ISEE 1 and 2 observations of ion distributions at the plasma sheet: tail lobe boundary

Observations of plasma ions (0.075--29 keV) at or near the plasma sheet: tail lobe boundary made with the ISSE 1 and 2 spacecraft are the principal topic of this paper. We describe crossings of this boundary that occurred during various levels of geomagnetic activity at distances X/sub SM/ = -7 to -13 R/sub E/, near local midnight, and 2 R/sub E/ or more above the estimated location of the tail current sheet. Fluxes of ions directed along the magnetic field lines (referred to as ion beams) with kinetic energies of 1->29 keV are commonly observed during crossings of this boundary. The ion beams nearest the edge of the plasma sheet are directed earthward, while deeper within the plasma sheet, additional ion beams, directed tailward, are encountered, usually resulting in bidirectional or counterstreaming ion distributions. The ion distribution functions for these clearly indicate that the tailward directed beams are reflected versions of the earthward directed beams, returning after mirroring in the stronger magnetic field near the Earth