中村 正人

The first measurement of He II (304 Angstrom) emission in the dayside magnetosphere has been done by the EUV scanner onboard Planet-B. A magnetic storm with the minimum Dst of -70 nT occurred during the observation. Before the initial phase of the magnetic storm, the average He+ density of similar to1 cm(-3) in the dayside magnetosphere was observed. During the main phase, the average He+ density rapidly increased up to 4-20 cm(-3). During the recovery phase, the average density smoothly decreased to the lowest density, which is due to the daily ionospheric filling. The total loss of the ions to the outer magnetosphere is estimated by 3.6x10(30) ions. This is sufficiently large to populate the plasma sheet.

We report observations of magnetospheric convection by the beam instrument, EFD-B. on Geotail. The region analyzed in this study is mainly the afternoon sector of the magnetosphere between L = 9.7 - 11.5. When the instrument is operated, electron beams are emitted from guns and some of them return to detectors attached to the main body of the satellite. However, we find that the return beams are often spread over a wide range of satellite spin phase angles, so that the calculated convection is unreliable. In order to remove noisy data, we set up suitable selection criteria. We infer that the convection strength is of the order of 20 km/s. The convection has generally westward and outward components. This indicates that the plasma located at the satellite positions is being convected toward the magnetopause. Moreover, the obtained convection is highly variable because standard deviations are comparable to the strength. We then compare the convection estimated by the beam instrument with that by the particle instrument, LEP. We find that the convections derived from the two instruments are positively correlated, with correlation coefficients above 0.7. The analysis reported here is expected to be useful in the interpretation of the multi-spacecraft data from the Cluster II mission.

The extreme ultraviolet scanner onboard the Planet-B spacecraft has taken EUV images (He II 304 Angstrom) of both the duskside plasmasphere and nearby outer magnetosphere on September 9-10, 1998. Even during a relatively quiet period the images have revealed that there is a significant amount of escape of plasmaspheric ions from the outer plasmasphere toward the magnetosphere. Such a phenomenon is not expected from the conventional teardrop model of the plasmasphere. Plasma-spheric materials of at least 2.9x10(28) He+ ions were peeled off the main body of the plasmasphere within 12 hours. This quantity corresponds to an ion flux of 6.7 x10(24) ions/s under the assumption that the He+/H+ ratio is 10%. If geomagnetic storms occur twice a month, this flux is comparable to the predictions for a stormtime transport. This suggests that the plasmasphere may provide the magnetosphere with significant amount of plasmas even when the geomagnetic activity is low.

The first optical measurement of He+ ions in the plasma sheet was successful by using the XUV scanner onboard Planet-B. The intensity of He II (304 Angstrom) was similar to0.02 Rayleigh. This result encourages future imaging missions for the whole magnetosphere by detecting He II (304 Angstrom) emission. This is also a proof of the existence of cold plasmas in the plasma sheet. The existence of cold ions brings a new question: How can they remain cold in the plasma sheet?

Cold dense plasma with the ionospheric origin is often observed in the outer magnetosphere with L values as large as 10. We have examined the electric field data accompanied by the cold dense plasma. The electric field data are obtained by the direct measurement of the drift motion of electrons released from electron guns. We get westward components of convection. In addition, there is an AC component of electric field in the ULF range larger than the DC component. If such a large variation of electric field exists in the vicinity of the stagnation point, the plasmaspheric plasma is expected to how away to the outer magnetosphere. Then we have a test particle simulation by adding a potential variable in time. There is a particle outflow accompanied by the AC variation of electric field in the ULF range. If there are more complicated variations of electric field, it is possible that particles flow out frequently. (C) 2000 Elsevier Science Ltd. All rights reserved.

A high precision quadrupole mass filter has been developed using the zero-method control circuit. With this method, peak values of a control voltage are compared with a high-precision reference voltage and detected by a diode and signal-producing resistance connected in series. The dc component of the signal is selectively amplified and fed back to a module in the amplifying system of the rf voltage so as to minimize the voltage in excess of the reference voltage. An instrument has been constructed and the doublet of He-4(+)-D-2(+) has been successfully separated. (C) 2000 American Institute of Physics. [S0034-6748(00)00903-5].

