was found to have the least occurrence of the CEJ. It is suggested that activities that support strong EEJ inhibits the occurrence of the CEJ. Percentage of occurrence of the CEJ varied with seasons across the longitudes. The order of seasonal variation of morning occurrence does not tally with the evening occurrence order at any station. A semiannual equinoctial maximum in percentage of morning occurrence of the CEJ was obtained at Huancayo and Addis Ababa. Only Addis Ababa recorded equal equinoctial maxima in percentage of evening occurrence of the CEJ. The seasonal distribution of the occurrences of the CEJ at different time regimes implies a seasonal variability of causative mechanisms responsible for the occurrence of the CEJ.

DOI： 10.5194/angeo-35-535-2017

Language：English   Publishing type：Research paper (scientific journal)  

The present paper investigates the asymmetrical variability of the location of the north and the south equatorial ionization anomaly (EIA) crests in the East-Asian sector, along with their association with simultaneous observations of equatorial electrojet (EEJ) strength, geomagnetic activity index, and solar flux index during the 2002-2003 sudden stratospheric warming (SSW) event. Analysis of these observations indicates the existence of a large-scale quasi 16-day periodic meridional movement in both EIA crests, and also reveals a strong correlation between the quasi 16-day oscillation in geomagnetic latitudes of the EIA crest and EEJ strength. The latitude of the northern/southern EIA crest and the EEJ strength indicate that obvious synchronous periodic oscillations were in-phase in the northern and southern hemisphere when the SSW occurred. In addition, it is also found that both the EIA crest location and amplitude of the periodic movement of the EIA locations exhibit hemispheric asymmetry. The amplitude of the periodic movement of the EIA location in the southern hemisphere is larger than that of the northern hemisphere, and the southern EIA crest is further off from the equator than the north one. Understanding these asymmetries requires a combination of mechanisms that involve at least trans-equator meridional winds and the position of a sub-solar point; however, potential disturbances in neutral winds associated with the SSW may additionally complicate the equatorial ionospheric dynamics.

DOI： 10.1007/s11430-016-0096-y

Language：English   Publishing type：Research paper (scientific journal)  

We examined the longitudinal variability of the equatorial electrojet (EEJ) and the occurrence of its counter electrojet (CEJ) using the available records of the horizontal component H of the geomagnetic field simultaneously recorded in the year 2009 (mean annual sunspot number R_z Combining double low line 3.1) along the magnetic equator in the South American, African, and Philippine sectors. Our results indicate that the EEJ undergoes variability from one longitudinal representative station to another, with the strongest EEJ of about 192.5nT at the South American axis at Huancayo and a minimum peak of 40.7nT at Ilorin in western Africa. Obtained longitudinal inequality in the EEJ was explicable in terms of the effects of local winds, dynamics of migratory tides, propagating diurnal tide, and meridional winds. The African stations of Ilorin and Addis Ababa registered the greatest % of CEJ occurrence. Huancayo in South America, with the strongest electrojet strength, was found to have the least occurrence of the CEJ. It is suggested that activities that support strong EEJ inhibits the occurrence of the CEJ. Percentage of occurrence of the CEJ varied with seasons across the longitudes. The order of seasonal variation of morning occurrence does not tally with the evening occurrence order at any station. A semiannual equinoctial maximum in percentage of morning occurrence of the CEJ was obtained at Huancayo and Addis Ababa. Only Addis Ababa recorded equal equinoctial maxima in percentage of evening occurrence of the CEJ. The seasonal distribution of the occurrences of the CEJ at different time regimes implies a seasonal variability of causative mechanisms responsible for the occurrence of the CEJ.

DOI： 10.5194/angeo-35-535-2017

Language：English   Publishing type：Research paper (scientific journal)  

Meridional movement of northern and southern equatorial ionization anomaly crests in the East-Asian sector during 2002–2003 SSW
The present paper investigates the asymmetrical variability of the location of the north and the south equatorial ionization anomaly (EIA) crests in the East-Asian sector, along with their association with simultaneous observations of equatorial electrojet (EEJ) strength, geomagnetic activity index, and solar flux index during the 2002–2003 sudden stratospheric warming (SSW) event. Analysis of these observations indicates the existence of a large-scale quasi 16-day periodic meridional movement in both EIA crests, and also reveals a strong correlation between the quasi 16-day oscillation in geomagnetic latitudes of the EIA crest and EEJ strength. The latitude of the northern/southern EIA crest and the EEJ strength indicate that obvious synchronous periodic oscillations were in-phase in the northern and southern hemisphere when the SSW occurred. In addition, it is also found that both the EIA crest location and amplitude of the periodic movement of the EIA locations exhibit hemispheric asymmetry. The amplitude of the periodic movement of the EIA location in the southern hemisphere is larger than that of the northern hemisphere, and the southern EIA crest is further off from the equator than the north one. Understanding these asymmetries requires a combination of mechanisms that involve at least trans-equator meridional winds and the position of a sub-solar point; however, potential disturbances in neutral winds associated with the SSW may additionally complicate the equatorial ionospheric dynamics.

DOI： 10.1007/s11430-016-0096-y

Language：English   Publishing type：Research paper (scientific journal)  

A kink-like neutral sheet oscillation event observed by Cluster between 1436 and 1445 UT on 15 October 2004 has been investigated. The oscillations with periods between 40 and 60 s, observed at (−13.1, 8.7, −0.5) RE, are dominant in BX and BY. And they propagate mainly duskward with a velocity of (86, 147, 46) km/s. Their periods and velocity can be explained by the magnetic double-gradient instability. These oscillations are accompanied by strong field-aligned currents (FACs), which prefer to occur near the strongly tilted current sheet, and local maximum FAC tends to occur near the neutral sheet. The FACs show one-to-one correlated with a high-latitude Pi2 pulsation event recorded by KTN and TIK stations with a delay time of ~60 and 90 s, respectively. Both the Pi2 and oscillations propagate westward with a comparative conjunctive speed. These findings suggest a strong relation between the FACs and Pi2, and we infer that the Pi2 is caused by the FACs. The periods of the FACs are modulated by the oscillations but not exactly equal, which is one possible reason that the period of the Pi2 caused by the FACs could be different from the oscillations. We speculate that a current circuit between the plasma sheet and ionosphere can be formed during strongly tilted current sheet, and successive tilted current sheet could generate quasiperiodic multiple FAC systems, which can generate high-latitude Pi2 pulsations and control their periods.

DOI： 10.1002/2016JA023370

Language：English   Publishing type：Research paper (scientific journal)  

A kink-like neutral sheet oscillation event observed by Cluster between 1436 and 1445 UT on 15 October 2004 has been investigated. The oscillations with periods between 40 and 60s, observed at (-13.1, 8.7, -0.5) R-E, are dominant in B-X and B-Y. And they propagate mainly duskward with a velocity of (86, 147, 46)km/s. Their periods and velocity can be explained by the magnetic double-gradient instability. These oscillations are accompanied by strong field-aligned currents (FACs), which prefer to occur near the strongly tilted current sheet, and local maximum FAC tends to occur near the neutral sheet. The FACs show one-to-one correlated with a high-latitude Pi2 pulsation event recorded by KTN and TIK stations with a delay time of -60 and 90s, respectively. Both the Pi2 and oscillations propagate westward with a comparative conjunctive speed. These findings suggest a strong relation between the FACs and Pi2, and we infer that the Pi2 is caused by the FACs. The periods of the FACs are modulated by the oscillations but not exactly equal, which is one possible reason that the period of the Pi2 caused by the FACs could be different from the oscillations. We speculate that a current circuit between the plasma sheet and ionosphere can be formed during strongly tilted current sheet, and successive tilted current sheet could generate quasiperiodic multiple FAC systems, which can generate high-latitude Pi2 pulsations and control their periods.

DOI： 10.1002/2016JA023370

Language：English   Publishing type：Research paper (scientific journal)  

MAGDAS (Magnetic Data Acquisition System) is a magnetometer initiated by Kyushu University used to study space weather. The latest of real-time Magnetic Data Acquisition System of Circum-pan Pacific Magnetometer Network was successfully installed at University Malaysia Sabah (UMS) Sabah, Malaysia by International Center for Space Weather Science and Education, ICSWSE, Kyushu University, Japan. This is the second magnetometer under MAGDAS after Langkawi station (LKW) (geographic latitude and longitude: 6.30º, 99.78º and geomagnetic latitude and longitude: -2.32º, 171.29º). This paper reports on first analysis of geomagnetic observation of MAGDAS at Sabah station. The geomagnetic components: H, D, Z and total F obtained from MAGDAS’s magnetometer are presented. The diurnal variation of geomagnetic elements during initial measurement at Sabah indicates a good variation pattern. The validation of amplitude variation with the nearest station also significantly relative.

DOI： 10.5013/IJSSST.a.17.41.30

Language：English   Publishing type：Research paper (scientific journal)  

Longitudinal profile of the equatorial electrojet current and its dependence on solar activity
The equatorial electrojet (EEJ) is a strong eastward ionosphere current flowing in a narrow band along dip equator. It is flowing eastward in the dayside of equatorial E region of about 600 km wide and within 90 to 130 km altitude. The objectives of this research is to study the variations Equatorial Electrojet (EEJ) current measured from ground_based magnetometer. The data were provided by magnetometer network such as Magnetic Data Acquisition System (MAGDAS)/Circum-pan Pacific Magnetometer Network (CPMN), Indian Institute of Geomagnetism (IIG), International Real-time Magnetic Observatory Network (INTERMAGNET). We also request the data from the single observation station such as Fuquene (FUQ). The analysis is carried out using the EUEL index and normalization technique is applied to yield better results with the aids of CM4 current model. This involves fourteen stations in South America, African, Indian and Southeast Asian sectors. The analysis done are including longitudinal and solar activity dependence where the years revolves are 2008 and 2011. These two years represents the minimum phases of solar activity (2008) and inclining phases of solar activity (2011). The results obtained show that EEJ varies with longitudes and solar activity. EEJ current is found to be highest at the American sector and lowest between African and Indian sector. This current is also highest in inclination phase during 2011 compared to 2008 (solar minimum).

DOI： 10.1166/asl.2017.8372

Language：English   Publishing type：Research paper (scientific journal)  

Latitudinal variation of ionospheric currents in Southeast Asian sector
The equatorial electrojet (EEJ) is a current system caused by the enhanced ionospheric conductivity near the dayside magnetic dip equator. In this study, we attempt to obtain the latitudinal profile of Sq and total current component (EEJ+Sq) and consequently attain the EEJ latitudinal variation. Analysis is carried on using magnetic data recorded at six stations in Southeast Asian sector. The data used are equatorial electrojet index, EUEL, which were calculated from geomagnetic northward, H, component on geomagnetically quiet days. We further compared latitudinal profile of Sq and total current during solar minimum (2008) and solar maximum (2014) in order to study their solar cycle dependence. Furthermore, we examined the EEJ circuit for both solar activity period. Result shows that as the EEJ strength increase with solar activity, it contribute to higher increase in total current during 2014, and consequently contribute to significant difference in the total current latitudinal profile during 2008 and 2014. On the other hand, the EEJ latitudinal variation illustrates the current circuit where the reverse current was observed to start on the flanks of dip equator at about 5° and terminate at less than 10° latitude in both hemispheres.

DOI： 10.1166/asl.2017.8358

Language：English   Publishing type：Research paper (scientific journal)  

The Equatorial Electrojet (EEJ) is a narrow band of electrons flowing from east to west at daytime at low latitudes. The electron current produces a magnetic field variation that can be measured at different latitudes. In this work, we have used the data analysis in order to quantify the solar and lunar contributions to those variations and study the morphology of the EEJ current.

DOI： 10.1017/S1743921318000662

Language：English   Publishing type：Research paper (scientific journal)  

We investigated evolution of ionospheric currents during sudden commencements using a ground magnetometer network in conjunction with an all-sky imager, which has the advantage of locating field-aligned currents much more accurately than ground magnetometers. Preliminary (PI) and main (MI) impulse currents showed two-cell patterns propagating antisunward, particularly during a southward interplanetary magnetic field (IMF). Although this overall pattern is consistent with the Araki (solar wind sources of magnetospheric ultra-low-frequency waves. Geophysical monograph series, vol 81. AGU, Washington, DC, pp 183-200, 1994. doi:10.1029/GM081p0183) model, we found several interesting features. The PI and MI currents in some events were highly asymmetric with respect to the noon- midnight meridian; the post-noon sector did not show any notable PI signal, but only had an MI starting earlier than the pre-noon MI. Not only equivalent currents but also aurora and equatorial magnetometer data supported the much weaker PI response. We suggest that interplanetary shocks impacting away from the subsolar point caused the asymmetric current pattern. Additionally, even when PI currents form in both pre- and post-noon sectors, they can initiate and disappear at different timings. The PI currents did not immediately disappear but coexisted with the MI currents for the first few minutes of the MI. During a southward IMF, the MI currents formed equatorward of a preexisting DP-2, indicating that the MI currents are a separate structure from a preexisting DP-2. In contrast, the MI currents under a northward IMF were essentially an intensification of a preexisting DP-2. The magnetometer and imager combination has been shown to be a powerful means for tracing evolution of ionospheric currents, and we showed various types of ionospheric responses under different upstream conditions.

DOI： 10.1186/s40623-016-0517-y

Language：English   Publishing type：Research paper (scientific journal)  

To clarify the effect of the dawn and dusk terminators on Pi2 pulsations, we statistically analyzed the longitudinal phase and amplitude structures of Pi2 pulsations at middle- to low-latitude stations (GMLat = 5.30°-46.18°) around both the dawn and dusk terminators. Although the H (north-south) component Pi2s were affected by neither the local time (LT) nor the terminator location (at 100 km altitude in the highly conducting E region), some features of the D (east-west) component Pi2s depended on the location of the terminator rather than the LT. The phase reversal of the D component occurred 0.5-1 h after sunrise and 1-2 h before sunset. These phase reversals can be attributed to a change in the contributing currents from field-aligned currents (FACs) on the nightside to the meridional ionospheric currents on the sunlit side of the terminator, and vice versa. The phase reversal of the dawn terminator was more frequent than that of the dusk terminator. The D-to-H amplitude ratio on the dawn side began to increase at sunrise, reaching a peak approximately 2 h after sunrise (the sunward side of the phase reversal region), whereas the ratio on the dusk side reached a peak at sunset (the antisunward side). The dawn-dusk asymmetric features suggest that the magnetic contribution of the nightside FAC relative to the meridional ionospheric current on the dusk side is stronger than that on the dawn side, indicating that the center of Pi2-associated FACs, which probably corresponds to the Pi2 energy source, tends to be shifted duskward on average. Different features and weak sunrise/sunset dependences at the middle-latitude station (Paratunka, GMLat = 46.18°) can be attributed to the larger annual variation in the sunrise/sunset time and a stronger magnetic effect because of closeness from FACs. The D-to-H amplitude ratio decreased with decreasing latitude, suggesting that the azimuthal magnetic field produced by the FACs in darkness and the meridional ionospheric current in sunlight also decreased with decreasing latitude.

DOI： 10.1186/s40623-016-0514-1

Language：English   Publishing type：Research paper (scientific journal)  

The auroral intensification at the poleward boundary of the auroral oval is often considered to be the ionospheric manifestation of the distant reconnection. In the present study, however, we propose that the poleward boundary intensifications (PBIs) are initiated by ionospheric polarization due to fast polar cap flows, which are known to be well correlated with PBIs. The current continuity at the ionosphere can be described in two different ways, that is, the reflection of an Alfvén wave and the closure of Pedersen and Hall currents with field-aligned currents (FACs). The required consistency between the two approaches sets a framework for modeling the ionospheric polarization, and we numerically test the aforementioned idea focusing on an induced upward FAC as indicative of PBIs. The results show that in case the polar cap flow channel approaches the auroral oval perpendicularly from poleward, (i) upward and downward FACs are induced at the poleward boundary to the west and east of the longitudinal center of the flow channel, respectively; (ii) those induced FACs extend much wider in longitude than the flow channel; (iii) the peak densities of those induced FACs are significantly larger than those of the incident FACs; (iv) those induced FACs are distributed almost symmetrically in longitude, indicating that the Pedersen polarization dominates the Hall polarization; and (v) if the polar cap flow inclined dawnward (duskward), an upward (downward) FAC is induced first. These results are consistent with the reported characteristics of PBIs, which are rather difficult to explain otherwise.

