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.