齋藤 義文

A type of whistler waves termed "long-duration whistler waves" (LDWW) in ELF range (1 - 64 Hz) in the magnetosheath is studied according to the wave characteristics and generation based on data observed by the search coil magnetometer onboard the Geotail satellite. LDWW are band-limited emissions near the lower hybrid frequency typically lasting several tens of minutes. Orientations of wave normal are determined from waveform data of vector magnetic field with an assumption of plane waves and are observed to be fairly well organized in a certain duration of LDWW. Ambiguity in propagation directions is removed by comparison of the phase relation between magnetic components and one component of electric field. The propagation vectors of LDWW are statistically aligned along the "Parker spiral" but are primarily reversed in the dusksides and in the dawnsides of the magnetosheath that is, one is simward, and the other is antisunward. This outstanding asymmetry strongly suggests that the bow shock region is the common soxirce region with waves propagating away from the bow shock along the draped magnetic field in the magnetosheath. Electrons composing a type of distribution function in flat-topped shape are concurrently observed during a series of LDWW events and are likely to yield a favorite condition for LDWW propagating a long path free from attenuation caused by wave particle interactions. Copyright 1997 by the American Geophysical Union.

The behavior of the tail field and plasma during a moderate substorm with onset at ∼1120 UT on Dec. 13, 1994 is examined using data from the ISTP spacecraft GEOTAIL and WIND, synchronous orbit and the ground. Five substorms were observed on this day while GEOTAIL was located near the center of the tail at Xgsm = -46 Re. In each substorm the field and plasma variations were similar to those observed during substorms by spacecraft closer to the earth. A southward turning of the IMF caused accumulation of lobe flux, development of a more tail-like field, and eventually an expansion phase and its consequences. The ∼1120 UT onset immediately followed a northward turning which ended a lengthy interval in which the IMF was alternately northward and southward. Shortly after the onset a flux rope passed GEOTAIL with a delay consistent with its formation at 20-30 Re several minutes earlier than a Pi 2 burst began at midnight. Immediately after the onset the lobe field decreased and the plasma sheet disappeared. During the substorm recovery phase the plasma sheet reappeared with plasma moving earthward. The plasma data show that the tailward flow is a combination of convecting and streaming plasma. All of the substorms exhibited multiple onsets. The main onset of each can be determined by a combination of negative bay onsets, Pi 2 bursts, synchronous field-aligned currents, dispersionless particle injection, and midlatitude positive bays. In one event the flux rope at 46 Re arrived before dispersionless injection at synchronous orbit suggesting that reconnection in the tail begins at or before major onsets. In fact most of the major onsets were preceded by pseudo breakups early in the growth phase, and weak tailward flows carrying a weak vertical field fluctuating about zero. These observations suggest that reconnection begins in the middle tail early in the growth phase and that it is substantially intensified or begins again at another location at the expansion onset. ©1997 COSPAR. Published by Elsevier Science Ltd.

During Oct. 18-20, 1995, the WIND and GEOTAIL spacecraft recorded passage of a large magnetic cloud. Soon after entering into the cloud, plasma/field/wave measurements on GEOTAIL showed that the earth's bow shock appeared about 10 Re upstream from the nominal position ((9, -23, -2) Re in GSE). This shock expansion were apparently triggered by the arrival of the magnetic cloud: A high Alfvén velocity within the magnetic cloud, which was due to the combination of high magnetic field intensity and low plasma density, made the magnetosonic Mach number of the bow shock small, and in turn resulted in the rapid outward expansion of the bow shock. We discuss the transient phenomena caused by this rapid expansion and sequential bow shock crossings associated with the solar wind condition inside this magnetic cloud. ©1997 COSPAR. Published by Elsevier Science Ltd.

We report on auroral-like electrons detected frequently in the plasma sheet. These electrons are characterized by their highly collimated bi-directional features along the field lines that are extended up to keV energy ranges. In this paper, we concentrate on samples obtained in the dawn-flank plasma sheet. Despite their detected locations in the proximity of the interface to the low-latitude boundary layer (LLBL), the energy spectra and the highly collimated features show striking contrast with the well-known bi-directional thermal electrons in the LLBL. The distribution function shape suggests that these electrons are related with auroral energization processes. It is reported that the electrons are frequently detected accompanying fluctuating flows of a few hundreds km/s. Implications of this observation to the auroral energization processes in the dawn/prenoon sectors are discussed. © 1997 Published by Elsevier Science Ltd.

Observations of slow shocks in the Earth's magnetotail at the plasma sheet-lobe boundaries have been well documented. We restudy the magnetic field data of two slow shocks: one was observed from Geptail on January 17, 1994 at XGSE = -92 RE, and another was observed from ISEE-3 on February 2, 1983 at AGSE = -220 RE. In both cases, the slow shock layer was followed by an adjoining rotational discontinuity layer on the postshock side. Compound structures each composed of a slow shock layer and an adjoining rotational discontinuity layer have been recently observed in interplanetary space from Wind, Geotail and Imp-8. Because the two successive discontinuities are very close to each other, the compound structure looks like a new kind of MHD discontinuity. It may be called a double discontinuity. Since double discontinuities exist not only in interplanetary space but also in the magnetotail region, they could be a general MHD structure in space plasma. Copyright 1997 by the American Geophysical Union.

