COMMISSION H: WAVES IN PLASMAS
Edited by Yoshiharu Omura and Toshimi Okada
Based on papers published from November of 1998 to October of 2001, we compiled majorachievements in the field of plasma waves and related studies made by Japanese scientists andtheir collaborators. We categorize the studies into two groups. One is based on observations
and experiments, and the other is theories and computer simulations. Studies in each category
are further divided into several sections. Each section provides a specific summary of importantscientific achievements rather than a comprehensive report of the whole research activities ofJapanese Commission H. On the other hand, the reference list attached at the end is intended
to be used as a database of all papers we have collected from the Japanese Commission H
members.
H1. Space Observation and Experiments of Plasma Waves
H1.1 Hydromagnetic and ULF Wave Phenomena
The magnetic field data from the Engineering Test Satellite -VI (ETS-VI) have been analyzed
to investigate the occurrence distributions of pulsations in Pc3-5 frequency ranges in themagnetosphere. Although the azimuthal Pc4 pulsations on the nightside start at almost the sametime as substorm onsets, they are different from Pi2 pulsations in the magnetosphere. Nose et al.
[1998a] suggest that the azimuthal Pc4 pulsations on the nightside are excited through coupling
to the fast mode Alfvén waves which were launched at substorm onset.
Using data from the polar orbiting Dynamic Explorer (DE)-1 and -2 satellites and aground-based station, Nose et al. [1998b] investigated electron precipitation phenomena
accompanying Pc 5 pulsations. They found that the resonance structure has a small scalecomparable to the ion acoustic gyroradius, then kinetic Alfvén waves having electric fields
parallel to the ambient magnetic field can arise.
Nose et al. [1999] investigated statistical characteristics of dayside Pi2 pulsations observed at
Mineyama (25.5° geomagnetic latitude) from November 1994 through June 1996, using analgorithm to detect Pi2 pulsations which was introduced in [Nose et a., 1998c]. They found thatthe magnetospheric cavity model can explain most of observational results.
H1.2 Generation and Propagation of ELF/VLF Waves
Recent theoretical work has suggested that the energy release in magnetic reconnection ismediated by electrons in waves called ‘whistlers’ which move much faster for a given
perturbation of the magnetic field because of their smaller mass. During reconnection theplasma forms a ‘magnetic nozzle’ like the nozzle of a hose, and the reconnection rate iscontrolled by how fast plasma can flow out of the nozzle. The whistler velocity and associatedplasma velocity both increase as the ‘nozzle’ becomes narrower. A narrower nozzle therefore
no longer reduces the total plasma flow – the outflow is independent of the size of the nozzle.
Deng and Matsumoto [2001] reported observations demonstrating that reconnection in themagnetosphere is driven by whistlers, in good agreement with the theoretical predictions.
Many ELF chorus emissions were observed by the Plasma Wave Instrument (PWI) onboard
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GEOTAIL spacecraft around the equatorial plane in the dayside outer magnetosphere. Alongwith the chorus observations, the pitch angle anisotropy of the cyclotron resonant electrons wasmeasured by the Comprehensive Plasma Instrument (CPI). Yagitani et al. [1999] showed thatthe observed anisotropy was too small to generate simultaneously observed chorus emissions
via linear cyclotron resonance. This is possibly caused by the pitch angle diffusion of cyclotron
resonant electrons by the generated chorus emissions.
Electrons injected into the nightside magnetosphere during the expansion phase of a substorm
generate whistler-mode waves near the equator which propagate downwards along
field-aligned paths and through the ionosphere to be observed on the ground as substorm
chorus evens. Smith and Nagano [2001] presented observations of such events recorded
around Halley station (L~4) in Antarctica. Combined with a full-wave code to model the
propagation of the waves through and under the ionosphere, the observations were used toestimate the apparent position and motion of the regions where the SCE waves enter the
Earth-ionosphere waveguide during the substorm progress.
Rafalsty et al. [1999] studied penetration of electromagnetic waves into the seawater on thebasis of wide-band synchronous measurements of ELF/VLF atmospherics on the sea surface
and underwater. They found that the sea layer behaves as a spatial filter for modes in the
Earth-Ionosphere waveguide.
