SUBSTORMS WITH MULTIPLE INTENSIFICATIONS - POST-ONSET PLASMA SHEET THINNINGS IN THE MORNING SECTOR OBSERVED BY PROGNOZ-8

We present observations of plasma sheet thinnings in the dawn sector o f the near-Earth tail (8 less than or equal to R < 11 R(E)) recorded b y Prognoz-8 during 2 substorms with multiple intensifications. In one case (on 23 July 1981), prolonged substorm activity in the compressed magnetosphere (at similar to 5nPa solar wind dynamic pressure) was rel ated td persistent strongly southward (B-Z=15 nT) interplanetary magne tic field. Plasma sheet dropouts began similar to one hour after the m ain onset. Cold plasma populations (the low latitude boundary layer/ma ntle) were seen in a stretched magnetic configuration deep in the magn etosphere. There were signatures of local acceleration/heating of cold plasma above the retreating/expanding hot plasma sheet. In the case o f 27 July 1981, the same phenomena were recorded at a low similar to 1 nPa solar wind dynamic pressure. The distribution of the duration of total 13 plasma sheet dropouts was in the range of 1 to 15 minutes, wi th the majority of events lasting less than 5 minutes. Our data are co mplementary to the recent reports on GEOS-2 and CRRES energetic partic le dropouts in the morning sector.