Edited by Isamu Nagano

H1. Observation of Space Plasma Waves and Related Phenomena

H1.1. Wave Probing of Space Plasmas

With the successful operation of the GEOTAIL spacecraft, observation data from the onboard Plasma Wave Instrument (PWI) [Matsumoto et al., 1994a] have revealed space plasma processes in the distant geomagnetotail and the dayside magnetopause regions. A wide range of plasma wave phenomena, such as Broadband Electrostatic Noise (BEN), Continuum Radiation (CR) or Terrestrial Myriametric Radiation (TMR), Auroral Kilometric Radiation (AKR), etc., has been studied. Wave forms of BEN in the geomagnetic tail were first detected by the onboard Wave Form Capture (WFC) receiver. Matsumoto et al. [1994b] showed that most of the BEN in the plasma sheet boundary layer (PSBL) are not continuous broadband noises but are composed of a series of isolated spiky pulses having a special form termed as "Electrostatic Solitary Waves (ESW)". Comparison between the BEN observations and plasma measurements shows that the uppermost frequency of the BEN is closely related to the temperature of the flowing ions in the PSBL [Kojima et al., 1994]. Examples of quasistatic electric field measurements have illustrated some characteristics of spiky electric fields and their relationship to low-frequency waves in the PSBL and plasma sheet [Cattell et al., 1994]. Hashimoto et al. [1994] showed that some of TMR intensified and broadened in frequency in good correlation with AKR. They named such radiation as Auroral Myriametric Radiation (AMR). Correlation and difference between AMR and TMR, and the intensity and the bandwidth of the AMR were discussed. The location of the reconnection line and plasmoid size in the tail were estimated using AKR data by Murata et al. [1995]. They showed that the most probable location of the plasmoid edge is around -60 Re in the GSE x direction. The estimated size of the plasmoids ranges from 10 to 50 Re. Hammond et al. [1995] presented the first observations of a CME by Ulysses and GEOTAIL spacecraft separated substantially in heliographic latitude. Both see similar plasma features during an interval in which GEOTAIL was in the deep magnetosheath and Ulysses was in the solar wind at 5 AU. From GEOTAIL particle data, slow-mode shocks were also reported [Saito et al., 1995]. Sugiyama et al. [1995a] have analyzed the diffuse upstream ions and concluded these had a bow shock origin. The detailed features of acceleration and heating of cold ion beams in the PSBL have been analyzed by Hirahara et al. [1994].

The Akebono (EXOS-D) satellite continues to provide high quality radio wave observation data. Wave normal directions and delay time of Omega navigation signals were measured continuously over one hour. Based on these data and electron density obtained along the trajectory a trial was made to obtain the global plasma distribution using ray tracing and fitting method [Sawada et al., 1993; Kimura et al., 1995]. The rising tone solar radio bursts in the decametric wavelength range have been observed associated with strong solar flares. The detailed analyses showed that a large isolatedly closed loop is formed by magnetic reconnection and is pushed out with a speed of 400-500 km/s toward outer coronal region [Aoyama and Oya, 1994].

H1.2. Hydromagnetic and ULF Wave Phenomena

The origin and propagation of the regular and irregular ULF magnetic pulsations have been investigated with the observation made with GEOTAIL and other satellites and at multiple ground stations.

Based on GEOTAIL observation Sugiyama et al. [1995b] found that low frequency upstream waves are only excited near the foreshock boundary and attenuate in the far upstream region from the bow shock at a distance of about 60 Re which would imply that there is not a lot of additional free energy in the upstream region. Nakamura et al. [1994] analyzed the Pc 5 pulsation observed in the dayside magnetosphere. Three cases observed at the dawnside, sub-solar, and duskside regions were explained by the magnetic field line standing pulsation. Takahashi et al. [1994a] analyzed the compressional Pc 3 magnetic pulsation and accompanying electric field oscillations, which are consistent with fast magnetosonic waves propagating earthward. Their result strongly supports the view that ULF waves generated near the quasi-parallel portion of the bow shock propagate into the magnetosphere.

