Geosynchronous orbit lies in the transition region between the inner, dipole-dominated, and outer, solar-wind-dominated regions of the magnetosphere. Therefore, it 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 magnetospheric plasma analyzers (MPA) reveal numerous aspects of the global magnetospheric response:
On January 10 the geosynchronous satellites entered the nightside plasma sheet at unusually early local times, indicating that the magnetospheric convection was strong. This was confirmed by the encounter with cold plasmaspheric plasma near local noon and even prior to local noon. Under normal conditions, such plasma is typically observed in the dusk region, but it is commonly seen closer to noon during convection enhancements associated with storm sudden commencements [cf., Elphic et al., Evolution of Plasmaspheric Ions at Geosynchronous Orbit During Times of High Geomagnetic Activity, Geophys. Res. Lett.,23, 2189, 1996]. This plasma corresponds to the outer layer of the plasmasphere, which is stripped away by the enhanced convection and carried to the dayside magnetopause. The cold plasmaspheric plasma appeared to be moving westward across the front side of the magnetosphere, consistent with similar previous events.
On the nightside, the geosynchronous satellites found that the magnetospheric field was strongly tilted away from its nominal dipole orientation, indicating that the cross-tail currents were strong and located close to the Earth, further indicative of strong convection. Consistent with the strengthened tail current sheet, there was enhanced substorm activity, producing energized plasma injections on the night side.
This activity lasted from roughly 0400 to 1900 UT on Jan. 10 and then subsided. The hot electrons produced by the substorm activity were conducive to surface charging of satellites, with temperatures sufficient to produce surface potentials of a few hundred volts to greater than a kilovolt. One MPA instrument directly observed surface charging of its host satellite to several hundred volts. Precipitation of substorm-energized electrons in the post-midnight region also gave rise to upflowing auroral ions with energies from a few eV to several keV.
On January 11, the geosynchronous satellite near 10 LT observed crossings of the magnetopause into the magnetosheath near 0200 UT and again near 0300 UT. These crossings appear to be associated with the arrival of the peak solar wind density and hence strong dynamic pressure on the magnetosphere, but they are unusual in that the interplanetary field appeared to be northward at the time. Previous studies have indicated that both compression by strong dynamic pressure and erosion of dayside magnetic flux by reconnection with southward IMF are needed to bring the magnetopause in past geosynchronous orbit.
Following the magnetopause crossings on January 11, magnetospheric activity appeared to abate. The convection field strength appeared to return to more normal values, as indicated by the location of the electron plasma sheet. Furthermore, the tail magnetic field was found to be much less stretched, and substorm activity subsided. No dense plasmasphere was seen at all on this day, consistent with the erosion and loss of the outer portion during the earlier enhanced convection interval. Low densities of cold ions arriving from the ionosphere to refill the plasmasphere were observed on the dayside.
Unlike a number of similar events that we have observed in the past, the relatively high solar-wind densities present for this event did not produce a superdense plasma sheet [cf., Borovsky et al., The Superdense Plasma Sheet: Plasmaspheric Origin, Solar-Wind Origin, or Ionospheric Origin?, J. Geophys. Res., in press, 1997]. We believe that this is probably due to a "closed magnetospheric gate," caused by northward IMF, during the times of high solar-wind density. WIND data would help confirm this suggestion. The lack of a superdense plasma sheet during the interval of enhanced convection suggests that no strong ring current would be formed and, hence, that Dst should not be driven strongly negative by this event [cf., Kozyra et al., "The Effects of a Superdense Plasma Sheet on Ring Current Evolution and Coupling to the Plasmasphere," Huntsville96 Conference on Encounter Between Global Observations and Models in the ISTP Era, Guntersville, Alabama, Sept. 1996].
Finally, with relevance to the failure of the Telstar 401 satellite, on neither Jan. 10 nor Jan. 11 did the MPA instruments observe any strong enhancements in directly penetrating background, which suggests there was no significant increase in either the relativistic electrons responsible for deep dielectric charging or the highly energetic ions that can produce single-event upsets.