SUPERPOSED EPOCH ANALYSIS OF THE SUBSTORM PLASMA SHEET

More than 43,000 plasma and magnetic field measurements with the AMPTE/IRM satellite in the magnetotail are used for a superposed epoch analysis of the conditions in the near-Earth plasma sheet around 39 major substorm onsets. In the central plasma sheet, the magnetic field elevation, the ion bulk speed, and the ion temperature all show marked rises during the substorm expansion phase. Somewhat less pronounced increases of all three parameters are found in the plasma sheet boundary layer. But here the substorm effects are typically seen during the recovery phase. In the near-Earth neutral line substorm model, the latter finding would indicate that the recovery phase begins when reconnection has proceeded to plasma sheet boundary layer field lines. Since the temporal evolution of ion temperature and ion density are anticorrelated especially in the central plasma sheet, we argue that the strong plasma heating during substorms occurs in a nonadiabatic fashion.