DOI： 10.1002/2016JA023143

Language：English   Publishing type：Research paper (scientific journal)  

Ultralow frequency (ULF) waves generated in the ion foreshock are a well-known source of Pc3-Pc4 waves (7–100 mHz) observed in the dayside magnetosphere. We use data acquired on 10 April 2013 by multiple spacecraft to demonstrate that ULF waves of upstream origin can propagate to the midnight sector of the inner magnetosphere. At 1130–1730 UT on the selected day, the two Van Allen Probes spacecraft and the geostationary ETS-VIII satellite detected compressional 20 to 40 mHz magnetic field oscillations between L ∼ 4 and L ∼ 7 in the midnight sector, along with other spacecraft located closer to noon. Upstream origin of the oscillations is concluded from the wave frequency that matches a theoretical model, globally coherent amplitude modulation, and duskward propagation that is consistent with expected entry of the upstream wave energy through the dawnside flank under the observed interplanetary magnetic field. The oscillations are attributed to magnetohydrodynamic fast-mode waves based on their propagation velocity of ∼300 km/s and the relationship between the electric and magnetic field perturbations. The magnitude of the azimuthal wave number is estimated to be ∼30. There is no evidence that the oscillations propagated to the ground in the midnight sector.

DOI： 10.1002/2016JA022958

Language：English   Publishing type：Research paper (scientific journal)  

We present solar quiet (Sq) variation of the horizontal (H) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the SqH magnitude that enveloped the geomagnetic latitudes between 21.13°N (Fayum FYM) in Egypt and 39.51°S (Durban DRB) in South Africa was observed, while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north-south asymmetry of the SqH, which is indicative of higher SqH magnitude in the Northern Hemisphere compared to the Southern Hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ) was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern Hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean, and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector.

DOI： 10.1002/2016JA022857

Language：English   Publishing type：Research paper (scientific journal)  

In this study, we examined middle- and low-latitude Pi2 events to address the following two issues regarding the well-known substorm current wedge (SCW) model for Pi2 pulsations: (1) the center of the SCW, which is estimated using the Pi2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern; and (2) although ideally Pi2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultraviolet Imager data. We assumed that the ionospheric footprint of the upward field-aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle-latitude station Zyryanka (ZYK; GMLAT = 59.6°) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle-latitude Pi2. Our results suggest that the circular shapes of Pi2 polarization curves are caused by the delayed driven Alfvénic waves that are superimposed on the geomagnetic northward components of SCW oscillations.

DOI： 10.1002/2015JA021698

Language：English  

Eastward-expanding auroral surges observed in the post-midnight sector during a multiple-onset substorm
We present three eastward-expanding auroral surge (EEAS) events that were
observed intermittently at intervals of about 15 min in the post-midnight
sector (01:55-02:40 MLT) by all-sky imagers and magnetometers in northern
Europe. It was deduced that each surge occurred just after each onset of a
multiple-onset substorm, which was small-scale and did not clearly expand
westward, because they were observed almost simultaneously with Pi 2 pulsations
at the magnetic equator and magnetic bay variations at middle-to-high latitudes
associated with the DP-1 current system. The EEASs showed similar properties to
omega bands or torches reported in previous studies, such as recurrence
intervals of about 15 min, concurrence with magnetic pulsations with amplitudes
of several tens of nanotesla, horizontal scales of 300-400 km, and occurrence
of a pulsating aurora in a diffuse aurora after the passage of the EEASs.
Furthermore, the EEASs showed similar temporal evolution to the omega bands,
during which eastward-propagating auroral streamers occurred simultaneously in
the poleward region, followed by the formation of north-south-aligned auroras,
which eventually connected with the EEASs. Thus, we speculate that EEASs may be
related to the generation process of omega bands. (Continued)

DOI： 10.1186/s40623-015-0350-8

Language：English   Publishing type：Research paper (scientific journal)  

In this study, we examined middle- and low-latitude Pi2 events to address the following two issues regarding the well-known substorm current wedge (SCW) model for Pi2 pulsations: (1) the center of the SCW, which is estimated using the Pi2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern; and (2) although ideally Pi2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultraviolet Imager data. We assumed that the ionospheric footprint of the upward field-aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle-latitude station Zyryanka (ZYK; GMLAT=59.6 degrees) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle-latitude Pi2. Our results suggest that the circular shapes of Pi2 polarization curves are caused by the delayed driven Alfvenic waves that are superimposed on the geomagnetic northward components of SCW oscillations.

DOI： 10.1002/2015JA021698

Language：English   Publishing type：Research paper (scientific journal)  

The prolonged southward IMF-B<inf>z</inf> event of 2–4 May 1998: Solar, interplanetary causes and geomagnetic consequences
A detailed investigation is carried out to understand the prolonged (∼44 h) weakly southward interplanetary magnetic field (IMF-Bz) condition during 2–4 May 1998. In situ observations, during the period, show the passage of an expanding magnetic cloud embedded in an interplanetary coronal mass ejection (ICME), followed up by a shock and an interplanetary discontinuity driven by another ICME. It is the arrival of the ICMEs and the upfront shocks that caused the prolonged southward IMF-Bz condition. The magnetic configuration of the source regions of the IMF associated with the ICME interval was also examined, which showed open magnetic field structures, emanating from a small active region on the north of the heliospheric current sheet (HCS). The structures remained constantly to the north of the HCS, both on 29 April and 1 May, suggesting no change in their polarity. The draping of these outward directed radial field lines around the propagating CMEs in the shocked plasma explains the observed polarity changes of the IMF-Bz at 1 AU. In addition, multiple enhancements were also detected in the geomagnetic field variations, which showed a distinct one-to-one correspondence with the density pulses observed at 1 AU, during 0700–1700 UT on 3 May. The spectral analyses of both the variations showed the same discrete frequencies of 0.48, 0.65, and 0.75 mHz, demonstrating that the solar wind density enhancements can cause detectable global geomagnetic disturbances. The observations, thus, provide a deeper insight into the possible causes and geomagnetic consequences of a prolonged weakly southward IMF-Bz condition.

DOI： 10.1002/2015JA022185

Language：English   Publishing type：Research paper (scientific journal)  

A detailed investigation is carried out to understand the prolonged (approximate to 44h) weakly southward interplanetary magnetic field (IMF-B-z) condition during 2-4 May 1998. In situ observations, during the period, show the passage of an expanding magnetic cloud embedded in an interplanetary coronal mass ejection (ICME), followed up by a shock and an interplanetary discontinuity driven by another ICME. It is the arrival of the ICMEs and the upfront shocks that caused the prolonged southward IMF-B-z condition. The magnetic configuration of the source regions of the IMF associated with the ICME interval was also examined, which showed open magnetic field structures, emanating from a small active region on the north of the heliospheric current sheet (HCS). The structures remained constantly to the north of the HCS, both on 29 April and 1 May, suggesting no change in their polarity. The draping of these outward directed radial field lines around the propagating CMEs in the shocked plasma explains the observed polarity changes of the IMF-B-z at 1 AU. In addition, multiple enhancements were also detected in the geomagnetic field variations, which showed a distinct one-to-one correspondence with the density pulses observed at 1 AU, during 0700-1700 UT on 3 May. The spectral analyses of both the variations showed the same discrete frequencies of 0.48, 0.65, and 0.75 mHz, demonstrating that the solar wind density enhancements can cause detectable global geomagnetic disturbances. The observations, thus, provide a deeper insight into the possible causes and geomagnetic consequences of a prolonged weakly southward IMF-B-z condition.

DOI： 10.1002/2015JA022185

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

We present three eastward-expanding auroral surge (EEAS) events that were observed intermittently at intervals of about 15 min in the post-midnight sector (01:55-02:40 MLT) by all-sky imagers and magnetometers in northern Europe. It was deduced that each surge occurred just after each onset of a multiple-onset substorm, which was small-scale and did not clearly expand westward, because they were observed almost simultaneously with Pi 2 pulsations at the magnetic equator and magnetic bay variations at middle-to-high latitudes associated with the DP-1 current system. The EEASs showed similar properties to omega bands or torches reported in previous studies, such as recurrence intervals of about 15 min, concurrence with magnetic pulsations with amplitudes of several tens of nanotesla, horizontal scales of 300-400 km, and occurrence of a pulsating aurora in a diffuse aurora after the passage of the EEASs. Furthermore, the EEASs showed similar temporal evolution to the omega bands, during which eastward-propagating auroral streamers occurred simultaneously in the poleward region, followed by the formation of north-south-aligned auroras, which eventually connected with the EEASs. Thus, we speculate that EEASs may be related to the generation process of omega bands. On the other hand, the EEASs we observed had several properties that were different from those of omega bands, such as greater eastward propagation speed (3-4 km/s), shorter associated magnetic pulsation periods (4-6 min), and a different ionospheric equivalent current direction. The fast eastward propagation speed of the EEASs is consistent with the speed of eastward expansion fronts of the substorm current wedge reported in previous studies. The difference in the ionospheric current between the EEASs and omega bands may be caused by a large temporal variation of the surge structure, compared with the more stable wavy structure of omega bands.

DOI： 10.1186/s40623-015-0350-8

Language：English   Publishing type：Research paper (scientific journal)  

The characteristics of the F2 layer parameters NmF2 and hmF2 over Dibrugarh (27.5° N, 95° E, 17° N geomagnetic, 43° dip) measured by a Canadian Advanced Digital Ionosonde (CADI) for the period of August 2010 to July 2014 are reported for the first time from this low mid-latitude station lying within the daytime peak of the longitudinal wave number 4 structure of equatorial anomaly (EIA) around the northern edge of anomaly crest. Equinoctial asymmetry is clearly observed at all solar activity levels whereas the midday winter anomaly is observed only during high solar activity years and disappears during the temporary dip in solar activity in 2013 but forenoon winter anomaly can be observed even at moderate solar activity. The NmF2/hmF2 variations over Dibrugarh are compared with that of Okinawa (26.5° N, 127° E, 17° N geomagnetic), and the eastward propagation speed of the wave number 4 longitudinal structure from 95° E to 127° E is estimated. The speed is found to be close to the theoretical speed of the wave number 4 (WN4) structure. The correlation of daily NmF2 over Dibrugarh and Okinawa with solar activity exhibits diurnal and seasonal variations. The highest correlation in daytime is observed during the forenoon hours in equinox. The correlation of daily NmF2 (linear or non-linear) with solar activity exhibits diurnal variation. A tendency for amplification with solar activity is observed in the forenoon and late evening period of March equinox and the postsunset period of December solstice. NmF2 saturation effect is observed only in the midday period of equinox. Non-linear variation of neutral composition at higher altitudes and variation of recombination rates with solar activity via temperature dependence may be related to the non-linear trend. The noon time maximum NmF2 over Dibrugarh exhibits better correlation with equatorial electrojet (EEJ) than with solar activity and, therefore, new low-latitude NmF2 index is proposed taking both solar activity and EEJ strength into account.

DOI： 10.1186/s40623-015-0355-3

Language：English   Publishing type：Research paper (scientific journal)  

In this study, we constructed an empirical model of the equatorial electrojet (EEJ), including local time and longitudinal dependence, based on simultaneous data from 12 magnetometer stations located in six longitude sectors. An analysis was carried out using the equatorial electrojet index, EUEL, calculated from the geomagnetic northward H component. The magnetic EEJ strength is calculated as the difference between the normalized EUEL index of the magnetic dip equator station and the normalized EUEL index of the off-dip equator station located beyond the EEJ band. Analysis showed that this current is always strongest in the South American sector, regardless of local time (LT), and weakest in the Indian sector during 0900 and 1000 LT, but shifted to the African sector during 1100 to 1400 LT. These longitude variations of EEJ roughly follow variations of the inversed main field strength along the dip equator, except for the Indian and Southeast Asian sectors. The result showed that the EEJ component derived from the model exhibits a similar pattern with measured EEJ from ground data during noontime, mainly before 1300 LT.

DOI： 10.1186/s40623-015-0373-1

Language：English   Publishing type：Research paper (scientific journal)  

We studied the orbit of an electron revolving around an infinitely massive nucleus of a large classical Hydrogen atom subject to an AC electric field oscillating perpendicular to the electron’s circular orbit. Using perturbation theory in geometric algebra, we show that the equation of motion of the electron perpendicular to the unperturbed orbital plane satisfies a forced simple harmonic oscillator equation found in Lorentz dispersion law in Optics. We show that even though we did not introduce a damping term, the initial orbital position and velocity of the electron results to a solution whose absorbed energies are finite at the dominant resonant frequency ω = ω0; the electron slowly increases its amplitude of oscillation until it becomes ionized. We computed the average power absorbed by the electron both at the perturbing frequency and at the electron’s orbital frequency. We graphed the trace of the angular momentum vector at different frequencies. We showed that at different perturbing frequencies, the angular momentum vector traces epicyclical patterns.

Language：English   Publishing type：Research paper (scientific journal)  

High-resolution (1-s) geomagnetic data from the MAGDAS Network, Kyushu University, Japan, were examined to detect any local anomalous geomagnetic behavior that is possibly linked with the eruption of the Aso volcano, Kyushu Island, Japan, during 2011. Geomagnetic data recorded in the vicinity of the Aso volcano at the Kuju (KUJ) station, Japan, were analyzed and compared with those recorded at the Darwin (DAW) station in Australia, near the conjugate point of the KUJ station. The KUJ station is located about 20 km northeast of the Aso volcano. The amplitude range of daily variations was calculated and examined for the three geomagnetic components (H-, D-, and Z-components) along with the amplitude of ultra–low-frequency emissions in the Pc3-4 ranges (Pc3; 10–45 s and Pc4; 45–150 s). Data possessing and analysis reveal some local anomalous geomagnetic variations at the KUJ station during the 2011 volcanic activities of the Aso volcano. On the other hand, no anomalous changes were observed at the DAW station during that time. Therefore, the observed anomalous geomagnetic variations are considered to be of a local volcanic origin.

DOI： 10.1007/s12517-015-1858-8

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Magnetic and ionospheric disturbances in the far eastern region of Russia during the magnetic storm of 5 April 2010 are studied using data of geophysical stations operated by IKFIA SB RAS and IKIR FEB RAS. By performing wavelet analysis of experimental data, the wavelet powers of geomagnetic perturbations at different stations are estimated, in an attempt to investigate the dynamical development of a geomagnetic storm. It is shown that, though weak geomagnetic disturbances were present prior to the main phase of magnetic storm, the variations of the magnetic field during a storm development were found to be rather strong. The highest intensity of geomagnetic disturbances during the interplanetary shock at the Earth's magnetosphere was observed at KTN (L~9) while at ZYK (L~4) strongest geomagnetic perturbations
occurred during the magnetospheric substorm with the onset at 09:03 UT. Large geomagnetic fluctuations were recorded at TIX and CHD (L~5-6), when the High-Intensity Long-Duration Continuous AE Activity (HILDCAA) was observed on 6 April 2010. Ionospheric conditions at YAK (L~3.4) and PET (L~2.2) were characterized by a pre-storm enhancement in the electron density in the F2 layer on 4 April 2010 and prolonged negative phase of the ionospheric storm during the main and recovery phases of magnetic storm on 6-8 April 2010. These experimental results underscore the importance of multiinstrumental observations and provide clues to the complex interactive processes.

Language：English   Publishing type：Research paper (other academic)  

This paper presents variations of Ultra Low Frequency (ULF) and Very Low Frequency (VLF) during moderate geomagnetic storm using ground based observatories, Magnetic Data Acquisition System (MAGDAS)/Circum-Pan Magnetic Network (CPMN) magnetometer and Atmospheric Weather Electromagnetic System for Observation Modeling and Education (AWESOME) VLF receiver. In this analysis, the recorded ULF horizontal (H)-component are extracted from MAGDAS magnetometer at Yap Island (YAP), Federated States of Micronesia, Davao (DAV), Philippines, and Tirunelveli (TIR), India stations. VLF amplitude data were observed from VLF receiver at UKM while the transmitters are located at Katabomman (VTX), India, North West Cape (NWC), Australia and Lualualei, Hawaii (NPM), USA. The observations of the space weather conditions in this study are divided into two categories: quiet day (10 April 2010) and moderate geomagnetic storm (6 April 2010). The results show that the ULF and VLF variations are significantly affected during geomagnetic storm which geomagnetic activities are closely related with the solar parameters.