Nickolaenko and Hayakawa [1999] demonstrated that multiple electromagnetic pulses
originating from staircase (multiply bent) strokes turn into a single pulse when propagation in
the Earth-Ionosphere caving due to ELF radio wave dispersion and attenuation. Regarding the
propagation mode of electromagnetic waves, Singh and Hayakawa [2001] critically examined
the relative merits of ducted and non-ducted propagation of low-latitude whistlers in the lightof work done mostly in the Asian countries.
Yoshida et al. [2000] investigated correlation between ion outflows observed by Akebono and
EISCAT in the night side auroral region. They proposed that ELF waves were excited by aplasma instability due to upward thermal electron beam, and the wave caused transverse ion
heating at the top of the ionosphere.
H1.3 Electrostatic Waves Excited by Electrons
Matsumoto et al. [1999] performed data analysis of electrostatic waves and plasma particles inthe deep magnetotail respectively obtained by Plasma Wave Instrument and Comprehensive
Plasma Instrument onboard the GEOTAIL spacecraft. When the GEOTAIL spacecraft
experienced multiple crossings of the plasma sheet boundary layer, broadband electrostatic
noise (BEN) and Langmuir wave were observed alternatively. The dynamic frequency
spectra of BEN are very bursty in time, and their waveforms are a series of electrostatic solitary
waves (ESW). The LW is observed when an enhancement of an electron flux is found on ahigh-energy tail of a relatively cold velocity distribution function of the major thermal electrons.
The ESW, on the other hand, are observed in the presence of a hot thermal electron distributionfunction, in which electrons responsible for the ESW are embedded.
Omura et al. [1999a] studied waveforms of electrostatic solitary waves (ESW) andcorresponding electron velocity distribution functions observed by the Geotail spacecraft.
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When they observed a series of ESW in the plasma sheet boundary layer of the Earth's
magnetotail, they found enhanced fluxes of high-energy electrons flowing along the ambient
magnetic field. They found good correlation between the propagation direction of the ESW andthe direction of the enhanced high-energy electron flux. This supports the model proposed bycomputer simulations that ESW are formed by the electrons modulated through the
bump-on-tail instability. The enhanced electron flux isregarded as the diffused electron beam after the saturation of the instability.
Kasahara et al. [2001] investigated waveforms of electrostatic broadband noise in ion heating
region observed by Akebono. It is found that the waves can be classified into continuous noise
and impulsive noise. They also showed the spatial distribution of the continuous noise
statistically depending on local time, geomagnetic activity, and season.
Pickett et al. [1999] studied cusp energetic particle (CEP) events using simultaneous wave datafrom Polar and Akebono and magnetometer data from ground stations. They suggested
presence of coherent electrostatic structures that were highly localized along the ambient
magnetic field. Some of cusp waves may be indicators of the reconnection process taking placethrough the cusp.
Tsurutani et al. [2001a, b] made a statistical survey of the ELF/VLF electrostatic andelectromagnetic plasma waves detected within the polar cap boundary layer, and found that two
types of intense electric waves were present: solitary bipolar pulses (electron holes) and ~kHz
electric turbulence. They argued that the PCBL waves were most likely a consequence ofauroral zone field-aligned current instabilities, while the currents had in turn been ascribed tobe due to magnetospheric convection driven by the solar wind.
Usui et al. [1999a, b] analyzed the electron environment associated with the TP emissions,
intense bursts of ECH waves called TP emissions which occurred sporadically andintermittently in time in the dayside magnetosphere. Field analysis shows that, due to theoccurrence of an intense Pc-5 pulsation, there was E × B drift flow which was switching its
direction in time near the magnetopause. The GEOTAIL/CPI data shows the electron density
fluctuated corresponding to the drift flow variation. Overall, TP emissions tend to occur duringa transient time interval including a time when the density of cold electrons below 100 eV takes
a minimum value. They concluded that the sporadic and intermittent signature of TP emissions
can be due to the change of the electron environment driven by the intense field pulsation.
H1.4 Electromagnetic Waves Excited by Electrons
Hashimoto et al. [1999a,b] reported a new kind of terrestrial continuum that appears to begenerated inside the Earth's plasmasphere. It has been detected by the Geotail satellite at anorbital distance of 10 to 30 Re in the dayside and evening sectors of the magnetosphere. Thispreviously undetected emission, which will be called A kilometric continuum, is found to
consist of slowly drifting narrowband signals at a frequency of 100 to 800 kHz, corresponding
to the plasma frequency inside the plasmasphere at an altitude extending down to only a few
thousand kilometers in the topside equatorial region of the Earth's ionosphere. Unlike normal
continuum, kilometric continuum is found to occur only near the magnetic equator, where it
appears to be unrelated to magnetic activity and is emitted within a well-defined beam within
only ~10 of the magnetic equator.
