DYNAMICS OF THE PLASMASPHERE IN THE DUSK SECTOR AS OBSERVED IN THE OUTER IONOSPHERE

Direct measurements of the thermal plasma parameters in the topside io nosphere reveal variations of the plasmasphere boundary in the dusk se ctor. The ACTIVE satellite's nearpolar orbits at altitudes of 500 - 18 00 km around winter solstice 1989 were used to study the bulge region of the plasmasphere during intervals with different levels of geomagne tic agitation. The narrow, sharply defined trough in electron concentr ation corresponding to the plasmapause under quite conditions situated at L = 6 - 7 moved to lower L-values with increasing geomagnetic acti vity. This narrow trough can be found in all main ion constituents. Du ring periods of moderate geomagnetic activity, following the onset of a weak magnetic storm, a portion of the plasmaspheric bulge region was separated from the main plasmaspheric body. This can be seen in the o uter ionosphere as an inner narrow trough at lower L-value. Troughs in light ions need no longer coincide with this in electron concentratio n. He+ is the most sensitive constituent reflecting the dusk sector pl asmaspheric situation at this altitude.