Relativistic electron scattering by electromagnetic ion cyclotron fluctuations: Test particle simulations

Relativistic electron scattering by electromagnetic ion cyclotron (EMIC) fluctuations is studied using test particle computations coupled to the results of a hybrid simulation code. The enhanced EMIC fluctuations are derived from a 1-D, self-consistent hybrid simulation model and are due to the growth of the Alfven cyclotron instability driven by the ion temperature anisotropy, T-i perpendicular to > T-i parallel to (where the subscripts perpendicular to and parallel to refer to directions perpendicular and parallel to the background magnetic field, respectively), in a magnetized, homogeneous, collisionless plasma with a single ion species. The test particle computations follow the motion of relativistic test electrons in the input EMIC fluctuations. The time evolution of the mean square pitch angle change of the test electrons is calculated and used to determine the pitch angle diffusion coefficient. Finally, the results are compared with quasi-linear diffusion theory. The diffusion coefficients given by the test particle computations agree with the ones from quasi-linear theory very well except for large-amplitude waves (delta B/B-0 >= 0.03 in the case presented, where dB is the wave magnetic field amplitude and B-0 is the background magnetic field) when the weak turbulence approximation in quasi-linear theory breaks down. Quasi-linear theory overestimates the pitch angle diffusion coefficient for large-amplitude waves and may, consequently, overestimate the pitch angle diffusion of relativistic electrons in the radiation belts at high L values.