In this paper, we report on highly asymmetric spectrum of electrons observed at the boundary of postplasmoid plasma sheet (PPPS). The data were obtained when GEOTAIL made repeated crossings of the boundary after encountering with a tailward flowing plasmoid at XGSM ∼ -46RE. In the spectrum, electrons in 0.1 - 1 keV energy range are seen to flow earthward along the lobe-like field lines into the PPPS counter to more energetic components (ions and electrons) leaking from the PPPS. In the boundary region, earthward streaming electrons are observed even when the energetic leaking components almost disappear, which makes us characterize, this region more generally by tailward flowing field aligned currents (FAC). By referring to hybrid code (ion particles, massless electron fluid) results, we propose that the earthward flowing electrons sustain the FAC away from the X-line (tailward if a spacecraft is located tailward of the X-line), which is a part of a Hall current loop built-up in the course of magnetic reconnection. Copyright 1997 by the American Geophysical Union.

The nightside magnetosphere under the northward IMF condition is not flat at the distance of 10 to 15 Re but reveals a variety of interesting features. The magnetospheric boundary layer consists of the tailward flowing outer layer and the earthward flowing inner layer. In the dawn as well as the dusk flank regions there are large amplitude oscillations with periods of 8 to 15 min which show the character of the slow mode or the mirror mode waves. Moreover, around the midnight meridian the ions and electrons are injected earthward at intervals of 1 to 2 hours. The westward drift of the injected ions produces the energy-dispersive ion signatures in the dusk region. The electric field Ey is positive on average at this distance while in the distant tail it has been found to be negative on average. These observations of Ey are compatible with the magnetic reconnection in the magnetotail when the twisting of the neutral sheet is taken into account. Copyright 1997 by the American Geophysical Union.

It has been known that an efficient re-acceleration of energetic storm ions can occur between a propagating interplanetary shock and the bow shock. However, it has not been known whether a similar event could occur for electrons. In this paper, we shall report the first observational evidence of re-acceleration of energetic electrons at the front of an interplanetary shock. ©1997 COSPAR. Published by Elsevier Science Ltd.

Geotail has surveyed the dayside magnetopause in the equatorial plane and studied the dayside reconnection of the interplanetary magnetic field (IMF) with the magnetospheric field lines. When the IMF is directed southward leakage of ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) are observed. Reconnection is shown to be responsible for the leakage. A variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions is explained by the ''velocity filter effect'' model with a finite source region. In addition to such conditions, reconnection occurs also when the IMF has low-inclination angles, i.e., the condition between the southward and northward IMF conditions. Between the magnetosphere and the magnetosheath, two types of boundary region develop, i.e., the inner-LLBL and the outer-LLBL. The inner-LLBL is characterized by the bidirectional cold electrons and trapped cold ions coexisting with hot magnetospheric plasma. The field lines are regarded to be closed. The outer-LLBL is, on the other hand, open to the magnetosheath. It is characterized by the uni-directional cold electrons escaping to the magnetosheath. Newly penetrating solar wind ions are overtaking the trapped cold ions. The formation of the outer-LLBL is explained by the high-latitude reconnection at the equatorward regions of the cusp. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

A case study of the low-latitude boundary layer (LLBL) on the dawnside (7-9 MLT) is reported. As in previous studies, the LLBL structure is well organized if it is taken to consist of two parts, the outer-LLBL and the inner-LLBL. The inner-LLBL is where the mixing of the magnetosheath and the magnetospheric plasma is taking place on closed field lines. The outer-LLBL is where magnetosheath-like plasma is flowing tailward. Detailed analysis of particle signatures, together with the information that IMF B-y was the dominant component directed persistently dawnward for this interval, suggests an interpretation that the outer-LLBL is on reconnected open field lines. The positions of the reconnection sites relative to the spacecraft, and the dynamics of the flux tubes subsequent to reconnection to form the observed outer-LLBL, are also discussed. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

Structure of the distant magnetotail (x<-150 Re) in the y-z plane is studied statistically on the basis of the plasma and magnetic field data obtained by the GEOTAIL spacecraft. In order to reduce the scatter due to time variations in the solar wind direction, a coordinate system is introduced whose x-axis is taken parallel to the hourly direction of the solar wind measured in the upstream region. Our analysis shows that the distant tail has almost the same dimension in y and z directions for average IMF conditions. In other words, there is no indication of the tail flattening along the y axis for average magnitude of IMF By. The distant tail current sheet is found to be tilted by 20 degrees on average either clockwise or anticlockwise depending on the sign of the IMF By. However, the location of the core lobe, which is defined to be the region of maximum probability of observing the lobe, is not tilted from the z axis. This means that the IMF By effect is not an overall rotation of the tail structure but involves a skewing of the internal structure. The tilt angle of the current sheet is larger when the IMF is directed northward. The large tilt angle observed for the northward IMF implies that the northward magnetic field component in the tail does not necessarily mean the closure of field lines across the current sheet; this finding thus alleviates the difficulty regarding the net tailward transport of positive Bz during quiet times raised earlier. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

