The GEOTAIL/Low Energy Particle (LEP) observations have revealed detailed features of acceleration and heating of cold ion beams in the plasma sheet boundary layer (PSBL). In the lobe region, the cold ion beams are flowing tailward and nearly along magnetic field lines with small perpendicular drift toward the plasmas sheet. Upon entering the plasma sheet, these cold beams are heated and accelerated up to several keV/q in the PSBL where a high-speed ion flow is observed separately and simultaneously, and finally assimilated into the hot component of the plasma sheet proper. It should be noted that the acceleration of the cold ion beams in the PSBL is observed in the direction perpendicular to the magnetic field, and the perpendicular velocities of the cold ion beams during the acceleration coincide well with those of the high-speed ion beams.

This fact suggests that the perpendicular acceleration is due to an increase of the E¿B drift speed in the PSBL as particles move from the lobe to the central plasma sheet. The electric field intensity for the observed E¿B drift motion is estimated as 2--5 mV/m. The direction of the electric field inferred from the ion motion in the PSBL is mainly in the south-to-north direction rather than the dawn-to-dusk direction which is generally thought to be typical in the tail lobe. The direction of the acceleration is at times observed to change drastically, which suggests that the electric field direction fluctuates significantly as well. ¿American Geophysical Unio 1994