||Yoshikawa A., and R. Fujii, Earth’s Ionosphere: Theory and Phenomenology of Cowling Channels, Electric Currents in Geospace and Beyond (eds A. Keiling, O. Marghitu, and M. Wheatland), John Wiley & Sons, Inc, Hoboken, N.J.,, doi: 10.1002/9781119324522.ch25, 2018.01, 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 (FAC).
The Cowling effect is now well known as enhancement of current flow in the direction of the primary electric field by the secondary Hall current [Chapman, 1956]. The Cowling effect was first investigated by Cowling  in connection with the solar atmosphere. The generation mechanism [Cowling and Boreger, 1948] was adopted to account for equatorial electrojet [Hirono, 1950, Untied, 1967] and auroral electrojet [Boström, 1964]. The Cowling effect has been investigated theoretically and observationally [e.g., Baujohann, 1983; Yasuhara et al., 1985; Haerendel, 2008, Amm et al., ; Amm and Fujii, 2008; Marghitu et al., 2011].
Figure 1 shows traditional picture of two-dimensional Cowling channel model elongated along east-west direction [e.g., Baumjohann, 1983], in which ionospheric Hall and Pedersen conductivity are height-integrated. The primary westward electric field (E1) drives northward Hall current and westward Pedersen current. The southward secondary field (E2) is generated so that the Pedersen current closes the primary Hall current between the conductivity gradients. The secondary Hall current flows in the same direction as the primary Pedersen current and forms the electrojet system.
Generally, it is difficult to specify polarization effects in the ionosphere from ground-based data alone. These data only allow to infer the resultant total electrodynamic fields, but cannot track back the chain of cause and consequence that led to the physical situation which then causes these observed total fields. Thus, using ground-based data alone in most cases we can only state whether an observed situation is consistent or not with the expectations from an “active” polarization effect.
To quantify the Cowling effect, we need to know the relative strength of the polarization electric field to total electric field and to what extent it cancels (closes) the primary Hall current. This problem is complementary to the question: How much curl-free Hall current flows out to the magnetosphere as FAC?
In order to reply to this problem provided by Amm et al., , modeling of Cowling channel has been further developed.
To describe the Cowling channel, Amm et al.,  and Fujii et al.,  introduce a parameter called the Cowling efficiency. It is defined as a ratio how much of Hall current is confined inside the ionosphere by the secondary Pedersen current excited by the polarization electric field. Definition of Cowling efficiency is practically important. It provides a general way to calculate quantitatively the polarization electric field, if the Cowling efficiency, the conductance, and either primary or the total electric fields are known [Amm et al., 2013].
It has been suggested that to identify the Cowling efficiency for specific phenomenon, one needs to know the impedance of the magnetospheric circuit, which completes the current circuit in the M-I system via FAC [e.g., Fujii et al., 2011]. However, it is questionable to assign a magnetospheric impedance for steady state because the M-I system is always changing dynamically.
The M-I coupling process via shear Alfven waves has been used to investigate the nonstationary FAC closure by ionospheric conducting current [e.g., Scholer, 1970]. Assuming specific electric field configurations of an incident wave, Glaβmeier  and Itonaga and Kitamura  have shown that a secondary polarization field due to gradients of Hall conductance can appear in the reflected wave. Actually, the Alfven wave approach can be used to describe not only local and dynamical phenomena but also more generally global quasi-static M-I coupling processes [Yoshikawa et al., 2010]. Therefore it is very important to understand the how the shear Alfven wave interacts with the Cowling channel.
Yoshikawa et al., [2011a] give a general theory about M-I coupling, independent of specific geometries or specific situations. This theory, based on the Alfven waves used in a way of a basis function for the M-I coupling process, is later applied in Yoshikawa et al., [2013a] and Yoshikawa et al., [2013b] specifically to a Cowling channel situation, but can be applied for any general case.
Most of Cowling channel models introduced so far rely on a thin-sheet ionosphere [e.g., Baujohann, 1983]. However, in a realistic ionospheric E-layer, a vertical distribution of the Pedersen conductivity and Hall conductivity has maximum peak around 125 km altitude and around 110 km altitude, respectively [e.g., Richmond and Thayer, 2000]. In order properly consider the ionospheric current closure, one also takes into account the ionospheric thickness [Amm et al., 2008]. One step in this direction is to assume that the Pedersen and Hall current flow thin layers at different altitudes [Fujii et al., 2011; Amm et al., 2011; Yoshikawa et al., 2011].