DOI： 10.1109/IconSpace.2015.7283762

Language：English   Publishing type：Research paper (other academic)  

In the E layer of equatorial ionosphere, the equatorial electrojet (EEJ) current interacts with the global Sq current and caused abnormally large amplitude of geomagnetic field component measured at the magnetic dip equator station. In this study, we attempted to separate EEJ and Sq at dip equator using the EUEL index derived from MAGDAS/CPMN stations. Moreover, we normalized the observation data to overcome the uncertainties due to the latitudinal variation of both EEJ and Sq currents which are neglected in most of previous studies. This new approach allowed the study of longitudinal dependence of EEJ. Analysis is performed using data from stations at East and West of Southeast Asian sector for a year of 2011. Additionally, we examined the solar activity dependence of EEJ during this inclining phase of solar cycle.

DOI： 10.1109/IconSpace.2015.7283763

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

The space–time characteristics of geomagnetic pulsations during a sudden impulse on August 4, 2010 have been analyzed using ground-based and satellite observations. It has been indicated that two types of geomagnetic pulsations with different spatial extensions, oscillation frequencies, and generations were observed at that time. It has been found that geomagnetic pulsations with identical oscillation frequencies (∼4.5 mHz) at different latitudes were observed, with a maximal amplitude in the dusk sector. Oscillations with close frequencies were registered in the solar wind in the IMF B<inf>z</inf> component. Higher-frequency (7–10 mHz) pulsations dependent on latitude were registered on the dawn side. It is assumed that geomagnetic pulsations with frequencies of ∼4.5 mHz were caused by oscillations penetrating from the interplanetary medium, and higher-frequency pulsations were Alfvén resonance oscillations generated during the compression of the magnetosphere. An asymmetric oscillation amplitude distribution relative to noon was caused by the IMF orthospiral orientation in this event.

DOI： 10.1134/S001095251504005X

Language：English   Publishing type：Research paper (scientific journal)  

Spatial variability of solar quiet fields along 96° magnetic meridian in Africa: Results from MAGDAS
We have used chains of Magnetic Data Acquisition System (MAGDAS) magnetometer records of the horizontal (H) and vertical (Z) magnetic field intensities during September 2008 to August 2009 (year of deep minimum) across Africa to study their variability during the quietest international days, which coincidently associated with the sudden stratospheric warming (SSW) event in January 2009. This selection of the most international quiet days is indicative of 80&#37; that are strongly associated with days when unusually strong and prolonged sudden SSW event occurs in January 2009. Interestingly, in January, a significant magnitude depletion of solar quiet (Sq) equivalent current was observed near noon hours around the magnetic equator (Addis Ababa, ABB) compared to any other months along with a consistent significantly reduced value across the Northern Hemisphere and moderate decrease at the Southern Hemisphere. Also, we found that Nairobi and Dar es Salaam at the Southern Hemisphere, which are close to ABB (dip equator), are strongly prone to westward electric field compared to the magnetic equator and Khartoum at the Northern Hemisphere. Significant negative values of MSq(Z) magnitudes observed near noon hours at Hermanus indicate the presence of induced currents that suggest ocean effects along with reversal to significant positive values in the afternoon, which subsided before 1800-LT in almost all the months, indicate stronger influence of ionospheric currents. On seasonal variability of Sq(H), a slight depression at ABB during September equinox is one of the evidences of seasonal Sq focus shift. Latitudinal variability of Sq near-noon hours was also investigated. Key Points Abnormal intensification of electric currents Latitudinal sections of the African stations Highest and positive magnitude in February

DOI： 10.1002/2014JA020728

Language：English   Publishing type：Research paper (scientific journal)  

We have used chains of Magnetic Data Acquisition System (MAGDAS) magnetometer records of the horizontal (H) and vertical (Z) magnetic field intensities during September 2008 to August 2009 (year of deep minimum) across Africa to study their variability during the quietest international days, which coincidently associated with the sudden stratospheric warming (SSW) event in January 2009. This selection of the most international quiet days is indicative of 80&#37; that are strongly associated with days when unusually strong and prolonged sudden SSW event occurs in January 2009. Interestingly, in January, a significant magnitude depletion of solar quiet (S-q) equivalent current was observed near noon hours around the magnetic equator (Addis Ababa, ABB) compared to any other months along with a consistent significantly reduced value across the Northern Hemisphere and moderate decrease at the Southern Hemisphere. Also, we found that Nairobi and Dar es Salaam at the Southern Hemisphere, which are close to ABB (dip equator), are strongly prone to westward electric field compared to the magnetic equator and Khartoum at the Northern Hemisphere. Significant negative values of MSq(Z) magnitudes observed near noon hours at Hermanus indicate the presence of induced currents that suggest ocean effects along with reversal to significant positive values in the afternoon, which subsided before 1800 LT in almost all the months, indicate stronger influence of ionospheric currents. On seasonal variability of S-q(H), a slight depression at ABB during September equinox is one of the evidences of seasonal S-q focus shift. Latitudinal variability of Sq near-noon hours was also investigated.

DOI： 10.1002/2014JA020728

Language：English   Publishing type：Research paper (scientific journal)  

We examined Pi2 pulsations observed simultaneously at low-latitude stations (L = 1.15 - 2.33) around the dawn terminator. Those Pi2 pulsations observed in the sunlit region were polarized in the azimuthal (D, positive eastward) direction. We found that the D component oscillations in the dark and sunlit regions were in antiphase, whereas the H component oscillated in phase. A statistical analysis indicates that these D component phase reversals occurred about 0.5 h sunward of the dawn terminator at 100 km in altitude, corresponding to the highly conducting E layer. The azimuthal polarization and D component phase reversals related to the dawn terminator cannot be explained by the existing models of low-latitude Pi2s (e.g., cavity resonance or substorm current wedge oscillations). Similar D component phase reversals were also found on the dusk side although the amplitude of the D component is smaller than that of the H component. We suggest that the meridional ionospheric current in the sunlit region adjacent to the dawn terminator drives the D component oscillations in antiphase with those D oscillations produced by the oscillatory field-aligned current (FAC) on the postmidnight side. The meridional current is expected to form a part of a current system that extends from the postmidnight FAC to the equatorial Cowling current. The D component oscillations in the Northern and Southern Hemispheres are also in antiphase, indicating that the current system is symmetric with respect to the equator. Key Points We examined Pi2 pulsations observed at low latitudes around the dawn terminator Phase reversals of the east-west component occur near the dawn terminator We suggest that meridional ionospheric current drives the east-west oscillation

DOI： 10.1002/2013JA019691

Language：English   Publishing type：Research paper (scientific journal)  

Pi2 pulsations observed around the dawn terminator
We examined Pi2 pulsations observed simultaneously at low-latitude stations (L = 1.15 - 2.33) around the dawn terminator. Those Pi2 pulsations observed in the sunlit region were polarized in the azimuthal (D, positive eastward) direction. We found that the D component oscillations in the dark and sunlit regions were in antiphase, whereas the H component oscillated in phase. A statistical analysis indicates that these D component phase reversals occurred about 0.5 h sunward of the dawn terminator at 100 km in altitude, corresponding to the highly conducting E layer. The azimuthal polarization and D component phase reversals related to the dawn terminator cannot be explained by the existing models of low-latitude Pi2s (e.g., cavity resonance or substorm current wedge oscillations). Similar D component phase reversals were also found on the dusk side although the amplitude of the D component is smaller than that of the H component. We suggest that the meridional ionospheric current in the sunlit region adjacent to the dawn terminator drives the D component oscillations in antiphase with those D oscillations produced by the oscillatory field-aligned current (FAC) on the postmidnight side. The meridional current is expected to form a part of a current system that extends from the postmidnight FAC to the equatorial Cowling current. The D component oscillations in the Northern and Southern Hemispheres are also in antiphase, indicating that the current system is symmetric with respect to the equator. Key Points We examined Pi2 pulsations observed at low latitudes around the dawn terminator Phase reversals of the east-west component occur near the dawn terminator We suggest that meridional ionospheric current drives the east-west oscillation

DOI： 10.1002/2013JA019691

Language：English   Publishing type：Research paper (scientific journal)  

CDF data archive and integrated data analysis platform for ERG-related ground data developed by ERG Science Center (ERG-SC)
The Exploration of energization and Radiation in Geospace project Science Center (ERG-SC) has developed the science data file archive and integrated data analysis tool for the ground network observation data of the ERG project. We have organized data design consortiums to develop and elaborate the standard metadata and data variable sets for each type of the ERG-related ground data being archived as Common Data Format (CDF) files. The integrated data analysis software for the project has also been developed on the basis of the Space Physics Environment Data Analysis Software (SPEDAS) which works in concert with the CDF data file repository. The software tools, which are provided as plug-in libraries for SPEDAS, are made available to the international science community so that scientists and students are ready to proceed to integrated studies combining the ground data with other satellite and simulation data seamlessly. The integrated data analysis software can make scientific activities more productive and help the ERG project achieve the scientific goals.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.asr.2014.11.010

Language：English   Publishing type：Research paper (scientific journal)  

We have examined simultaneous ULF activity in the Pi2 and Pc3 bands at the near-equatorial magnetic stations in South America from SAMBA and MAGDAS arrays and low-orbiting CHAMP satellite during its passage over this meridional network. At the nighttime, both Pi2 and Pc3 waves in the upper ionosphere and on the ground are nearly of the same magnitude and in-phase. At the same time, the daytime Pc3 pulsations on the ground and in space are nearly out-of-phase. Comparison of observational results with the theoretical notions on the MHD wave interaction with the system ionosphere-atmosphere-ground suggests that nighttime low-latitude Pi2 and Pc3 wave signatures are produced by magnetospheric fast compressional mode. The daytime near-equatorial Pc3 waves still resist a quantative interpretation. These waves may be produced by a combination of two mechanisms: compressional mode leakage through the ionosphere, and by oscillatory ionospheric current spreading towards equatorial latitudes.

DOI： 10.1016/j.asr.2014.11.010

Language：English   Publishing type：Research paper (scientific journal)  

Eleven strong Sumatran earthquakes, with their epicenter less than 550 km away from the Kototabang (KTB) geomagnetic station (2007-2012), were studied to examine the occurrence of anomalous ultra-low frequency emissions (ULF-EM). Anomalous ULF signals, possibly associated with the earthquake’s precursors, were determined by the Welch ratio S_Z/S_H at 0.06 Hz at the KTB station. These ULF anomalies were then compared with geomagnetic data observed from two reference stations in Darwin and Davao, to prevent misinterpretation of global geomagnetic disturbances as precursors. This study aims to analyze the relationship between earthquake magnitude and hypocenter radius, and seismic index against lead time during ULF-EM anomalies. We used the polarization ratio Welch method in terms of power spectrum density to evaluate the geomagnetic data by overlapping windows and applying fast Fourier transform (FFT). The results showed anomalous variations in onset and lead time, determined using the standard deviation controlling the S_Z/S_H power pattern. Our positive correlation between lead time of ULF emission and earthquake magnitude as well as between lead time and seismic index. It shows a negative correlation between hypocenter distances to KTB station against lead time.

DOI： 10.5614/j.math.fund.sci.2015.47.1.7

Language：English   Publishing type：Research paper (scientific journal)  

The solar zenith angle (SZA) dependence of the conductance is studied and a simple theoretical form for the Hall-to-Pedersen conductance ratio is developed, using the peak plasma production height. The European Incoherent Scatter (EISCAT) radar observations at Tromsø (67 MLAT) on 30 March 2012 were used to calculate the conductance. The daytime electric conductance is associated with plasma created by solar extreme ultraviolet radiation incident on the neutral atmosphere of the Earth. However, it has been uncertain whether previous conductance models are consistent with the ideal Chapman theory for such plasma productions. We found that the SZA dependence of the conductance is consistent with the Chapman theory after simple modifications. The Pedersen conductance can be understood by approximating the plasma density height profile as being flat in the topside E region and by taking into account the upward gradient of atmospheric temperature. An additional consideration is necessary for the Hall conductance, which decreases with increasing SZA more rapidly than the Pedersen conductance. This rapid decrease is presumably caused by a thinning of the Hall conductivity layer from noon toward nighttime. We expressed this thinning in terms of the peak production height in the Chapman theory. Key Points Solar zenith angle dependence of the conductance is theoretically studied The dependence is different between the Hall and the Pedersen conductances The Hall conductivity layer thins from noon to sunset

DOI： 10.1002/2014JA020665

Language：English   Publishing type：Research paper (scientific journal)  

A wavy current sheet event observed on 15 October 2004 between 1235 and 1300 UT has been studied by using Cluster and ground-based magnetometer data. Waves propagating from the tail center to the duskside flank with a period ∼30 s and wavelength ∼1 RE are superimposed on a flapping current sheet, accompanied with a bursty bulk flow. Three Pi2 pulsations, with onset at ∼1236, ∼1251, and ∼1255 UT, respectively, are observed at the Tixie station located near the foot points of Cluster. The mechanism creating the Pi2 (period ∼40 s) onset at ∼1236 UT is unclear. The second Pi2 (period ∼90 s, onset at ∼1251 UT) is associated with a strong field-aligned current, which has a strong transverse component of the magnetic field, observed by Cluster with a time delay ∼60 s. We suggest that it is caused by bouncing Alfvén waves between the northern and southern ionosphere which transport the field-aligned current. For the third Pi2 (period ∼60 s) there is almost no damping at the first three periods. They occur in conjunction with periodic field-aligned currents one-on-one with 72 s delay. We suggest that it is generated by these periodic field-aligned currents. We conclude that the strong field-aligned currents generated in the plasma sheet during flapping with superimposed higher-frequency waves can drive Pi2 pulsations on the ground, and periodic field-aligned currents can even control the period of the Pi2s. Key Points Flapping superimposed on waves (∼30 s) are accompanied with BBF during substorm The waves propagate from the tail center to the duskside flank Field-aligned currents drive Pi2s on the ground, even control their period

DOI： 10.1002/2014JA020526

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/s40623-014-0146-2

Language：English   Publishing type：Research paper (scientific journal)  

Space weather study has increasingly attracts the attention of many scientists to explore the interaction between solar activity and geomagnetic activity. Observation on Pc5 geomagnetic pulsations with periods ranging from 150-600seconds due to solar wind perturbations at the equatorial region currently not widely explored. In this paper, we will briefly discuss the data processing methods involve in order to analyze the geomagnetic data observed by magnetometer from geomagnetic observation stations; Tirunelveli (TIR), India, Langkawi (LKW), Malaysia and Yap Island (YAP), Federated States of Micronesia which are located at equatorial region. The explanation of the processing methods is based on the 24-hour data with 1 second sampling interval extracted during quiet and disturbed days. The result indicates that the higher amplitude of H geomagnetic field is recorded during daytime and maintain during nighttime at all analyzed stations. In addition, Pc5 ULF pulsations are corresponding well with the solar wind perturbations.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/2014SW001110

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.nrjag.2014.09.001

Language：English   Publishing type：Research paper (scientific journal)  

Multiple instrumental observations including GPS total electron content (TEC), foF2 and hmF2 from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14-17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30h below -10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N2 measurements and long-lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm-enhanced density plume in the initial phase. In addition, multiple short-lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) Bz in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset. Key Points Multiple PPEF occur when the IMF Bz was stable southwardThe negative ionospheric phase oscillated in the American sectorInhibitation of EIA was caused by O/N2 changes and DDEF

DOI： 10.1002/2014JA020273

Language：English   Publishing type：Research paper (scientific journal)  

Multiple instrumental observations including GPS total electron content (TEC), f(o)F(2) and h(m)F(2) from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14-17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30 h below -10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N-2 measurements and long-lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm-enhanced density plume in the initial phase. In addition, multiple short-lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) B-z in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset.

DOI： 10.1002/2014JA020273

Language：English   Publishing type：Research paper (scientific journal)  

We investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high-temporal (8s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field-aligned irregularities in the sporadic Es region. They had the same period of 110s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated similar to 8.0mV/m in the F region and similar to 2.0mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from 6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were -56 degrees and -45 degrees, and the amplitude ratios were 2.7x105m/s and 8.4x105m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfven waves must be taken in account.

