ACCELERATION AND HEATING OF COLD ION-BEAMS IN THE PLASMA SHEET BOUNDARY-LAYER OBSERVED WITH GEOTAIL

The GEOTAIL/Low Energy Particle (LEP) observations have revealed detai led features of acceleration and heating of cold ion beams in the plas ma sheet boundary layer (PSBL). In the lobe region, the cold ion beams are flowing tailward nearly along magnetic field lines with small per pendicular drift toward the plasma sheet. Upon entering the plasma she et, these cold ion beams are heated and accelerated up to several keV/ q in the PSBL where a high-speed ion now is observed separately and si multaneously, and finally assimilated into the hot component of the pl asma sheet proper. It should be noted that the acceleration of the col d ion beams in the PSBL is observed in the direction perpendicular to the magnetic field, and the perpendicular velocities of the cold ion b eams during the acceleration coincide well with those of the high-spee d ion beams. This fact suggests that the perpendicular acceleration is due to an increase of the E x B drift speed in tile PSBL as particles move from the lobe to the central plasma sheet. The electric field in tensity for the observed E x B drift motion is estimated as 2-5 mV/m. The direction of the electric field inferred from the ion motion in th e 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 chang e drastically, which suggests that the electric field direction fluctu ates significantly as well.