BRAKING OF HIGH-SPEED FLOWS IN THE NEAR-EARTH TAIL

We have studied possible braking mechanisms of high-speed ion flows in the near-Earth central plasma sheet for radial distances between 9 an d 19 Earth Radii (R-E) on the basis of observations made by the AMPTE/ IRM satellite. Flows with velocities in excess of 400 km/s are almost always Earthward for this range, indicating that the source of the flo ws is beyond 19 R-E. Though the occurrence rate of the highspeed flows substantially decreases when the satellite comes closer to the Earth, high-speed flows with velocities higher than 600 km/s are still obser ved. We suggest that the high-speed flows are stopped at a clear bound ary between the regions of dipolar field and tail-like field in the pl asma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which i s estimated from the observations by assuming a pressure balance, is 6 .7 nT. The inertia current caused by the braking of the flow and the c urrent caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation o f the substorm current wedge.