An extreme ultraviolet (XUV) scanner on board the Mars orbiter, Planet-B, observed the terrestrial plasmasphere while it was in a parking orbit around the earth. The image was available only for the dusk side of the plasmasphere. Nonetheless, this was the first image from the outside of the plasmasphere. Future missions, such as the NASA IMAGE mission in 2000 as wells the Japanese plasmaspheric imaging telescope on SELENE in 2003 will provide further information on this region.

The objective of the present study is to examine the timing of various onset-associated signatures and address the cause-and-effect relationship between the formation of a near-Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = -30.3 R-E in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950-km/s) tailward convection flow accompanied with southward B-z (< -10 nT) indicates that the near-Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (approximate to 1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near-Earth reconnection process as a major role in the substorm dynamics.

Mars Imaging Camera (MIC) on board PLANET-B, Japanese Mars mission, is a small, compact and lightweight imager. It features three-color linear CCD aligned with the spacecraft's spin axis and is designed to take two-dimensional images of Mars and its satellites using the spacecraft's spin. The total field of view (FOV) of the camera is 360 degree (around the spin axis)x 54.2 degree (along the array). MIC takes one image of designated area in the total FOV in one spin. The angular resolution of the camera is about 400 urad/pixel which corresponds to a spatial resolution of about 60 m at the periapsis of 150 km-altitude. Image data can be compressed by JPEG lossy image compression algorithm. Compression chip set is supplied from CNES of France under international collaboration. Among three imaging modes for inflight operation of MIG, the autonomous tracking mode is prepared for the encounter with Phobos or Deimos. In this mode, MIC will track the object moving relative to the spacecraft by analyzing its motion. A series of pre-flight tests of MIC was performed. In this test, the hardware performance and the functions of MTC were verified and the data for radiometric and geometric corrections were obtained. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.

Along the magnetopause skimming orbit oil October 17, 1992, the Geotail spacecraft observed intense bursts of electron cyclotron harmonic waves called "totem pole" (TP) emissions which occurred sporadically and intermittently in time in the dayside magnetosphere. The present paper focuses on the analysis of the electron environment associated with the TP emissions. Field analysis shows that, because of the occurrence of an intense Pc5 pulsation, there was E x B drift flow which was switching its direction in time near the magnetopause. The Geotail/comprehensive plasma instrumentation data show that the electron density fluctuated corresponding to the drift flow variation. Overall, TP emissions tend to occur during a transient time interval including a time when the density of cold electrons below 100 eV takes a minimum value. In the interval from 2225 to 2235 UT, TP emissions were well correlated with the density decrease of the cold electrons. In the interval including a stagnation time of the low switching front the Sun-duskward to the tail-dawnward direction, the density of the cold electrons became minimum, and simultaneously, TP emissions were observed. Contrary to the decrease of the cold electrons, the electron density increased st energies higher than 100 eV ill association with the TP emissions. Namely, the TP emissions were observed when the density ratio of the hot electrons to the cold ones (n(h)/n(c)) increased. In the interval from 2252 to 2302 UT, TP emissions were found during a transition time of the cold electron density. We call conclude 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.