The origin of hot and high speed plasmas observed by the GEOTAIL spacecraft in the magnetotail is discussed in terms of slow shock acceleration and heating. We find that the bulk flow energy for the hot and high speed plasma in the tail plasma sheet is larger than the thermal energy, and there is an lower limit on the ratio of the thermal to bulk flow energy 2p/rho upsilon(2). The lower boundary of 2p/rho upsilon(2) is about 0.2-0.4, and the ratio is independent of the plasma temperature in the range of several 100 eV to 10 keV. It is believed that the magnetic reconnection associated with a slow shock can produce the hot and high speed plasma, though the observed lower limit cannot be explained by a standard slow shock heating and acceleration process, because the lower limit of 2p/rho upsilon(2) obtained by the standard slow shock Rankine-Hugoniot relation is 0.4 even for the strong slow shock limit. In order to explain the observed lower boundary of 2p/rho upsilon(2), we study the Rankine-Hugoniot relations by taking into account of non-standard MHD effects such as a temperature anisotropy and a heat flux. The observed lower limit can be explained by a slow shock including the temperature anisotropy and the heat flux effects. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

The structure of the plasma sheet in the distant magnetotail observed by the Geotail satellite is examined. We found that the observed structure of the plasma sheet is often different from the standard Harris-type plasma sheet [Harris, 1962]. The observed structure can be expressed as a double-peaked current sheet which has a pair of localized electric currents away from the neutral sheet, i.e., a geometrically thick zero magnetic field region attached to the thin boundary layer with a large magnetic field gradient. This type of the plasma sheet is often observed in the distant magnetotail at radial distances of -50 > X(GSM)/R(E) > -125. We discuss a possible model to explain the formation of the double-peaked current sheet in terms of large-scale magnetic reconnection associated with slow shocks.

The ion distribution function in the upstream region of slow-mode shocks observed in the Earth's magnetotail are investigated. We have found the existence of a region called the ''foreshock region'' upstream of slow-mode shocks that can be clearly distinguished from the main dissipation region of the slow-mode shocks. This foreshock region is characterized by counterstreaming ions: backstreaming ions that flow from the shock surface toward the upstream region along the magnetic field and the lobe cold ions that flow from the upstream region into the shock surface. Backstreaming ions may be hot plasma sheet ions escaping from the downstream region toward the upstream region. The incident cold ions are heated about 3% - 20% of the total ion heating throughout the slow-shock transition in this foreshock region. Wave-particle interaction with the electromagnetic ion cyclotron waves generated by the counterstreaming ions is a candidate of the cold ion heating mechanism in the foreshock region.

In the lobe/mantle region at similar to 159 R(E) amay from the Earth during a geomagnetically disturbed period, we have found the coexistence of three ion species, H+, He++, and O+, streaming tailward with nearly the same flow velocity similar to 200-500 km/s. Both H+ and O+ ions are detected almost continuously from near plasma sheet to near magnetopause region. From a positive correlation between the proton density and their velocity component parallel to the magnetic field V-II+ parallel to, we conclude that most of protons have come from the solar wind. The existence of He++ further supports this conclusion, which implies the importance of solar wind contribution to the magnetotail. The existence of O+, on the other hand, suggests that the ions of ionospheric origin have mixed with those of solar wind origin. The lack of positive correlation between O+ density and V-o+ parallel to is consistent with the idea that O+ ions have some source mechanism different from that of protons. Simultaneously: curious velocity differences are also observed: V-o+ parallel to appears to he often faster than V-II+ parallel to by Delta V-parallel to = 20-30 km/s. This observation may provide a key for further discussion.

The Geotail spacecraft frequently observes cold dense ion flows (CDIFs) streaming tailward in the magnetotail lobe region (X(GSM)similar to-10 to -210 R(E)). The density is often quite high (similar to 1/cm(3)), comparable to or larger than that in the plasma sheet, and the tailward speed is from 50 to 500 km/s. The CDIFs sometimes contain multiple (two or three) energy-per-charge components. The lowest- and highest-energy components are identified as H+ and O+, respectively. The intermediate-energy component is identified as He+, Therefore a part of the CDIFs in the distant lobes is believed to be of ionospheric origin. A possible source candidate is the upward flowing ions (UFIs) outgoing from the dayside amoral region or polar wind escaping from the polar cap and accelerated along the magnetic field direction. The high proton to O+ density ratio in the CDIFs suggests that the major component of the CDIFs originates from the solar wind and that the cold ion beams of ionospheric origin merge with the solar wind component penetrating into the magnetosphere at the mantle or the flank of the tail lobe. In some observations, the H+ ions continuously stream tailward both in the tail lobe and in the magnetosheath, and they couple with the solar wind ions through the magnetopause. Although the O+ and He+ ion flows cannot be recognized so clearly in the magnetosheath, weak O+ fluxes are sometimes present in the magnetosheath near the magnetopause. This implies that plasma of ionospheric origin may leak from the tail to the magnetosheath. The multicomponent CDIF events make it possible to discuss the sources and transport processes of the tail lobe plasma, and their behavior in or near the magnetopause provides important clues to the interactions between the lobe and the magnetosheath.

