LAUSR

Abstract. Long-term variations of the relativistic (~ MeV) electrons in the Earth's radiation belt are explored to study seasonal features of the electrons. An L-shell dependence of the seasonal variations of the electrons is revealed for the first time. A clear ~ 6-month periodicity is identified for 1.5–6.0 MeV electron fluxes in the L-shells between ~ 3.0 and ~ 5.0, representing two peaks per year. The two-peak variation is strong during solar cycle descending to minimum phases, with weaker/no variations during solar maximum. The peaks are largely asymmetric in amplitude. These are not essentially equinoctial: sometimes the peaks are shifted to solstices and sometimes one annual peak is only observed. No such seasonal features are prominent for L   5.0. The results imply varying solar/interplanetary drivers of the radiation belt electrons at different L-shells. This has a potential impact on the modeling of space environment. Plausible solar drivers are discussed.