Based on a case study of Pc 3 pulsation observed with AMPTE/CCE satellite and five ground stations, Takahashi et al. [1994b] found evidence for the generation of compressional MHD modes in the magnetosphere by bow-shock associated magnetosheath ULF waves and their subsequent propagation across L-shells to field lines conjugate to the ground stations. Compressional magnetic pulsations with irregular waveforms and periods longer than 150 s have been studied by using data from AMPTE/CCE and GEOS 5 and 6 in the dayside magnetosphere and from the Kakioka station. Observations indicate that some nightside pulsations in the Pi 3 band have daytime origins [Matsuoka et al., 1995].

The dayside enhancement of ULF wave activity observed near the polar cusp has been studied by Sakurai et al. [1993] in relation with variations of both the ground and the interplanetary magnetic field. They showed that the cusp might be a source region of magnetic energy penetrating into the magnetosphere, and resonant oscillations of ULF waves are clearly observed at the auroral latitude. Kato et al. [1994] used Pc 5 pulsation observed at three stations in Iceland and three stations in Antarctica to estimate the geomagnetic conjugate point of Syowa Station. Statistical result of the conjugate point concentrates around that calculated from the Tsyganenko model within a distance of one fifth wave length.

Magnetic data from the STEP 210 degree magnetic meridian chain stations have been analyzed. Yumoto et al. [1994a] have investigated if global cavity-mode and localized field-line oscillations can be excited in the inner magnetosphere by interplanetary impulses (Sc/Si). To study the origin of packet structure and phase skips, the phase-time structure of low latitude magnetometer data over the Pc 3 range for a typical equinoctial day has been examined. Preliminary results suggest that local effects are important in determining the detailed phase structure and there is no evidence of phase skips occurring simultaneously at widely spaced stations [Menk and Yumoto, 1994]. The amplitude and phase of H- and D-components recorded at three stations along the chain suggest that Pi 2 pulsations consist of several different modes in the structured magnetosphere [Yumoto et al., 1994b].

H1.3. ELF/VLF Wave Phenomena

Wave forms observed by the ELF-search coil magnetometer onboard GEOTAIL show that the observed polarization of foreshock whistler mode waves has strong dependency on the angle between directions of their propagation and that of solar wind flow [Hayashi et al., 1994].

A particular type of ELF emission has been observed with multi-band spectral structure around the local gyrofrequency of oxygen ion when the Akebono satellite passes near the equatorial plane of the inner plasmasphere during the main phase [Liu et al., 1994]. Modified Alfven mode emissions are frequently observed by the Akebono satellite around the geomagnetic equator. The wave normal direction is measured to be almost perpendicular to the magnetic meridian plane. By ray tracing, the emissions are interpreted as trapped in the equatorial plane by a sharp density gradient as in the plasmapause [Kasahara et al., 1994].

Quasi-monochromatic waves with an electrostatic nature in the lower hybrid frequency range have been identified in the PSBL by the wave form observations in 0-25 Hz band from the double probe onboard the GEOTAIL spacecraft. Simultaneous measurements of low energy ions also indicate that the wave coincides with the transition of the ion-energy from 0.1 keV/e to 10 keV/e [Okada et al., 1994]. The WFC receiver onboard GEOTAIL was also used for detailed analysis of the frequency-time structure of the low-frequency CR near the lower cutoff. CR in the lobe region, especially in the vicinity of the PSBL, is sometimes accompanied by intense electrostatic electron-cyclotron-harmonic (ECH) (n+1/2) waves. These suggest that such a low-frequency CR in the distant tail region is most likely to be generated from the ECH waves near the PSBL, and trapped within the lobe region [Nagano et al., 1994a].