The nature of the dawn-dusk component of the motion of magnetic field lines in the plasma sheet is studied at the distances of 40 to 160 Re in active times. In the presence of the By component of the magnetic field, even the flow which is purely parallel to the tail axis (which is taken to be the x direction) produces an apparent velocity of field lines in the y direction. However, the more substantial cause of the dawn-dusk motion is the rotation of magnetic field lines. In the region of the earthward flow, the magnetic field lines rotate in such a way as to become closer to the plane which contains the earth's dipole moment that is, the tilting of field lines caused by the presence of By becomes smaller as field lines move closer to the Earth, In the region of the tailward flow, the magnetic field lines rotate from northward to southward in a clockwise or counterclockwise sense depending on the sign of By as they move tailward. The net convection in the dawn or dusk direction, if it existed, is masked by this rotational motion in the plasma sheet. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

The distances to the magnetic neutral lines in the magnetotail are derived from the observations of the convection in geomagnetically active times (Kp larger than 3). Observations at seven orbit segments of GEOTAIL that are distributed from x =-36 to -169 Re and longer than 24 hours are used. For each data set the ratio between the earthward-moving northward magnetic flux and the tailward-moving southward magnetic flux is derived. The ratio varies systematically with distance, and suggests that the distant neutral line tends to be located at about 140 Re while the near-Earth neutral line tends to be formed inside 50 Re- Mainly the open field lines are reconnected at the distant neutral line, while mainly the closed field lines are reconnected at the near-Earth neutral line. The northward magnetic flux transported earthward from the distant neutral line is several times larger than the net southward flux transported tailward from the near-Earth neutral line the open magnetic flux that is reconnected at the distant neutral line is substantially greaterthan that reconnected at the near-Earth neutral line. Observations of the field-aligned component of the flow velocity suggests that sometimes these two neutral lines exist together and produce the quasi-stagnant plasmoid.

This letter reports on leakage ions from the low latitude boundary layer (LLBL) to the magnetosheath boundary layer (MSBL) as observed by Geotail at the dayside magnetopause. A reconnection of the interplanetary magnetic field (IMF) with magnetospheric field lines is shown to be responsible for the leakage. Based on analysis of a variation in lower cut-off levels occurring in the velocity distribution function of the leakage ions, we propose a "velocity filter effect" model with a finite source region to explain this variation. The location of the reconnection region is subsequently calculated to be 2.2RE north of the geomagnetic equatorial plane under the proposed velocity filter effect model.

Nishida et al. (1986) proposed the category of quasi-stagnant plasmoids on the basis of ISEE-3 observations. Their quasi-stagnant plasmoids occurred during intervals of low geomagnetic activity, and the electron plasma moment data suggested that the plasmoids moved tailward very slowly (&lt 300 km/s). This paper reports for the first time the low-energy (32 eV/e-43 keV/e) plasma ion signatures of a quasi-stagnant plasmoid, which was observed with Geotail at X=-170 RE on October 15, 1993. On this day Kp was generally quiet, but Geotail observed two prominent bipolar Bz perturbations identifiable as plasmoids. The first plasmoid moved tailward very slowly (~250 km/s), and it was associated with a Pi2 onset possibly localized at high latitudes and with a gradual enhancement in the power of Aurora] Kilometric Radiation (AKR) emissions. On the other hand, the second plasmoid moved tailward fairly rapidly (~500 km/s), and it was associated with a Pi2 onset observable at wide range of latitudes and with, a sharp enhancement in the AKR power. Thus the first plasmoid falls into the category of quasi-stagnant plasmoids but the second plasmoid would notj this is consistent with the previous observations by Nishida et al. that quasi-stagnant plasmoids are sometimes followed by substorm onset-related plasmoids. A unique feature of the plasma ions in the first plasmoid is that the earthward field-aligned beam was observed first and then the tailward field-aligned beam was observed, as the spacecraft entered into the plasmoid. This time sequence is rarely observed for plasmoids during geomagnetically active times. It is explained in the framework of the quasi-stagnant plasmoid model. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

A new method to estimate the spatial structure of the moving magnetohydrodynamic (MHD) system is presented. We integrate the Faraday's induction equation, [Formula Omitted], assuming that the system is one-dimensional. The spatial gradient direction, along which the integration is made, is determined by minimizing the residue of the integrated Faraday's equation against the observed magnetic field. This new method can be regarded as an extension of the conventional magnetic minimum variance method. Sample data analyses using this new method for magnetotail events are also presented. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

A new method to estimate the spatial structure of the moving magnetohydrodynamic (MHD) system is presented. We integrate the Faraday's induction equation, ∂B→/∂t = -rot E→, assuming that the system is one-dimensional. The spatial gradient direction, along which the integration is made, is determined by minimizing the residue of the integrated Faraday's equation against the observed magnetic field. This new method can be regarded as an extension of the conventional magnetic minimum variance method. Sample data analyses using this new method for magnetotail events are also presented.

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term 'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic By fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the By/Bz field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic Bx field is very small, but significant By and/or Bz fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

