M F Thomsen, J E Borovsky, D J McComas, L A Weiss, S Maurice, and G D Reeves (all at MS D466, Los Alamos National Laboratory, Los Alamos, NM 87545; 505-667-1210; e-mail: mthomsen@lanl.gov)

Situated at the transition between the inner, dipole-dominated, and outer, solar-wind-dominated regions of the magnetosphere, geosynchronous orbit is an ideal location from which to observe the magnetosphere's response to the January 10-11 passage of a coronal mass ejection. Geosynchronous observations from three longitudinally separated Los Alamos satellites reveal numerous aspects of the global magnetospheric response, including evidence of enhanced magnetospheric convection, strong cross-tail currents, enhanced substorm activity, and erosion and loss of the outer plasmasphere during the first 18 hours after the shock arrival, when the IMF was southward. After the northward turning of the IMF there was a considerable quieting of convection and substorm activity; early on the 11th, however, there was a strong compression of the magnetopause to inside of 6.6 Re due to an interval of very high solar wind density. Unlike a number of similar events that we have observed in the past, the high solar-wind densities for this event did not produce a superdense plasma sheet at geosynchronous orbit, probably because, during the times of high solar-wind density, the "magnetospheric gate" was closed due to northward IMF. The lack of a superdense plasma sheet during the intervals of enhanced convection (southward IMF) suggests that no strong ring current would be formed and, hence, that Dst would not be driven strongly negative by this event, in agreement with the preliminary values derived by the World Data Center C in Kyoto. Finally, the strong compression of the magnetosphere produced a rapid and spectrally-hard increase in the high-energy (>1 MeV) outer belt electrons that disappeared after the pressure subsided; then over the course of the next several days the relativistic electron fluxes increased appreciably, but with a softer spectrum, in the manner commonly seen following high-speed solar-wind stream interfaces.