DOI： 10.1002/2012JA018585

Language：English   Publishing type：Research paper (scientific journal)  

We investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high-temporal (8 s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field-aligned irregularities in the sporadic Es region. They had the same period of 110 s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated ~8.0 mV/m in the F region and ~2.0 mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from 6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were -56° and -45°, and the amplitude ratios were 2.7 × 105 m/s and 8.4 × 105 m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfvén waves must be taken in account. Key Points Midlatitude SuperDARN radar observed Pi2 pulsations in the E and F regions The electric field of Pi2 pulsations in the ionosphere are estimated The conception of pure cavity mode is not sufficient ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2012JA018585

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1186/1880-5981-66-20

Language：English   Publishing type：Research paper (scientific journal)  

Ground magnetic effects of the equatorial electrojet simulated by the TIE-GCM driven by TIMED satellite data
Quiet time daily variations of the geomagnetic field near the magnetic equator due to the equatorial electrojet are simulated using the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) and compared to those observed by ground-based magnetometers. Simulations are run both with and without tidal forcing at the height of the model lower boundary (∼97km). When the lower boundary forcing is off, the wind that generates an electromotive force in the model is primarily the vertically nonpropagating diurnal tide, which is excited in the thermosphere due to daytime solar ultraviolet heating. The lower boundary tidal forcing adds the effect of upward propagating tides, which are excited in the lower atmosphere and propagate vertically to the thermosphere. The main objective of this study is to evaluate the relative importance of these thermospherically generated tides and upward propagating tides in the generation of the equatorial electrojet. Fairly good agreement is obtained between model and observations when the model is forced by realistic lower boundary tides based on temperature and wind measurements from the Thermosphere-Ionosphere- Mesosphere Energetics and Dynamics (TIMED) satellite, as determined by Wu et al. (2012). The simulation results show that the effect of upward propagating tides increases the range of the geomagnetic daily variation in the magnetic-northward component at the magnetic equator approximately by 100&#37;. It is also shown that the well-known semiannual change in the daily variation is mostly due to upward propagating tides, especially the migrating semidiurnal tide. These results indicate that upward propagating tides play a substantial role in producing the equatorial electrojet and its seasonal variability. Key Points The ground magnetic effect of the equatorial electrojet is simulated Upward propagating tides explain about 50&#37; the magnetic effect The semiannual variation is mainly due to upward propagating tides ©2014. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/2013JA019487

Language：English   Publishing type：Research paper (scientific journal)  

Since the discovery of the radiation belt decades ago, there still remain some fundamental questions as to which one is the mechanism responsible for the acceleration of electrons. Groundbased Pc5 magnetic pulsation during the process of increasing of 2-MeV electron fluxes has been analyzed. First, a filter bandpass in the period range of 150-600 seconds has been used to localize the Pc5 waves. Second, we then applied a wavelet transform procedure, whereby the Morlet function as a mother wavelet was selected to analyze Pc5 wave packets. First, we show that dynamic pressure of solar wind controls the power of Pc5 magnetic pulsations. Second, by performing a cross-spectrum analysis of Pc5 wavelet during electron radiation belts we show that the wavelet power of Pc5 magnetic pulsations which is associated with a maximum wavelet cross spectrum show a similar change of Pc5 pulsations occurs during radiation belt events. Increasing of electron fluxes which is initiated by the presence of large power of Pc5 magnetic pulsations has been observed. This indicates that Pc5 magnetic pulsations could play a role in the acceleration and transport mechanism of the electron radiation belt. Also, 4-5 days from the beginning of increasing of electron fluxes we observed globally, a depression in the power of Pc5 magnetic pulsations as well as a monotonically decreasing of the solar wind dynamic pressure. On the other hand, during the end period of the electron belt, we also observed a sudden increasing of Pc5 power. We suggest that during the expansion periode of the outer electron radiation belt outward to interplanetary electron belt pressure that will reduce the solar wind dynamic pressure and consequently a decrease occurs in the power of Pc5 magnetic pulsation. And, in the end period of the electron radiation belt the electron fluxes back to its normal level and consequently a sudden increase of the Pc5 solar wind dynamic pressure occurs and that sudden increase also drives the sudden increasing power of Pc5 magnetic pulsations.

DOI： 10.14716/ijtech.v5i3.612

Language：English  

DOI： 10.1016/j.srt.2013.11.007.

Language：English   Publishing type：Research paper (scientific journal)  

Decomposition of horizontally extended current system into components in the polarization processes and extraction of Cowling channel defined in the companion paper embedded in the total current system are numerically demonstrated. We successfully visualize the background and polarization components in the magnetosphere-ionosphere coupling process by using the proposed theoretical framework. As a fundamental response, the polarization charge produced by the Pedersen current divergence has a role to cancel and intensify the ambient background electric field inside and outside the high-conductivity band, respectively. In contrast, the polarization charge produced by the Hall current divergence has a role to rotate the electric field from the background electric field, which causes a meandering of ionospheric convection flow along the boundary of a high-conductivity band. The Hall and Pedersen currents are always perpendicular to each other. They never close each other when conductances are homogeneous, but they can do that at the conductivity gradient region. This is the reason why a Hall polarization charge is induced and a resultant Cowling channel is formed. Key Points Illustration of Cowling channel Hall and Pedersen current close each other at the conductivity gradient region Identify the Hall polarization field and Alfven conductance ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50513

Language：English   Publishing type：Research paper (scientific journal)  

We present the first complete formulation of the coupling between the ionospheric horizontal currents (including Hall currents) and the field-aligned currents (FAC) via shear Alfven waves, which can describe the formation of a Cowling channel without any a priori parameterization of the secondary (Hall polarization) electric field strength. Our theory reorganizes the Cowling channel by "primary" and "secondary" fields. Until now there are no theoretical frameworks, which can derive these separated components from observed or given total conductance, electric field, and FAC distributions alone. But when a given incident where Alfven wave is considered as the driver, the reflected wave can be uniquely decomposed into the primary and secondary components. We show that the reflected wave can, depending on actual conditions, indeed carry FAC that connect to divergent Hall currents. With this new method, we can identify how large the secondary electric field becomes, how efficiently the divergent Hall current is closed within the ionosphere, and how much of the Hall current continues out to the magnetosphere as FAC. In typical ionospheric situations, only a small fraction of FAC is connected to Hall currents at conductance gradients, i.e., the secondary field is relatively strong. But when conductances are relatively low compared with Alfven conductance and/or horizontal scales smaller than ~10 km, the Hall FAC may become significant. Key Points Formation of Cowling channel coupling to the shear Alfven wave Identify the polarization field is generated to close the Hall current Our theory separates the Cowling channel into primary and secondary components ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50514

Language：English   Publishing type：Research paper (other academic)  

Associative relationship between magnetospheric ULF waves in the Pc 5 range (1.7-6.7 mHz) and high energetic particles on the geosynchronous orbit has been investigated in detail in past studies. In particular, there has been interest in the efficiency of global Pc 5 waves, i.e., those having small m-numbers, in accelerating particles [1]. In this paper we investigated this feature by using energetic electron fluxes at geosynchronous orbits, and by using low-latitude global-mode Pc 5 pulsations: We used superposed epoch analyses to show correlations.

DOI： 10.1109/IconSpace.2013.6599453

Language：English   Publishing type：Research paper (scientific journal)  

A one-dimensional kinetic model is constructed to simulate the electron acceleration by inertial Alfven waves. The electrons are divided into cold and hot electrons and treated separately. Cold components are described by the fluid equation and hot ones by the Vlasov equation, both carrying field-aligned currents. Intense variation of Alfven speed has been introduced by inclusion of cold electrons. The model results show that the exponential decrease of the plasma density plays a key role, which leads to the sharp gradient of both Alfven velocity and electron inertial length. When Alfven waves encounter this sharp gradient at lower altitudes, the electrons accelerated by the waves become super-Alfvenic, and the width of burst structures becomes much wider than the electron inertial length. Consequently, the background electrons carry the oppositely field-aligned current due to plasma oscillation. It is demonstrated that the current carried by the electrons exceeding the wavefront is balanced by the reverse current carried by background electrons. This mechanism can be used to reasonably explain observations of the electron bursts accompanied by little net field-aligned current. Furthermore, our simulation indicates another type of Alfven wave reflection due to mirror force and wave-particle interaction. Key Points Kinetic model which separates the electrons into cold and hot parts Explain the electron bursts accompanied by little net field-aligned current Another type of Alfven wave reflection due to wave-particle interaction ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50355

Language：English   Publishing type：Research paper (scientific journal)  

A one-dimensional kinetic model is constructed to simulate the electron acceleration by inertial Alfven waves. The electrons are divided into cold and hot electrons and treated separately. Cold components are described by the fluid equation and hot ones by the Vlasov equation, both carrying field-aligned currents. Intense variation of Alfven speed has been introduced by inclusion of cold electrons. The model results show that the exponential decrease of the plasma density plays a key role, which leads to the sharp gradient of both Alfven velocity and electron inertial length. When Alfven waves encounter this sharp gradient at lower altitudes, the electrons accelerated by the waves become super-Alfvenic, and the width of burst structures becomes much wider than the electron inertial length. Consequently, the background electrons carry the oppositely field-aligned current due to plasma oscillation. It is demonstrated that the current carried by the electrons exceeding the wavefront is balanced by the reverse current carried by background electrons. This mechanism can be used to reasonably explain observations of the electron bursts accompanied by little net field-aligned current. Furthermore, our simulation indicates another type of Alfven wave reflection due to mirror force and wave-particle interaction.

DOI： 10.1002/jgra.50355

Language：English  

The response of the dayside equatorial electrojet to step-like changes of IMF B<inf>Z</inf>
The equatorial electrojet (EEJ) is driven by zonal electric fields, which are known to be well correlated with the interplanetary electric field and therefore with the interplanetary magnetic field (IMF) BZ component. In the present study, we investigate how the equatorial H magnetic component, and therefore the EEJ, responds to step-like changes of IMF BZ. The reduction of southward IMF BZ (northward turning) and that of northward IMF BZ (southward turning) are examined separately. The result shows that for the northward turnings, the EEJ immediately starts to weaken with the accuracy of the estimates of the travel times of the IMF changes. The time constant of the response is much longer, and the equatorial H component decreases continuously by 40 nT for 30 min after the northward turnings. In contrast, the response of the EEJ to the southward turnings is far less clear in both magnitude and timing, and it does not depend on whether or not IMF BZ actually becomes southward. The difference in the EEJ response to the northward and southward turnings reflects at least partially the fact that the magnetosphere-ionosphere system is more sensitive to IMF B Z when IMF is southward than northward. It is suggested that the electric field penetrates from the polar region to the dip equator through a global current system that connects the auroral electrojets and the EEJ, and the ionospheric conductance in the polar region may play an important role in the formation of such a current system. Key Points EEJ starts to weaken within 1 min from the arrival of northward IMF turnings. The time constant of EEJ response to northward turnings is ~30 minutes. The response of EEJ to southward turnings is not clear in magnitude or timing. ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50318

Language：English   Publishing type：Research paper (scientific journal)  

We devise an approach to calculate the polarization electric field in the ionosphere, when the ionospheric conductances, the primary (modeled) or the total (measured) electric field, and the Cowling efficiency are given. In contrast to previous studies, our approach is a general solution which is not limited to specific geometrical setups, and all parameters may have any kind of spatial dependence. The solution technique is based on spherical elementary current (vector) systems (SECS). This way, we avoid the need to specify explicit boundary conditions for the searched polarization electric field of its potential which would be required if the problem was solved in a differential equation approach. Instead, we solve an algebraic matrix equation, and the implicit boundary condition that the divergence of the polarization electric field vanishes outside our analysis area is sufficient. In order to illustrate our theory, we then apply it to two simple models of auroral electrodynamic situations, the first being a mesoscale strong conductance enhancement in the early morning sector within a relatively weak southward primary electric field, and a morning sector auroral arc with only a weak conductance enhancement, but a large southward primary electric field at the poleward flank of the arc. While the significance of the polarization electric field for maximum Cowling efficiency is large for the first case, it is rather minor for the second one. Both models show that the polarization electric field effect may not only change the magnitude of the current systems but also their overall geometry. Furthermore, the polarization electric field may extend into regions where the primary electric field is small, thus even dominating the total electric field in these regions. For the first model case, the total Joule heating integrated over the analysis area decreases by a factor of about 4 for maximum Cowling efficiency as compared to the case of vanishing Cowling efficiency. Furthermore, for this case the resulting total electric field structurally shows a strong resemblance to that frequently observed during auroral omega band events. ©2013. American Geophysical Union. All Rights Reserved.

DOI： 10.1002/jgra.50254

Language：English  

The strong inhomogeneities in plasma parameters in the ionosphere and adjacent regions can trap waves in the upper end of the ULF range (Pc1/Pi1). The topside ionosphere is characterized by a rapidly increasing Alfvén speed with a scale height on the order of 1000 km. Shear-mode Alfvén waves in this region can be partially trapped at frequencies in the 0.1-1.0 Hz range. The same structure can trap fast-mode compressional waves in this frequency band. Since these waves can propagate across magnetic field lines, this structure constitutes a waveguide in which energy can propagate at speeds comparable to the Alfvén speed, typically on the order of 1000 km/s. Hall effects in the ionosphere couple these two wave modes, so that the introduction of a field-aligned current by means of a shearmode Alfvén wave can excite compressional waves that can propagate in the waveguide. In the limit of infinite ionospheric conductivity, these waves are isolated from the atmospheric fields; however, for finite conductivity, ionospheric and atmospheric waves are coupled. Transverse magnetic modes in the atmosphere can propagate at ULF frequencies and form global Schumann resonances with the fundamental at 8 Hz. It has been suggested that signals that propagate at the speed of light through this atmospheric waveguide can rapidly transmit signals from the polar region to lower latitudes during sudden storm commencements.

DOI： 10.1029/169GM19

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the characteristics of poleward moving auroral arcs (PMAAs) observed with the all-sky monochromatic imager at the South Pole Station (74.3 degrees CGLAT) in the interval 08:00-11:00 MLT on July 8, 2004. During this period, Pc 5 geomagnetic pulsations are also observed as well as quasi-stationary auroral patches (QSAPs), which keep their forms, luminosities, and locations for up to several hours. It is demonstrated that the PMAAs, Pc 5 pulsations in the north-south component, and oscillations in the drift of the QSAPs in the east-west direction have similar dominant periods and are well correlated with each other. The oscillatory ionospheric plasma drifts accompanying the PMAAs are extracted from the horizontal motion of the QSAPs and are found to propagate poleward with the PMAAs. The phase relations of the PMAAs and Pc 5 pulsations in the northward component with respect to the westward drift velocity of the QSAPs are close to anti-phase and in-phase, respectively. To examine whether these phenomena can be interpreted by field line resonances (FLRs), we perform a numerical simulation using a horizontally two-dimensional model that consists of the auroral acceleration region and the ionosphere with time-evolution conductivity. The results indicate that the observed data can be well explained by the FLR model with realistic physical parameters, except for the phase relation of the PMAAs with respect to the Pc 5 pulsations and the QSAPs.

DOI： 10.1029/2012JA017899

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the characteristics of poleward moving auroral arcs (PMAAs) observed with the all-sky monochromatic imager at the South Pole Station (74.3°CGLAT) in the interval 08:00-11:00 MLT on July 8, 2004. During this period, Pc 5 geomagnetic pulsations are also observed as well as quasi-stationary auroral patches (QSAPs), which keep their forms, luminosities, and locations for up to several hours. It is demonstrated that the PMAAs, Pc 5 pulsations in the north-south component, and oscillations in the drift of the QSAPs in the east-west direction have similar dominant periods and are well correlated with each other. The oscillatory ionospheric plasma drifts accompanying the PMAAs are extracted from the horizontal motion of the QSAPs and are found to propagate poleward with the PMAAs. The phase relations of the PMAAs and Pc 5 pulsations in the northward component with respect to the westward drift velocity of the QSAPs are close to anti-phase and in-phase, respectively. To examine whether these phenomena can be interpreted by field line resonances (FLRs), we perform a numerical simulation using a horizontally two-dimensional model that consists of the auroral acceleration region and the ionosphere with time-evolution conductivity. The results indicate that the observed data can be well explained by the FLR model with realistic physical parameters, except for the phase relation of the PMAAs with respect to the Pc 5 pulsations and the QSAPs.

DOI： 10.1029/2012JA017899

Language：English   Publishing type：Research paper (scientific journal)  

A study of latitudinal dependence of Pc 3-4 amplitudes at 96° magnetic meridian stations in Africa

Language：English   Publishing type：Research paper (scientific journal)  

We have analyzed an event on 14 February 2003 in which Cluster satellites and the CPMN ground magnetometer chain made simultaneous observations of a Pi 2 pulsation along the same meridian. Three of the four Cluster satellites were located outside the plasmasphere, while the other one was located within the plasmasphere. By combining the multipoint observations in space and the multipoint observations on the ground, we have obtained a detailed L-profile of the Pi 2 signatures, which has not been done in the past. In addition, we have used a method called Independent Component Analysis (ICA) to separate out other superposed waves with similar spectral components. The result shows that the wave phase of the Pi 2 was the same up to L &sim; 3.9 (corresponding to the plasmasphere), became earlier up to L &sim; 4.1 (corresponding to the plasmapause boundary layer), and showed a delaying tendency up to L &sim; 5.9 (corresponding to the plasmatrough). This systematic phase pattern, obtained for the first time by a combination of a ground magnetometer chain and multisatellites along a magnetic meridian with the aid of ICA, supports the interpretation that a Pi 2 signal propagated from a farther source and reached the plasmasphere.