Prior to the observation of fast tailward plasma flows, earthward plasma Sows are often observed in the distant magnetotail. By studying the evolution of the ion distribution functions for the transition flow from earthward to tailward flow, we find a multi-layer structure of beamlike plasma flows in the plasma sheet boundary layer, PSBL: 1) earthward, anisotropic beam plasma can be observed in the outer region of PSBL, i.e., the plasma lobe side of the PSBL; and 2) the counterstreaming ions, which consist of earthward and tailward beam-like plasma flows, are observed in the inner region of the PSBL; i.e., the plasma sheet side. Inside the plasma sheet, an isotropic, hot ion plasma can be observed. These characteristics are similar to those measured in the PSBL closer to the Earth, and the tailward anisotropic ion beams observed in the near-Earth magnetotail are thought to be the mirrored counterpart of the earthward ion beam. In the distant, magnetotail, however, it is not easy to explain the tailward beam as the mirrored counterpart, because the reflected ion beams at the Earth will be absorbed into the plasma sheet before the reflected ions can travel to the distant magnetotail. We propose another model of two active magnetic reconnection sites. The tailward ion beams come from a near-Earth reconnection site which is embedded in the distant magnetotail reconnection site ejecting the earthward flow. © 1996, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Prior to the observation of fast tailward plasma flows, earthward plasma flows are often observed in the distant magnetotail. By studying the evolution of the ion distribution functions for the transition flow from earthward to tailward flow, we find a multi-layer structure of beam-like plasma flows in the plasma sheet boundary layer, PSBL: 1) earthward, anisotropic beam plasma can be observed in the outer region of PSBL, i.e., the plasma lobe side of the PSBL; and 2) the counterstreaming ions, which consist of earthward and tailward beam-like plasma flows, are observed in the inner region of the PSBL; i.e., the plasma sheet side. Inside the plasma sheet, an isotropic, hot ion plasma can be observed. These characteristics are similar to those measured in the PSBL closer to the Earth, and the tailward anisotropic ion beams observed in the near-Earth magnetotail are thought to be the mirrored counterpart of the earthward ion beam. In the distant magnetotail, however, it is not easy to explain the tailward beam as the mirrored counterpart, because the reflected ion beams at the Earth will be absorbed into the plasma sheet before the reflected ions can travel to the distant magnetotail. We propose another model of two active magnetic reconnection sites. The tailward ion beams come from a near-Earth reconnection site which is embedded in the distant magnetotail reconnection site ejecting the earthward flow.

It is found that the structure of the plasma sheet in the distant magnetotail observed by the GEOTAIL satellite can be often expressed as a doubly humped current sheet which has a pair of localized electric currents away from the neutral sheet, i.e., a geometrically thick zero magnetic field region attached to the thin boundary layer with a large magnetic field gradient. The observed structure of the plasma sheet is different from the standard Harris-type plasma sheet. We discuss a possible model to explain the formation of the doubly humped current sheet in terms of large-scale magnetic reconnection associated with slow shocks.

GEOTAIL data from the low-latitude flanks of the magnetotail at -30R(E) < X(GSM) < -15R(E) are analyzed. An example presented in this paper, which is representative of a group of the cases at the dawnside, shows that the region is characterized by highly varying magnetic field and the appearance of cold (<1 keV) and dense (>0.5/cc) plasma. While some of these cold-dense plasma are detected to flow tailward at 300-500 km/s, others are found to flow only slowly tailward or even sunward sometimes. Electrons of >100 eV are seen to be depleted in the tailward flowing part, suggesting that these data are taken when the satellite was located on open field lines. When the sunward flows are detected, the denisty tends to show an intermediate value, and thermal electrons (100-500 eV) are found to show bi-directional anisotropy. This suggests that part of the magnetosheath plasma is captured on closed field lines at the inner part of the low-latitude boundary layer (LLBL) in the tail flanks. Another study inside the plasma sheet at (X(GSM), Y-GSM) = (-17, -12)R(E) (a position well inside the magnetopause) shows a 2.5-hours period of the plasma sheet filled with cold (500 eV) and dense (2/cc) plasma. Bi-directional thermal electrons are seen to accompany this population. This may imply that relatively unheated plasma from the LLBL of the near-Earth magnetotail is filling a substantial part of the plasma sheet.

Based on GEOTAIL/LEP observations in the distant magnetotail, this paper reports on several new features of velocity distribution functions of electrons and ions within a plasmoid and at its boundary. Here we use the term 'plasmoid' in a wider meaning than usual in spite of the presence of significant magnetic B-y fields. In the lobe, as expected from MHD simulations of magnetic reconnection processes, cold plasmas are pushed away from the plasma sheet before the arrival of plasmoids, while after the plasmoid passage the convection is enhanced toward the normal direction to the plasma sheet. The cold ions flow into the plasmoid along magnetic field lines, are heated and accelerated perpendicularly at the boundary, and finally merge with hot plasmas deeper inside the plasmoids. Deep inside plasmoids, however, the ion distribution functions often show the existence of counterstreaming ion beams, while the simultaneously measured electron distribution functions show a flat-top distribution. It is noted that the presence of the counterstreaming ions is a fine structure along magnetic field lines inside the whole distribution convecting tailward with speeds of 500-900 km/s. The relative velocity of the two components along the magnetic field line reaches 1000-1500 km/s, which is much higher than the local Alfven speed. Each component has an anisotropic distribution with respect to its center in the velocity space; the perpendicular temperature is several times higher than the parallel temperature. We conclude that these counterstreaming ions are most likely of lobe origin, and they have not had time enough for thermalization. They might have entered the plasmoid from the northern and southern lobes, being heated and accelerated through slow-mode shocks at the boundaries. Hence, these field lines are open, and both ends are connected to the northern and southern lobes. This phenomenon is observed predominantly in the latter part of the plasmoid after southward turning of the magnetic field, especially after the plasma bulk speed has increased stepwise and the B-y/B-z field magnitudes have attained the peak value. It is also observed even near the neutral sheet, where the magnetic B-x field is very small, but significant B-y and/or B-z fields exist. Since the tailward flow speed becomes faster associated with the above phenomenon, these open field lines would be draped around the leading (core) part of plasmoids. The compression due to the draping may increase the field intensity.

