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 and 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 flows is beyond 19 R E . Though the occurrence rate of the high‐speed flows substantially decreases when the satellite comes closer to the Earth, high‐speed flows with velocities higher than 600 km/s are still observed. We suggest that the high‐speed flows are stopped at a clear boundary between the regions of dipolar field and tail‐like field in the plasma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which is 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 current caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation of the substorm current wedge.