The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), a low-altitude and polarorbiting satellite, has provided a long-term global picture of the radiation belts since its launch on July 3, 1992. While the inner belt (L2) electrons vary on solar cycle, semiannual, and solar rotation time scales, and with geomagnetic storms. Recently developed models of predicting MeV electron at geostationary orbit [Li et al., 2001> and the Dst index [Temerin and Li, 2001> based on solar wind measurements are used to examine the cause of the prominent semiannual variations of outer belt electrons and the Dst index. The equinoctial effect (the angle between the Earth's dipole and the flow direction of the solar wind) contributes most to the semiannual variation of the Dst and MeV electrons deep in the inner magnetosphere (L<5). The semiannual variation of MeV electrons at geostationary orbit is attributed mostly to the semiannual variation of solar wind velocity. ¿ 2001 American Geophysical Union