LOW-ENERGY ION SPECTRAL PEAKS DETECTED BY CRRES IN THE PLASMA SHEET

We have examined energy-versus time color spectrograms compiled by the low-energy plasma analyzer (LEPA) during 140 orbits of the Combined R elease Radiation Effects Satellite (CRRES). Over the period of interes t, the apogee of CRRES' orbit precessed from near dawn to near midnigh t. During more than half of the orbits LEPA detected low-energy ion sp ectral peaks (LISPs) that fell into two categories: isotropic and fiel d aligned. Isotropic LISPs were detected most frequently in the 0200-0 500 magnetic local time (MLT) sector and relatively high levels of Kp. They always were detected in the company of >10 keV plasma sheet elec trons and appeared in one or, at most, two LEPA energy channels. Field -aligned LISPs were evenly distributed over the sampled MLT sector and magnetic activity Levels. They could be either bidirectional or monod irectional. They too were detected along with plasma sheet electrons w hose spectra may or may not contain significant fluxes at energies gre ater than or similar to 10 keV. Although some field-aligned LISPs were detected in one or two energy channels, others were quite spread in e nergy. Isotropic LISPs are interpreted as signatures of CRRES charging . Field-aligned LISPs cannot be due to charging. On the basis of other measurements at geostationary orbit, we interpret them as being of io nospheric origin. Their detection in the postmidnight sector suggests that they were initially accelerated perpendicular to (ion conics) rat her than along (ion beams) the Earth's magnetic field.