The classical picture illustrated in Figure 1 describes divergence-free approximation of auroral electrojet. However, a longitudinal boarder of Cowling channel is also important for considering finite aurora arc formation and Harang reversal region [Harang, 1947; Heppner, 1972; Marghitu et al., 2011], where the auroral electrojet is diverging.
Amm et al.,  give a review of the work available in the literature until 2008 regarding following aspects of ionospheric electrodynamics and Magnetosphere-Ionosphere (M-I) coupling:
-Polarization effect in the ionosphere (often referred to as “Cowling effect)”
-Inductive effect in the ionosphere
-The effect of the three-dimensional (3D) nature of the ionosphere for ionospheric electrodynamics
-The consequences of the above mentioned aspects to M-I coupling.
Marghitu,  provides an excellent review for auroral arc electrodynamics, by considering the 1D thin uniform arc, the 2D thick uniform arc, and the non-uniform arc. The various arc features are assembled together in a tentative 3D arc model.
The purpose of this chapter is to review the recent development of Cowling channel model after Amm et al.,  and Marghitu, . Recent work provide an extension of theoretical description of the classical Cowling channel with respect to the following aspects: 1) Taking into account the 3D nature of 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 Cowling channel, but also at its zonal boundaries. Using this improved model, schematically illustrated in Figure 2 with Cowling efficiency description, we discuss current closure and their energy principle for evolution of Cowling channel. Energy flow inside the Cowling channel and impact of polarization effect on Joule dissipation in more general M-I coupling scheme are also provided. In addition, we also clarify how shear Alfven wave interacts to the Cowling channel and their application to the global magnetosphere-ionosphere coupling simulations..
||Akimasa Yoshikawa, Akiko Fujimoto, Akihiro Ikeda, Teiji Uozumi, Shuji Abe, Monitoring of Space and Earth electromagnetic environment by MAGDAS project
Collaboration with IKIR-Introduction to ICSWSE/MAGDAS project, E3S Web of Conferences, 10.1051/e3sconf/20172001013, 20, 2017.10, 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..
||S. Imajo, Akimasa Yoshikawa, T. Uozumi, Shin Ohtani, A. Nakamizo, P. J. Chi, Application of a global magnetospheric-ionospheric current model for dayside and terminator Pi2 pulsations, Journal of Geophysical Research, 10.1002/2017JA024246, 122, 8, 8589-8603, 2017.08, 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..
||S. Ohtani, Akimasa Yoshikawa, The initiation of the poleward boundary intensification of auroral emission by fast polar cap flows
A new interpretation based on ionospheric polarization, Journal of Geophysical Research, 10.1002/2016JA023143, 121, 11, 10,910-10,928, 2016.11, 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 Alfvï¿½n 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..
||Toshitaka Tsuda, Mamoru Yamamoto, Hiroyuki Hashiguchi, Kazuo Shiokawa, Yasunobu Ogawa, Satonori Nozawa, Hiroshi Miyaoka, Akimasa Yoshikawa, A proposal on the study of solar-terrestrial coupling processes with atmospheric radars and ground-based observation network, Radio Science, 10.1002/2016RS006035, 51, 9, 1587-1599, 2016.09, 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..
||M. G. Cardinal, Akimasa Yoshikawa, Hideaki Kawano, Huixin Liu, Masakazu Watanabe, S. Abe, T. Uozumi, G. Maeda, Tohru Hada, K. Yumoto, Capacity building
A tool for advancing space weather science, Space Weather, 10.1002/2014SW001110, 12, 10, 571-576, 2015.01.
||Run Shi, Huixin Liu, Akimasa Yoshikawa, Beichen Zhang, Binbin Ni, Coupling of electrons and inertial Alfven waves in the topside ionosphere, Journal of Geophysical Research, 10.1002/jgra.50355, 118, 6, 2903-2910, 2013.01, 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.
||O. Amm, R. Fujii, Heikki Antero Vanhamaki, Akimasa Yoshikawa, A. Ieda, General solution for calculating polarization electric fields in the auroral ionosphere and application examples, Journal of Geophysical Research, 10.1002/jgra.50254, 118, 5, 2428-2437, 2013.01, 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..
||Akimasa Yoshikawa, O. Amm, Heikki Antero Vanhamaki, R. Fujii, Illustration of Cowling channel coupling to the shear Alfven wave, Journal of Geophysical Research, 10.1002/jgra.50513, 118, 10, 6405-6415, 2013.01, 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.
