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.
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 > XGSM/RE > −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.
[1] We present in situ observations consistent with the ballooning mode in the vicinity of the magnetic equator at X GSM = À10 to À13 R E prior to substorm-associated dipolarization onsets. The ballooning instability is expected to have a wavevector along the Y direction and to give variation to the curvature of the ambient magnetic field lines. The magnetic field fluctuations appearing in the B x component are transported by the ambient plasma drift in the Y direction. A discrete frequency band would be identified in time series data if the mode has a discrete wavelength. The ballooning mode of this property was identified at the magnetic equator a few min before dipolarization onsets only when the plasma b was large (20 to 70). Using low-energy ion velocity data, we show that the mode has almost zero frequency in the plasma rest frame so that w sc $ k y Á v y , where w sc is the frequency in the spacecraft frame, and k y and v y are the wavenumber and the ambient plasma flow in the Y direction, respectively. This enables us to estimate the wavelengths of the ballooning mode, which were found to be of the order of the ion Larmor radius.
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.
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