This paper reports the result of a coordinated data analysis of a morningside Pc5 event observed at different altitudes in the magnetosphere and also on the ground. The event took place during 1400-1500 UT of April 29, 1993. The Geotail satellite was located in the boundary region and observed a 5-min quasi-periodic magnetic oscillation. The oscillation was mostly transverse to the background magnetic field. A 90 degrees phase lag between the magnetic field and electric field variations was not clear, suggesting that the oscillation was not a standing wave and that Geotail was located in or close to the excitation region. The plasma flow vector rotated clockwise on the equatorial plane viewed from the north as expected for a magnetospheric surface wave on the morningside. At geosynchronous altitude, the GOES satellites also observed a 5-min magnetic oscillation but with a significantly smaller amplitude than at the Geotail position. Five-minute magnetic oscillations were also detected at Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) and Magnetometer Array for Cusp and Cleft Studies (MACCS) ground stations in the same local time sector as the satellites, even equatorward of a region 2 field-aligned current observed by the Freja magnetometer data. From the phase analysis of ground signatures, the wave is inferred to propagate westward (antisunward) at a velocity of 18 degrees in longitude per minute. The propagation speed mapped to the equator, 400 km/s, is in the range of the expected flow speed of the magnetosheath. It is inferred that in the present event, the Kelvin-Helmholtz instability at the magnetopause, rather than at the inner edge of the boundary layer, excited an oscillation at the single frequency in a large area from the boundary region to deep inside the magnetosphere.

We have built an X-ray ultraviolet (XUV) scanner on board Mars orbiter NOZOMI (Planet-B). This scanner has the He I and II, emissions from the Martian atmosphere and ionosphere as its main target. These EUV emissions provide important information for the study of both Martian geological history and the interaction between solar wind and the Martian ionosphere. The XUV scanner will be operated in the parking orbit around the earth and also in the transfer orbit to Mars, where the terrestrial plasmasphere and interplanetary emissions will be studied.

Geotail observed a fast tailward flow burst (a speed of >600 km/s) with southward B-z in the Earth's magnetotail at a radial distance of 15.5 R-E on March 30, 1995. Ions in this burst consisted of a single-component plasma showing convection motion, and these ions were confined near the neutral sheet. This flow burst was likely associated with a pseudobreakup rather than a major substorm onset. In this event, magnetic reconnection appeared to take place only for the field lines near the neutral sheet. The magnetic reconnection was quenched soon and resulted in a short-lived tailward flow burst embedded in the plasma sheet.

Geotail was at the magnetopause near the sub-solar point when the magnetic cloud struck the earth's magnetosphere on January 10, 1997. Reconnection between the southward IMF and the magnetospheric field lines occurred at the dayside magnetopause as observed by Geotail. The signatures of the reconnection were, i) accelerated plasma flow from the reconnection site which reached up to 500 km/s, ii) ion heating through the reconnection process, and iii) a velocity filter effect seen in the ion distribution function in the magnetosheath boundary layer. This reconnection may be related to the large substorm onset observed at geosynchronous orbit.

DC electric fields were measured by the Electric Field Detector - Beam experiment (EFD-B) onboard the sounding rocket S-310-26 above the E-region ionosphere during the Sporadic-E Experiment over Kyushu (SEEK) campaign. The sounding rocket was launched into the ionosphere when a strong quasiperiodic (QP) echo structure appeared. The EFD-B measured both the electric field and the magnetic field from X+169 s until X+232 s (altitude of 167 km to 173 km). Accuracy of the measurements were 0.19 mV/m for the electric field and 1.9 nT for the magnetic field. The magnetic field measurements were in very good agreement, with a model field (IGRF 95). The electric fields, which were projections of the ionospheric electric fields to the altitude of the observation point along the magnetic field line, reached to 11.5 mV/m at maximum. The observed electric field is much larger than expected, and is associated with the 4 km periodic structure of the electric field which may be related to the atmospheric gravity wave activity in the E region.

