The low energy particle (LEP) instrument onboard GEOTAIL is designed to make comprehensive observations of plasma and energetic electrons and ions with fine temporal resolution in the terrestrial magnetosphere (mainly magnetotail) and in the interplanetary medium. It consists of three units of sensors (LEP-EA, LEP-SW and LEP-MS) and a common electronics (LEP-E). The Energy-per-charge Analyzers (EA) measure three-dimensional velocity distributions of electrons (with EA-e) and ions (with EA-i), simultaneously and separately, over the. energy-per-charge range of several eV/q to 43 keV/q. Emphasis in the EA design is laid on the large geometrical factor to measure tenuous plasma in the magnetotail with sufficient counting statistics in the high-time-resolution measurement. On the other hand, the Solar Wind ion analyzer (SW) has smaller geometrical factor, but fine angular and energy resolutions, to measure energy-per-charge spectra of the solar wind ions. In both EA and S W sensors, the complete three-dimensional velocity distributions can only be obtained in a period of four spins, while the velocity moments up to the third order are calculated onboard every spin period (nominally, 3 sec). The energetic-ion Mass Spectrometer (MS) can provide three-dimensional determinations of the ion composition. In this paper, we describe the instrumentation and present some examples of the inflight measurements.

Abstract. In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 RE geocentric distance apogee in the northern hemisphere and 1.8 RE perigee in the southern hemisphere. The Thermal Ion Dynamics Experiment (TIDE) on POLAR has allowed sampling of the three-dimensional ion distribution functions with excellent energy, angular, and mass resolution. The Plasma Source Instrument (PSI), when operated, allows sufficient diminution of the electric potential to observe the polar wind at very high altitudes. In this paper, we describe the results of a survey of the polar wind characteristics for H + , He + , and O + as observed by TIDE at -5000 km and -8 RE altitudes over the polar cap during April-

[1] We propose a model for the energy dispersion of electron precipitation associated with pulsating auroras, considering the wave-particle interactions with propagating whistler mode waves from the equator. Since the resonant energy depends on the magnetic latitude, the pitch angle scattering of different energy electrons can occur continuously along the field line. Considering the energy-dependent path length and the precipitation start time of the precipitating electrons, the transit time of whistler mode waves, and the frequency drift, we calculated the precipitation of electrons observed at the topside ionosphere. Note that higher energy electrons precipitate into the ionosphere of the opposite hemisphere earlier than lower energy electrons. As a result, an energy dispersion of precipitating electrons is observed at the topside ionosphere, even though the modulation of low energy electrons occurs prior to that of high energy electrons. Using the model, we conducted a time-of-flight (TOF) analysis of precipitating electrons observed by the REIMEI satellite, assuming an interaction with the whistler mode chorus rising tone. Our TOF analysis suggests that the modulation region of the pitch angle scattering is near the magnetic equator, whereas previous models expected that the modulation region is far from the magnetic equator. The estimated parameters, such as wave-frequency and latitudinal distribution of the modulation region, are consistent with previous statistical studies of whistler waves at the magnetosphere.

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) on SELENE (Kaguya) has completed its ∼1.5-year observation of low-energy charged particles around the Moon. MAP-PACE consists of 4 sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measured the distribution function of low-energy electrons in the energy range 6 eV-9 keV and 9 eV-16 keV, respectively. IMA and IEA measured the distribution function of low-energy ions in the energy ranges 7 eV/q-28 keV/q and 7 eV/q-29 keV/q. All the sensors performed quite well as expected from the laboratory experiment carried out before launch. Since each sensor has a hemispherical field of view, two electron sensors and two ion sensors installed on the spacecraft panels opposite each other could cover the full 3-dimensional phase space of low-energy electrons and ions. One of the ion sensors IMA is an energy mass spectrometer. IMA measured mass-specific ion energy spectra that have never before been obtained at a 100 km altitude polar orbit around the Moon. The newly observed data show characteristic ion populations around the Moon. Besides the solar wind, MAP-PACE-IMA found four clearly distinguishable ion populations on the dayside of the Moon: (1) Solar wind protons backscattered at the lunar surface, (2) Solar wind protons reflected by magnetic anomalies on the lunar surface, (3) Reflected/backscattered protons picked-up by the solar wind, and (4) Ions originating from the lunar surface/lunar exosphere.

We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low-altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified abovẽ 3 keV. Moreover, stable precipitations at~1 keV are found for the PsA. A "precipitation gap" is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave-particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on-off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.

In Earth's environment, the observed polar outflow rate for O(+) ions, the main source of oxygen above gravitational escape energy, corresponds to the loss of approximately 18% of the present-day atmospheric oxygen over 3 billion years. However, part of this apparent loss can actually be returned to the atmosphere. Examining loss rates of four escape routes with high-altitude spacecraft observations, we show that the total oxygen loss rate inferred from current knowledge is about one order of magnitude smaller than the polar O(+) outflow rate. This disagreement suggests that there may be a substantial return flux from the magnetosphere to the low-latitude ionosphere. Then the net oxygen loss over 3 billion years drops to approximately 2% of the current atmospheric oxygen content.

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 XGSE "'• -30RE and XGS E -,• -210RE. 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 •he 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.

Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell-Boltzman statistics), is a fundamental question in studies of astrophysical and laboratory plasmas. Astrophysical plasmas are often so tenuous that binary collisions can be ignored, and it is not clear how thermal equilibrium develops for these 'collisionless' plasmas. One example of a collisionless plasma is the Earth's plasma sheet, where thermalized hot plasma with ion temperatures of about 5 x 10(7) K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earth's plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earth's shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.