LOW-ENERGY PARTICLE LAYER OUTSIDE OF THE PLASMA SHEET BOUNDARY

The ISEE spacecraft in the geomagnetic tail frequently crossed the high-latitude boundary of the plasma sheet. On a number of these crossings on the morningside (between 15 R(E) and 22 R(E)) the ISEE instruments detected an enhanced population of low-energy electrons and ions immediately adjacent to the plasma sheet boundary layer (PSBL). The electrons in this low-energy layer (LEL) have energies less than a few hundred eV, and they are aligned along the magnetic field direction propagating in the tailward direction. The ions have energies less than 100 eV and are also streaming along the magnetic field direction but in the earthward direction. These particles are clearly distinguished from the bulk of the particles in the plasma sheet and the PSBL. These observations may help clarify where the various particle features in the geomagnetic tail map to in the ionosphere. It is suggested that the LEL maps to the soft (< 1 keV) electron precipitation region poleward of the plasma sheet boundary.