Quantifying event-specific radial diffusion coefficients of radiation belt electrons with the PPMLR-MHD simulation

The relativistic electrons trapped in Earth's outer radiation belt are highly dynamic. Understanding the acceleration and deceleration mechanisms for these MeV electrons is interested to the magnetospheric and space weather community. Radial diffusion is one of the critical mechanism for explaining the acceleration, transport, and loss of radiation belt. A key parameter to quantify the effects of radial diffusion is the radial diffusion coefficients. Characterizing and estimating the azimuthal mode structure of ULF waves are required for calculating the radial diffusion coefficients. Inadequate satellite measurements lead to difficulties in acquiring mode structure. With the global PPMLR MHD model we simulate two consecutive storms in December 2015, a moderate storm on December 14-15 and a strong storm on December 19-22, and calculate the radial diffusion coefficient from the simulated ULF waves. Our results show that even though the strong storm leads to more enhanced magnetic and electric power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ULF waves. For both storms, the total magnetic and electric power is better correlated with the solar wind dynamic pressure in the storm initial phase and more correlated with AE index in the recovery phase. The magnetic wave power is shown to be mostly distributed in low mode numbers while the electric power spreads over a wider range of modes. Furthermore, the magnetic and electric power spectral densities are higher at higher L regions, with a stronger L dependence in the magnetic spectra. The estimated radial diffusion coefficient based on MHD fields shows that inside the magnetopause the contribution from electric fields is larger than or comparable to that from magnetic fields, and our event-specific MHD-based radial diffusion coefficient can be smaller than some previous empirical radial diffusion coefficient estimations by more than an order of magnitude. At last, by validating against in situ observations from MMS spacecraft, our MHD results are found to generally well reproduce the total magnetic fields and wave power for both storms, while the electric power is underestimated in the MHD simulations.