DOI： 10.5194/angeo-29-1663-2011

Language：English   Publishing type：Research paper (scientific journal)  

An empirical model of the quiet daily geomagnetic field variation
An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp <= 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity.

DOI： 10.1029/2011JA016487

Language：English   Publishing type：Research paper (scientific journal)  

In order to correctly describe the dynamical behavior of the magnetosphere-ionosphere (MI) coupling system and the scale-dependent auroral process, we develop a synthesis formulation that combines the process of (1) the inverse Walen separation of MHD disturbance into parallel- and antiparallel-propagating shear Alfven wave to the ambient magnetic field, (2) the shear Alfven wave reflection process including (3) the scale-dependent electrostatic coupling process through the linearized Knight relation, (4) two-layer ionosphere model, and (5) dynamic conductance variations. A novel procedure that applies the inverse Walen relation to the incompressional MHD disturbances at the inner boundary of the MHD region enables to extract the component of the shear Alfven wave incident to the ionosphere. The extracted incident electric field supplies an electromotive force for the generation of the MI coupling system, and the reflected electric field is generated such that it totally satisfies the synthesis MI-coupling equation. A three-dimensional ionospheric current system is represented by a two-layer model in which the Pedersen and the Hall current are confined in the separated layers, which are connected by field-aligned currents driven by the linear current-voltage relation between two layers. Hence, our scheme possibly reproduces two types of the scale-dependent MI-decoupling process of the perpendicular potential structure: due to the parallel potential drop at the auroral acceleration region and the other due to the parallel potential differences inside the ionosphere. Our newly formulation may be well suited for description of scale-dependent auroral process and mesoscale ionospheric electrodynamics interlocked with the dynamical development of magnetospheric processes.

DOI： 10.1029/2011JA016460

Language：Japanese   Publishing type：Research paper (scientific journal)  

An Algorithm for Detecting Precursory Events from Time Series Data
一般に，前兆現象は突発現象にそのものに比べて非常に目立ちにくく，その開始時刻は曖昧である．従来よく用いられてきた変化点検出法を適用した場合，このような微小で緩慢な変化は見逃されやすい．Tokunagaら(2010)では，Ideら(2005)の提案した特異スペクトル分析を応用した変化点検出法（SST）を，多次元データを用いたアルゴリズム（MSST）へと拡張することで，鋭敏に前兆現象の開始時刻を推定できることを示した．MSSTは，緩慢な変化も検出できる鋭敏な手法であるが，実データへの適用では誤検出が問題になる．本稿では，突発現象の大まかな開始時刻をあらかじめ検出し，さらに検出された時刻の前後で前兆現象の開始時刻と終了時刻を個別に探索することで，前兆現象を鋭敏に検出でき，かつMSST単体よりも誤検出を劇的に減少させることができることを示す．In general, precursory events are observed as minute and less-visible fluctuations preceding an onset of massive fluctuations of extraordinary phenomena. Hence, existing change-point detection methods most likely overlook precursory events. Tokunaga, et al. (2010) extended the method for detecting the change-points, Singular Spectrum Transformation (SST) proposed by Ide, et al. (2005), to the multivariable SST (MSST). Although MSST can detect minute changes, we have to reduce false positives because real world data includes non-stationary trends and measurement noise. In this paper, we propose the algorithm to detect precursory events in an off-line manner. Our algorithm consists of three kinds of change-point detection methods. We show that the number of false positive reduce drastically by combining three different types of change-point detection methods.

Language：English   Publishing type：Research paper (scientific journal)  

To predict an occurrence of extraordinary phenomena, such as earthquakes, failures of engineering systems and financial market crushes, it is important to identify precursory events in time series. However, existing methods are limited in their applicability for real world precursor detections. Recently, Ide and Inoue [1] have developed an SSA-based change-point detection method, called singular spectrum transformation (SST). SST is suitable for detecting various types of change-points, but real world precursor detections can be far more difficult than expected. In general, precursory events are observed as minute and less-visible fluctuations preceding an onset of massive fluctuations of extraordinary phenomena and therefore they are easily over-looked. To overcome this point, we extend the conventional SST to the multivariable SST. The originality of our strategy is in focusing on synchronism detections of precursory events in multiple sequences of univariate time series. We performed some experiments by using artificial data and showed the superiority of multivariable SST in detecting onset of precursory events. Furthermore, the superiority is also shown statistically in determining the onset of precursory events by using real world time series.

Language：English   Publishing type：Research paper (scientific journal)  

In this report we present a temporal relationship between ground Pi2 and auroral kilometric radiation (AKR). We analyzed six isolated substorm events, which were observed by the MAGDAS/CPMN ground magnetometer network and the plasma wave instrument onboard the Polar satellite. We found that the time derivative of the height-integrated AKR power and the ground Pi2 D component had the same periodicity and that the two were synchronized with each other. When the D component fluctuated with the same (opposite) polarity as the magnetic bay variation, the AKR power tended to increase (decrease) during the corresponding interval. An isolated substorm event (AE similar to 40 nT), which occurred around 10: 19 UT on 24 January1997, was selected for a detailed study. The behavior of the Pi2 event can be interpreted by the substorm current wedge (SCW) and Pi2 propagation models. It is confirmed that the midlatitude and high-latitude D component oscillations can be treated as a proxy of the SCW oscillations, whereas the H component oscillations exhibited some phase shifts by the propagation delay of the Pi2 waves. That is, the temporal relation between the time derivative of the AKR power and the ground Pi2 suggests that the height-integrated AKR power was modulated coherently with the SCW oscillations.

DOI： 10.1029/2010JA016042

Language：English   Publishing type：Research paper (scientific journal)  

In this report we present a temporal relationship between ground Pi2 and auroral kilometric radiation (AKR). We analyzed six isolated substorm events, which were observed by the MAGDAS/CPMN ground magnetometer network and the plasma wave instrument onboard the Polar satellite. We found that the time derivative of the height-integrated AKR power and the ground Pi2 D component had the same periodicity and that the two were synchronized with each other. When the D component fluctuated with the same (opposite) polarity as the magnetic bay variation, the AKR power tended to increase (decrease) during the corresponding interval. An isolated substorm event (AE ∼ 40 nT), which occurred around 10:19 UT on 24 January1997, was selected for a detailed study. The behavior of the Pi2 event can be interpreted by the substorm current wedge (SCW) and Pi2 propagation models. It is confirmed that the midlatitude and high-latitude D component oscillations can be treated as a proxy of the SCW oscillations, whereas the H component oscillations exhibited some phase shifts by the propagation delay of the Pi2 waves. That is, the temporal relation between the time derivative of the AKR power and the ground Pi2 suggests that the height-integrated AKR power was modulated coherently with the SCW oscillations.

DOI： 10.1029/2010JA016042

Language：English   Publishing type：Research paper (other academic)  

To predict an occurrence of extraordinary phenomena, such as earthquakes, failures of engineering system and financial market crushes, it is important to identify precursory events in time series. However, existing methods are limited in their applicability for real world precursor detections. Recently, Ide and Inoue [1] have developed an SSA-based change-point detection method, called singular spectrum transformation (SST). In this paper, we extend the SST so that it is applicable for real world precursor detections, focusing on the wide applicability of the conventional SST. Although the SST is suitable for detecting various types of change-points, detecting precursors can be far more difficult than expected because, in general, real world time series contains measurement noise and non-stationary trends. Furthermore, precursory events are usually observed as minute and less-visible fluctuations preceding an onset of massive fluctuations of extraordinary phenomena and therefore they are easily over-looked. To overcome this, we extend the conventional SST to the multivariable SST, focusing on the synchronism detection of precursory events in multiple sequences of univariate time series. First, we would like to define the problem setting of real world precursory detections and consider its difficulties. Second, the multivariable SST is introduced. Third, we apply SST to geomagnetic time series data and show the multivariable SST is more suitable than the conventional SST for real world precursor detections. Finally, we show further experimental results using artificial data to evaluate the reliability of SST-based precursor detections.

Language：Others  

DOI： 10.1007/s10120-010-0559-y, 10.1016/j.ejca.2016.04.012_references_DOI_UX2wletTCS6Zbzczafu5vBKIFKF, 10.1634/theoncologist.2019-0601_references_DOI_UX2wletTCS6Zbzczafu5vBKIFKF, 10.3892/ol.2011.525_references_DOI_UX2wletTCS6Zbzczafu5vBKIFKF, 10.1200/jco.18.01

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1029/2009JA014397

Language：English   Publishing type：Research paper (scientific journal)  

Alfvenic-coupling algorithm for global and dynamical magnetosphere-ionosphere coupled system
This study proposes a new formula that describes a dynamical magnetosphere-ionosphere (MI) coupling system through the field-aligned current (FAC) closure and electrostatic potential connectivity. In the past, MI coupling processes were described as either "inductive'' or "static.'' The inductive coupling scheme is based on the reflection of MHD waves at the ionosphere, whereas in the static coupling scheme the electrostatic potential is determined through the FAC closure. In contrast, in our new formulation these two schemes are combined by the "Alfvenic-coupling'' algorithm. The concept of the Alfvenic coupling is as follows. When a distribution of ionospheric current is changed from the background condition through the mapping of magnetospheric disturbances and/or by the change of ionospheric conductivity, an ionospheric reflection electric filed is instantaneously generated to satisfy the current continuity condition. The electrostatic potential of this reflection field also feeds back to the magnetosphere and excites shear Alfvenic disturbances in the magnetosphere. Therefore, the current continuity condition has to include the FAC of Alfvenic disturbances. In this sense, the reflection potential is not only a modification of the ionospheric potential but also a source of inductive Alfvenic disturbances. Quantitative estimation of feedback components to the magnetosphere reveals that our new formulation is more suitable to global MI simulations than the static MI coupling, which has been the only scheme used in the past.

DOI： 10.1029/2009JA014924

Language：English   Publishing type：Research paper (scientific journal)  

Phase relation between Pi2-associated ionospheric Doppler velocity and magnetic pulsations observed at a midlatitude MAGDAS station
We examined the correlation between nighttime Pi2 pulsations detected simultaneously by a frequency modulated continuous wave (FM-CW) (HF) radar and by a ground magnetometer, both located at a midlatitude (L = 2.05) Magnetic Data Acquisition System station. Eighty-three Pi2 events were observed during the 43 day period from 23 September 2006 to 4 November 2006. The variations of the ground magnetic H component and ionospheric Doppler velocity (V*) exhibited high coherence for 80&#37; of the 83 Pi2 events, for about a half of which the H and V* variations have the same dominant frequency. For such events, V* led H by 90 degrees in phase, in the midnight sector of 2230-0300 LT. The average Ey (east-west electric field) amplitude derived from V* is 0.27 mV/m. The 90 degrees phase delay was not found for the five events that were observed near dusk and dawn. The phase relation of H and V* for Pi2s in the midnight sector may be explained in terms of the radial standing structure of compressional waves, i.e., cavity mode oscillation.

DOI： 10.1029/2009JA014397

Language：English  

In order to understand the electro-magnetic phenomenona in space, the Space Environment Research Center (SERC) of Kyushu University is deploying the MAGnetic Data Acquisition System of the Circum-pan Pacific Magnetometer Network (MAGDAS/CPMN). In this network, we also installed FM-CW (Frequency Modulated Continuous Wave) radars at Paratunka, Russia (PTK: Magnetic Latitude = 45.8°, Magnetic Longitude = 221.6°), Sasaguri, Japan (SAS: M. Lat. = 23.2°, M. Lon. = 199.6°), and Manila, Philippines (MNL: M. Lat. = 4.19°, M. Lon. = 192.4°) to detect the ionospheric electric field variations. The FM-CW radar is a kind of HF (High Frequency) radar that can measure the ionospheric Doppler velocity, from which we can calculate the ionospheric electric fields. In the present paper we will introduce our FM-CW radar array observations and its preliminary scientific results. The results are as follows: (1) At the time of SC, dusk-to-dawn and subsequent dawn-to-dusk electric field penetrate into the low-latitude ionosphere. (2) Pi 2-associated ionospheric electric field at L = 2.05 is a manifestation of the plasmaspheric cavity mode. (3) Ground Pc 5 pulsation seems to be driven by ionospheric electric field at low and equatorial region.

DOI： 10.1142/9789812838209_0027

Language：English   Publishing type：Research paper (scientific journal)  

The Mercury magnetospheric orbiter (MMO) of the Japanese-European BepiColombo mission carries a dual-sensor magnetometer, MMO/MGF. The sensors are of the classical fluxgate type mounted on a boom. For redundancy, each sensor carries its own electronics and is connected to a different data processing unit. MMO/MGF can sample the magnetic field at a rate of up to 128 Hz. The resulting comparatively high time resolution of the magnetic field measurements, i.e., down to 8 ms, will be necessary when studying the dynamics of and processes within the Hermean magnetosphere, since the Mariner 10 observations have shown that their typical time scales are much shorter than in the Earth's magnetosphere, by about a factor of 30. The high time resolution will also be very useful for studying the evolution of the still young solar wind plasma as well as interplanetary shocks at 0.3-0.46 AU. Of course, MMO/MGF is also well-prepared to assist the sister magnetometer aboard the Mercury planetary orbiter, MPO/MAG, in measuring Mercury's intrinsic magnetic field, in particular by helping to distinguish between temporal fluctuations and spatial variations.

DOI： 10.1016/j.pss.2008.05.019

Language：English   Publishing type：Research paper (scientific journal)  

We analyzed ground magnetometer data for the 10 International Quiet Days during 1996-2007. The data were obtained from 19 stations along 210°magnetic meridian (MM) of the Circum-pan Pacific Magnetometer Network (CPMN) covering both the Northern Hemisphere and Southern Hemisphere. From the daily variations of the geomagnetic field, we deduced the latitude-local time (LAT-LT) diagram of the equivalent Sq current system, which can be regarded as the superposition of the following three current systems: Sqo, Sq 1, and Sq2. The Sq0, Sq1, and Sq2 current systems are equivalent current systems for the yearly average, annual variation, and semiannual variation of the Sq field, respectively. We have examined temporal and spatial features of these current systems. The principal features are as follows: (1) The total current intensities of the Sq1 and Sq2 current systems are about 35&#37; and 15&#37; of that of the Sq0 current system, respectively. (2) The Sq0 and Sq2 current systems have a dayside vortex in each hemisphere, while the Sq1 current system has a single vortex centered at the equatorial region in the morning sector (∼1000 LT). Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009JA014638

Language：English   Publishing type：Research paper (scientific journal)  

Equivalent current systems for the annual and semiannual S-q variations observed along the 210 degrees MM CPMN stations
We analyzed ground magnetometer data for the 10 International Quiet Days during 1996-2007. The data were obtained from 19 stations along 210 degrees magnetic meridian (MM) of the Circum-pan Pacific Magnetometer Network (CPMN) covering both the Northern Hemisphere and Southern Hemisphere. From the daily variations of the geomagnetic field, we deduced the latitude-local time (LAT-LT) diagram of the equivalent S-q current system, which can be regarded as the superposition of the following three current systems: Sq(0), Sq(1), and Sq(2). The Sq(0), Sq(1), and Sq(2) current systems are equivalent current systems for the yearly average, annual variation, and semiannual variation of the S-q field, respectively. We have examined temporal and spatial features of these current systems. The principal features are as follows: (1) The total current intensities of the Sq(1) and Sq(2) current systems are about 35&#37; and 15&#37; of that of the Sq(0) current system, respectively. (2) The Sq(0) and Sq(2) current systems have a dayside vortex in each hemisphere, while the Sq(1) current system has a single vortex centered at the equatorial region in the morning sector (similar to 1000 LT).