The different kinds of VLF/ELF emissions (chorus, auroral hiss, periodic emissions), have been reviewed [Hayakawa and Smith, 1993; Sazhin et al., 1993; Sazhin and Hayakawa, 1994]. Also, the generation mechanisms of VLF/ELF emissions have been examined by means of the direction finding [Hayakawa, 1993a]. VLF/ELF radio noise in the inner plasmasphere has been reviewed [Hayakawa, 1993b, 1993c], and the generation mechanism of hiss-triggered emission has been investigated by the combined data of spectral analysis and direction finding [Hayakawa, 1993d; Hattori and Hayakawa, 1994a, 1994b].

The narrow-band plasmapause hiss observed by ISIS-1 and -2 has been analyzed statistically. The occurrence peak variations with latitude and MLT were explained by the plasmapause contraction during the magnetic storm and the inflow direction of energetic electrons drifting from the magnetotail, respectively. The hiss seems to be generated by cyclotron instabilities near the plasmapause [Ondoh, 1993, 1995]. Based on DE-1 data, plasma irregularities scale and density were obtained from the upper limit frequency of the polar hiss. Whistler-triggered hiss was observed in the low-latitude magnetosphere. Its generation mechanism was discussed in terms of linear and nonlinear spectrum broadening. Impulsive VLF waves were observed in association with strong ELF hiss in the night and seem to be electrostatic whistler-mode waves. A chorus with a narrower gap frequency was observed in the recovery phase which implies a reduction by one fifth of the local magnetic field from the quiet time [Ondoh and Nakamura, 1994, 1995]. Wave normal directions of V-shaped hiss and saucers have been measured by the Akebono satellite. The directions are downgoing for hiss and upgoing for saucers with a large wave normal angle close to the resonance angle. The characteristics was reproduced by ray tracing [Kasahara et al., 1995]. Nishino and Tanaka [1994] have analyzed low-latitude VLF hiss observed at Moshiri and Kagoshima associated with the severe magnetic storm during October 19-21, 1989. The hiss exhibited an increase of frequency of maximum energy with local time. Injected 5 keV electrons were estimated to contribute to the generation of hiss at dawn time equatorial region.

Nonlinear wave-wave interactions have been studied in the subauroral ionosphere on the basis of ISIS-2 satellite observations of Siple station VLF signals [Ohnami et al., 1993]. The corresponding theoretical consideration was given [Trakhtengertz and Hayakawa, 1993].

The location and extent of the exit areas at the ionospheric base of Siple VLF signal have been deduced from the wave intensity measured at six ground stations in Canada. The magnetic field intensity and polarization distributions are in close agreement with the results of a mathematical evaluation and a full wave calculation [Ikeda et al., 1995]. The structure of ducts for mid-latitude whistlers and their ionospheric transmission have been studied by Takahashi et al. [1993] based on the direction finding. Subionospheric propagation of low-latitude whistlers has been studied by Ohta et al. [1994] and Hayakawa et al. [1995a], whose characteristics are found to be very close to those of tweek sferics [Hayakawa et al., 1994; Hayakawa et al., 1995b]. Propagation characteristics of whistlers in the Jovian ionosphere and magnetosphere have been examined [Nagai et al., 1993; Hayakawa, 1995].

The radio emissions associated with earthquakes have been observed on the Intercosmos-24 satellite [Molchanov et al., 1993, 1994a]. The ionospheric perturbation associated with earthquakes as detected by subionospheric VLF propagation has been reported by Hayakawa and Sato [1994].

H1.4. LF/MF/HF Wave Phenomena

Low-latitude propagation characteristics of LF whistler-mode signals have been analyzed by observing Decca navigation signals from Hokkaido, Japan, at its geomagnetically conjugate area Birdsville, Australia. The Doppler broadening and polarization of the signals observed around sunset suggest the propagation in a small number of ducts, while those of the signals at nighttime suggest the existence of multiple clustered ducts. Emission of the LF signals from the ducts has been discussed by calculating ionospheric penetration losses for the two cases [Iwata et al., 1995a; Iwata and Tanaka, 1995]. By comparing the energetic electrons observed by the NOAA-6 satellite with the whistler-mode Decca signals received around Birdsville, and VLF/ELF emissions observed at Moshiri and Kagoshima, Iwata et al. [1995b] concluded that the penetration of energetic electrons deep into the magnetosphere may be deduced from ground-based measurements of resonant waves at low latitudes.

