Behavior of MeV electrons at geosynchronous orbit during last two solar cycles

A comparison of MeV electron measurements at geosynchronous orbit, GEO, with solar wind shows that the MeV electron prediction model developed for GEO using data from the declining phase of solar cycle 22 (1995-1996) works well for the declining phase of solar cycle 23 (2006-2008), indicating that the MeV electron flux has a predictable and systematic response to the solar wind. The same comparison for solar maximum (2000-2003) shows that the model works less well partly because it does not match the high flux cutoff seen in the data and partly because it does not reproduce the sudden drops in flux that occur when the magnetopause is close to GEO. The model also reproduces the nonlinear correlation of the solar wind speed with the log of the MeV electron flux seen at GEO. An examination of 15 yr of solar wind and the MeV electron data shows that geomagnetic activity driven by a southward orientation of the interplanetary magnetic field, IMF, is a necessary condition for MeV electron enhancements at GEO and that high-speed solar wind are not necessary. The reason that high-speed solar wind is almost always associated with the enhancement of MeV electrons is mainly because high-speed solar wind almost always has some southward components of the IMF.