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Lobe-trapped continuum radiation (LTCR) has been observed in the distant magnetotail regions
by GEOTAIL spacecraft. On the basis of the statistical analysis with the spectrum andwaveform data of the LTCR, Takano et al. [2000] suggested that the LTCR source regions arelocated around the plasma sheet boundary layer (PSBL) of the tail region.
Through Geotail observations Matsumoto et al. [2000] have revealed that the terrestrial
continuum radiation observed in the magnetotail consists of several components that are
generated at different locations through different processes. "The continuum-like radiation" isclassified into the following components: classical nonthermal continuum radiation generated
in the dawnside and dayside plasmapause, short-lived enhancement of the nonthermal
continuum radiation generated at the nightside plasmapause, auroral myriametric radiation
generated at the auroral plasma cavity, lobe-trapped continuum radiation generated at theplasma sheet boundary layer (PSBL), and narrowband 2fp radiation generated throughbeam-plasma instability at the PSBL.
Kasaba et al. [2000] performed statistical studies of the direction finding of 2fp radiation andthe spatial distribution of plasma waves and energetic particles in the terrestrial electron
foreshock observed by Geotail. The "2fp radio source" is likely to be in the leading region of
the electron foreshock where the most intense Langmuir waves are observed. The Langmuir
wave activities and the population of energetic electrons gradually decrease in the region
beyond 10 RE from the contact point between the tangential IMF lines and the bow shock.
They also investigated the influence of the solar wind conditions on the activities in theelectron foreshock. A positive correlation of the 2fp radio activity is confirmed with the solar
wind kinetic energy flux and a decrease of 2fp radio activity with decreasing IMF cone angle
resulting in IMF lines tangent to the far flank of the bow shock. The 2fp radio activity is more
affected by both parameters than the amplitude of Langmuir waves is affected.
Morioka et al. [2001] found the unexpected dumbbell type pitch angle distribution ofrelativistic electrons in the inner radiation belt from the observation of the Akebono (EXOS-D)
satellite. They investigated the spatial distribution of pitch angle anisotropy, and revealed thatthe dumbbell distribution exists only around the magnetic equator. They also found that the
dumbbell distribution appears almost concurrent with the strong UHR emissions, which havebeen known as Equatorial Plasma Wave Turbulence (EPWAT) phenomena generated at themagnetic equator. Though the mechanism for sustaining unstable pitch angle distribution has
not been clear, they suggested the relationship between the EPWAT emissions and thedumbbell distributions.
Plasma wave observation by the Akebono satellite contributed to the modeling of theplasmaspheric plasma distribution. Kimura et al. [2001] reviewed the modeling method using
propagating wave characteristics obtained by the satellite and ray tracing. This method can beapplied to get a long period variation of plasmaspheric density profile for one solar cycle.

H1.5 Observational and Experimental TechniquesElectron density measurements in the lower ionosphere were carried out during 1975-1992 byusing sounding rockets launched at KSC (Kagoshima Space Center in Japan). Nagano and4Okada [2000] estimated low electron densities in the D-region, by comparing the intensity ofground-based VLF and MF radio signals observed by the rockets in the ionosphere, with that
calculated by a full wave treatment.
Fukami and Nagano [2000] analyzed the height pattern of an HF radio wave intensity received
by a sounding rocket in the daytime ionosphere about 1000 km away from the transmitter.
The step-like damping structures of the HF wave observed at about 170 km and 200 km
altitudes were found to be caused by reflections of one-hop and two-hop modes of an ordinary
wave. By using these reflection heights, they estimated the lower ionospheric electron density
profile, which was consistent with the ionogram traces actually measured at the rocket
launching site.
Very long (several tens of meters) wire dipole antennas have been used to observe
low-frequency (< MHz) AC electric field onboard spacecraft. Accurate measurements of theelectric field require the accurate evaluation of the effective length of a wire dipole immersed
in space plasmas. By using the whistler-mode wave observations with GEOTAIL spacecraft,
Imachi et al. [2000] demonstrated that the effective length is almost a half of the dipole's
tip-to-tip length at ELF/VLF frequencies, as conventionally assumed in the past studies.
