Equinoctial and solstitial averages of magnetospheric relativistic electrons: A strong semiannual modulation

Data from SAMPEX, POLAR, and other spacecraft shows that high energy electrons (E ≳ 1 MeV) vary in a highly coherent way throughout the entire outer radiation zone of the Earth (2.5 ≲ L ≲ 6.5). These data are used to perform long‐term analysis of the flux variations of E>2 MeV electrons throughout the outer trapping zone. Averages are performed over 3‐month intervals centered on the spring and fall equinoxes and on the winter and summer solstices. Modulation is found such that equinoctial fluxes of electrons are nearly a factor of three larger than the solstitial fluxes based upon a superposed epoch analysis for 1992–1999. These semiannual modulations of relativistic electron fluxes are compared with concurrent solar wind data and are examined in terms of prior models of geomagnetic activity modulation. It is concluded that both southward IMF component merging (Russell‐McPherron) and Kelvin‐Helmholtz instability considerations (Boller‐Stolov) may play important roles.