Normal-Angle Dependence of the Interaction between Radiation Belt Electrons and Fast Magnetosonic Waves

Fast magnetosonic (MS) waves have been suggested to contribute significantly to radiation belt electron dynamics via Landau resonance and transit time scattering. The corresponding quasi-linear diffusion coefficients in pitch angle, energy and particularly cross-pitch-angle-energy are calculated from the gyro-averaged test-particle simulations. It is found that the cross diffusion coefficient is an effective indicator to differentiate between the contributions of resonant and non-resonant mechanisms. Furthermore, the dependence of diffusion coefficients on the normal angle of MS waves are parametrically investigated. Numerical results show that the increasing normal angle can lead to the shrinking of the Landau resonance region and the expansion of the transit-time scattering region. With the increasing normal angle, the pitch angle diffusion coefficients decrease significantly (by about two orders of magnitude), while the other two diffusion coefficients have a relatively limited decrease (within one order of magnitude). For arbitrary normal angles, the magnitude of cross diffusion coefficients is comparable to the other diffusion coefficients, suggesting that the cross diffusion is indispensable in the kinetic simulation works.