Relativistic electron loss due to ultralow frequency waves and enhanced outward radial diffusion

Using the THEMIS and GOES satellites and ground-based magnetometers, the loss of outer zone radiation belt electrons through the magnetopause in response to ultralow frequency (ULF) waves is examined. A 2 orders of magnitude decrease in >2 MeV electron flux observed at geosynchronous orbit, starting at 00 UT on 25 June 2008, is attributed to a rapid (1–4 h) nonadiabatic loss process. ULF waves were observed by the THEMIS-A, -D, and -E probes in the afternoon-to-dusk sector from the magnetopause to geosynchronous altitude. Estimates of the electron resonant energies indicate strong drift resonant interactions occurring between the energetic electrons and the observed waves. The rate of outward radial diffusion was estimated for MeV electrons using the observed ULF wave azimuthal electric field and compressional magnetic field and the diffusion time (~2.5 h) was found to be in good agreement with the observed time for nonadiabatic flux decreases at geosynchronous orbit. The magnetopause was compressed inside of its nominal position because of increased solar wind dynamic pressure. The electron loss is interpreted as a combination of magnetopause shadowing (from the compressed magnetosphere) and enhanced outward diffusion from ULF wave-particle drift resonant interactions. The enhanced day-night asymmetry of the MeV electron drift path from the compression suggests that enhanced losses may have also occurred around local noon as well as in the afternoon-to-dusk sector.