Quantifying radial diffusion coefficients of radiation belt electrons based on global MHD simulation and spacecraft measurements

Radial diffusion is one of the most important acceleration mechanisms for radiation belt electrons, which can be enhanced from drift-resonant interactions with large-scale fluctuations of the magnetosphere's magnetic and electric fields (Pc5 range of ULF waves). In order to physically quantify the radial diffusion coefficient, D-LL, we run the global Lyon-Fedder-Mobarry (LFM) MHD simulations to obtain the mode structure and power spectrum of the ULF waves and validate the simulation results with available satellite measurements. The calculated diffusion coefficients, directly from the MHD fields over a Corotating Interaction Region (CIR) storm in March 2008, are generally higher when solar wind dynamic pressure is enhanced or AE index is high. In contrary to the conventional understanding, our results show that inside geosynchronous orbit the total diffusion coefficient from MHD fields is dominated by the contribution from electric field perturbations, rather than the magnetic field perturbations. The calculated diffusion coefficient has a physical dependence on mu (or electron energy) and L, which is missing in the empirical diffusion coefficient, D-LL(Kp) as a function of Kp index, and D-LL(Kp) are generally greater than our calculated D-LL during the storm event. Validation of the MHD ULF waves by spacecraft field data shows that for this event the LFM code reasonably well-reproduces the B-z wave power observed by GOES and THEMIS satellites, while the E-phi power observed by THEMIS probes are generally underestimated by LFM fields, on average by about a factor of ten.