Magnetic field and plasma observations by GEOTAIL in the distant tail at x = -200 R(E) are studied for a geomagnetically quiet interval when IMF Bz is predominantly northward and /Byl/ is larger than Bz on average. In the distant tail during this interval, Bz is northward and Ey is directed dusk-to-dawn on average. This combination of Bz and Ey does not seem to represent the tailward convection of the closed field lines, because Ey is much weaker in the lobe than in the plasma sheet so that the field lines would be piled up at the plasma sheet boundary if they were closed. The observations suggest instead that the plasma and field lines are convected parallel to the neutral sheet across the tail at the same time as they flow tailward. The direction of this cross-tail convection depends on the polarity of IMF By and is antisymmetric with respect to the neutral sheet, which can be twisted by tens of degrees under the influence of IMF By. A consistent picture is obtained from observations both inside the tail and at the tail magnetopause. This convection profile agrees in topology with the cusp reconnection model, but it occurs mainly in the plasma sheet while only the lobe field lines are expected to be involved according to this model. Observations show that not only the cold-dense ions which can be linked directly with the entrant solar wind plasma but also the hot-tenuous ions in the plasma sheet take part in the convection.

We have identified slow-mode shocks between the plasma sheet and lobe in the midtail to distant-tail regions by using three-dimensional magnetic field data and three-dimensional plasma data including density, velocity, temperature, and heat flux of both ions and electrons observed by the GEOTAIL satellite. Analyzing the data obtained between September 14, 1993, and February 16, 1994, we have found 303 plasma sheet-lobe boundary crossings at distances between X(GSE) similar to -30R(E) and X(GSE) similar to -210R(E). Thirty-two out of these 303 boundaries are identified as slow-mode shocks, We have found back streaming ions on the upstream side of the slow-mode shocks, which may be important in understanding the dissipation mechanism of the slow shocks in collisionless plasma, We have also found acceleration of cold ions between the upstream and the downstream of the slow-mode shocks, These cold ions are often observed in the lobe, and they are usually flowing tailward. Upon entering the plasma sheet, they are accelerated and rotate around the magnetic field and at times show ring-shaped velocity distributions, These ions may reflect the kinetic structure of slow-mode shocks, Slow shocks are at times observed also on the front side of plasmoids, These slow shocks on the front side of plasmoids have a different orientation from that of the ordinary slow shocks observed at the plasma sheet-lobe boundaries, which suggests an existence of ''heart''-shaped plasmoids predicted by a numerical simulation.

A statistical study is made on the reconnection process in the distant tail by using the GEOTAIL observations at x approximate to -150 Re and -95 Re in geomagnetically active times nith Kp greater than or equal to 3. Occurrence frequencies of various combinations of the field polarity and the convection direction suggest that at least 75% of the reconnection of open field lines occurs inside 150 Re and at least 39% occurs inside 95 Re. At 150 Re the convection of closed field lines constituting the viscous-like cell is at most 25% of the convection generated by the reconnection of open field lines. The reconnection rate of open field lines varies across the tail and is a few times higher near the tail axis than at the flanks.

GEOTAIL observation in the distant dusk upstream region showed the existence of the diffuse upstream ions (5-43 keV/q) when the interplanetary magnetic field was nearly radial and separated from the nearest magnetopause by greater than or similar to 20-30 R(E). Since even with the Bohm limit the perpendicular diffusion was not fast enough to transport the magnetospheric ions to the spacecraft, we conclude that these diffuse ions had a bow shock origin.

We report an observation of the singly-charged oxygen ions (O+) in the plasma sheet at X(GSE) similar to -60Re. Observed structures in the intensity and angular distribution of O+ ions correlated with a localized high-density region and large-scale changes in the proton flow direction. The average energy of O+ was much higher than that of the main component (H+). The tailward flowing high-density region containing O+ ions caused the drastic flow variations in the north-south meridional plane in the plasma sheet. The anisotropic O+ distribution probably by finite gyroradius effect was observed just before the density enhancement, while the O+ flow direction was the same as that of the proton bulk flow in the high-density region.

The GEOTAIL/Low Energy Particle (LEP) observations have revealed detailed features of acceleration and heating of cold ion beams in the plasma sheet boundary layer (PSBL). In the lobe region, the cold ion beams are flowing tailward nearly along magnetic field lines with small perpendicular drift toward the plasma sheet. Upon entering the plasma sheet, these cold ion beams are heated and accelerated up to several keV/q in the PSBL where a high-speed ion now is observed separately and simultaneously, and finally assimilated into the hot component of the plasma sheet proper. It should be noted that the acceleration of the cold ion beams in the PSBL is observed in the direction perpendicular to the magnetic field, and the perpendicular velocities of the cold ion beams during the acceleration coincide well with those of the high-speed ion beams. This fact suggests that the perpendicular acceleration is due to an increase of the E x B drift speed in tile PSBL as particles move from the lobe to the central plasma sheet. The electric field intensity for the observed E x B drift motion is estimated as 2-5 mV/m. The direction of the electric field inferred from the ion motion in the PSBL is mainly in the south-to-north direction rather than the dawn-to-dusk direction which is generally thought to be typical in the tail lobe. The direction of the acceleration is at times observed to change drastically, which suggests that the electric field direction fluctuates significantly as well.