Wavelet analysis is suitable for investigating waves, such as Pi2 pulsations, which are limited
in both time and frequency. Nose et al. [1998c] have developed an algorithm to detect Pi2
pulsations by wavelet analysis. They tested the algorithm and found that the results of Pi2
detection are consistent with those obtained by visual inspection. The algorithm is applied in a
project which aims at the nowcasting of substorm onsets.
Elie et al. [1999] proposed use of neural network on the forthcoming satellite DEMETER to
identify the coherent VLF signal like whistlers.
Goto et al. [2000a, 2000b, 2001] proposed a direction finding method based on the wave
distribution function (WDF) by using the concept of the energy function. The method wasapplied to the Akebono data and the derived results were in the acceptable range fromtheoretical viewpoints.
Kasahara et al. [2000] proposed a new computational technique for extracting particular wave
phenomena from enormous data sets. Applying this method to the Akebono VLF wave data,some kinds of plasma waves were automatically classified and the spatial distribution of chorus
emission was revealed.
Kojima et al. [2000] developed an Automatic Waveform Selection (AWS) method for
identifying the electrostatic solitary waves observed by Plasma Wave Instrument (PWI)
onboard the Geotail spacecraft. They defined an ESW index using the AWS method. The
ESW index allows us to perform statistical analyses by calculating it from a huge amount of
Geotail data sets.
In order to measure the Martian plasma waves in the frequencies from dc to 32 kHz, the low
frequency plasma waves analyzer (LFA) onbaord the PLANRT-B (NOZOMI) spacecraft has
been developed. The scientific objectives and system configuration of LFA are5

comprehensively presented [Matsumoto et al.,1998].
Ishisaka, et al., [1999a, b; 2001] have derived an empirical relation to estimate the very low
electron density in the magnetosphere ranging from 0.001 to 50 /cc by coordinating thespacecraft potential and the plasma wave such as the continuum radiation observed by
GEOTAIL.
H2. Theory and Computer Experiments on Plasma Waves
H2.1 Wave Instabilities
Ueno [2001] derived a dispersion relation for the lower-hybrid-drift instability including theeffects of magnetic curvature associated with transverse electromagnetic perturbations. To
account for the curvature drift, an alternative method proposed by Nakamura [1997] wasapplied to obtain the perturbed distribution function. The previous treatment, i.e., simulating
the curvature drift by a virtual gravitational drift, was found to be considerably inaccurate.
When an ambient magnetic field has a curvature so that the curvature drift is directed opposite
the grad-B drift, the maximum growth rate increases as long as the radius of curvature is larger
than a certain value, while the rate decreases for a sharper magnetic field curvature.
Kono and Tanaka [2000] has formulated a theory of spiral structure formation to show thatspiral structures are rather basic entities in magnetized rotating plasmas subjected to various
kind of instabilities such as collisional drift wave instability, flute mode instability due tocentrifugal force, and Kelvin-Helmhotz instability. The characteristic features of spiralstructures observed experimentally in ECR plasmas are reproduced by our theory.
A new analytical approach to cyclotron instability of electron beams with sharp gradients invelocity space is developed by Trakhtengerts et al. [1999] , with taking into account magnetic
field in- homogeneity and non-stationary behavior of the electron beam velocity. Under these
conditions, the conventional hydrodynamic instability of such beams is drastically modified
and second-order resonance effects become important. Hobara et al. [1999, 2000] andDemekhov et al.[2000] studied the properties of electron beams formed by cyclotroninteractions between radiation belt electrons and a quasi-monochromatic whistler wave packet
from a ground VLF transmitter. The beams are formed due to trapping of the electrons at theforward edge of the wave packet, their acceleration inside the wave packet, the escape of theaccelerated electrons from the moving backward edge of the wave packet, and their following
free motion in an in homogeneous magnetic field. A combination of these processes provide
the main features of the spatial-temporal evolution of the beams, which are investigated bothanalytically and numerically.
Ishiguro et al. [1999] developed a 2-1/2-dimensional open boundary particle simulation modeland studied the current-driven electrostatic ion-cyclotron instability and related D.C. potential
difference. Fresh streaming electrons are injected smoothly from th boundaries at each time
step, avoiding unphysical accumulation of charged particles in front of the boundaries. As acurrent-driven electrostatic ion cyclotron instability grows, a d. c. potential difference along themagnetic field lines is created.