DOI： 10.1029/2009JA014638

Language：English   Publishing type：Research paper (scientific journal)  

Equivalent current systems for the annual and semiannual S-q variations observed along the 210 degrees MM CPMN stations
We analyzed ground magnetometer data for the 10 International Quiet Days during 1996-2007. The data were obtained from 19 stations along 210 degrees magnetic meridian (MM) of the Circum-pan Pacific Magnetometer Network (CPMN) covering both the Northern Hemisphere and Southern Hemisphere. From the daily variations of the geomagnetic field, we deduced the latitude-local time (LAT-LT) diagram of the equivalent S-q current system, which can be regarded as the superposition of the following three current systems: Sq(0), Sq(1), and Sq(2). The Sq(0), Sq(1), and Sq(2) current systems are equivalent current systems for the yearly average, annual variation, and semiannual variation of the S-q field, respectively. We have examined temporal and spatial features of these current systems. The principal features are as follows: (1) The total current intensities of the Sq(1) and Sq(2) current systems are about 35&#37; and 15&#37; of that of the Sq(0) current system, respectively. (2) The Sq(0) and Sq(2) current systems have a dayside vortex in each hemisphere, while the Sq(1) current system has a single vortex centered at the equatorial region in the morning sector (similar to 1000 LT).

DOI： 10.1029/2009JA014638

Language：English   Publishing type：Research paper (scientific journal)  

The objective of this study is to understand better the propagation of Pi 2 waves in the nighttime region. We examined Pi 2 oscillations that showed high correlation between high- and low-latitude Magnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network stations (correlation coefficient: |γ| ≥ 0.75). For each horizontal component (H and D) we examined the magnetic local time (MLT) dependence of the delay time of high-latitude Pi 2 oscillations that corresponds to the highest correlation with the low-latitude Pi 2 oscillation. We found the delay time of the high-latitude H showed remarkable MLT dependence, especially in the premidnight sector: we found that in the premidnight sector the high-latitude H oscillation tends to delay from the low-latitude oscillation (<100 s). On the other hand, the delay time of the high-latitude D oscillation was not significant (∼±10 s) in the entire nighttime sector. We propose a Pi 2 propagation model to explain the observed delay time of high-correlation highlatitude H. The model quantitatively explains the trend of the event distribution. We also examined the spatial distribution of high-correlation Pi 2 events relative to the center of auroral breakups. It was found that the high-correlation Pi 2 events tend to occur away from the center of auroral breakups by more than 1.5 MLT. The present result suggests that the high-correlation H component Pi 2 oscillations at high latitude are a manifestation of forced Alfvén waves excited by fast magnetosonic waves. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009JA014163

Language：English   Publishing type：Research paper (scientific journal)  

The objective of this study is to understand better the propagation of Pi 2 waves in the nighttime region. We examined Pi 2 oscillations that showed high correlation between high- and low-latitude Magnetic Data Acquisition System/Circum Pan-Pacific Magnetometer Network stations (correlation coefficient: vertical bar gamma vertical bar >= 0.75). For each horizontal component (H and D) we examined the magnetic local time (MLT) dependence of the delay time of high-latitude Pi 2 oscillations that corresponds to the highest correlation with the low-latitude Pi 2 oscillation. We found the delay time of the high-latitude H showed remarkable MLT dependence, especially in the premidnight sector: we found that in the premidnight sector the high-latitude H oscillation tends to delay from the low-latitude oscillation (<100 s). On the other hand, the delay time of the high-latitude D oscillation was not significant (similar to +/- 10 s) in the entire nighttime sector. We propose a Pi 2 propagation model to explain the observed delay time of high-correlation high-latitude H. The model quantitatively explains the trend of the event distribution. We also examined the spatial distribution of high-correlation Pi 2 events relative to the center of auroral breakups. It was found that the high-correlation Pi 2 events tend to occur away from the center of auroral breakups by more than 1.5 MLT. The present result suggests that the high-correlation H component Pi 2 oscillations at high latitude are a manifestation of forced Alfven waves excited by fast magnetosonic waves.

DOI： 10.1029/2009JA014163

Language：English   Publishing type：Research paper (scientific journal)  

The solar flare effect (SFE) at dip equatorial latitudes sometimes shows the counter-Sq variation around local noon. In this paper, we refer to this type of SFE as SFE*. Observations obtained by the Circum-pan Pacific Magnetometer Network (CPMN) reveal some previously unknown characteristics of SFE*. Some of these features are the following: (1) the counter-Sq SFE variation is observed in limited longitude (local time) and latitude sectors (i.e., LT ≈ 1030-1300, DipLat. ≲, 1.5°) (2) outside this limited region, the amplitude of SFE(Z/) and preflare S q(H) have a linear relationship (3) SFE* is observed during the significant depression of the horizontal magnetic field at dip equatorial latitudes, which indicates the presence of westward electric fields whose origin remains to be understood. Copyright 2009 by the American Geophysical Union.

DOI： 10.1029/2009JA014124

Language：English   Publishing type：Research paper (scientific journal)  

The solar flare effect (SFE) at dip equatorial latitudes sometimes shows the counter-S-q variation around local noon. In this paper, we refer to this type of SFE as SFE*. Observations obtained by the Circum-pan Pacific Magnetometer Network (CPMN) reveal some previously unknown characteristics of SFE*. Some of these features are the following: (1) the counter-S-q SFE variation is observed in limited longitude (local time) and latitude sectors (i.e., LT approximate to 1030-1300, vertical bar DipLat.vertical bar less than or similar to 1.5 degrees), (2) outside this limited region, the amplitude of SFE(H) and preflare S-q(H) have a linear relationship, and (3) SFE* is observed during the significant depression of the horizontal magnetic field at dip equatorial latitudes, which indicates the presence of westward electric fields whose origin remains to be understood.

DOI： 10.1029/2009JA014124

Language：English   Publishing type：Research paper (scientific journal)  

The solar flare effect (SFE) at dip equatorial latitudes sometimes shows the counter-S-q variation around local noon. In this paper, we refer to this type of SFE as SFE*. Observations obtained by the Circum-pan Pacific Magnetometer Network (CPMN) reveal some previously unknown characteristics of SFE*. Some of these features are the following: (1) the counter-S-q SFE variation is observed in limited longitude (local time) and latitude sectors (i.e., LT approximate to 1030-1300, vertical bar DipLat.vertical bar less than or similar to 1.5 degrees), (2) outside this limited region, the amplitude of SFE(H) and preflare S-q(H) have a linear relationship, and (3) SFE* is observed during the significant depression of the horizontal magnetic field at dip equatorial latitudes, which indicates the presence of westward electric fields whose origin remains to be understood.

DOI： 10.1029/2009JA014124

Language：English   Publishing type：Research paper (bulletin of university, research institution)  

Language：English   Publishing type：Research paper (scientific journal)  

Traditionally, due to observational constraints, ionospheric modelling and data analysis techniques have been devised either in one dimension (e.g. along a single radar beam), or in two dimensions (e.g. over a network of magnetometers). With new upcoming missions like the Swarm ionospheric multi-satellite project, or the EISCAT 3-D project, the time has come to take into account variations in all three dimensions simultaneously, as they occur in the real ionosphere. The link between ionospheric electrodynamics and the neutral atmosphere circulation which has gained increasing interest in the recent years also intrinsically requires a truly 3-dimensional (3-D) description. In this paper, we identify five major science questions that need to be addressed by 3-D ionospheric modelling and data analysis. We briefly review what proceedings in the young field of 3-D ionospheric electrodynamics have been made in the past to address these selected question, and we outline how these issues can be addressed in the future with additional observations and/or improved data analysis and simulation techniques. Throughout the paper, we limit the discussion to high-latitude and mesoscale ionospheric electrodynamics, and to directly data-driven (not statistical) data analysis. © 2008 European Geosciences Union.

DOI： 10.5194/angeo-26-3913-2008

Language：English   Publishing type：Research paper (scientific journal)  

The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22-46°) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network (CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180°. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfvén waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the DIH ratio is also observed just after dawn at very low latitudes (22°). As Alfvén waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes. Copyright 2007 by the American Geophysical Union.

DOI： 10.1029/2007JA012585

Language：English   Publishing type：Research paper (scientific journal)  

The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22-46 degrees) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network ( CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180 degrees. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfven waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the D/H ratio is also observed just after dawn at very low latitudes (22 degrees). As Alfven waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes.

DOI： 10.1029/2007JA012585

Language：English   Publishing type：Research paper (scientific journal)  

The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22-46 degrees) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network ( CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180 degrees. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfven waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the D/H ratio is also observed just after dawn at very low latitudes (22 degrees). As Alfven waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes.

DOI： 10.1029/2007JA012585

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the source region of Pi 2 and its propagation mechanism in the magnetosphere, we have developed a three-dimensional (3D) propagation model of MHD waves in the magnetosphere and compared it with observational results presented by Uozumi et al. [Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes. J. Geophys. Res. 109, A08203, doi:10.1029/2003JA009898]. We have assumed the propagation mechanism of Pi 2 as follows: fast mode waves are firstly generated at the Pi 2 source region, propagate from there in a 3D manner, and then excite shear Alfvén mode waves via mode conversion. Thus excited Alfvén mode waves then propagate to the Earth, and are observed as Pi 2 pulsations at ground stations. Based on the above propagation, we have assumed the propagation path of Pi 2 starting from a Pi 2 source region located on the magnetic equatorial plane to a ground station, and calculated the Alfvén transit time or time of flight (TOF) of MHD waves propagating in the magnetosphere. The model for the 3D spatial distribution of the Alfvén speed and the fast mode speed (assumed identical, under cold-plasma condition) is constructed based on realistic magnetic field and plasma density models of the magnetosphere. Time differences in model TOFs among our ground stations have been compared with the observed differences in the Pi 2 maximum-power times among the ground stations presented by Uozumi et al. [Propagation characteristics of Pi 2 magnetic pulsations observed at ground high latitudes. J. Geophys. Res. 109, A08203, doi:10.1029/2003JA009898]. As a result, it is suggested that the most probable source location of Pi 2 is 9 RE and 22.5 MLT on the equatorial plane in the magnetotail. This is the first study that deduced the Pi 2 source location in a quantitative manner. The consistency between the observational and the numerical estimation result supports our assumption shown in the above as regards the energy transfer mechanisms of Pi 2s globally observed in the high-latitude region. It is suggested that the generation of fast mode waves in the near-Earth region at the onsets of substorms is the essential process for the global Pi 2 occurrence. © 2006.

DOI： 10.1016/j.pss.2006.03.016

Language：English   Publishing type：Research paper (scientific journal)  

The Earth's inner magnetosphere (inside 10 Re) is a region where particle energy increases to the relativistic energy range. This region is very important as a laboratory where high-energy particle acceleration can be directly measured in a dipolar field configuration, as well as for human activities in space including space weather prediction. Despite abundant in situ satellite measurements, this region has been "missing" because of several difficulties arising from the measurements, such as high-energy particle contamination of low-energy particle measurement, protection against the possible incidence of radiation belt particles on the satellite, and the difficulties of measuring three-dimensional particles over a broad energy range, from a few electron volts to more than 10 MeV. In this paper, we address important scientific topics and propose a possible configuration of small satellites termed Energization and Radiation in Geospace (ERG), which would provide new insights into the dynamics of the inner magnetosphere and strongly contribute to the International Living With a Star project.

DOI： 10.1016/j.asr.2005.05.089

Language：English  

Language：English   Publishing type：Research paper (scientific journal)  

During 28-31 October, 2003, a series of coronal mass ejections hit the magnetosphere and triggered two consecutive large storms. Three ground magnetometers at L=1.32-1.41 recorded field-line resonances (FLRs) during this interval. The FLR frequencies decreased from 0600 LT on 31 October 2003 during the main phase of the second storm until 12 LT when the recovery phase of this storm began. After the decrease, the FLR frequencies returned to its pre-storm value (at 0600 LT on 31 Oct.) in a few hours. The measured decrease in the FLR frequency suggests a relative increase in mass density along the field lines during the magnetic storm. On the other hand, the total electron content (TEC) data suggest that the ionospheric plasma number density during this storm was similar to that during quiet times. A possible explanation for the increase in mass density would be an outflow of the heavy ions (e.g., O+) from the ionosphere to the plasmasphere.

DOI： 10.1186/BF03351959

Language：English   Publishing type：Research paper (scientific journal)  

The north-south asymmetry of the amplitude of ULF pulsations in the Pc 3-5 band is studied using magnetic field data from the magnetically conjugate stations at L similar to 5.4: Kotzebue (KOT) in the northern hemisphere and Macquarie Island (MCQ) in the southern hemisphere. We obtained the following results for the northward (H) component of magnetic pulsations: (1) The north to south power ratio shows a maximum in the northern winter and a minimum in the northern summer. This "seasonal variation'' is stronger at higher frequencies (Pc 3 and Pc 4 frequencies). (2) The north to south power ratio for the Pc 4 and Pc 5 frequency band is basically greater than 1.0 for all seasons. This "positive offset'' is stronger at lower frequencies. The "seasonal variation'' implies that the magnetohydrodynamic (MHD) waves incident from the magnetosphere are more strongly shielded when the ionospheric conductivity is higher. The "positive offset'' may result from the difference of the background magnetic field intensity between KOT and MCQ.

DOI： 10.1029/2003JA010242

Language：English   Publishing type：Research paper (scientific journal)  

Local time distribution of low and middle latitude ground magnetic disturbances at sawtooth injections of 18-19 April 2002
[1] During a magnetic storm of 18 April 2002, quasi-periodic variations of the low-energy electron flux were observed by the LANL satellites at geosynchronous orbit; this phenomenon has been called the "sawtooth event." During this event, on the ground, magnetic bays and Pi 2 pulsations took place corresponding to each enhancement of the particle flux, and they had features typical to usual substorms. However, unlike typical substorms, the ACE satellite observed no apparent northward turnings of the IMF corresponding to the sawtooth event. In this study, we used ground magnetic data from middle- and low-latitude stations which are distributed widely in the longitudinal direction, selected from the CPMN (Circum-Pan Pacific Magnetometer Network) and INTERMAGNET stations and compared the magnetic variations during the sawtooth event with that of the typical substorm (Lester et al., 1984). We found that the local time distribution of the polarization axis of the Pi 2 pulsations show a good agreement with that for a typical substorm, except that the local time width of the expected current wedge was 12 hours. On the other hand, the H component is predominant in the amplitude of the magnetic bay on the ground; the distribution of the H component also suggests a 12-hour-wide current wedge, which did not develop much in time. From these features, it is suggested that a current wedge was formed during this sawtooth event, and it generated the Pi 2 pulsations on the ground. However, the local time width of the current wedges is much wider than typical substorms, and its uniqueness causes the ground features different from typical substorm-associated magnetic variations on the ground. Copyright 2005 by the American Geophysical Union.

DOI： 10.1029/2004JA010734

Language：English   Publishing type：Research paper (scientific journal)  

[1] During a magnetic storm of 18 April 2002, quasi-periodic variations of the low-energy electron flux were observed by the LANL satellites at geosynchronous orbit; this phenomenon has been called the "sawtooth event.'' During this event, on the ground, magnetic bays and Pi 2 pulsations took place corresponding to each enhancement of the particle flux, and they had features typical to usual substorms. However, unlike typical substorms, the ACE satellite observed no apparent northward turnings of the IMF corresponding to the sawtooth event. In this study, we used ground magnetic data from middle- and low-latitude stations which are distributed widely in the longitudinal direction, selected from the CPMN (Circum-Pan Pacific Magnetometer Network) and INTERMAGNET stations and compared the magnetic variations during the sawtooth event with that of the typical substorm (Lester et al., 1984). We found that the local time distribution of the polarization axis of the Pi 2 pulsations show a good agreement with that for a typical substorm, except that the local time width of the expected current wedge was 12 hours. On the other hand, the H component is predominant in the amplitude of the magnetic bay on the ground; the distribution of the H component also suggests a 12-hour-wide current wedge, which did not develop much in time. From these features, it is suggested that a current wedge was formed during this sawtooth event, and it generated the Pi 2 pulsations on the ground. However, the local time width of the current wedges is much wider than typical substorms, and its uniqueness causes the ground features different from typical substorm-associated magnetic variations on the ground.

DOI： 10.1029/2004JA010734

Language：English   Publishing type：Research paper (scientific journal)  

In order to investigate the characteristics of Pi 2 propagation observed in the high-latitude region, ground magnetometer data obtained at high-latitude CPMN stations were analyzed. The power of magnetic perturbation, (DeltaH)(2) + (DeltaD)(2), were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotel'nyy (KTN: MLAT = 69.94degrees, MLON = 201.02degrees) reached the maximum amplitude earlier than those at lower-latitude station Tixie (TIK: MLAT = 65.67degrees, MLON = 196.88degrees), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67degrees, MLON = 212.12degrees) and Kotzebue (KOT: MLAT = 64.52degrees, MLON = 249.72degrees). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been similar to22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward ( westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT ( set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210degrees magnetic meridian. Our results show that the low-latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high-latitude Pi 2s in addition to low-latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher-latitude region, for studying substorm onset timing issues.

