New magnetospheric results from the SAMPEX mission

Abstract

Results are described from energetic particle detectors onboard the Solar, Anomalous, and Magnetospherie Particle Explorer (SAMPEX) satellite. Electron data are shown for energies E>400 keV in the outer zone of electron trapping (I,>3). The processes by which electrons are accelerated to very high energies (E>I MeV) are discussed. Data are sorted according to L-values and are compared with concurrent solar wind and geomagnetic conditions. Data from SAMPEX are also compared to GOES and UARS measurements. It is found that high-speed solar wind streams drive the acceleration and recirculation of electrons throughout the outer zone on time scales of one day (or less). Very high time resolution measurements from SAMPEX show the very sporadic nature of magnetosphere-atmosphere coupling processes. INWRODUCTIONRelatively few studies have been able to examine the longterm variation of magnetospheric electrons over a broad range of energies and L-values. Gussenhoven et al. [1987] showed the average pattern of E > 1.0 MeV electrons using dosimeters onboard the Defense Meteorological Satellite Program (DMSP) spacecraft at 840 km altitude; they saw evidence of 27-day recurrences of flux enhancements throughout one year (1984) of their observations. Williams [1966] also found clear evidence of 27-day periodicity in electrons with E > 280 keV and E _> 1 MeV. Recent studies [e.g., lmhofet al., 1991] have been able to examine several months of low-altitude electron (E>I.0 MeV) data and compare this with L=6.6 measurements, lmhof et al. found a good correlation between intense precipitation spikes at low altitude and trapped flux levels at geostationary orbit. Very recently, lmhof et al. [1994] showed that electron fluxes with E>I MeV track one another very well at low and high altitudes and that such high energy electrons seem to be accelerated on a rapid (<1 day) time scale.It is of great interest and importance in magnetospheric physics to understand basic particle acceleration and loss mechanisms and to determine the variation time scales for high-energy electrons throughout the Earth's radiation belts. It has been noted that the relativistic electrons could provide a significant coupling mechanism between the magnetosphere and the Earth's middle atmosphere [Baker et al., 1987b;Callis et al., 1991]. Relativistic electrons in the Earth's magnetosphere are also of considerable practical importance because of their deleterious effect on spacecraft subsystems [Gussenhoven et at., 1987;Baker et al., 1994b].We present here measurements of relativistic electron flux variations as determined with sensitive sensor systems onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). Approximately the fLrst two years of data are considered using several different energy thresholds and covering all relevant L-values. The data show abrupt electron enhancements throughout the outer magnetosphere on time scales of order one day. This suggests a coherent global behavior for electron acceleration. It is concluded that the o...