Observation of magnetospheric relativistic electrons accelerated by Pc-5 ULF waves

Magnetic field, electric field and energetic electron measurements from CRRES, GOES-6 and -7 satellites are used to investigate the production of up to 2 MeV electron fluxes through a storm sudden commencement (SSC) event on August 27, 1991. Strong Pc-5 oscillations, whose electric field is mainly along the radial direction, and up to an order-of-magnitude enhancement in relativistic electron fluxes occurred simultaneously in a two-hour period. The enhanced electron fluxes are found to have a pancake-like pitch angle distribution, which is consistent with the fluxes being accelerated near the equatorial plane. The electron acceleration is shown to result from drift-resonant interactions with the toroidal-mode Pc-5 ULF waves having wave frequencies three times the electron drift frequency. In view of the L-variation of ULF wave frequencies, electron acceleration to relativistic energies becomes more effective in the geosynchronous region.