Precipitation of relativistic electrons by interaction with electromagnetic ion cyclotron waves

On August 20, 1996, balloon-borne X-ray detectors observed an intense X-ray event as part of a French balloon campaign near Kiruna, Sweden, at 1532 UT (1835 magnetic local time), on an L shell of 5.8. The energy spectrum of t his event shows the presence of X rays with energies >1 MeV, which are best accounted for by atmospheric bremsstrahlung from monoenergetic similar to 1.7 MeV precipitating electrons. Ultraviolet images from the Polar satellit e and energetic particle data from the Los Alamos geosynchronous satellites show the onset of a small magnetospheric substorm 24 min before the start of the relativistic electron precipitation event. Since the balloon was sou th of the auroral oval and there was no associated increase in relativistic electron flux at geosynchronous altitude, the event is interpreted as the result of selective precipitation of ambient relativistic electrons from th e radiation belts. Pitch angle scattering caused by resonance with electrom agnetic ion cyclotron mode waves is the most likely mechanism for selective precipitation of MeV electrons. A model is presented in which wave growth is driven by temperature anisotropies in the drifting substorm-injected pro ton population. The model predicts that this wave growth and resonance with similar to 1.7 MeV electrons will occur preferentially in regions of densi ty >10 cm(-3), such as inside the duskside plasmapause bulge or detached pl asma regions. The model predictions are consistent with the location of the balloon, the observed energies, and the timing with respect to the substor m energetic particle injection.