On the occasion of the perihelion passage of Halley's comet in early 1985, the Sakigake spacecraft made observations in the distance close to the comet nucleus. The plasma wave turbulence and the shock wave positions have been studied. A report has been made on the plasma phenomena in the outer regions of the coma [Oya, 1993].

Periodic bursts of very intense electrostatic electron cyclotron harmonic waves (ESCH) have been discovered by Sakigake at the PSBL. The bursts are enhanced with the period around 1.5 min suggesting large volume of plasma cloud periodically flowing toward the tail region. There are also periodic ESCH wave of about 300 s suggesting the velocity space modulation in the plasma flow [Oya et al., 1994].

H1.5.Observational Techniques

Matsumoto et al. [1994a] described the GEOTAIL PWI subsystems and their functions. They also reported some initial results of observations made during the traversal of the geomagnetic tail and a skimming pass of the dayside magnetopause.

Further studies have been made on the wave distribution function for magnetospheric VLF/ELF waves by means of regularization and generalized cross validation [Hayakawa, 1993a; Yamaguchi et al., 1994] and also by a Bayesian approach [Hirari and Hayakawa, 1995]. Sakamoto et al. [1995] reported a technique to determine the wave normal only from the amplitude of the signals which does not need such a wide band telemerty as required in the usual technique based on the phase information. Rafalsky et al. [1995] have presented a one-site distance-finding technique for locating lightning discharge.

H2. Active Space Experiments and Laboratory Experiments on Plasma Waves

H2.1. Wave Propagation

Akebono has observed ELF wave emissions from the lower ionosphere which was heated by a ground based high power HF transmitter with an ELF frequency amplitude modulation. The intensity of the ELF signals was confirmed theoretically by a full wave method [Kimura et al., 1994; Yagitani et al., 1994].

H2.2. Wave Generation and Instabilities

A drastic potential drop along a collisionless magnetized plasma flow has been observed in the presence of localized large-amplitude electron plasma waves generated externally [Sato et al., 1993]. Anomalous plasma transport across a magnetic field and electron heating were experimentally observed when axisymmetric electromagnetic fields in the lower-hybrid range of frequencies were applied to a fully-ionized plasma column [Hatakeyama et al., 1994]. In a single-ended Q-machine the effects of a radial electric field on current-driven low frequency instabilities was investigated experimentally [Iizuka and Schrittwieser, 1993]. It has been observed that the emission of electromagnetic wave around the electron cyclotron frequency is strongly enhanced when double layers are formed in magnetized collisionless plasmas [Mieno et al., 1994]. Effects of rotating rf electric fields on density profile of a magnetized plasma column have been investigated in connection with particle drifts due to traveling waves around the ion cyclotron frequency [Sato et al., 1994]. Ding et al. [1994] have performed measurements on plasma responses to electron injections from a floated emissive endplate into a magnetized collisionless plasma column produced in a single-ended Q machine. Measurements have been performed on ion cyclotron oscillations induced by a small emissive disc plate floating in a magnetized collisionless plasma column [Aoyama et al., 1994].

H2.3. Nonlinear Effects

ISY-METS rocket experiment has been conducted to verify a newly developed microwave energy transmission system for space use and to study nonlinear effects of the microwave in the ionospheric plasma. An intense microwave was transmitted by an active phased array antenna. The wave (HF) observation indicated that natural plasma waves are enhanced by the microwave energy through nonlinear interaction [Akiba et al., 1993].

