Spectral properties of the Alfven cyclotron instability: Applications to relativistic electron scattering

One-dimensional hybrid simulations of the Alfven-cyclotron instability in magnetized, homogeneous, collisionless, electron-proton plasmas are carried out to investigate the spectral properties of the resulting electromagnetic ion cyclotron (EMIC) fluctuations. The protons are initialized with a bi-Maxwellian velocity distribution (T-p perpendicular to > T-p parallel to, where the subscripts refer to directions relative to the background magnetic field) to drive the instability. The spectra of the resulting EMIC fluctuations are characterized in terms of the wave number corresponding to the peak energy spectral density and the spectral width, both of which agree with the prediction of linear dispersion theory using instantaneous simulation values of plasma parameters. By requiring the electrons on the edge of the loss cone to be in cyclotron resonance with the wave at peak energy spectral density, the approximate condition for fast loss of relativistic electrons in the outer radiation belt due to pitch angle scattering by EMIC waves is derived. In addition, the enhanced EMIC fluctuations in the hybrid simulations are used as input waves in test particle computations to study the pitch angle scattering of relativistic electrons. The results show that fast loss of geophysically interesting relativistic electrons (<= 2 MeV) is favored in regions of relatively high plasma densities, relatively cool proton temperatures, and relatively low magnetic fields. However, the results also suggest that the loss enhancement of geophysically interesting relativistic electrons by reduced proton temperatures can be weakened and complicated by the reduction in energy densities of the enhanced EMIC fluctuations corresponding to cooler proton temperatures.