Estimating relativistic electron pitch angle scattering rates using properties of the electromagnetic ion cyclotron wave spectrum

An EMIC wave event observed by the CRRES spacecraft during an active period on 11 August 1991 was studied in order to estimate electron minimum interaction kinetic energy E-min and using quasilinear theory, to calculate the resonant scattering rate D-alpha alpha. The wave packet semibandwidth delta omega/2 pi full-width half maximum ranged from 0.06 Hz to 0.27 Hz. Resonant scattering was assumed to occur over the frequency interval omega(m) - delta omega to omega(m) + delta omega. Assuming typical stormtime ion concentrations, the use of realistic wave spectral properties when compared to only using the central wave frequency wm results in 3 to 4 times as many wave packets that are able to interact with relativistic electrons below similar to 2 MeV. Values of D-alpha alpha associated with two of the wave packets, where E-min falls to within the 1-2 MeV energy range, were comparable to the limit of strong diffusion suggesting enhanced electron precipitation. CRRES observed an similar to 1 order of magnitude decrease in the 1-2 MeV electron flux levels during the EMIC wave interval. It is suggested that this flux decrease was due to EMIC waves pitch angle scattering the relativistic electrons. The EMIC waves were observed near the start of the main phase of a geomagnetic storm. This study strengthens the suggestion that relativistic electron scattering by EMIC waves can compete with the Dst effect as a mechanism of decreasing relativistic electron fluxes from the outer zone during magnetic storms.