Fast tailward ion flows with strongly southward magnetic fields are frequently observed near the neutral sheet in the premidnight sector of the magnetotail at 20-30 R-E for substorm onsets in Geotail observations. These fast tailward flows are occasionally accompanied by a few keV electrons. With these events, we study the structure and dynamics of magnetic reconnection. The plasma sheet near the magnetic reconnection site can be divided into three regions: the neutral sheet region (near the neutral sheet with the absolute magnitude of B-x of < 5 nT), the boundary region (near the plasma sheet/tail lobe boundary with the absolute magnitude of B-x is near or > 10 nT), and the off-equatorial plasma sheet (the rest). In the neutral sheet region, plasmas are transported with strong convection, and accelerated electrons show nearly isotropic distributions. In the off-equatorial plasma sheet, two ion components coexist: ions being accelerated and heated during convection toward the neutral sheet and ions flowing at a high speed almost along the magnetic field. In this region, highly accelerated electrons are observed. Although electron distributions are basically isotropic, high-energy (higher than 10 keV) electrons show streaming away from the reconnection site along the magnetic field line. In the boundary region, ions also show two components: ions with convection toward the neutral sheet and field-aligned ions flowing out of the reconnection region, although acceleration and heating during convection are weak. In the boundary region, high-energy (10 keV) electrons stream away, while medium-energy (3 keV) electrons stream into the reconnection site. Magnetic reconnection usually starts in the premidnight sector of the magnetotail between X-GSM = -20 R-E and X-GSM = -30 R-E prior to an onset signature identified with Pi 2 pulsation on the ground. Magnetic reconnection proceeds on a timescale of 10 min. After magnetic reconnection ends, adjacent plasmas are transported into the postreconnection site, and plasmas can become stationary even in the expansion phase.

A case study of the structure of the low-latitude boundary layer (LLBL) on dawnside (0600-0900 MLT) is reported. It is shown that the LLBL consists of two regions, the sheath-like region and the mixing region. The sheath-like region is where cold ions from the magnetosheath are flowing tailward. Detailed study has shown that this region is produced by reconnection and subsequent draping of open field lines over the dayside magnetosphere [Fujimoto et al., 1997]. The mixing region, on the other hand, is characterized by mixing of cold ions from the magnetosheath with the magnetospheric hot ions. Thermal electrons (< 500 eV) enhanced bidirectionally along field lines are detected to accompany these mixed ions. The balanced bi-directional flux is taken to indicate the closed topology of the field lines. An interesting finding is that the flow direction in this region tends to be sunward. Flow in the mixing region and the adjacent plasma sheet is faster sunward than in the ring current region, suggesting that these two constitute a channel of sunward returning convection in the dayside outer magnetosphere for this particular case. More new information on the mixing region from the present study is the ion distribution function showing a three-component feature and anticorrelation between plasma density and the degree of electron bidirectional heating. These observational facts would offer clues for understanding of the mixing region formation, which still remains open.

We have studied plasma sheet behavior in the near-Earth magnetotail during the substorm expansion and recovery phases using Geotail plasma and magnetic field observations. In association with onset of the expansion phase, tailward plasma flows form with southward magnetic fields in the plasma sheet beyond 20 RE- These tailward flows do not necessarily continue during the whole expansion phase. After the tailward flows, plasma sheet plasmas become almost stationary. These observations imply that magnetic recon-nection ceases in or near the onset site. In the recovery phase, fast earthward flows appear intermittently in the plasma sheet/tail lobe boundary. The source of these earthward flows appears to be disconnected from the near-Earth magnetic reconnection process. Stationary plasmas, which are often cold (&lt 2 keV) and dense (&gt 0.4 cm-3), are observed near the equatorial current sheet of the plasma sheet in the expansion and recovery phases. These plasmas are likely transported from the dawnside and/or the duskside of the plasma sheet just after the end of magnetic reconnection.

This paper describes the outline of the electron beam boomerang experiment carried out on the Geotail spacecraft. The experiment uses an electron beam artificially emitted from the satellite to measure the three dimensional electric field as well as the magnitude of the magnetic field. Unlike the Geos electron beam experiment, The boomerang experiment measures the time of flight of the electron beam returning to the spacecraft, which improves the measuring accuracy significantly.

We present the specifications of the Mars Imaging Camera (MIC) on the Planet-B spin-stabilized spacecraft, and key scientific objectives of MIC observations. A non-sun-synchronous orbit of Planet-B with a large eccentricity of about 0.87 around Mars provides the opportunities (1) to observe the same region of Mars at various times of day and various solar phase angles with spatial resolution of about 60 m from a distance of 150 km altitude (at periapsis), and (2) to monitor changes of global atmospheric conditions on Mars near an apoapsis of 15 Mars radii. In addition, (3) several encounters of Planet-B with each of the two Martian satellites are scheduled during the mission lifetime of two years from October 1999 to observe their shapes and surface structures with three color filters, centered on 450, 550, and 650 nm. (4) A search for hypothetical dust rings along the orbits of two satellites will be tried from the forward-scattering region of sunlight.