||Akimasa Yoshikawa, O. Amm, Heikki Antero Vanhamaki, A. Nakamizo, R. Fujii, Theory of Cowling channel formation by reflection of shear Alfven waves from the auroral ionosphere, Journal of Geophysical Research, 10.1002/jgra.50514, 118, 10, 6416-6425, 2013.01, 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.
||R. Fujii, O. Amm, Heikki Antero Vanhamaki, Akimasa Yoshikawa, A. Ieda, An application of the finite length Cowling channel model to auroral arcs with longitudinal variations, Journal of Geophysical Research, 10.1029/2012JA017953, 117, 11, 2012.12, 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..
||Heikki Antero Vanhamaki, Akimasa Yoshikawa, O. Amm, R. Fujii, Ionospheric Joule heating and Poynting flux in quasi-static approximation, Journal of Geophysical Research, 10.1029/2012JA017841, 117, 8, 2012.01, 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..
||Akimasa Yoshikawa, O. Amm, Heikki Antero Vanhamaki, R. Fujii, A self-consistent synthesis description of magnetosphere-ionosphere coupling and scale-dependent auroral process using shear Alfvén wave, Journal of Geophysical Research, 10.1029/2011JA016460, 116, 8, 2011.01, 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..
||O. Amm, R. Fujii, K. Kauristie, A. Aikio, Akimasa Yoshikawa, A. Ieda, Heikki Antero Vanhamaki, A statistical investigation of the Cowling channel efficiency in the auroral zone, Journal of Geophysical Research, 10.1029/2010JA015988, 116, 2, 2011.01, 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..
||Y. Yamazaki, K. Yumoto, M. G. Cardinal, B. J. Fraser, P. Hattori, Y. Kakinami, J. Y. Liu, K. J.W. Lynn, R. Marshall, D. McNamara, T. Nagatsuma, V. M. Nikiforov, R. E. Otadoy, M. Ruhimat, B. M. Shevtsov, K. Shiokawa, S. Abe, T. Uozumi, Akimasa Yoshikawa, An empirical model of the quiet daily geomagnetic field variation, Journal of Geophysical Research, 10.1029/2011JA016487, 116, 10, 2011.01, 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..
||R. Fujii, O. Amm, Akimasa Yoshikawa, A. Ieda, Heikki Antero Vanhamaki, Reformulation and energy flow of the Cowling channel, Journal of Geophysical Research, 10.1029/2010JA015989, 116, 2, 2011.01, 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..
||Akimasa Yoshikawa, A. Nakamizo, O. Amm, Heikki Antero Vanhamaki, R. Fujii, Y. M. Tanaka, T. Uozumi, K. Yumoto, S. Ohtani, Self-consistent formulation for the evolution of ionospheric conductances at the ionospheric e region within the M-I coupling scheme, Journal of Geophysical Research, 10.1029/2011JA016449, 116, 9, 2011.01, 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..
||Tanaka T., A. Nakamizo, A. Yoshikawa , S. Fujita, H. Shinagawa, H. Shimazu, T. Kikuchi, K. Hashimoto, Substorm convection and current system deduced from the global simulation, J. Geophys. Res., 10.1029/2009JA014676, 115, A05220, J. Geophys. Res. 115, A05220, doi:10.1029/2009JA014676, 2010.12.
||Yoshikawa A., H. Nakata, A. Nakamizo, T. Uozumi, M. Itonaga, S. Fujita, K. Yumoto, and T. Tanaka, Alfvenic-coupling algorithm for global and dynamical magnetosphere-ionosphere coupled system, J. Geophys. Res., 10.1029/2009JA014924, 115, A04211, J. Geophys. Res., 115, A04211, doi:10.1029/2009JA014924, 2010.03.
||Yoshikwa A., H. Nakata, A. Nakamizo, T. Uozumi, M. Itonaga, and K. Yumoto, A new magnetospherere- ionosphere coupling scheme for temporal and global magnetospheric MHD simulations, Mem. Fac. Sci., Kyushu Univ., Ser. D, Earth ＆ Planet.Sci.,Vol, XXXII, No2, 87-94, XXXII, 2, 87-94, Vol, XXXII, No2, 87-94, 2009.03.
||O. Amm, A. Aruliah, S. C. Buchert, R. Fujii, J. W. Gjerloev, A. Ieda, T. Matsuo, C. Stolle, Heikki Antero Vanhamaki, Akimasa Yoshikawa, Towards understanding the electrodynamics of the 3-dimensional high-latitude ionosphere
Present and future, Annales Geophysicae, 10.5194/angeo-26-3913-2008, 26, 12, 3913-3932, 2008.11, 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..