DOI： 10.1029/2003JA009898

Language：English   Publishing type：Research paper (scientific journal)  

[1] In order to investigate the characteristics of Pi 2 propagation observed in the high-latitude region, ground magnetometer data obtained at high-latitude CPMN stations were analyzed. The power of magnetic perturbation, (ΔH)2 + (ΔD)2, were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotel'nyy (KTN: MLAT = 69.94°, MLON = 201.02°) reached the maximum amplitude earlier than those at lower-latitude station Tixie (TIK: MLAT = 65.67°, MLON = 196.88°), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67°, MLON = 212.12°) and Kotzebue (KOT: MLAT = 64.52°, MLON = 249.72°). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been ∼22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward (westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT (set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210° magnetic meridian. Our results show that the low-latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high-latitude Pi 2s in addition to low-latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher-latitude region, for studying substorm onset timing issues. Copyright 2004 by the American Geophysical Union.

DOI： 10.1029/2003JA009898

Language：English   Publishing type：Research paper (scientific journal)  

It has been generally accepted that there are two models for the propagation mechanism of Pc3 pulsations observed on the ground near the magnetic equator, as follows: (1) Compressional waves, propagating along the equatorial plane of the magnetosphere across the ambient magnetic field lines, arrive at the equatorial ionosphere and couple with the magnetic perturbations on the ground through the ionosphere (compressional wave model); (2) Alfvén waves or compressional waves, propagating into the high-latitude ionosphere, generate large-scale ionospheric current oscillations there, and then they leak to the low latitude and cause Pc3 pulsations near the magnetic equator (ionospheric current model). In order to clarify which of these models is more suitable for Pc3 pulsations on the ground near the magnetic equator, the longitudinal structures of their coherence, amplitude, and phase were statistically studied by using three longitudinally separated subequatorial stations, CRI (Φ = 3.01°, A = 273.44°), GUA (Φ = 5.60°, A = 215.50°), and MUT (Φ = 6.24°, A = 192.17°). We found a nearly inphase structure in the 0730-1700 LT sector and a nearly 180° phase shift across 0730 LT for high-coherence Pc3 events in the H component. The longitudinal phase structure can be explained by the ionospheric current model; in this case, the 180° phase shift across 0730 LT means a meridional current along the dawn terminator supplied from the source at higher latitudes. The compressional wave model cannot explain the 180° phase shift across 0730 LT if the ionosphere is assumed as an infinitely thin sheet; however, the compressional wave model still remains as a candidate if we treat the ionosphere as that having a finite thickness. The data at hand do not let us conclude which of the two models is more appropriate; still, the longitudinal phase structure, shown in this paper, is a new finding that has never been reported and is important for understanding how Pc3 pulsations propagate from the magnetosphere to the equatorial ionosphere. Copyright 2004 by the American Geophysical Union.

DOI： 10.1029/2003JA009903

Language：English   Publishing type：Research paper (scientific journal)  

[1] It has been generally accepted that there are two models for the propagation mechanism of Pc3 pulsations observed on the ground near the magnetic equator, as follows: ( 1) Compressional waves, propagating along the equatorial plane of the magnetosphere across the ambient magnetic field lines, arrive at the equatorial ionosphere and couple with the magnetic perturbations on the ground through the ionosphere ( compressional wave model); ( 2) Alfven waves or compressional waves, propagating into the high-latitude ionosphere, generate large-scale ionospheric current oscillations there, and then they leak to the low latitude and cause Pc3 pulsations near the magnetic equator ( ionospheric current model). In order to clarify which of these models is more suitable for Pc3 pulsations on the ground near the magnetic equator, the longitudinal structures of their coherence, amplitude, and phase were statistically studied by using three longitudinally separated subequatorial stations, CRI (Phi = 3.01degrees, Lambda = 273.44degrees), GUA (Phi = 5.60degrees, Lambda = 215.50degrees), and MUT (Phi = 6.24degrees, Lambda = 192.17degrees). We found a nearly inphase structure in the 0730 - 1700 LT sector and a nearly 180degrees phase shift across 0730 LT for high-coherence Pc3 events in the H component. The longitudinal phase structure can be explained by the ionospheric current model; in this case, the 180degrees phase shift across 0730 LT means a meridional current along the dawn terminator supplied from the source at higher latitudes. The compressional wave model cannot explain the 180degrees phase shift across 0730 LT if the ionosphere is assumed as an infinitely thin sheet; however, the compressional wave model still remains as a candidate if we treat the ionosphere as that having a finite thickness. The data at hand do not let us conclude which of the two models is more appropriate; still, the longitudinal phase structure, shown in this paper, is a new finding that has never been reported and is important for understanding how Pc3 pulsations propagate from the magnetosphere to the equatorial ionosphere.

DOI： 10.1029/2003JA009903

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

Excitation of a Hall-current generator by field-aligned current closure, via an ionospheric, divergent Hall-current, during the transient phase of magnetosphere-ionosphere coupling
To clarify the process by which an ionospheric current system is formed by field-aligned current (FAC) closure in the ionosphere, an inclusive formulation of magnetosphere-ionosphere (MI) coupling is undertaken. The "Hall-current generator", which is excited during the transient phase of MI coupling, plays a crucial role in the formation of the ionospheric rotational-current system. It extracts energy from the FAC system through the divergent Hajl-current and pumps it into the rotational Hall-current. The energy of the rotational current accumulates as an evanescent poloidal magnetic field, associated with the ionospheric surface wave. This accumulated energy is also fed back to the FAC system through the change in energy flow of the Hall-current generator. It is found that there is a typical timescale for the rotational-current system to accumulate or extract the poloidal magnetic energy of ionospheric surface waves. This depends on the inductance of the rotational-current system and the effective conductivity of the ionospheric rotational current. This characteristic timescale becomes the cause of an ionospheric inductive effect, such as a time delay or phase lag between the source electromagnetic field of the FAC and the corresponding poloidal magnetic field on the ground. The latter causes an inductive shielding effect on the amplitude of the geomagnetic disturbance. Numerical simulation has been able to explain the details of the physical process that occurs when the incident FAC is developing and decaying, and how the energy and current are redistributed into the other elements during the transient MI-coupling process Copyright 2002 by the American Geophysical Union.

DOI： 10.1029/2001JA009170

Language：English   Publishing type：Research paper (scientific journal)  

To clarify the process by which an ionospheric current system is formed by field-aligned current (FAC) closure in the ionosphere, an inclusive formulation of magnetosphere-ionosphere (MI) coupling is undertaken. The "Hall-current generator'', which is excited during the transient phase of MI coupling, plays a crucial role in the formation of the ionospheric rotational-current system. It extracts energy from the FAC system through the divergent Hall-current and pumps it into the rotational Hall-current. The energy of the rotational current accumulates as an evanescent poloidal magnetic field, associated with the ionospheric surface wave. This accumulated energy is also fed back to the FAC system through the change in energy flow of the Hall-current generator. It is found that there is a typical timescale for the rotational-current system to accumulate or extract the poloidal magnetic energy of ionospheric surface waves. This depends on the inductance of the rotational-current system and the effective conductivity of the ionospheric rotational current. This characteristic timescale becomes the cause of an ionospheric inductive effect, such as a time delay or phase lag between the source electromagnetic field of the FAC and the corresponding poloidal magnetic field on the ground. The latter causes an inductive shielding effect on the amplitude of the geomagnetic disturbance. Numerical simulation has been able to explain the details of the physical process that occurs when the incident FAC is developing and decaying, and how the energy and current are redistributed into the other elements during the transient MI-coupling process.

DOI： 10.1029/2001JA009170

Language：English   Publishing type：Research paper (scientific journal)  

[1] A diagrammatic method to intuitively describe the inductive response of wave fields in the magnetosphere-ionosphere-atmosphere-Earth (MIAE) electromagnetically coupled system is developed. The coupling process of magnetohydrodynamic (MHD) waves and atmospheric electromagnetic waves, which interact with an anisotropically conducting thin sheet ionosphere, can be understood in terms of a process of redistribution of the induced current of wave modes. On the basis of the current conservation law and Faraday's law of electromagnetic induction, a redistribution process of the source-induced current of wave modes to the secondary wave is simply illustrated. The purpose of this paper is to propose a methodology to describe the inductive response of a complex electromagnetically coupled system that is divided by a conducting boundary layer. As an example, we derive the reflection coefficient and the mode conversion ratio of MHD waves at an anisotropic, inductive ionosphere by using the concept of induced current redistribution. We also demonstrate that the redistribution process of wave modes under the law of conservation of induced current also satisfies an energy conservation law in the MIAE system. However, the true merit of this method is its applicability to the highly inhomogeneous region, in which many types of waves exist and interact through the arbitrary boundary layers.

DOI： 10.1029/2000JA000405

Language：English   Publishing type：Research paper (scientific journal)  

[i] A diagrammatic method to intuitively describe the inductive response of wave fields in the magnetosphere-ionosphere-atmosphere-Earth (MIAE) electromagnetically coupled system is developed. The coupling process of magnetohydrodynamic (MHD) waves and atmospheric electromagnetic waves, which interact with an anisotropically conducting thin sheet ionosphere, can be understood in terms of a process of redistribution of the induced current of wave modes. On the basis of the current conservation law and Faraday's law of electromagnetic induction, a redistribution process of the source-induced current of wave modes to the secondary wave is simply illustrated. The purpose of this paper is to propose a methodology to describe the inductive response of a complex electromagnetically coupled system that is divided by a conducting boundary layer. As an example, we derive the reflection coefficient and the mode conversion ratio of MHD waves at an anisotropic, inductive ionosphere by using the concept of induced current redistribution. We also demonstrate that the redistribution process of wave modes under the law of conservation of induced current also satisfies an energy conservation law in the MIAE system. However, the true merit of this method is its applicability to the highly inhomogeneous region, in which many types of waves exist and interact through thearbitrary boundary layers. Copyright 2002 by the American Geophysical Union.

DOI： 10.1029/2000JA000405

Language：English   Publishing type：Research paper (scientific journal)  

Hall-induced inductive shielding effect on geomagnetic pulsations
A new formula describing the inductive response of the magnetosphere-ionosphere-atmosphere-Earth electromagnetic coupled system to the time development of the field-aligned current (FAC) source-field is developed. Using this new formula, the Hall-induced inductive shielding effect (ISE) on geomagnetic pulsation is investigated. The ISE is caused by the reduction of the amplitude of total rotational current because of the generation of "inductive" rotational current by the inductive part of ionospheric divergent electric field, which originates the existence of divergent Hall current. It will be shown that the ISE is more efficient for high-frequency pulsations, large horizontal scale phenomena and high-conducting ionospheric conditions. Quantitative analysis strongly suggests that the ionospheric conductivity in the dayside hemisphere can easily reach and sometimes exceed the turning point of geomagnetic pulsations of which frequencies are in the Pc 3 ∼ 4 pulsations range.

DOI： 10.1029/2001GL013610

Language：English   Publishing type：Research paper (scientific journal)  

A new formula describing the inductive response of the magnetosphere-ionosphere-atmosphere-Earth electromagnetic coupled system to the time development of the field-aligned current (FAC) source-field is developed. Using this new formula, the Hall-induced inductive shielding effect (ISE) on geomagnetic pulsation is investigated. The ISE is caused by the reduction of the amplitude of total rotational current because of the generation of "inductive" rotational current by the inductive part of ionospheric divergent electric field, which originates the existence of divergent Hall current. It will be shown that the ISE is more efficient for high-frequency pulsations, large horizontal scale phenomena and high-conducting ionospheric conditions. Quantitative analysis strongly suggests that the ionospheric conductivity in the dayside hemisphere can easily reach and sometimes exceed the turning point of geomagnetic pulsations of which frequencies are in the Pc 3 ∼ 4 pulsations range.

Language：English   Publishing type：Research paper (scientific journal)  

How does the ionospheric rotational Hall current absorb the increasing energy from the field-aligned current system?
It has been well recognized that field-aligned current (FAC) systems lose their energy in the ionosphere through the Joule dissipation that is caused by their closure via the ionospheric Pedersen current, and that the ionospheric Hall current cannot contribute the total energy dissipation. However, it is also true that the rotational Hall current is excited by the incident FAC, and it radiates a Poynting vector that grows a poloidal-type magnetic field. Even if the Hall effect cannot do work on an external system, what does its contribution to the accumulation of poloidal magnetic energy really mean? In this paper, it is clarified that the divergent Hall current, excited during the transient phase of magnetosphere-ionosphere coupling, closes via the FAC and produces a Hall current generator, which pumps up the energy of the FAC system to increase the ionospheric rotational Hall current (together with its associated poloidal magnetic field).

DOI： 10.1029/2001GL014125

Language：English   Publishing type：Research paper (scientific journal)  

[1] The present paper deals with the transient behavior of MHD perturbations in the inner magnetosphere induced by an impulsive localized eastward current (source current) as a model of Pi2 pulsations in the magnetosphere. The magnetospheric model consists of a dipole magnetic field, plasmasphere, ionosphere with Pedersen conductivity, and a free outer boundary. The source current is an impulsive magnetospheric current at the onset of the substorm current wedge and is distributed around the equatorial plane of L = 10 with +/-2 hour longitudinal extent around midnight. The numerical results allow us to track variation in the expected Pi2 pulsation signals in both local time and L. The poloidal-mode wave exhibits plasmasphere virtual resonance, resulting in large amplitudes around midnight, weakening toward dayside. The toroidal-mode wave is excited as a field line resonance immediately after the wave front of the poloidal-mode wave crosses regions where the radial gradient of V-A is steep. The toroidal-mode wave has largest amplitude at the local time of the east/west edge of the source current. The duration of this wave is similar to5 min. In the middle plasmasphere where the radial gradient of the V-A is smaller, the poloidal-mode wave tends to predominate over the toroidal-mode wave. These numerical results are consistent with satellite observations, in so far as the day-night asymmetry of Pi2 pulsations and the observation of transient toroidal waves.

DOI： 10.1029/2001JA900137

Language：English   Publishing type：Research paper (scientific journal)  

The present paper deals with the transient behavior of MHD perturbations in the inner magnetosphere induced by an impulsive localized eastward current (source current) as a model of Pi2 pulsations in the magnetosphere. The magnetospheric model consists of a dipole magnetic field, plasmasphere, ionosphere with Pedersen conductivity, and a free outer boundary. The source current is an impulsive magnetospheric current at the onset of the substorm current wedge and is distributed around the equatorial plane of L = 10 with ±2 hour longitudinal extent around midnight. The numerical results allow us to track variation in the expected Pi2 pulsation signals in both local time and L. The poloidal-mode wave exhibits plasmasphere virtual resonance, resulting in large amplitudes around midnight, weakening toward dayside. The toroidal-mode wave is excited as a field line resonance immediately after the wave front of the poloidal-mode wave crosses regions ? where the radial gradient of VA is steep. The toroidal-mode wave has largest amplitude at the local time of the east/west edge of the source current. The duration of this wave is ̃5 min. In the middle plasmasphere where the radial gradient of the VA is smaller, the poloidal-mode wave tends to predominate over the toroidal-mode wave. These numerical results are consistent with satellite observations, in so far as the day-night asymmetry of Pi2 pulsations and the observation of transient toroidal waves. Copyright 2002 by the American Geophysical Union.

DOI： 10.1029/2001JA900137

Language：English   Publishing type：Research paper (scientific journal)  

Propagation property of the 2D-coupled MHD wave induced from an impulsive current is studied by using a magnetospheric model having the dipole magnetic field, tile ionosphere with Pedersen conductivity, and a free outer boundary. This study is the first step toward realistic modeling of Pi2 pulsation generated by the current wedge model. Numerical calculation reveals that the ionosphere controls not only damping of the field-line resonance oscillation but also its frequency. The field-line resonance oscillation exhibits the fundamental mode structure along the field line in the plasmasphere. In high latitudes, there are waves bouncing between the ionospheres. This suggests that the waveform in the middle- and low-latitudes is different from that in high-latitudes.

