Energetic electrons, 50 keV to 6 MeV, at geosynchronous orbit: Their responses to solar wind variations

Abstract

Using simultaneous measurements of the upstream solar wind and of energetic electrons at geosynchronous orbit, we analyze the response of electrons over a wide energy range, 50 keV to 6 MeV, to solar wind variations. Enhancements of energetic electron fluxes over this whole energy range are modulated by the solar wind speed and the polarity of the interplanetary magnetic field (IMF). The solar wind speed seems to be a dominant controlling parameter for electrons of all energy. Electron enhancements occur after solar wind speed enhancements with a time delay that increases with energy and that also depends on the average polarity of the IMF. The electron enhancements have a shorter delay if the IMF Bz < 0 and a longer delay if the IMF Bz > 0 during the solar wind speed enhancement. The dependence on solar wind condition varies for different energy electrons, with lower‐energy electrons (<200 keV) responding more to the polarity of the IMF and higher energy electrons (>1 MeV) responding more to the solar wind speed. The variations of different energy electrons are well correlated among themselves. For five years, 1995–1999, the correlation coefficients of 1.1–1.5 MeV electrons with lower‐energy electrons, 50–75, 105–150, 225–315, and 500–750 keV, are 0.55, 0.64, 0.74, and 0.90. This correlation is enhanced if a time shift proportional to their energy difference is included. The optimal time shifts and the corresponding correlation coefficients for the four lower energy electrons are 36, 32, 13, and 7 hours and 0.75, 0.77, 0.81, and 0.92, respectively.