MAGNETOSPHERIC PLASMA ANALYZER - INITIAL 3-SPACECRAFT OBSERVATIONS FROM GEOSYNCHRONOUS ORBIT

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously sinc e late 1989, 1990, and 1991. These identical instruments provide for t he first time simultaneous plasma observations from three widely space d geosynchronous locations. The MPA instruments measure the three-dime nsional velocity space distributions of both electrons and ions with e nergies between approximately 1 eV/q and approximately 40 keV/q. MPA c apabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecra ft to synthesize a synoptic view of the morphology of the magnetospher e at geosynchronous orbit over a 6-week interval in early 1992. The MP A observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the coo l, dense plasmasphere (13.1% of the data); (2) a warmer, less dense pl asma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combinati on of plasma trough and plasma sheet (18.6%); (5) an empty trough regi on, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low-latitude boundary layer (0.7 %); and (7) the lobe (0.3%). The local time distributions of these reg ions are examined. For example, as suggested by previous authors, we f ind that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times on ly the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous sp acecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simu ltaneous data sets at different local times allow us to at least parti ally separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapa use and magnetopause as they pass over geosynchronous orbit. As expect ed, the plasmapause is found to have a highly variable shape, at vario us times showing (1) a stable dusk side bulge, (2) a variable bulge wh ich expands, contracts, and moves, (3) an overall expansion and contra ction of the plasmasphere, and (4) even more complicated behavior whic h is best accounted for by large-scale structure of the plasmapause an d/or disconnected plasma blobs. During the 6 weeks of data examined, t he magnetosheath was encountered on several occasions at synchronous o rbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm th is asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth-Sun line.