中村 正人

Compressional Pc 3 magnetic pulsations and accompanying electric field oscillations were observed by the GEOTAIL satellite in the dayside magnetosphere on October 17-18, 1992. When interpreted in terms of MHD wave modes, the oscillations in the fields are consistent with fast magnetosonic waves propagating Earthward. This result lends strong support to the view that ULF waves generated near the quasi-parallel portion of the bow shock propagate into the magnetosphere and are observed as compressional Pc 3 pulsations in the dayside magnetosphere. The Earthward component of Poynting flux associated with the pulsations is similar to 5 x 10(-8) W/m(2).

This paper presents multiple magnetopause crossings observed with Geotail on October 17, 1992. On this day Geotail skimmed the magnetopause in the dawn flank sector (the spacecraft had a speed of similar to 0.7 km/s normal to the nominal magnetopause) and crossed the magnetopause more than 45 times in four hours. When the magnetopause crossings were observed, the interplanetary magnetic field was southerly directed. For the large number of crossings we have examined the magnetopause normal directions by using the minimum variance analysis technique, and as a result we have confirmed in a statistical sense a clear inbound/outbound dependence of the magnetopause orientation consistent with the tailward-moving recurrent bumps in the magnetopause. In addition, there is an implication that the northward (southward) flow along the magnetopause occured for the inward (outward) directed magnetic field component normal to the magnetopause.

This paper reports a peculiar series of events observed with Geotail on October 24, 1992 at X similar or equal to -80 R(E). On this day a combination of a bipolar perturbation in B-y, and a double-peaked perturbation in B-Z, signature of a flux rope, was observed. Following this flux rope, bipolar B-z events, identifiable as Traveling Compression Regions (TCRs), were recurrently observed. The axis of the flux rope was deduced to be field-aligned. Energetic ions (electrons) streamed earthward (tailward) in the flux rope. During the TCRs, the ExB flow showed a bipolar north-then-south per turbation, consistent with expected lobe plasma motion. The TCRs corresponded to weak ground magnetic disturbances and Pi2 waves. The observations can be explained by a model in which quasi-stagnant plasmoids (Nishida et al., 1986) are recurrently generated.

It is found that dense plasma is a very common plasma regime in the earth's magnetotail, especially beyond 150R(E) behind the earth. Plasma density in the tail lobe inferred from spacecraft potential measurements is often comparable with that in the tail current sheer. This observation suggests that the tail lobe is rich in cold plasmas. In the distant current sheet, plasma density sometimes exceeds 1 cm(-3). The high-density current sheet accompanies northward magnetic field, coming out from both the dawn and dusk tail flanks. Two possible models are discussed; one is highly stretched LLBL (low latitude boundary layer) that spreads to the whole current sheet in the distant tail, and the other is disappearance of the distant magnetotail during northward IMF.

The Geotail spacecraft remained close to the dayside magnetopause from October 17 to October 18, 1992. During the intervals of this very unique orbit when the spacecraft was within the magnetopause, Pc5 pulsations were observed almost continuously in the magnetic field and in the electric field. The signature of the pulsations agrees with the theory of field line standing oscillations viith the mode being the fundamental. The observed Pc5 pulsations were regarded to be triggered by sudden compressions and expansions of the magnetic field. The resulted displacement of the field line reached a maximum of 2Re.

The Geotail satellite quasi‐periodically crossed the dawn flank of magnetopause more than ten times during an interval of 1.5 hours on November 4, 1992. Magnetopause crossings were characterized by quasi‐periodic pulses of a sawtooth wave form in the magnetic field and the plasma flow components tangential to the magnetopause. The magnetic field strength in the magnetosheath was larger than that in the magnetosphere. The direction of magnetic field outside the magnetopause current layer was northward with antisunward tilt, indicating the draping of magnetic field on the magnetopause. Boundary normals of wavy magnetopause systematically incline sunward on the upstream side, while they tend to incline antisunward with considerable deviation on the downstream side. Comparison with other multiple crossing events suggests that the November 4 event exhibits wavy structure of the dawn flank magnetopause associated with the northward interplanetary magnetic field. Copyright 1994 by the American Geophysical Union.

