Detection of localized, plasma‐depleted flux tubes or bubbles in the midtail plasma sheet

Recent studies have shown that most Earthward transport hi 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‐instrument observations from the ISEE1 and ISEE 2 spacecraft to argue that when the plasma sheet becomes thick and close to its equilibrium state, the plasma and magnetic field signatures of high‐speed flow events are consistent with the theoretically predicted signatures of plasma‐depleted flux tubes or “bubbles” [ Pontius and Wolf , 1990; Chen and Wolf , 1993]. These signatures consist of a decrease in the plasma pressure and an increase in the B z ‐component of the magnetic field accompanying the high speed flow. We show that the Earthward moving bubbles are separated from the plasma ahead of them by a sharp tangential discontinuity. The layer ahead of the bubbles exhibits flow and magnetic field shear consistent with flow around an Earthward moving obstacle. The bubble is in approximate total pressure balance with the surrounding medium. 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.