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

Using the global Lagrangian version of the piecewise parabolic method-magnetohydrodynamic (PPMLR-MHD) model, we simulate two consecutive storms in December 2015, a moderate storm on 14-15 December and a strong storm on 19-22 December, and calculate the radial diffusion coefficients (D-LL) from the simulated ultralow frequency waves. We find that even though the strong storm leads to more enhanced B-z and E power than the moderate storm, the two storms share in common a lot of features on the azimuthal mode structure and power spectrum of ultralow frequency waves. For both storms, the total B-z and E 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. B-z wave power is shown to be mostly distributed in low mode numbers, while E power spreads over a wider range of modes. Furthermore, the B-z and E power spectral densities are found to be higher at higher L regions, with a stronger L dependence in the B-z spectra. The estimated D-LL 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 D-LL can be smaller than some previous empirical D-LL estimations by more than an order of magnitude. At last, by validating against in situ observations from Magnetospheric Multiscale spacecraft, our MHD results are found to generally well reproduce the total B-z fields and wave power for both storms, while the E-phi power is underestimated in the MHD simulations.