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 geosynchronous orbit in association with magnetopause crossings and storm sudden commencements (SSCs). The occurrence frequency distributions at geosynchronous orbit of both magnetopause-associated and SSC-associated plasmaspheric ions is peaked near 1400 LT, with an overall range from 1000 LT to beyond 1800 LT. This is greatly skewed from the average plasmaspheric distribution at 6.6 R_E, which peaks closer to 1800 LT. The evolution of SSC-associated plasmaspheric ions is tracked using a superposed epoch analysis: lower-activity SSCs produce minor changes from the pre-SSC local time distribution; after geomagnetically-effective 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) the prompt penetration of a convection electric field.