Long term measurements of radiation belts by SAMPEX and their variations

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), a low- altitude and polar-orbiting satellite, has provided a long-term global pict ure of the radiation belts since its launch on July 3, 1992. While the inne r belt (L < 2) protons appear to have only a solar cycle variation, the out er radiation belt (L > 2) electrons vary on solar cycle, semiannual, and so lar rotation time scales, and with geomagnetic storms. Recently developed m odels of predicting MeV electron at geostationary orbit [Li et al., 2001] a nd the Dst index (Temerin and Li, 2001) based on solar wind measurements ar e used to examine the cause of the prominent semiannual variations of outer belt electrons and the Dst index. The equinoctial effect (the angle betwee n 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 g eostationary orbit is attributed mostly to the semiannual variation of sola r wind velocity.