DOI： 10.1029/2000GL000108

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

A wave equation describing the generation of field-aligned current (FAC) in the magnetosphere is derived. The equation has four source terms. The first and second terms represent the effects of inhomogeneous Alfven speed (VA) and curvilinear magnetic field line, respectively. The perpendicular perturbation inertial current produces the perturbation FAC via these effects. Around the magnetic equator in the region of dipolar magnetic field where VA is inversely proportional to the power of the radial distance from the Earth's center, the first and second terms have magnitudes of the same order and their signs are identical. The first term dominates over the second one around the region where the gradient of VA is sharp and vice versa around the position where the stretched field line intersects the magnetic equator. The third and fourth terms are related to the diamagnetic current. When the unperturbed magnetic pressure has an inhomogeneous distribution, the perpendicular diamagnetic current due to the perturbation of the plasma pressure yields the perturbation FAC (third term). When the perpendicular diamagnetic current flows in the unperturbed state, the perturbations of the magnetic and plasma pressures also bring about the perturbation FAC (fourth term). In the case of β ~ 1, the third and fourth terms have magnitudes of the same order. If the disturbance bears a diamagnetic property, this would be especially the case. However, if the disturbance propagates perpendicularly to the ambient magnetic field, the perturbation FAC would be little generated by the fourth term. © 2000, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.

DOI： 10.1186/BF03351654

Language：English   Publishing type：Research paper (scientific journal)  

To investigate the generation and propagation mechanisms of Pi 2 magnetic pulsations, we have analyzed magnetic field data from the 210° magnetic meridian (MM) stations. We used 50 Pi 2 events that were simultaneously observed at seven stations along the 210° MM during January 1995, and focused our analysis on associated magnetic energy, ((ΔH)²⁺(ΔD)²)/μ<inf>0</inf>. The times when the amplitude of the magnetic energy attained the maximum (T(max)) were compared among these stations. We found that T(max) has a latitudinal dependence, especially at higher latitudes, which has not been previously reported. At Kotel'nyy (L=8.50) on the poleward side of the auroral region, T(max) occurred an average of 21 seconds earlier than T(max) at Guam (L=1.01), and often as much as one minute earlier. The existence of latitudinal variation has implications for interpretation of issues related to timing of substorm onset; it is necessary to consider the global features of Pi 2 events in the study of auroral and magnetospheric substorms.

DOI： 10.1029/1999GL010767

Language：English   Publishing type：Research paper (scientific journal)  

The nature of reflection and mode conversion of MHD waves at the high-latitudinal inductive ionosphere is analyzed, based on the current conservation law of wave modes. The term "inductive ionosphere" refers to the nonzero rotational electric field or nonzero compressional magnetic field in the reflection process of shear Alfvén waves on the ionosphere. The finite rotational electric field causes mutual induction between the divergent and rotational current systems at the ionosphere. The one-step Hall effect for the divergent electric field of the shear Alfvén wave produces a rotational Hall current and excites the ionospheric surface compressional wave. The Hall effect for the rotational electric field of an ionospheric surface compressional wave produces a divergent Hall current (two-step Hall effect), which feeds back the compressional magnetic energy to the reflected field-aligned current. We find that the renormalization of the ionospheric rotational electric field to the reflection process of the shear Alfvén wave causes some peculiarities in the distribution of ionospheric currents and mode-converted wave magnetic fields. Such peculiarities become particularly obvious in the high-conducting ionosphere. For example, in the ionospheric current distributions, a considerable component of the ionospheric divergent current is accounted for by the divergent Hall current. The rotational Hall and Pedersen currents cancel each other out and lead to zero total ionospheric rotational current. The amplitude of the poloidal magnetic field transmitted from the toroidal magnetic field of the incident shear Alfvén wave shows a nonlinear dependence on ΣH/ΣP. It also shows a new type of effective ionospheric shielding effect in the ΣP/ΣA parameter space for a fixed ΣH/ΣP condition. We assert that the inductive response of the ionosphere should become an indispensable concept for reflection, mode conversion, transmission, and generation of various phenomena relating to the field-aligned current system. Copyright 2000 by the American Geophysical Union.

DOI： 10.1029/1999ja000159

Language：English   Publishing type：Research paper (scientific journal)  

Language：English  

Ground magnetic perturbations associated with the standing toroidal mode oscillations in the magnetosphere-ionosphere system
The behavior of toroidal mode oscillations of standing Alfvén waves (refer to as standing Alfvén oscillations) in the coupled magnetosphere-ionosphere system is investigated using a trapezoid-shape magnetosphere model. It is found that the magnetic perturbation is transmitted across the ionosphere differently in the two cases where the ionospheric electric field perturbation is static (Pedersen conductivity > Hall conductivity) and where it is inductive (Pedersen conductivity < Hall conductivity). It is noted that the ionospheric Hall current for the inductive condition shields the magnetic field perturbation. The north-south asymmetry of the conjugate ground magnetic perturbations is calculated by using a trapezoid model with the ionospheric and magnetospheric parameters based on the IGRF and IRI. It is revealed that the ionospheric electric field is almost static for the fundamental mode oscillation, whereas inductive for the higher harmonic ones. It is also found that the north-south asymmetry of the ground magnetic perturbations depends not only on the L-value but also on magnetic longitude; this is because the ionosphere and magnetic field conditions are not uniform as a function of longitude.

DOI： 10.1186/BF03351669

Language：English   Publishing type：Research paper (scientific journal)  

Eigenmode analysis of field line oscillations interacting with the ionosphere-atmosphere-solid earth electromagnetic coupled system
In order to understand the effect of ionosphere on the localized toroidal magnetic shell oscillation (field line oscillation), we have constructed a rectangular box model for the magnetosphere-ionosphere-solid earth system including the anisotropically conducting ionosphere and performed the eigenmode analysis of the magnetohydrodynamic (MHD) waves in the model. We have found that the eigenmodes of field line oscillation are effectively controlled by the height-integrated Pedersen and Hall conductivities, meaning that the field-aligned current (FAC) in the magnetosphere is closed to both the Pedersen current and the Hall current. The divergent Hall current, that is, the Hall current, which closes the FAC, is a new found current that is driven by the Hall effect of the ionospheric inductive (rotational) electric field. The divergent Hall current brings into sharp relief by considering the ionosphere to be inductive, and it inevitably requires the electromagnetic coupling between the magnetosphere, ionosphere, atmosphere, and solid earth. The nature of the ionospheric current associated with the standing shear Alfven mode is also analyzed in detail by using the concept of wave reflection and mode conversion at the inductive ionosphere. In particular, we emphasize that the magnetic flux sustained by the rotational Hall current, that is, accompanied by the radiation of fast magnetosonic wave to the magnetosphere and atmospheric poloidal magnetic wave to the atmosphere, should be taken into account for magnetosphere-ionosphere coupling of FAC.

DOI： 10.1029/1999ja900287, 10.1186/bf03351669_references_DOI_QZjTXhdUbFP8cCtoeB591eezRJP, 10.1029/2011ja016460_references_DOI_QZjTXhdUbFP8cCtoeB591eezRJP, 10.1186/bf03351736_references_DOI_QZjTXhdUbFP8cCtoeB591eezRJP, 10.1186/bf03352541_references_DOI_QZjTXhdU

Language：English   Publishing type：Research paper (scientific journal)  

By assuming a realistic distribution of the height-integrated Cowling conductivity along the noon meridian, the transient response of the non-uniform equatorial ionosphere to plane compressional MHD waves is examined numerically. The direct incidence of an SC-type disturbance (or DL-field) on the ionosphere produces no preliminary reverse impulse (PRI) changes around the dayside equator, supporting the view that the equatorial PRI is caused by an extension of the ionospheric current responsible for the high-latitude PRI. The rise of the ground DL-field at the dip equator lags behind those in the off-equatorial region. This is a new finding which has not yet been reported observationally. When a magnetic pulsation is incident on the dayside ionosphere, the phase of the ground magnetic perturbation at the dip equator also lags behind those in the off-equatorial region. The phase lag at the dip equator obtained numerically agrees approximately with those observed for daytime Pi2 pulsations. However, the amplitude of the ground magnetic perturbation obtained numerically is depressed at the dip equator. Such a depression is contrary to the observational fact that the pulsation amplitude is enhanced at the dayside equator. Since there exists a sharp increase of the ionospheric zonal current at the dayside equator, the Pi2 pulsations around the dayside equator may also be due to the ionospheric current of polar origin. © 1998 Elsevier Science Lid. All rights reserved.

DOI： 10.1016/S1364-6826(97)00110-7

Language：English   Publishing type：Research paper (scientific journal)  

In order to clarify the wave characteristics of Pi2 and Pc 4-5 magnetic pulsations around the dip equator, we analyzed magnetic data from the latitudinally dense magnetometer array in Brazil. We found that the phase difference between Pi2 pulsations observed at globally separated low-latitude stations is small, whereas Pi2 pulsations observed within the dayside dip equator region of ±2° latitude show phase lags of about 30° ∼ 50° behind those in the off-dip equator region. Pc 4-5 magnetic pulsations at the dip equator also show the same phase character. Pi2 amplitudes are enhanced in the equatorial region, where the phase lags of pulsations must be associated with the enhancement of ionospheric conductivity. The equatorial phase lags can be explained by invoking the induction effect of the equatorial enhanced ionospheric current above the good conductor Earth.

Language：English   Publishing type：Research paper (scientific journal)  

Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210° magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, and the phase lags ∼ 34° behind those at low-latitude (magnetic latitude Φ = 19.5-46.2°) stations. On the other hand, during nighttime the amplitude of Pi 2 pulsations at the dip equator is depressed, and the phase lags ∼ 18° behind those at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off-dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.

DOI： 10.1029/97GL02146

Language：English   Publishing type：Research paper (scientific journal)  

One-dimensional transient response of the inner magnetosphere at the magnetic equator is investigated using two models of altitude distribution of the Alfvén speed V_A. The present paper concentrates on the transfer function of the system under consideration and its poles, which govern the transient response of the system. The poles, which are mathematical counterparts of the cavity resonances, appear owing to the inhomogeneity of V_A and their locations depend on the altitude distribution of VA as well as the position of external source (or outer boundary of the inner magnetosphere). Even if there exists no strong Alfvén velocity gradient at the outer boundary, an observable cavity-mode oscillation in the Pi2 range can be excited because of the existence of a strong gradient of the plasmapause within the inner magnetosphere. However, the existence of a strong gradient at the outer boundary brings about a long-lived nature of the cavity-mode oscillation as well as calls some new poles into existence. While the surface of the solid earth forms the inner boundary at which the almost perfect reflection of wave takes place, the ionosphere is of secondary importance as a reflector of wave. The existence of the solid earth plays an essential role in the observability of the compressional oscillation arising from the cavity resonance all over the inner magnetosphere. The real part of each pole has a negative value, meaning that the cavity-mode oscillation decays with a damping factor of absolute value of the real part of the pole. Such a damping is primarily due to the leakage of energy through the outer boundary of the inner magnetosphere.

Language：English   Publishing type：Research paper (scientific journal)  

Under a model of altitude distribution of the Alfvén speed VA, one-dimensional transient response of the inner magnetosphere at the magnetic equator to earthward propagating impulse- and step-like MHD disturbances is considered. The waveforms of transient compressional oscillations due to these disturbances at some L shells are directly simulated by a numerical inversion of the Laplace transform with orthonormal Laguerre functions. The present paper concentrates on the analysis of waveforms. Then, it is verified that the compressional oscillations are due to the poles of the system under consideration. The oscillation arising from the cavity resonance all over the inner magnetosphere is most dominant. However, its amplitude becomes smaller as the characteristic time scale T of an incident disturbance grows large, and it is negligibly small for T greater than several times of eigenperiod of the resonance. On the other hand, when T is relatively small (e.g., T ≲ 10 s), the oscillations due to the cavity resonances trapped around the trough in VA are outstanding. It is also found that the relative phase between the cavity-mode oscillations all over the inner magnetosphere at the earth's surface and another L shell increases monotonically with L when the inner magnetosphere has no strong gradient or a strong positive gradient of VA at its outer boundary. However, the relative phase is nearly zero and nearly 180° inside and outside a specific L shell, respectively, when the inner magnetosphere has a strong negative gradient at its outer boundary. The one-dimensional cavity-mode type resonance of the inner magnetosphere is certainly a cause of equatorial Pi2 pulsations. However, some constituents of the Pi2's may be not cavity-mode oscillations but quasi-steady-state oscillations forced by some damped sinusoidal waves incident on the outer boundary of the inner magnetosphere.

Language：English   Publishing type：Research paper (scientific journal)  

Reflection of shear Alfvén waves at the ionosphere and the divergent Hall current
A general expression for the reflection and mode conversion of MHD waves at the ionosphere is derived. On the basis of the expression, the effect of ionospheric divergent Hall current (Hall part of the induction current associated with the inductive electric field of fast magnetosonic wave) on localized toroidal oscillation is examined. When the horizontal scale of localized oscillation is of the order of several times of the height of ionosphere, the inductive electric field can play a significant role in the reflection of shear Alfvén waves with longer periods (e.f., - 100 s) in the high-latitude region, especially, in the auroral zone. Then, the contribution of the divergent Hall current to the field-aligned one can be no longer neglected and so the eigenfrequency of localized toroidal oscillation is effectively controlled by the height-integrated Hall conductivity in the ionosphere.

DOI： 10.1029/95GL03580

Language：English   Publishing type：Research paper (scientific journal)  

A general expression for the reflection and mode conversion of MHD waves at the ionosphere is derived. On the basis of the expression, the effect of ionospheric divergent Hall current (Hall part of the induction current associated with the inductive electric field of fast magnetosonic wave) on localized toroidal oscillation is examined. When the horizontal scale of localized oscillation is of the order of several times of the height of ionosphere, the inductive electric field can play a significant role in the reflection of shear Alfven waves with longer periods (e.g., similar to 100 s) in the high-latitude region, especially, in the auroral zone. Then, the contribution of the divergent Hall current to the field-aligned one can be no longer neglected and so the eigenfrequency of localized toroidal oscillation is effectively controlled by the height-integrated Hall conductivity in the ionosphere.

Event date： 2023.12

Language：English   Presentation type：Oral presentation (general)  

Venue：San Francisco   Country：United States  

プラズマのマルチスケール結合が本質となるオーロラ物理学について、量子アルゴリズムの適用可能性と、そのロードマップについて議論を行う。

Event date： 2023.12

Language：English   Presentation type：Oral presentation (general)  

Venue：San Francisco   Country：United States  

Event date： 2023.12

Language：English   Presentation type：Oral presentation (general)  

Venue：San Francisco   Country：United States  

Event date： 2023.12

Language：English  

Venue：San Francisco   Country：United States  

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Event date： 2023.12

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Venue：San Francisco   Country：United States  

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Event date： 2023.11

Language：English   Presentation type：Oral presentation (general)  

Venue：つくば国際会議場   Country：Japan  

Event date： 2023.11

Language：English  

Venue：つくば国際会議場   Country：Japan  

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Event date： 2023.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：京都大学生存圏研究所   Country：Japan  

Event date： 2023.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：大阪大学豊中キャンパス 南部ホール   Country：Japan  

Event date： 2023.10

Language：Japanese  

Venue：大阪大学豊中キャンパス 南部ホール   Country：Japan  

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Event date： 2023.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Bangi   Country：Malaysia  

Event date： 2023.10

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Venue：Bangi   Country：Malaysia  

Event date： 2023.10

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Venue：Penang   Country：Malaysia  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学青葉山キャンパス   Country：Japan  

Event date： 2023.9

Language：Japanese  

Venue：東北大学青葉山キャンパス   Country：Japan  

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Event date： 2023.9

Language：Japanese  

Venue：東北大学青葉山キャンパス   Country：Japan  

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Event date： 2023.9

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Venue：東北大学青葉山キャンパス   Country：Japan  

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Event date： 2023.9

Language：Japanese  

Venue：東北大学青葉山キャンパス   Country：Japan  

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Event date： 2023.8

Language：English   Presentation type：Oral presentation (general)  

Venue：SAPPORO   Country：Japan  

Event date： 2023.8

Language：English  

Venue：SAPPORO   Country：Japan  

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Event date： 2023.7 - 2023.8

Language：English   Presentation type：Oral presentation (general)  

Country：Singapore  

Event date： 2023.7 - 2023.8

Language：English   Presentation type：Oral presentation (general)  

Country：Singapore  

Event date： 2023.5 - 2023.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Berlin   Country：Germany  

Event date： 2023.5 - 2023.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Berlin   Country：Germany  

Event date： 2023.5

Language：English   Presentation type：Oral presentation (general)  

Venue：幕張メッセ,千葉県幕張市   Country：Japan  

Event date： 2023.5

Language：English   Presentation type：Oral presentation (general)  

Venue：幕張メッセ,千葉県幕張市   Country：Japan