DETECTION OF LOCALIZED, PLASMA-DEPLETED FLUX TUBES OR BUBBLES IN THE MIDTAIL PLASMA SHEET

Recent studies have shown that most Earthward transport in the midtail , high-beta plasma sheet takes place in the form of short-lived, high- speed plasma flow bursts. Bursty bulk flows are observed both when the plasma sheet is thin, such as during substorm expansion, and when it is thick, such as during substorm recovery. We present multi-instrumen t observations from the ISEE 1 and ISEE 2 spacecraft to argue that whe n the plasma sheet becomes thick and close to its equilibrium state, t he plasma and magnetic field signatures of high-speed flow events are consistent with the theoretically predicted signatures of plasma-deple ted flux tubes or ''bubbles'' [Pontius and Wolf, 1990; Chen and Wolf, 1993]. These signatures consist of a decrease in the plasma pressure a nd an increase in the B-z-component of the magnetic field accompanying the high speed flow, We show that the Eathward moving bubbles are sep arated from the plasma ahead of them by a sharp tangential discontinui ty. The layer ahead of the bubbles exhibits flow and magnetic field sh ear consistent with flow around an Earthward moving obstacle, The bubb le is in approximate total pressure balance with the surrounding mediu m, We show that there is a systematic difference in the orientation of the discontinuity measured at ISEE 1 and 2, implying a small (about 1 -3 R(E)) cross-tail size of the bubbles.