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Ishiguro and Sato [1999] investigated formation of dc potential structure due to current-driven
electrostatic ion-cyclotron instabilities by means of an open boundary two-and-a-halfdimensional electrostatic particle cell (PIC) simulation model. A dc potential difference in built
up as a fluctuation associated with an ion-cyclotron wave induced by streaming electrons
grows. A V-shaped dc potential structure is created for a bell-shaped electron stream, while thedc potential does not have a structure across the magnetic field for a uniform electron stream.
According to the recent observations by several satellites, FAST satellite observed very strong
solitary waves in the downward current regions of the mid-altitude auroral zone, and Polarspacecraft also succeeded in detecting solitary waves in the polar region. These solitary waves
are reported to have isolated two-dimensional structures. Miyake et al. [2000] studied theformation mechanism of the isolated two-dimensional solitary waves via two-dimensional
electrostatic particle simulations. They demonstrated that such isolated two-dimensional
solitary waves can be generated by a simple electron two-stream instability.
Kasaba et al. [2001] performed computer simulations of the self-consistent nonlinear evolutionof electrostatic and electromagnetic 2fp waves excited by electron beams with electromagnetic
particle code. In both one- and two-dimensional periodic systems an electrostatic 2fp wave isgenerated at twice the wave number of forward propagating Langmuir waves by wave-wave
coupling. This wave grows with the forward propagating Langmuir wave in the nonlinear stageof the simulations. The electrostatic 2fp wave in the simulations is saturated at about -20 ~ -30
dB of that of the Langmuir waves. It is larger than the value expected from observations in theterrestrial electron foreshock. The electromagnetic 2fp wave is only excited in two-dimensional
systems. The magnitude of the electromagnetic 2fp wave is correlated with the backward
propagating Langmuir wave, not with the electrostatic 2fp wave. This result suggests that theelectromagnetic 2fp wave is excited by the wave-wave coupling of forward and backward
propagating Langmuir waves.
Matsukiyo et al. [1999] compared the relativistic electrostatic beam instabilities driven via the
Landau and the cyclotron resonances. The linear theory predicts that, when the electron beams
are non-relativistic or only weakly relativistic, the waves are generated predominantly via the
Landau interaction for a wide range of plasma parameters, except for a limited parameter
regime at oblique propagation angles. However, when the beam electrons are highly relativistic,
the cyclotron interaction becomes dominant. They discussed implications on plasma processes
in astrophysical environments.
H. 2.2 Wave Propagations
Ohta and Hayakawa [2000] performed three-dimensional ray-tracing taking into account thelatitudinal and longitudinal gradients of the ionosphere in order to explain the occurrence of
echo-train whistlers at very low latitudes.
From the analysis of the plasma wave observations by the Ulysses spacecraft, Nakagawa et al.
[2000] have investigated the dynamics of the Jovian Hectometric Radio Emissions (HOM),
which have been thought to be Jovian auroral radio emissions. They revealed that the HOM canbe classified into two components, which have different occurrence characteristics with respect
to Jovian System III longitude; (i) amplitude of emissions is related to the variation of the solar
wind pressure (sw-HOM), and (ii) amplitude of emissions is independent of solar wind7

parameters (nsw-HOM). They suggested that the source locations of sw- and nsw- HOM
emissions are L > 30 and L~12 respectively.
Misawa [2000] performed a 3D ray-tracing analysis on Io-related Jovian decametric radio
emission (Io-DAM) to investigate the condition for generation of Io-DAM emissions. The ray
tracing was calculated for the case of both R-X and L-O mode emissions using the most recent
Jovian global magnetic field model (VIP4). He compared the simulated occurrence probabilityof observable Io-DAM waves at ground with observations, and concluded that the observed
occurrence probability is not explained only by the initial wave condition at the source region(magnetoionic wave mode, ray direction, and source position), but additional conditions which
restrict the wave generation must be required.
A feasibility study of LFA plasma wave analyzer onboard NOZOMI spacecraft was made on
the detection possibility of Martian atmospherics during large dust storms. By the full wave
calculations using the realistic Martian ionospheric model and crustal magnetic anomalies,
Okada et al. [2001] showed the conditions for the LFA remote-sensing of the Martian
atmospheric discharge.