Data from the GEOTAIL Low Energy Particle (LEP) instrument and magnetometer (MGF) for a plasmoid event on October 8, 1993 when the spacecraft was located at X(GMS) similar to -142 R(E) were analyzed. The event started 16 minutes after a substorm onset with a tailward flow of electrons whose characteristic energy was several keV with an apparent energy dispersion. This was followed by the arrival of a beam of high energy ions, which also had a similar energy dispersion, and an enhancement of the magnetic field intensity. There was appreciably high flux of cold ions in the lobe region, and it was further enhanced at the front of the plasmoid. These cold ions were energized at the boundary of the plasmoid where a sharp decrease in the total magnetic field intensity was observed. It was found that the main part of the plasmoid had a significant dusk-to-dawn magnetic field, indicating the plasmoid had a flux rope, or a distorted closed-loop magnetic field structure. After passing the main part of the plasmoid, the spacecraft entered a region characterized by high energy tailward flowing ions and coexisting cold ions. The level of low frequency magnetic field fluctuations was relatively high, and there were some time variations in both high-energy and cold ion fluxes. Synthesizing these features, we conclude that our observations are consistent with the creation of a plasmoid and surrounding energetic particle layers following a substorm as predicted by the near-Earth neutral line model of a magnetospheric substorm.

Transitions from slow flow to fast tailward flow are studied in the distant plasma sheet at downtail distances of similar to 70 R(e) with the data from the GEOTAIL satellite. The speed of 300 km/s can be regarded as a reasonable measure that separates the fast and the slow flow. While the magnetic field tends to be northward during the slow flow, its polarity oscillates between northward and southward during the fast tailward flow. A case is studied in detail where the slow-to-fast transition of the flow velocity was observed when the spacecraft was very close to the neutral sheet, and it is seen that the pressure was balanced in the interface region between these flows. The observations are consistent with a model in which the fast tailward flow is associated with a plasmoid and the quiet plasma is pushed forward when a plasmoid approaches.

An excellent example of an onset of the right-handed ion/ion resonant instability is observed on October 8, 1993, when the Geotail was in the distant tail of X(GSM) = -142R(E). Preceding and following a passage of a plasmoid, magnetic noises having a right-hand circular polarity with the frequencies around the proton cyclotron frequency are detected. Especially, the event after the passage lasted for more than 10 min., and the ion plasma in this boundary layer was composed of a tailward streaming cold dense plasma and a warm tailward beam coming presumably from the plasma sheet, drifting at 1000 km/s relative to the former along the field line. We have used this distribution as a source function for a linear Vlasov analysis, and confirmed that the generation mechanism of the waves can well be explained in terms of the resonant interaction with the beam.

The ring-shaped velocity distribution of ions with energies of a few keV of the gyrating motion has at times been detected in the boundary region between the plasma sheet and the lobe in the distant tail at X(GSM) similar to -70 and -170Re. The normal direction of the ring-shaped distribution is almost parallel to the magnetic field, and the distribution moves tailward. In many cases the fast flowing ions of the Plasma Sheet Boundary Layer (PSBL) are observed simultaneously. The density of the ring is comparable to the density of the cold ion stream observed in the lobe. We discuss the possibility that these ring ions are generated from cold proton streams by the thermoelectric field that exists at the plasma sheet-lobe boundary.

We observed cold ion beams in the magnetotail lobe at X(GSM) approximately -42 Re in the initial operations of the Low Energy Particle (LEP) experiment onboard the GEOTAIL satellite on August 22, 1992, when multiple onsets of substorms took place. These ion beams generally consisted of protons and singly-charged oxygen ions (O+), flowing tailward with nearly the same velocities approximately 100-200 km/s. The H+ number density was generally of order 10(-2) CM-3, while the O+ density was of order 10(-3) CM-3 but at times increased sporadically by an order of magnitude. These ions presumably would be transported along magnetic field lines through the polar mantle from the dayside polar ionosphere and convected to the mid-magnetotail where the observation was made. That different ion species had equal velocities is consistent with the velocity filter effect due to E x B convection. Three-dimensional determinations of their velocity distributions have revealed off-ecliptic angles of the flow directions as large as several tens of degrees. We discuss this feature in terms of the enhanced convection velocity that is associated with the plasma-sheet thinning confined to a limited region in the Y direction. Characteristic changes in the flow velocity are also found in association with a substorm plasmoid passage.

Three plasma burst events were observed with the Low Energy Particle (LEP) instrument onboard GEOTAIL at X(GSM) = -42 R(E). Two of these events were detected when the spacecraft was in the plasma sheet. In these events, the first signature of the burst was the tailward flow of the accelerated electrons with energies of about 5 keV. This was followed by the appearance of the tailward streaming ions with energies > 20 keV. Importantly, these tailward streaming ions were observed together with the non-streaming ions of the plasma sheet which had not experienced acceleration or heating. Then the electron distribution function started to develop an elongated core component at < 1 keV and isotropic or perpendicularly heated structure at > 1 kev. These two events lasted for about two minutes. The spacecraft was in the tail lobe before the third event took place. In this event the first signature was the bistreaming electrons with peak energy at 2 keV. After a short interval of the flux decrease, hot electrons with energy approximately 3 keV appeared. The velocity distribution function of the ions had an incomplete shell structure with an average tailward velocity of approximately 900 km/sec. The non-streaming component was absent in this case. It is suggested that in the first two events the spacecraft was in the plasma jet emanating from the near Earth neutral line, while it traversed the edge of the plasmoid in the third event. The associated substorm onset can be identified for the first event after which the disturbance continued.

