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Tohru Hada Last modified date:2021.07.12

Professor / Department of Enviromental Fluid Science and Technology
Department of Advanced Environmental Science and Engineering
Faculty of Engineering Sciences

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 Reseacher Profiling Tool Kyushu University Pure
Academic Degree
Country of degree conferring institution (Overseas)
Yes Doctor
Field of Specialization
Space Environmental Fluid Dynamics, Space Plasma Physics, Nonlinear waves
Total Priod of education and research career in the foreign country
Outline Activities
Theoretical and numerical simulation studies on various phenomena in space and astrophysical plasma environments. Current research topics include:
-Nonlinear waves and turbulence in space plasma
-Structure and stability of collisionless shocks
-Diffusion and acceleration of cosmic rays
-Modeling of new generation electric plasma thrusters
Research Interests
  • The Earth in space environment
    keyword : Space weather, space environment
    2004.04~2015.03We conduct studies releavant to space weather and sun-earth relationship. .
  • next generation electric thrusters
    keyword : electric thrusters, helicon plasma
    2004.04~2015.03Theoreteical/numerical simulation studies on next generation plasma propulsion systems.
  • Nonlinear waves and turbulence in space plasmas
    keyword : nonlinear wave, MHD wave, turbulence, chaos
    1981.01~2015.03Study on nonlinear evolution of finite amplitude MHD waves in space/astro-plasma environment. Various parametric instabilities. Origin of phase coherence. Theory, simulation, and data analysis using spacecraft data (obtained by Japanese + EU projects)..
  • Collisionless shocks in space and astrophysical plasmas
    keyword : collisionless shock, shock stability, relativistic shock, Fermi acceleration, intermediate shock
    1981.01~2015.03Structure and stability of collisionless shock waves often found in space and astroplasma environment. Stability (discussion on its existence) of intermediate shocks..
  • Acceleration and diffusion of cosmic rays
    keyword : diffusion, acceleration, cosmic rays, energetic particles, Levy process
    1981.01~2015.03Effect of phase coherence of MHD turbulence on particle diffusion. Anomalous (super-/sub-)diffusion and Levy statistics, diffusive shock acceleration of energetic particles (cosmic rays). .
Current and Past Project
  • Plasma beta is a ratio between the plasma energy density and the magnetic field energy density. Plasma with large value of the beta is often found in space and astrophysical environments, and is also utilized in some of the nuclear fission experiments. On the other hand, there are only a few basic laboratory experimental studies conducted to study properties of the high beta plasma. In this project, we produce the high beta plasma using helicon device, and study various basic topics such as production and confinement efficiencies, physical processes at the null lines and null surfaces, penetration of electromagnetic plasma waves across the null surface, and chaotic orbits and the birth of anomalous resistivity near the cusp field. Numerical simulations are conducted along with corresponding laboratory experimental studies.
  • Electric propulsion is a form of spacecraft propulsion that uses electric energy to accelerate ionized gas or plasma propellant. Compared with chemical thrusters, electric thrusters can offer much higher fuel efficiency, so that many attractive future missions can be realized. However, conventional electric thrusters share an intrinsic weak point, namely a finite lifetime due to the erosion of electrodes that are in direct contact with dense plasma. The objective of our project is to develop a completely electrodeless (i.e., no components that exchange particles with the plasma propellant) plasma rocket engine by integrating and advancing our knowledge on electrodeless plasma production and acceleration. In our advanced-concept thruster, a background magnetic field is applied so as to avoid the propellant plasma to make contact with the thruster vessel. An efficient and convenient way to produce plasma in a magnetic field is to utilize the helicon wave. We will examine several different kinds of electrodeless thruster schemes, by laboratory experiments, theoretical modeling, and computer simulations. The ultimate goal is to achieve the exhaust velocity of 40 km/s and the propulsive efficiency of 50 %.