The helium emission monitor (BEM) is a newly developed extreme ultraviolet telescope adjusted to be sensitive to the He II emission line (304 Angstrom). It is developed as a prototype for upcoming plasmaspheric imagery missions. The HEM is designed to be compact and simple using a multilayer coated mirror, a thin metal filter, and a microchannel plate coated with CsI; it exhibits high sensitivity at 304 Angstrom (similar to 100 cps/R) and high angular resolution (2.5 degrees), and most importantly it is contamination free from other major emission lines, e.g., He I (584 Angstrom), O II (834 Angstrom), and H I (1216 Angstrom). We report on the initial observation of He II emission from the plasmasphere by the HEM carried on board sounding rocket S-520-19. We compare the observation with the theoretical prediction using a diffusive equilibrium model, the MSIS-86 model, and an equatorial electron density model. The model indicates a goad agreement with the observations using values for the He+ number density at the topside ionosphere of 3700 cm(-3) and for the equatorial temperature near the dawnside of the plasmapause of 8000 K.

Geotail plasma and field measurements at -95 R-E are compared with extensive ground-based, near-Earth, and geosynchronous measurements to study relationships between amoral activity and magnetotail dynamics during the expansion phases of two substorms, The studied intervals are representative of intermittent, moderate activity, The behavior of the aurora and the observed effects at Geotail for both events are harmonized by the concept of the activation of near-Earth X lines (NEXL) after substorm onsets with subsequent discharges of one or more plasmoids down the magnetotail, The plasmoids must be viewed as three-dimensional structures which are spatially limited in the dawn-dusk direction. Also, reconnection at the NEXL must proceed at variable rates on closed magnetic field lines for significant times before beginning to reconnect lobe flux, This implies that the plasma sheet in the near-Earth magnetotail is relatively thick in comparison with an embedded current sheet and that both the NEXL and distant X line can be active simultaneously, Until reconnection at the NEXL engages lobe flux, the distant X line maintains control of the poleward auroral boundary. if the NEXL remains active after reaching the lobe, the amoral boundary can move poleward explosively, The dynamics of high-latitude aurora in the midnight region thus provides a means for monitoring these processes and indicating when significant lobe flux reconnects at the NEXL.

The behavior of the tail field and plasma during a moderate substorm with onset at similar to 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 R-e. 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 similar to 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 R-e 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. (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.

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.

We developed normal incidence multilayer telescopes for observing EUV radiation from celestial objects. A telescope consists of spherical mirror coated with Mo/Si multilayer and a position sensitive detector made of microchannel plate on the focal plane. Three kinds of multilayers were deposited on three mirrors, which are optimized for observations of 130-140A, 170-180A and 270-330A, respectively. The characteristics of these telescopes were measured by using monochromatized synchrotron radiation. The peak reflectivity of multilayered mirrors was obtained to be nearly 60% at the peak wavelength of 133A and 172A and 10% at 300A at the normal incidence, as expected. The quantum efficiency of MCP detector was 20% in the wave-band 100-200A. These telescopes were on board a sounding rocket, S-520-19 to observe EUV emission from HZ43, a hot white dwarf, and galactic hot plasma.

Structure of the tail lobe magnetopause al an antisunward distance of about 70 Re was studied using the Geotail multiple boundary crossing data of August 16 and 17, 1993. The y component of the magnetic field during the crossings showed systematic variations consistent with temporal expansion of the tail in y direction. Since the geomagnetic activity during the crossings was high, these expansions are considered as caused by the passage of plasmoids. The magnetosheath plasma density near to the lobe boundary is found lower than that of undisturbed sheath. This region of depleted plasma density seem to extend several earth radii from the lobe boundary into the adjacent sheath and some smaller distance into the lobe. The transition from the low density lobe to the high density lobe occurs in a short time suggesting the presence of sharp inner boundary of the lobe boundary layer. Similarity in the density and the E x B flow velocity of the plasmas in the adjacent sheath and those in the lobe boundary layer suggests direct entry of sheath plasmas into the lobe across the magnetopause.