||T. Uozumi, K. Yumoto, K. Kitamura, S. Abe, Y. Kakinami, M. Shinohara, Akimasa Yoshikawa, Hideaki Kawano, T. Ueno, T. Tokunaga, D. McNamara, J. K. Ishituka, S. L.G. Dutra, B. Damtie, V. Doumbia, O. Obrou, A. B. Rabiu, I. A. Adimula, M. Othman, M. Fairos, R. E.S. Otadoy, A new index to monitor temporal and long-term variations of the equatorial electrojet by MAGDAS/CPMN real-time data
EE-index, Earth, Planets and Space, 10.1186/BF03352828, 60, 7, 785-790, 2008.01, 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..
||Y. M. Tanaka, K. Yumoto, Akimasa Yoshikawa, M. Itonaga, M. Shinohara, S. Takasaki, B. J. Fraser, Horizontal amplitude and phase structure of low-latitude Pc 3 pulsations around the dawn terminator A11308, Journal of Geophysical Research, 10.1029/2007JA012585, 112, 11, 2007.11, 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..
||Terumasa Tokunaga, Hiroko Kohta, Akimasa Yoshikawa, Teiji Uozumi, Kiyohumi Yumoto, Global features of Pi 2 pulsations obtained by independent component analysis, Geophysical Research Letters, 10.1029/2007GL030174, 34, 14, 2007.07, 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..
||R. L. Lysak, Akimasa Yoshikawa, Resonant cavities and waveguides in the ionosphere and atmosphere, Magnetospheric ULF Waves
Synthesis and New Directions, 2006, 10.1029/169GM19, 289-306, 2006.01, 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..
||Yuki Obana, Akimasa Yoshikawa, John V. Olson, Ray J. Morris, Brian J. Fraser, Kiyohumi Yumoto, North-south asymmetry of the amplitude of high-latitude Pc 3-5 pulsations
Observations at conjugate stations, Journal of Geophysical Research, 10.1029/2003JA010242, 110, A10, 2005.10, 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 ∼ 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..
||Obana, A. Yoshikawa, J.V. Olson, R.J. Morris, B.J. Fraser, S.I. Solovyev, and K. Yumoto, Techniques to investigate the ionospheric effect on ULF waves, Proceeding of The fifth Workshop on Applications of Radio Science (WARS) obart Australia, on Feb. 18-20, 2004, CD-ROM, H12, 2004.02.
||Abe S., K. Yumoto, H. Kawano, A. Yoshikawa, Y. Obana, S. I. Solovyev, D.G. Baishev, J.V. Olson, E.W. Worthington, and the Circum-pan Pacific Magnetometer Network Group, The Diagnosis of the Plasmapause by Ground Magnetometer Network Observation at Multiple Local Times, International Symposium on Information Science and Electrical Engineering 2003, Nov.13-14, 2003, ACROS Fukuoka, Fukuoka, Japan, 534-536, 2003.11.
||Kitamura. K., H. Kawano, S. Ohtani, A. Yoshikawa, K. Yumoto, and the Circum-pan Pacific Magnetometer Network Group, Quasi-periodic Substorms during Recovery Phase of Magnetic Storm for Space Weather Study, 354-357, 2003.11.
||Obana Y., A. Yoshikawa, J.V. Olson, R.J. Morris, B.J. Fraser, S.I. Solovyev and K. Yumoto, Environment Factors of PC 4 Amplitudes Observed at the CPMN Stations, International Symposium on Information Science and Electrical Engineering 2003, Nov.13-14, 2003, ACROS Fukuoka, Fukuoka, Japan, 256-258, 2003.11.
||Yoshikawa A. , H. Kohta, M.I tonaga, T. Uozumi, K. Yumoto, Inegrated Analysis of Coordinated Ground Magnetic Field Data for Space Weather Study, International Symposium on Information Science and Electrical Engineering 2003, Nov.13-14, 2003, ACROS Fukuoka, Fukuoka, Japan, 114-117, 2003.11.
||Takasaki S., H. Kawano, Y. Tanaka, A. Yoshikawa, M. Seto, M. Iizima, and K. Yumoto, Plasma Distribution in the Low-L part of the Plasma sphere during Magnetic Storms, International Symposium on Information Science and Electrical Engineering 2003, Nov.13-14, 2003, ACROS Fukuoka, Fukuoka, Japan, 253-255, 2003.11.
||Akimasa Yoshikawa, Yuki Obana, Manabu Shinohara, Masahiro Itonaga, Kiyohumi Yumoto, Hall-induced inductive shielding effect on geomagnetic pulsations, Geophysical Research Letters, 29, 8, 2002.04, 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..