  • Space plasma shocks and their associated phenomena are of primary importance in cosmic electrodynamics, due to their ubiquity in heliospheric and astrophysical contexts. They condition plasmas downstream and dramatically energize particles, which, for example, are believed to be the primary source of cosmic rays. The collisionless shocks observed near 1A.U. have provided a natural laboratory well suited for the study of these structures, since Earth's bow shock and traveling interplanetary shocks span a wide range of applicable parameter space. The extensive dataset of space plasma shock encounters collected by satellites within the past four decades has stimulated a tremendous theoretical effort, including an increasing number of sophisticated numerical experiments, in an attempt to understand associated processes. Much particle energization appears to have been explained by diffusive shock acceleration (DSA) models, but these weak turbulence methods are often not valid for the strong turbulence that is observed, and to date do not fully account for energetic particles seen in association with strongly perpendicular shocks (70o < theta_Bn < 90o). In addition, recent detailed examinations of 3- dimensional ion distributions in the foreshock exhibit non-thermal energetic tails that cannot be readily explained by diffusive processes. A similar conclusion is inferred from recent observations of the X-ray luminosity of gamma ray bursts. Coherent non-linear structures such as Short Large-Amplitude Magnetic Structures (SLAMS) are frequently seen in foreshocks upstream of quasi-parallel shocks (theta_Bn < 45o), but should not produce diffusive acceleration. Similarly, the nonlinear cyclic shock-reformation processes expected to occur should likewise alter the diffusive picture. We propose to bring together a diverse team of theorists, simulators and observers from space physics and astrophysics backgrounds to examine in detail the roles of strong turbulence and singular structures on the acceleration of particles near shocks. Observationally, our team will focus on energetic particle distributions produced by shocks and on further elucidating the descriptions of non-thermal particle distributions and the conditions leading to them. Our theorists will focus on recent applications of field theoretical methods to turbulence that hold promise for the study of heliospheric and astrophysical plasmas. Kinetic simulations will focus on modeling the roles of singular/non-linear structures in particle acceleration. We seek support from ISSI to provide opportunities for our multidisciplinary team to meet, discuss recent results and share ideas to guide ongoing work, since this endeavor will require occasional, but intensive, intercomminication. While individual members of our group will write independent papers on their findings, we propose to also jointly author a paper summarizing the progress from the team effort, which we anticipate will have significant impact for future work in space plasma particle acceleration applications.
Academic Activities
1. Tohru Hada, C. F. Kennel, T. Terasawa, Excitation of compressional waves and the formation of shocklets in the earth's foreshock, Journal of Geophysical Research, 92, 5, 4423-4435, 1987.05, Large-amplitude waves with typical frequencies of 0.01-0.05 Hz are often observed in the foreshocks of earth and other planets. Large-amplitude waves in the earth's foreshock are sometimes (but not always) observed in a highly time-developed form, either as steepened pulses or as discrete oscillatory wave packets of finite length. This implies that nonlinearities are strong enough to modify their waveforms before the solar wind carries them out the foreshock. The instabilities and steepening of upstream waves in the earth's foreshock caused by backstreaming ions are discussed in the first part of the paper. For typical foreshock “diffuse” ion distributions, right and left-hand polarized (RHP and LHP) waves propagating parallel to the local magnetic field are preferentially excited. Such noncompressional waves neither steepen nor grow fast enough to account for the amplitude polarizations and waveforms observed in the diffuse ion foreshock. Oblique waves develop a density compression and their magnetic field polarization is elliptical. Although these characteristics match the observations of the steepened waves in the diffuse ion zone, the growth rates of those waves oblique enough to steepen are too small to account for the observed amplitudes. On the other hand, the parallel propagating waves excited by the “reflected” ion distribution at the leading edge of the foreshock do grow fast enough, but do not steepen. We suggest that parallel propagating waves grow to finite amplitude in the “reflected” and “intermediate” ion zones of the earth's foreshock and refract as they are carried by the solar wind into the “diffuse” ion region, so that they become increasingly oblique and compressional. The more compressional they become, the more rapidly they steepen. Some steepen to the point where finite ion inertia dispersion wave creates a nonlinear wave train—a shocklet. Because wave refraction is less important in the very large foreshocks of interplanetary shocks, it is less likely that oblique, compressive, steepened waves will be generated in them, in agreement with observation. In the second part of this paper, we will simulate the time evolution of oblique low-frequency compressive waves using a one-dimensional hybrid code in which main ions are treated as superparticles, diffuse ions as a double-adiabatic fluid, and electrons as an isothermal fluid. Unlike conventional hybrid codes, parallel and perpendicular pressures of fluids are treated independently. This is necessary in order to well describe the compressional properties of obliquely propagating right-hand polarized waves in a high β plasma. We initialize the simulations with an oblique sinusoidal low-frequency wave of finite amplitude. Nonlinear steepening, formation of discrete dispersive wave packets, and subsequent ion cyclotron damping of the wave packets are found to occur..
2. Tohru Hada, Evolution of large amplitude Alfvén waves in the solar wind with β∼1, Geophysical Research Letters, 10.1029/93GL02704, 20, 22, 2415-2418, 1993.11, A new set of equations is obtained which describes evolution of finite amplitude, dispersive, elliptically polarized quasi‐parallel Alfvén waves, when the plasma and the wave amplitude satisfy the condition, |1‐CS 2/CA 2| < δB/B0, where CA and Cs are the Alfvén and the sound speed, respectively, and δB/B0 is the Alfvén wave amplitude normalized to the background magnetic field. When this condition is satisfied, the sound wave as well as the right‐ and left‐hand polarized Alfvén waves are nearly all degenerated. The solar wind plasma is a typical medium for the obtained set of equations to apply, since the plasma β is high, and the solar wind Alfvén waves have large amplitude. Modulational instability of a circularly polarized parallel Alfvén wave is discussed..