The electric field detector (EFD) on board GEOTAIL measures the electric field by two different techniques, one by the probe technique and the other by electron beam technique. The probe technique (EFD-P) gives electric field in the plane perpendicular to the satellite spin axis by measuring the voltage difference between the two sphere probes; each deployed 50 meters from the spacecraft in the opposite direction. The electron beam technique (EFD-B), measures the drift motion of the gyration center of artificially emitted electrons to obtain the electric field. The drift motion of the electrons is measured by two methods, one by measuring the drift motion itself and the other by measuring the time of the return flight of electrons to the spacecraft. To realize these measurements, EFD is equipped with two additional capabilities. One is the capability to measure the spacecraft potential relative to the ambient plasma and the other is that to control the spacecraft potential by emitting ions. The reliability of the electric field measurements can be improved greatly by employing the probe and beam techniques at the same time. The potential control of the spacecraft enables the plasma detectors onboard GEOTAIL to measure low energy ions which would otherwise be repelled by the positive potential of the spacecraft. This article describes the outline of the electric field experiments on GEOTAIL with emphasis on the principles of the measurements, the configuration of the hardware, the raw data processing, as well as the preliminary results from the initial operation with the intention of providing the basis for the studies which use the GEOTAIL electric field data.

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.

We have studied three-dimensional ion distribution functions obtained with high time resolution (every 4.5 s) in and near the plasma sheet boundary layer with the plasma instrument on AMPTE IRM. This multicase study in the plasma sheet boundary layer reveals that at times, both an earthward and a tailward high-speed ion component are observed. Comparing these two components, the earthward components have the larger densities, while the tailward components have higher velocities. Typically, the distribution function changes from this two-component highly anisotropic character to generally isotropic as the spacecraft moves from the lobe, across the plasma sheet boundary layer, and into the central plasma sheet. The high-speed components often deviate from simple crescent-shaped distributions and exhibit significant structure. During disturbed times, substantial flows perpendicular to the magnetic field are observed. In several of the reported cases an additional cold ion component of comparable density was observed whose bulk velocity perpendicular to the magnetic field sometimes differed dramatically from that of the high-speed components. It is speculated that these differences might be a signature of gyrophase bunching.

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.

In this paper we present a survey of the three-dimensional ion and electron distributions measured on AMPTE IRM near the neutral sheet in the near-Earth magnetotail (R almost-equal-to 10-19 R(E)). The data are classified according to substorm phases. Individual case studies of velocity distributions reveal that the ions in the neutral sheet consist of a single population. Fast and slow flows both occurred during the two extremes of magnetospheric activity, the substorm expansion phase and quiet times. Ion beams are occasionally observed, and their shapes in velocity space are similar to those observed in the plasma sheet boundary layer. A ring distribution is seen only twice in 50 events. The occurrence rate of fast flows is positively correlated with the global magnetic field elevation during disturbed times and anticorrelated with that elevation during quiet times. The global magnetic field elevation angle is related to the magnetic field configuration in the magnetotail. Therefore our results indicate a dependence of the flow speed within the neutral sheet on the magnetotail configuration at the point of observation.

Electric field detectors (EFD) on the EXOS-D (Akebono) satellite measure the vector electric field by both the standard double probe technique (EFD-P) and the newly developed ion beam technique (EFD-B). This paper describes the instrumentation and preliminary observed results obtained by the EFD-P such as the Langmuir characteristics of the probe in the plasmasphere and the convection electric field in the auroral ionosphere. An intense electric field has been observed between the region 1 and 2 current on the morning side. Close to the mid-day meridian, fluctuating electric fields that are likely to be related in terms of a cusp/cleft structure were observed. -from Authors

An ion beam technique to measure the DC electric field as well as the magnetic field in the ionosphere is being developed. A Lithium ion beam ejected from the spacecraft in the direction perpendicular to the magnetic field and some angle with the DC electric field returns to the origin after approximately one gyroperiod. There are two such directions, the beam ejected in one direction returns after a little longer time than the gyroperiod and the beam ejected in the other direction returns after a little shorter time than the gyroperiod. The deviation of the flight time of the beam from the gyroperiod gives the magnitude of the DC electric field and the two beam directions give the direction of the electric field. The magnetic field is given by making the average of the two flight times. A feasibility study of applying this technique to the measurement of the electric field and the magnetic field in the ionosphere is presented. A preliminary results of the laboratory test of the equipment is also given. © 1988.