H2.3 Shocks and Particle Acceleration
Terada and Machida [1999] studied supercritical perpendicular shock waves propagating in aplasma containing electrons and two positive ions (i.e., protons and alpha particles), using a
one-dimensional electromagnetic hybrid code. It is found that the shock with moderately highMach number becomes dynamic and changes its structure periodically, as the number density
ratio of heavy ions to protons is increased.
Particle acceleration processes at quasi-parallel shocks have been already widely discussed,
however, the very initial injection from thermal to supra-thermal energies is still controversial.
Sugiyama et al. [2001] showed that a non-linear wave-particle interaction at quasi-parallelshocks results in quick injection and quick further acceleration of ions to non-thermal (NT)energies. Instead of an ensemble of small amplitude random waves, a large-amplitude
monochromatic upstream wave is set to propagate into the shock transition layer and test
particle orbits are deterministically calculated. The resultant energy spectrum has the
exponential slope extending up to 70 times the upstream bulk flow energy with thecharacteristic energy being 7 times the upstream bulk flow energy. They have compared thisenergy spectrum with self-consistent hybrid simulation results and with Geotail satellite
observations in the upstream region to find reasonably good agreements.
Motivated by the observational evidences that large amplitude MHD waves in space often havespatially correlated wave forms, Kuramitsu and Hada [2000] discussed energy diffusion andacceleration of charged particles due to such waves, by performing test particle simulations. It
was found that, in a presence of locally correlated waves, charged particles can be efficiently
accelerated as they are successively mirror reflected by oppositely propagating wave packets, aprocess analogous to the Fermi acceleration. Statistical properties of the acceleration werediscussed in detail.
In a similar context, Krasnoselskikh et al. [1999] analyzed the acceleration and heating of
charged particles in a turbulence field with prescribed statistical properties, and identified the8relationship between the statistics of the field and that of the diffusion scaling law. The results
were discussed for possible applications in several branches of astrophysics and space physics.
Some of these theoretical expectations are analyzed by making comparison of observations andtest particle simulations of pitch angle diffusion and the parallel transport of energetic solar
wind helium ions by Tsurutani et al. [2000]. It was emphasized that the pitch angle diffusion
across the 90 degrees was a qualitatively different process from the diffusion at different angles,
and that inclusion of finite wave amplitude effect was essential.
H2.4 Nonlinear Effects
Traktengerts et al. [1999, 2001] estimated the role of the second-order cyclotron resonanceeffect in the self-consistent approach to the problem of triggered VLF emission is estimated.
The self-consistency includes the calculation of energetic electron beams produced by aquasi-monochromatic whistler wave packet and the analysis of secondary whistler wave
generation by this beam, with the magnetic field inhomogeneity taken into account, and wehave estimated the maximum amplification of secondary waves, as well as the dynamic
frequency spectrum of a triggered emission.
Krasovsky et al. [1999] studied an elementary event of interaction of two weak electrostatic
solitary waves associated with a deficit of the electrons trapped in the self-consistent potential
well. Consistent perturbation theory descriptive of the interaction dynamics is developed which
is based on the smallness of the trapped electron holes as compared with the length of theirshielding by plasma. It is shown that a violation of the adiabaticity of the particle motion andphase mixing process entail an irreversible deformation of the hole structure, effective heatingof the holes and the corresponding losses in the mechanical energy of the system. As a result of
the peculiar irreversibility, the "collisions" of the solitary waves take the character of inelastic
impacts, explaining the general tendency to merging of the phase density holes.
Shinohara et al. [2001] reported that rapid large scale magnetic field dissipation is observed ina full kinetic simulation of cross-field current instabilities in a collisionless plasma current
sheet even when the thickness of the current sheet is at ion-scale. The Kelvin-Helmholtz
instability caused by the velocity shear between the current-carrying ions and the cold
background ions excites lower hybrid drift instability at the edges of the undualated current
sheet. They showed that non-linear coupling between these two instabilities is responsible forthe observed rapid dissipation. The simulation result clearly illustrates the importance of
non-linear coupling between MHD and electron scale instabilities.
In usual MHD, it is assumed that a plasma is both isotropic and gyrotropic at any instant oftime. Noting that these are not always justified for applications in space plasmas which areessentially collisionless, Hada et al. [1999] proposed a new model to describe evolution of theplasma, by introducing externally two relaxation time scales, one for isotropization and anotherfor gyrotropization. This model includes both the MHD and the CGL formulations as limitingcases of strong coupling limits of the relaxation time scales. Linear wave dispersion relations
within the model are discussed.
