EVOLUTION OF PLASMASPHERIC IONS AT GEOSYNCHRONOUS ORBIT DURING TIMES OF HIGH GEOMAGNETIC-ACTIVITY

The evolution of the plasmasphere, the region of relatively dense cold plasma surrounding the Earth, is strongly dependent on magnetospheric activity. Here we report on plasmaspheric evolution as observed at ge osynchronous orbit in association with magnetopause crossings and stor m sudden commencements (SSCs). The occurrence frequency distributions at geosynchronous orbit of both magnetopause-associated and SSC-associ ated plasmaspheric ions is peaked near 1400 LT, with an overall range from 1000 LT to beyond 1800 LT. This is greatly skewed from the averag e plasmaspheric distribution at 6.6 R(E), which peaks closer to 1800 L T. The evolution of SSC-associated plasmaspheric ions is tracked using a superposed epoch analysis: lower-activity SSCs produce minor change s from the pre-SSC local time distribution; after geomagnetically-effe ctive SSCs, the ions appear almost immediately at earlier local times, spanning the late morning to dusk local time sector for hours. These observations are consistent with (1) a push of plasmaspheric material inward over the spacecraft due to magnetospheric compression and (2) t he prompt penetration of a convection electric field.