Reflection coefficients and excitation efficiencies of ion acoustic waves have been measured in the multicomponent plasma with negative ions. The reflection coefficient becomes larger for a negative bias of the grid, and is almost constant for a positive bias of the grid as the partial pressure of SF6 gas is increased [Ito and Nakamura, 1994]. Their nonlinear evolution has been studied using a double-plasma device. It was found that the phase velocity of the ion acoustic waves increases with the wave amplitude at a critical concentration of negative ions [Bailung and Nakamura, 1993].

H2.4. Experimental Techniques

A feasibility study has been given on a new orbiting power station (PSS) which supplies a power of order of 100 kW to orbiting customers. The unique feature of the proposed PSS is the use of a module which has a solar-cell array on one side and a microwave transmitting antenna array driven by FET amplifiers on the other side [Matsumoto et al., 1993].

H3. Theory and Computer Experiments on Plasma Waves

H3.1. Wave Propagation

A full wave technique has been developed to calculate ELF/VLF field distributions, both in the upper ionosphere and on the ground, radiated from a dipole source placed in the lower ionosphere. The technique is applied to investigate the propagation characteristics of ELF/VLF radiation from a polar electrojet current, which is illuminated by a powerful HF wave modulated at ELF/VLF frequencies transmitted from ionospheric heating facilities at high latitudes. Calculated field intensities are quite consistent with those actually observed by the Akebono satellite and ground VLF receivers during the ionospheric heating experiments at Tromso [Yagitani et al., 1994; Kimura et al., 1994] and at HIPAS [Nagano et al., 1994b]. The full wave calculation of the VLF waves radiated from a ground-based dipole antenna has also been successfully applied to explain signal intensity distributions in the ionosphere observed by Akebono over the Australian Omega station [Nagano et al., 1993].

Method of modeling of VLF wave propagation has been proposed taking into account of the amplitude [Molchanov et al., 1995a]. The effect of localized perturbation on subionospheric propagation has been studied by the finite element method [Baba and Hayakawa, 1995], and the influence of layered structure of the lower ionosphere has resulted in non-monochromatic spectrum behavior of ELF atmospheric noise [Molchanov et al., 1994c]. Omid et al. [1994] have calculated the excitation of electromagnetic waves in the ionosphere by a delta function current sheet. The propagation of radio emissions associated with earthquakes from an underground seismic source to the ionosphere has been studied by Molchanov et al. [1994b, 1995b].

Omura and Green [1993] presented an innovative and powerful new technique to be used in the interpretation of GEOTAIL plasma wave data when the spacecraft is in the deep geomagnetotail during potential reconnection events. At precisely the moment of reconnection, the magnetotail structure based on a MHD simulation was used in the ray tracing calculation of various plasma waves expected to be generated in the X point regions.

H3.2. Wave Generation and Instabilities

The physical mechanisms that might determine the damping rates and resonance widths of Pc 3-5 pulsations have been considered. Experimental support was found in the damping rate distribution of Psc 3 pulsations observed along the 210 degree chain stations [Yumoto et al., 1995]. The theoretical justification of the basic principles of Alfven field line resonance in two-dimensional inhomogeneous plasma has been given with a more correct treatment of the problem with the use of the Frobenius method. The singular behavior of the wave field near a resonant field line is maintained as well [Fedorov et al., 1995].

Particle-in-cell (PIC) simulation of electromagnetic pump wave propagation into an overdense, strongly magnetized plasma with a linear density gradient have been investigated. A localized three-wave interaction is observed involving the pump decaying into upper and lower hybrid waves. The results are relevant to stimulated electromagnetic emissions (SEE) [Goodman et al., 1994]. By computer experiments it was confirmed that the density nonuniformity is important for the excitation of the Broad Upshifted Maximum (BUM). Also, an excitation mechanism of the BUM was proposed by Ueda et al. [1994a].

The gyrophase organization of electron populations linearly couples the eigenmodes of gyrotropic parallel propagation in magnetoplasmas. The interaction, besides intensifying preexisting instabilities, excites new types of wave activity. Solutions of the nongyrotropic parallel dispersion equation and particle simulations illustrate instability enhancements and the stimulation of electrostatic and electromagnetic wave growth in media that are devoid of free energy sources [Brinca et al., 1993].