We present a series of tailward-to-Earthward particle anisotropy reversals observed by the GEOTAIL spacecraft in the distant magnetotail plasma sheet at a distance of \x\ approximate to-90R(E) during the 2.5-day period of 11/01/94 1200 UT to 13/01/94 2400 UT. The reversals were detected in the first order anisotropy of energetic protons and the first moment of the low energy plasma distribution function. The X-component of the cross-field flow exhibits the dearest indicator of a particle anisotropy reversal. A comprehensive examination of ground magnetometer and geosynchronous satellite data reveals that each distant tail anisotropy reversal occurred during the late expansion or recovery phase of a magnetospheric substorm. Out of 14 substorms during which adequate monitoring of the geomagnetic activity and the plasma sheet was possible all but possibly one exhibited a tailward-to-Earthward reversal in the cross field flow similar to 1-2 hours after substorm onset. The median time delay between substorm onset and the beginning of the subsequent Earthward convective flow was 94 min. Some uncertainty is associated with this time delay due to the commonly observed exits of the spacecraft to the lobe/mantle. The lower limit on the time delay based on the time of exit of the spacecraft to the lobe/mantle is 61 min. The implications of the particle anisotropy reversals for substorm dynamics are considered.

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 2.2R(E) north of the geomagnetic equatorial plane under the proposed velocity filter effect model.

A low and actively controlled electrostatic potential on the outer surfaces of a scientific spacecraft is very important for accurate measurements of cold plasma electrons and ions and the DC to low-frequency electric field. The Japanese/NASA Geotail spacecraft carries as part of its scientific payload a novel ion emitter for active control of the electrostatic potential on the surface of the spacecraft. The aim of the ion emitter is to reduce the positive surface potential which is normally encountered in the outer magnetosphere when the spacecraft is sunlit. Ion emission clamps the surface potential to near the ambient plasma potential. Without emission control, Geotail has encountered plasma conditions in the lobes of the magnetotail which resulted in surface potentials of up to about +70 V, The ion emitter proves to be able to discharge the outer surfaces of the spacecraft and is capable of keeping the surface potential stable at about +2 V. This potential is measured with respect to one of the electric field probes which are current biased and thus kept at a potential slightly above the ambient plasma potential. The instrument uses the liquid metal field ion emission principle to emit indium ions. The ion beam energy is about 6 keV and the typical total emission current amounts to about 15 mu A. Neither variations in the ambient plasma conditions nor operation of two electron emitters on Geotail produce significant variations of the controlled surface potential as long as the resulting electron emission currents remain much smaller than the ion emission current. Typical results of the active potential control are shown, demonstrating the surface potential reduction and its stability over time.

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.

Electric field data from the Geotail spacecraft provide an opportunity to extend the observations of spiky fields made by ISEE-1 to a region of the magnetosphere where quasistatic electric field measurements have not previously been made, to examine their possible importance in the dynamics of the middle and distant tail, and to test some hypotheses about their formation. In this paper, examples of large fields in the plasma sheet and its boundary at radial distances up to similar to 90 R(E) are presented. It is shown that three different types of large electric fields can occur: (1) spiky fields; (2) ''DC'' fields; and (3) waves at frequencies comparable to the lower hybrid frequency. There is usually a gradation between (1) and (3), and often large electric field spikes are embedded in regions of lower amplitude waves. The waves tend to occur in short (few to 10's of seconds) packets whose start and stop times are not always correlated with changes in the magnetic field and/or density (as indicated by the spacecraft potential). The peak frequency is often less than but comparable to the lower hybrid frequency in agreement with theories of lower hybrid drift waves in the magnetotail. The largest spikes are not always associated with the largest changes in the spacecraft potential and/or magnetic field. It is suggested that the spiky fields may represent the nonlinear development of the waves.