Three-dimensional variations of the plasma bulk flow direction in the plasma sheet at X(GSE) approximately -60Re were observed by the Low Energy Particle (LEP) instrument on board the GEOTAIL satellite. These variations have two components, namely a rapidly varying component of which the period is about 8 minutes and a more slowly varying component. Separating these two components, we have found a period when the rapidly varying component showed clear rotations while the direction of the slowly varying component nearly corresponded with the direction of the magnetic field. We have calculated the rotation axis of the rapidly varying component during this period and have found that the direction of the rotation axis changed by nearly 900 in a step-like motion while the vortex motion remained coherent in the plane perpendicular to the moving rotation axis. We discuss several possible mechanisms for these variations, MHD waves, vortex structures, and tail flapping motion with velocity shears.

Most of the BEN wave forms observed by GEOTAIL in the Plasma Sheet Boundary Layer (PSBL) appear as a series of isolated spiky pulses which are termed “Electrostatic Solitary Waves (ESW).” Comparison between the BEN observations and plasma measurements shows that the uppermost frequency of the ESW is closely related to the temperature of the flowing ions in the PSBL. We also find that the observed spike width of the ESW and the inter‐pulse time‐span change very rapidly on time scales ranging from a few milliseconds to a few hundreds of milliseconds, suggesting that the speed of the ESW potential changes very rapidly. Copyright 1994 by the American Geophysical Union.

Japan's spacecraft PLANET-B will be launched to Mars in 1998. An extreme-ultraviolet (EUV) measurement of 30.4-nm wavelength is proposed for an interplanetary He+ observation during the cruising phase from Earth to Mars. This measurement will help our understanding of the creation and loss of interplanetary He+. Another objective is to image the plasmasphere and magnetotail of the Earth. The EUV scanner we have designed for the PLANET-B mission will provide the opportunity to observe both interplanetary and magnetospheric He+. In addition, we describe another improved design of this EUV scanner, which covers wavelengths from 30 to 90 nm. This improved scanner will measure scattered light from helium (58.4 nm) and oxygen ions (83.4 nm) in addition to He II.

Japan's spacecraft PLANET-B will be launched to Mars. An EUV measurement of 30.4 nm wavelength is proposed for an interplanetary He II observation during the cruising phase from the Earth to the Mars. This measurement will help our understanding of the creation and loss of interplanetary He II. Another objective is the imaging of the plasmasphere and magnetotail of the Earth. The EUV scanner we have designed for PLANET-B mission will provide the opportunity to observe both interplanetary and magnetospheric He II.

Using low energy ion data obtained by Akebono satellite, we have calculated distribution functions of velocity-dispersed ion beams observed at the poleward edge of the auroral electron precipitation region. The calculated distribution functions can well be fitted by one-dimensional shifted-Maxwellians, whose bulk energy and temperature are several keV and several hundreds of eV, respectively. The bulk energy and temperature show a positive correlation, which may indicate that when the ions are accelerated to higher energy, they are heated to higher temperature simultaneously. We have also found a relation between the invariant latitude width of the observed ion beams divided by the square root of the temperature and their bulk velocity, which indicates that the source region of the ion beam is compact. These ion beams are obtained with high occurrence probability, suggesting that they are supplied from a steady X-type neutral line in the earth's magnetotail.

A sounding rocket S-520-12 was launched from Andoya, Norway at 02:06:00 U.T. on 26 February 1990, into pulsating aurora. Electron energy spectra were observed with a quadrispherical electrostatic analyzer (QESA). The rocket flew from one pulsating patch to another, and we observed the spectral variation of precipitating electron flux following this transition. Pulsation of particle flux was observed in the precipitating electrons above 4 keV and the spectrum was fitted with a power-law distribution, although the electrons with energy less than 4 keV did not show significant pulsation. We found that the pulsation periods obtained through Fourier analysis for the auroral emission recorded by ground TV camera and for the in-situ energy flux data of the precipitating electrons agreed well. However, the one-to-one correlation between the electron energy flux and the auroral intensity was relatively poor. We attributed this to the spatial nonuniformity of the boundary region between two pulsating patches, and to the unstable phase relationship between the dominant Fourier components of the auroral emission and the electron energy flux. This might be caused by the propagating and streaming nature of the pulsating aurora during the time of the observation. We also found low-energy electron precipitation at the boundary region between the two pulsating patches, which can be attributed to the acceleration of the electrons at an altitude of several thousand kilometers by upward-propagating kinetic Alfven waves. That wave might be generated in association with the ionospheric conductivity change caused by the precipitation of the auroral electrons and the resultant enhancement of ionization of the upper atmosphere.

Japan's spacecraft PLANET-B will be sent to the Mars in 1996. We are proposing the Martian ionosphere and the magnetosphere imaging using the extreme ultra violet (EUV) light on this mission. Our main target is the 84.3 nm light scattered by the oxygen ions. Interesting topics related to the imaging are; (1) the density profile of the oxygen ions in the Martian ionosphere, (2) oxygen ions which are reported to outflow from the nightside ionosphere to the Martian tail with about 1 keV energy, and (3) pick-up ions created at the dayside of the Mars. Also proposed is the measurement of the EUV light scattered by Helium ions in the interplanetary space during the cruise phase from the Earth to the Mars.