In a dusty plasma with streaming ions, it is known that a wake potential is produced
downstream of a dust particle due to ion-sound waves. The wake potential is a possible
candidate for producing attractive force between the dust particles. Nambu et al. [2000]
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discussed the effect of the external magnetic field imposed on the dusty plasma, and showedthat the amplitude of the wake potential was significantly reduced because the over-shielding
by streaming ions was inhibited by the presence of the external magnetic field.
Buti et al. [2000] studied nonlinear dynamics of large amplitude circularly polarized Alfvenic
wave packets using a hybrid model. They discussed the spatio-temporal evolution of the waves
including the collapse of the magnetic structures, the polarization switching from the LHP to
RHP, and the broadening of the wave packet. These theoretical results in mind, large amplitude
Alfven waves and structures were analyzed using the Ulysses data by Tsurutani et al. [2001c].
Discussed herein were the rotational discontinuities, the phase-steepened Alfven waves, and themagnetic field decreases (MD's) bounded by discontinuities. Hybrid simulation results suggestthat MDs may be a product of the evolution of nonlinear Alfven waves.
H2.5 Active Experiment and Spacecraft-Environment Interaction
Shklyar and Nagano [1998] developed a systematic approach to the problem of wave scattering
on plasma density irregularities. Treating the scattering process as a wave field spatial
evolution while crossing the scattering region, they showed that a spatial modulation of the
wave packet amplitude caused by spectral broadening in k space is a general consequence ofwave scattering. As an example of applications of theoretical consideration, they discussed
the relation between amplitude modulation and the apparent spectral broadening of VLF
transmitter signals observed by the Akebono satellite in the upper ionosphere.
Usui et al. [1999c] performed computer experiments on the measurement of reentry plasma
with radio waves with an electromagnetic particle code. They propose a remote measurementof the reentry plasma with the reflectmetry method which uses radio wave emission from thevehicle. In the experiments, we emit a pulse wave with no DC component from a vehicle
antenna toward a Gaussian reentry plasma put in front of the vehicle surface. The obtainedphase difference between the emitted and reflected waves is used in the reflectmetry to estimate
the spatial density profile. The spectrum of the reflected waves which has a cut-off frequency at
the maximum plasma frequency is used to estimate the maximum density in the plasma layer.
The results of the computer experiments revealed that the plasma parameters estimated with thereflectmetry agree very well with the ones actually used in the experiments.
H2.6 Te chniques of Data Analysis and Computer Experiments
Umeda et al. [2001] developed a new scheme to mask electromagnetic fields for absorbing
boundaries used in electromagnetic particle codes. The conventional masking method can
suppress reflection of various plasma waves by assigning large damping regions for absorbing
boundaries. It requires substantial computer memory and processing time. They introduced two
new parameters to control absorption of outgoing electromagnetic waves. The first parameter isa damping parameter to change the effective length of damping regions. The second parameteris a retarding parameter to change phase velocities of electromagnetic waves in the damping
regions. As a new masking method, we apply both damping and retarding factors. They also
found that the best absorption of outgoing waves is realized with a combination of a smaller
damping factor and a larger retarding factor. The new method allows us to reduce the size of
damping regions substantially.
A

two-dimensional(2d)

relativistic

electromagnetic
10

PIC

code

calledXOOPIC(Object-Oriented Particle-in-Cell code for X11-based Unix workstations) has been developedsince the early 1990s by PTSG (Plasma Theory and Simulation Group) in the University ofCalifornia at Berkeley. In order to minimize the redundancy in one-dimensional simulations,
Usui et al. [2000] developed a 1d-XOOPIC in which 1d arrays are used for field components aswell as particle positions. The formulations are also simplified for Maxwell's equations andcurrent deposition to the grid in the 1d code. To verify the function of the 1d code, we
performed simulations with typical models such as the Child-Langmuir current model andelectromagnetic wave propagation in plasma.
References
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Demekhov, A. G., V. Y. Trakhtengerts, Y. Hobara and M. Hayakawa [2000], “Cyclotron
amplification of whistler waves by nonstationary electron beams in an inhomogeneous
magnetic field,” Phys. Plasmas, vol. 7, no. 12, pp. 5153-5158.
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