3. Tohru Hada, Evolutionary conditions in the dissipative MHD system
Stability of intermediate MHD shock waves, Geophysical Research Letters, 10.1029/94GL02239, 21, 21, 2275-2278, 1994.01, The evolutionary conditions in the dissipative magnetohydrodynamic (MHD) system are discussed, and the results are applied to argue stability of MHD intermediate shock waves. When dissipative wave modes are included, arbitrary perturbations given to the shock can be expressed as a superposition of outgoing wave modes from the shock: thus, intermediate shocks are evolutionary in dissipative MHD. The matching conditions of incoming and outgoing wave modes at the shock give an under‐determined set of equations, which defines a unique solution when the minimum dissipation principle is imposed..
4. T. Hada, D. Koga, E. Yamamoto, Phase coherence of MHD waves in the solar wind, Space Science Review, 10.1023/A:1025506124402, 107, 1-2, 463-466, vol. 107, no.1-2, pp.463-466, 2003.01, Large amplitude MHD waves are commonly found in the solar wind. Nonlinear interactions between the MHD waves are likely to produce finite correlation among the wave phases. For discussions of various transport processes of energetic particles, it is fundamentally important to determine whether the wave phases are randomly distributed (as assumed in quasi-linear theories) or they have a finite coherence. Using a method based on a surrogate data technique and a fractal analysis, we analyzed Geotail magnetic field data (provided by S. Kokubun and T. Nagai through DARTS at the Institute of Space and Astronautical Science) to evaluate the phase coherence among the MHD waves in the earth’s foreshock region. The correlation of wave phases does exist, indicating that the nonlinear interactions between the waves is in progress..
5. Yasuhito Narita, Tohru Hada, Origin of nonlinear density fluctuations in the foreshock region, Earth, Planets and Space, 10.1186/s40623-018-0943-0, 70, 1, id.171(1)-id.171(7), 2018.10, The foreshock plasma exhibits large-amplitude disturbance in the plasma density and the magnetic field. The question of the density response to the magnetic field fluctuation is addressed and studied observationally and statistically using the in situ Cluster spacecraft data of the foreshock plasma. Three major findings are obtained. First, the density response is unique to the magnetic field fluctuation and is of the fast magnetosonic mode type. Second, the density response to the total magnetic energy density (simply subtracting by the mean field) exhibits a clear scaling to the beta-tilde parameter defined as the squared ratio of the sound speed to the Alfvén speed. We interpret that the total fluctuations mostly represent linear-mode waves, and the scaling law has a potential application to estimate the plasma parameter beta using the fluctuation data of the density and the magnetic field only. Third, the density response to the nonlinear (or large-amplitude) magnetic field fluctuation has a weak agreement with the theoretical expectation from the quasi-static balance with a larger degree of scattering in the data. We conclude that the foreshock plasma overall exhibits the linear-mode waves (fast mode) and a moderate degree of nonlinear fluctuations. The concept of the quasi-static balance is partly justified and applicable in the foreshock plasma..
1. T. Hada, F. Otsuka, S. Shinohara, H. Nishida, T. Tanikawa, I. Funaki, Development of electrode less helicon plasma thrusters, AP-RASC13 (Asia Pacific Radio Science Conference 2013), 2013.09.
2. T. Hada, Diffusive shock acceleration of cosmic rays with non-Gaussian transport, The 11th International School for Space Simulations, 2013.07, It is widely recognized that the diffusive shock acceleration (DSA) is the most likely acceleration process responsible for producing the observed power law cosmic ray spectrum, at least up to the so-called knee energies. One of the key elements of the DSA is the scattering of the cosmic rays by MHD turbulence, which is believed to exist both shock upstream and downstream. As the cosmic rays are repeatedly scattered by the MHD turbulence, they travel back and forth across the shock and energized by effectively compressed by the convergent background plasma flow.
While majority of past studies on the DSA employ quasi-linear type model for the cosmic ray diffusion, actual transport of the cosmic rays in plasma with MHD turbulence can be qualitatively different. In the quasi-perpendicular shock geometry, the cosmic ray diffusion may be sub-diffusive when the particles are trapped by the guiding field. In contrast, parallel diffusion of the cosmic rays may be considered super-diffusive when a time scale considered is less than the mixing (reflection) time scale.