||Akimasa Yoshikawa, How does the ionospheric rotational Hall current absorb the increasing energy from the field-aligned current system?, Geophysical Research Letters, 10.1029/2001GL014125, 29, 7, 2002.04, 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)..
||Akimasa Yoshikawa, 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, Journal of Geophysical Research, 10.1029/2001JA009170, 107, A12, 2002.03, 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.
||Yoshikawa, A., M. Itonaga and K. Yumoto, On the energy of the poloidal magnetic field near the ionosphere, Advances in Polar Upper Atmospheric Research, No.16, 45-58, 2002.01.
||Akimasa Yoshikawa, Kiyohumi Yumoto, Manabu Shinohara, Masahiro Itonaga, Diagrammatic method to describe the self-inductive response of the magnetosphere-ionosphere-atmosphere-Earth electromagnetically coupled system as a quasi-particle excitation, Journal of Geophysical Research, 10.1029/2000JA000405, 107, A5, 2002.01, [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..
||Masahiro Itonaga, Akimasa Yoshikawa, Shigeru Fujita, A wave equation describing the generation of field-aligned current in the magnetosphere, Earth, Planets and Space, 10.1186/BF03351654, 52, 7, 503-507, 2000.08, 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..
||Akimasa Yoshikawa, M. Itonaga, The nature of reflection and mode conversion of MHD waves in the inductive ionosphere
Multistep mode conversion between divergent and rotational electric fields, Journal of Geophysical Research, 105, A5, 10565-10584, 2000.05, 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..
||M. Itonaga, A. Yoshikawa, K. Yumoto, S. Fujita and H. Nakata, A study on the generation of field-aligned current in the magnetosphere, Mem. Fac. Sci., Kyushu Univ., Ser. D, Earth and Planet. Sci., Vol, XXXI, No1, 1-9, 2000.01.
||Itonaga, M., A. Yoshikawa and K. Yumoto, Transient response of the non-uniform equatorial ionosphere to compressional MHD waves, Journal of Atmospheric and Solar-Terrestrial Physics, 10.1016/S1364-6826(97)00110-7, 60, 2, 253-261, Vol.60, N0.2, 253-261, 1998.01.
||M. Itonaga, Akimasa Yoshikawa, K. Yumoto, One-dimensional transient response of the inner magnetosphere at the magnetic equator, 2. analysis of waveforms, Earth, Planets and Space, 49, 1, 49-68, 1997.02, 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..
||M. Itonaga, Akimasa Yoshikawa, K. Yumoto, One-dimensional transient response of the inner magnetosphere at the magnetic equator, 1. transfer function and poles, Earth, Planets and Space, 10.5636/jgg.49.21, 49, 1, 21-48, 1997.01, 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..
||M. Itonaga, Akimasa Yoshikawa, The excitation of shear alfvén wave and the associated modulation of compressional wave in the inner magnetosphere, Earth, Planets and Space, 10.5636/jgg.48.1451, 48, 11, 1451-1459, 1996.06, 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..
||Yoshikawa, A., and M. Itonaga, Reflection of shear Alfven waves at the ionosphere and the divergent Hall current, Geophysical Research Letters, 10.1029/95GL03580, 23, 1, 101-104, Vol.23, No.1, 101-104, 1996.01.
||Yoshikawa, A., M. Itonaga and T.-I.Kitamura, Effect of the ionospheric induction current on magnetohydrodynamic waves in the magnetosphere, Proceedings of the NIPR Symposium on Upper Atmosphere Physics, Vol.17, No.8, 49-59, 1995, 1995.01.
||Itonaga, M., T.-I.Kitamura and A. Yoshikawa, Interaction between hydromagnetic waves and the anisotropically conducting ionosphere, Journal of Geomagnetism and Geoelectricity, 47, 5, 459-474, Vol.47, No.5, 459-474, 1995, 1995.01.
||Yoshikawa, A., M. Itonaga and T.-I.Kitamura, On the coupled effect between the field aligned and ionospheric current, Proceeding of Eight International Symposium on Solar Terrestrial Physics, June 5-10, 1994, Sendai, Japan, 155-159, 1994.06.
||Itonaga, M., and A. Yoshikawa, Discrete spectral structure of low latitude and equatorial Pi2 pulsations,, Journal of Geomagnetism and Geoelectricity, Vol.44, No.3, 253-259, 44, 3, 253-259, 1992.01.