Both linear and nonlinear interactions between oblique whistler, electrostatic, quasi-upper hybrid mode waves and an electron beam were studied by linear analyses and electromagnetic particle simulations. Growth rates were first calculated. It has been found that there are four kinds of unstable mode waves for parallel and oblique propagation [Zhang et al., 1993].

The competing processes between the quasi-electrostatic whistler mode wave and the upper hybrid waves have been investigated. The results show that generation of the electrostatic plasma wave is mainly controlled by fp / fc and that upper hybrid waves become dominant only at less oblique wave normal angles when fp > fc. The simulation results give interpretation for the observation by the Akebono satellite [Watanabe and Oya, 1993].

Usui et al. [1993] has studied plasma responses to a high potential satellite in the electrodynamic tether system with the aid of two-dimensional electromagnetic particle computer experiments. The transient process of the plasma response and the spatial profiles of plasma density and current as well as the current-voltage characteristics of the satellite were examined. Okada et al. [1993] reported numerical experimental results on the electromagnetic environment around the spacecraft exposed to a high-speed plasma flow.

A review of computer simulations of the Kelvin-Helmholtz (K-H) instability at the magnetopause has been given. By means of a two-dimensional MHD simulation, it was shown that the observed rotation of the unperturbed magnetic field within the magnetopause leads to significant dependence of the magnetopause K-H instability on the orientation of the magnetosheath magnetic field [Miura, 1995]. The tangential stress due to the K-H instability at the magnetospheric boundary is found to be mostly caused by the Reynolds stress and is large enough for exciting a residual plasma convection inside the terrestrial magnetosphere [Miura, 1993]. Hybrid simulations of the MHD scale transverse K-H instability in a uniform plasma have been conducted. An enhanced mixing of ions across the shear layer was found to occur inside the vortex. The scale of the mixing process is determined by the fluid dynamics parameters and is anomalously quick/large compared to that due to the finite Larmor radius. The low latitude boundary layer just inside the magnetopause is the mixing layer formed possibly by this mechanism [Fujimoto and Terasawa, 1994].

Watanabe et al. [1993] have performed extensive three-dimensional computer simulations of the magnetosphere-ionosphere (M-I) coupling to study self-excitation of auroral arc like structure. It was found that V-shaped parallel potential structure is formed as a result of M-I coupling when an anomalous parallel resistivity is generated by a current-driven instability.

H3.3. Nonlinear Effects

In order to provide quantitative measure of the nonlinear effects of intense microwave beam on space plasmas, Matsumoto et al. [1995a] conducted computer simulations using an electromagnetic particle code. It turned out that the excited level of the Langmuir waves is smaller than that of the ECH waves, opposed to the theoretical prediction. Interpretation of this contradiction was given. Matsumoto et al. [1995b] have derived theoretical formulae which allow them to evaluate the coupling coefficients from the intense microwave to the nonlinearly excited plasma waves. Two cases were studied: one is for O-O-SX mode coupling, while the other for R-R-Langmuir three wave coupling.

Omura et al. [1994] presented simulations of the ESW as observed by GEOTAIL. They have found that ESW are generated as a result of the nonlinear coalescence of strong electrostatic waves excited by an electrostatic beam instability. This instability is driven by an electron beam drifting relative to the ions and other electrons drifting with the ions. Solitary waves in an ion-beam-plasma system were investigated theoretically by Nakamura and Ohtani [1995] using the pseudo-potential method, including finite temperatures of plasma ions and beam ions and it was found that a smaller solitary wave appears at the leading part of the pulse for the slow beam mode as a result of negative nonlinearity and anomalous dispersion