In the presentation, we first explain basics of the DSA process and the non-Gaussian transport of particles, and then discuss results of our test particle simulations of the DSA in which the scattering of particles is specified by several different diffusion models. Cosmic ray spectrum index as well as spatial profile of the cosmic ray intensity are evaluated and discussed for both sub-diffusive and super-diffusive cases. .
3. T. Hada, Y. Nariyuki, Y. Narita, Evaluation of higher order statistics of MHD turbulence using multi-spacecraft data, APPC12 (The 12th Asia Pacific Physics Conference of AAPPS), 2013.07, Magnetohydrodynamic (MHD) waves are ubiquitous in space plasma. In particular, those found in the foreshock region of the earth’s bowshock often have order of unity normalized wave amplitude of the magnetic field, and due to this large amplitude one has a possibility to directly observe nonlinear interaction among the waves. It is thus important to develop a robust and accurate method that can extract as much information as possible on the nonlinear behavior of the MHD waves, using the field and plasma data obtained from multi-point measurement. In this presentation we demonstrate that the higher-order statistics [1] of the MHD turbulence can be evaluated both in time and spatial (i.e., both in the frequency and the wave number) domains. Compared with the analysis in the time domain, that in the spatial domain is severely restricted due to a small number of data points, since the number of spacecraft in typical formation flights is less than five. However, the so-called Capon’s method has been successfully adopted in determination of the wave number spectra for Cluster experiments[2,3]. We show that the Capon’s method is also useful in evaluation of the bispectrum and the bicoherence[4]. Accuracy and robustness to the noise of the proposed method will be tested using data obtained by numerical simulations and also by the Cluster experiments.
[1] T. Dudok de Wit, in Space Plasma Simulation, eds., J. Bu ̈chner et al., Springer, Berlin (2003) [2] U. Motschmann et al., J. Geophys. Res., 101, 4961-4965, (1996)
[3] J. L. Pin ̧con and F. Lefeuvre, J. Geophys. Res., 96, 1789-1802, (1991)
[4] Y. Narita et al., Ann. Geophys., 26, 3389-3393, (2008).
4. T. Hada, F. Otsuka, S. Shinohara, H. Nishida, T. Tanikawa, I. Funaki, Research and development of electrodeless helicon plasma thrusters, IPELS013 (The 12th Int'l Workshop on the Interrelationship between Plasma Experiments in Laboratory and Space), 2013.07, Electric thruster is a form of spacecraft propulsion that uses electric energy to accelerate plasma propellant. Due to its large specific impulse, the electric thrusters are suited for long duration operations such as missions to outer planets. On the other hand, the performance of many of the conventional electric thrusters is severely limited by electrode wastage. In order to overcome this difficulty, we have been conducting the HEAT (Helicon Electrodeless Advanced Thruster) project to pursue research and development of electrodeless plasma thrusters.
In the presentation, we first briefly describe the background and the targets of the project, and then introduce the concepts of electrodeless plasma production using the so-called helicon waves (i.e., bounded whistler waves) and the electrodeless plasma acceleration via externally applied time-varying electromagnetic fields. In particular, we discuss some details on the three plasma acceleration schemes we consider: the Rotational Magnetic Field (RMF), the Rotational Electric Field (REF), and the Ponderomotive Acceleration (PA) schemes. Although the helicon plasma is collisional and dissipative, it shares many intrinsic features with space plasmas, implying that there are possibilities that people in space plasma community make substantial contributions in the field of electric thrusters. Theory and simulation results as well as recent laboratory experiments will be discussed.
5. Tohru Hada, Some modern analyses of space plasma waves, International Space Weather Initiative, 2012.09.
Membership in Academic Society
  • Association of Asia-Pacific Physical Societies
  • The Japan Society of Plasma Science and Nuclear Fusion Research
  • Japan Geoscience Union
  • Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS)
  • Review of Helicon High-Density Plasma:Production Mechanism and Plasma/ Wave Characteristics, Plasma and Fusion Research Vol. 13,1101014-1〜27(2018)
Educational Activities
Space Environmental Fluid Dynamics, Space Plasma Physics (regular lecture series + discussions)
Guest lectures at various foreign/domestic universities and academic institutes
Professional and Outreach Activities
Science Council of Japan, Division of Earth and Planetary Science, STPP subcommittee, chair (2015-)
International research and education on space weather through ICSWSE (2012- )
International collaboration at the ISSI (International Space Science Institute), (2008-2010)
Public lectures on space environment, space exploration, and plasma propulsion systems(2007-)
Contribution of an article on the Mars Explorer Project to a local newspaper in Los Angeles (2000)
International collaboration at the ISSI (International Space Science Institute), (1998-1999).