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 of the near-Earth tail (8 less than or equal to R < 11 R(E)) recorded by 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 related td persistent strongly southward (B-Z=15 nT) interplanetary magnetic field. Plasma sheet dropouts began similar to one hour after the main onset. Cold plasma populations (the low latitude boundary layer/mantle) were seen in a stretched magnetic configuration deep in the magnetosphere. There were signatures of local acceleration/heating of cold plasma above the retreating/expanding hot plasma sheet. In the case of 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, with the majority of events lasting less than 5 minutes. Our data are complementary to the recent reports on GEOS-2 and CRRES energetic particle dropouts in the morning sector.