Alfven waves play important roles in space and astrophysical plasmas. Further understanding of the nature of the nonlinear evolution may be essential in correctly describing micro-processes of these plasmas. While nonlinear development of Alfven waves in a one-dimensional closed system has been studied intensively, effort to study nonlinear Alfven waves in a more realistic plasma has only recently started. Using analytic theory and numerical simulations, Hada et al. [1994] discussed parametric instabilities of Alfven waves in two- (and three-) dimensions, and nonlinear evolution of Alfven waves subject to growth and damping. A set of equations was obtained to describe evolution of finite amplitude, dispersive, elliptically polarized quasi-parallel Alfven waves. The sound wave as well as the right- and left-hand polarized Alfven waves are nearly all degenerated. Modulational instability of a circularly polarized parallel Alfven wave has been discussed [Hada, 1993a]. The evolutionary conditions in the dissipative MHD system have been discussed by Hada [1994]. With emphasis on introducing relevant articles to interested readers, several important nonlinear aspects of Alfven waves have been briefly surveyed. In particular, mathematical and physical properties of the derivative nonlinear Schroedinger equation (DNLS), which describes the behavior of quasi-parallel weakly nonlinear and weakly dispersive MHD waves, were discussed [Hada, 1993b]. The properties of the DNLS equation were reviewed by Mjolhus and Hada [1995]: its physical significance, the exact solutions, its inverse scattering transform (IST), and the soliton formation processes. The process of dispersive steepening and a combined process of modulational instability and nonlinear Landau damping have been described, and the oblique two-parameter solitons were for the first time exhibited in detail. The IST was proposed to analyze nonlinear Alfven waves in a space plasma. Some numerical examples were given by Hada et al. [1993]. The IST describes the long-time evolution of quasi-parallel Alfven waves more efficiently than the Fourier transform.

The development of the fast reconnection mechanism has been systematically studied in a sheared field geometry. It was shown that the thin transition layers standing in the quasisteady fast reconnection configuration are divided into the intermediate wave region and the slow shock region [Ugai, 1993]. The temporal dynamics of magnetic reconnection was studied under a continuous local plasma injection. It was shown that the reconnection process is strongly influenced by the resistivity model [Ugai, 1994]. The temporal dynamics of MHD shocks, from a slow shock to a weak intermediate shock, was studied by one-dimensional MHD simulations with high resolution. It was argued that in a general noncoplanar situation the intermediate shock cannot stand stably [Ugai and Shimizu, 1994a]. The basic physical concept on the fast reconnection mechanism has been reviewed. For the evolution mechanism of fast reconnection, two theoretical models, the externally driven model and the spontaneous fast reconnection model, have been proposed. It was argued that the spontaneous model may be most applicable to catastrophic events observed in space plasmas [Ugai and Shimizu, 1994b]. The spontaneous model has been applied to the plasmoid dynamics. It was demonstrated that a large-scale plasmoid propagates at a speed somewhat less than the Alfven speed; also, along the plasmoid boundaries slow shocks stand [Ugai, 1995a]. Ugai [1995b] made simulations study on the physical mechanism by which magnetic energy is released into plasma energies. It was demonstrated that for the spontaneous fast reconnection mechanism the stored magnetic energy is effectively converted by the resulting large-scale motor effect. For the steady-state three-dimensional reconnection problem in a compressible plasma, Terasawa et al. [1993a] have found a new class of exact solutions, which consist of ideal-MHD regions, MHD singularities (slow shocks and rotational discontinuities), and a neutral line. This solution predicts the existence of plasma jetting parallel to the neutral line, in addition to the usual plasma jetting in the perpendicular direction. The solution has well-behaved velocity fields without any singularity.

Global MHD simulation of interaction between the solar wind and the earth's magnetosphere made clear a role of IMF orientation. The IMF orientation controls much of the interaction processes through magnetic reconnection. During southward IMF the energy inflow from the magnetosheath to magnetosphere increases due to enhancement of reconnection rate in the dayside magnetopause. For northward IMF the reconnection region moves to high latitude tail and the rate becomes small. The magnetic energy which is stored in lobes during southward IMF decreases and the plasma in the magnetosheath penetrates into flank magnetopause. Therefore, the northward turning of IMF gives a recovery of magnetosphere [Walker et al., 1993a, 1993b; Ogino et al., 1994, 1995].

