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≤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 ∼5nPa solar wind dynamic pressure) was related to persistent strongly southward (B z =−15 nT) interplanetary magnetic field. Plasma sheet dropouts began ∼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 ∼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.