On October 17, 1992, the Geotail satellite crossed the dawnside magnetopause approximately ill times. At the majority of these crossings, the. magnetic field and the normal component of the electric field were larger and the plasma. density was smeller in a low-latitude boundary layer than they were in either the nearby magnetosheath or in the magnetosphere. These results are interpreted in terms of the Kelvin-Helmholtz instability associated with velocity shear at the magnetopause. Consistent with this interpretation, it is shown that the low-latitude boundary layer was the region where the flow decreased from its magnetosheath to magnetospheric value. Evidence is presented that the magnetopause was locally oriented within less than 20 degrees of its nominal geometry on these crossings and that it moved with an amplitude of 1500-6000 kilometers. The thickness of the low-latitude boundary layer averaged 800 kilometers and the electric potential across it averaged 4 kilovolts with a spread of a factor of 2. Thus, the viscous interaction and similar processes are not significant contributors to magnetospheric convection.

Electric field, magnetic field, and spacecraft potential measurements on the Geotail satellite have confirmed earlier observations of relatively dense flowing plasmas in the magnetotail lobes. Within these flowing plasmas, density discontinuities of factors of 2 to 4 were observed. At least 15 of the Is magnetopause crossings that occurred during a 24-hour interval when the spacecraft was at GSM coordinates near X = -140 R(e), Y = 0, and Z = 15 R(e), were rotational discontinuities across which the magnetosheath plasma had direct entry into the tail lobes. Within the lobes near the rotational discontinuities, the spacecraft typically crossed a second boundary at which the plasma density decreased from its magnetosheath value by a typical factor of 1.5 to 3 while the magnetic field strength and flow speed increased. Equatorward of this density discontinuity, the plasma density and flow remained appreciable. At least 2 of the 18 magnetopause crossings may have been tangential discontinuities across which the density and magnetic field strength both changed. Within the lobes near these tangential discontinuities, significant densities of flowing plasma were observed. Since II of the 18 discontinuities were crossed within a time interval of less than 7 seconds, the boundary current layer was well-defined and very thin.

We have analyzed the magnetic and the electric field data of GEOTAIL obtained in the neutral sheet of the distant tail at \x\ = 210 R(e) when Kp was 0 or 1. In the distant neutral sheet, B-y is strongly dependent on IMF B-y and when averaged in each IMF sector B-y is much larger than B-z. The average B-z is a few times of 0.1 nT or less so that the precise value is critically dependent on the offset determination with an accuracy better than 0.1 nT. The drift velocity is directed anti-earthward and 300 to 400 km/s. A modified cusp reconnection model is proposed in which the tail field lines that reconnect with IMF on the surface of the tail are not open but traverse the neutral sheet. Magnetic torque is transmitted from the reconnection site along these tail field lines and produce the IMF-dependent B-y component in the neutral sheet.

The magnetic field observed at the neutral sheet crossings in the distant tail at \x\ = 200-210 R(e) can be classified into three types. At the sharp and isolated crossings (Type A), the B-z component is positive on average and the tailward extension of the LLBL seems to be the viable explanation, but the presence of large and IMF-dependent B-y component suggests that the field lines are not closed but are connected to IMF at both ends. When the neutral sheet crossings occur multiple times, Type B or C is observed. Type B is characterized by the bipolar magnetic signature and represents a plasmoid or flux rope. Type C is characterized by magnetic oscillations with typical time scales of a few tens of seconds. These oscillations may represent the magnetic loops or spirals produced by the tearing. In all cases, the direction of the drift motion derived from the electric and magnetic field data is directed predominantly tailward.

Quasi-monochromatic waves with an electrostatic nature in the lower hybrid frequency range have been identified in the plasma sheet boundary layer at X(GSM)=-49R(e) and -69R(e) by the wave-form observations in a band 0-25 Hz 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.