THE LOW-ENERGY PARTICLE (LEP) EXPERIMENT ONBOARD THE GEOTAIL SATELLITE

The low energy particle (LEP) instrument onboard GEOTAIL is designed t o make comprehensive observations of plasma and energetic electrons an d 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 electr onics (LEP-E). The Energy-per-charge Analyzers (EA) measure three-dime nsional velocity distributions of electrons (with EA-e) and ions (with EA-i), simultaneously and separately, over the energy-per-charge rang e of several eV/q to 43 keV/q. Emphasis in the EA design is laid on th e large geometrical factor to measure tenuous plasma in the magnetotai l with sufficient counting statistics in the high-time-resolution meas urement. On the other hand, the Solar Wind ion analyzer (SW) has small er geometrical factor, but fine angular and energy resolutions, to mea sure energy-per-charge spectra of the solar wind ions. In both EA and SW 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 (nomin ally, 3 sec). The energetic-ion Mass Spectrometer (MS) can provide thr ee-dimensional determinations of the ion composition. In this paper, w e describe the instrumentation and present some examples of the inflig ht measurements.