H3.4. Particle Acceleration, Heating and Diffusion

The mechanism for omega bands and torch structures of the postmidnight diffuse aurora has been studied with a two-dimensional electrostatic particle simulation. Electric dipoles can grow on the poleward boundary of the hot plasma, and result in the spatial modulation of the energetic electrons, which leads to the formation of omega bands and subsequent evolution into torch structures [Yamamoto et al., 1993a]. Convective generation of "giant" undulations on equatorward boundary of an evening diffuse aurora was also numerically simulated. Results show that the disturbance forms a series of Kelvin-Helmholtz waves along the polarized arc sheet [Yamamoto et al., 1993b, 1994]. It was shown that the evaluation of a westward-traveling surge can be identified with the aurora deformation due to the appearance of a westward plasma flow at the boundary between the open and closed field lines [Yamamoto et al., 1993c].

Current-voltage relationship including plasma flow along the mirror field has been investigated by Miura [1994]. Plasma structure and potential formation in a plasma flow along a magnetic mirror field have been investigated by a two-and-half dimensional electrostatic particle simulation [Ishiguro et al., 1993, 1995]. A particle simulation with plasma source has been carried out on plasma structures generated by an electron emissive electrode floated in collisionless plasma [Ishiguro and Sato, 1993]. Electrostatic particle simulation has been performed on ion cyclotron oscillations driven by applying a positive potential to a small electrode in a magnetized collisionless plasma [Ishiguro et al., 1994].

An overview of the stochastic ion acceleration process at the terrestrial bow shock and a summary of the remaining problems on the injection, acceleration, and spatial distribution was presented by Terasawa [1995]. The interaction between a plasmoid and cold ion beams in the tail lobe region was numerically investigated by Tsubouchi and Terasawa [1995]. They found out that the effective energization of ion beams takes place in the passage of a plasmoid, due to the electric field induced by the motion of the plasmoid. Results were successfully compared with the recent GEOTAIL observation. Nonlinear interaction between ions and MHD waves at cyclotron subharmonic frequencies has been studied by Terasawa et al. [1993b].

Plasma physics in a neutron star has been investigated and it was found that Hall current is of importance. Naito and Kojima [1994] calculated evolution of neutron-star magnetic-field with the Hall-current effect. The presence of the Hall current causes the couplings among different modes and the energy is transferred to different modes. It was concluded that the decaying features depend on the configuration of the fields. Since galactic cosmic rays are believed to be created in supernova remnants (SNRs), SNRs are expected to be a source of high energy gamma-ray through the decay of neutral pions produced by p-p collisions. Naito and Takahara [1994] calculated the gamma-ray spectra and showed that SNRs are observable in gamma-ray energy ranges with existing and future instruments.

Using Monte Carlo simulation, the phase-space distribution function of diffusively accelerated particles in shock waves with oblique magnetic field has been calculated. Large-angle scattering (LA) and pitch-angle scattering (PA) were examined as for diffusion process. The spectral index and the acceleration time scale have been obtained [Naito and Takahara, 1995a]. On the basis of Fermi acceleration in oblique shock waves, Naito and Takahara [1995b] obtained the spectral indices of accelerated particles from the average energy gain and the escape probability per cycle. Their results apply to any values of shock speed and obliqueness as far as the shock is subluminal.

H3.5. Techniques of Data Analysis and Computer Experiments

Ueda et al. [1994b] has studied the nonphysical increase of kinetic energy observed in electrostatic particle simulations. Their numerical experiments have revealed the characteristics of the numerical heating rate which is a function of a number of particle per grid and a number of grid points per Debye length.


The editor thanks S. Yagitani, X.-Y. Wu, and Y. Omura for their support in preparing this review article.


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