MHD-Test Particle Simulations of Electron Dynamics at GOES 16 and 18 During May and October 2024 Superstorms

The two largest storms of Solar Cycle 25, both initiated by CME shock compressions of the dayside magnetopause inside geosynchronous orbit, occurred on 10-11 May and 10-11 October 2024. Each storm produced a dropout in electron flux observed by GOES 16 and 18 when the spacecraft crossed into the magnetosheath, which occurred repeatedly as the spacecraft crossed the dayside magnetopause early and continued well into the recovery phase of the storms. By the end of the second day for both storms, geosynchronous flux had recovered to and slightly exceeded pre-storm levels. MHD-test particle simulations are performed using measurements from the Colorado Inner Radiation Belt Experiment CubeSat in LEO orbit, prior to both storms, and initial fluxes at GOES 16 and GOES 18 to weight test particle populations. Initially trapped test particles and those injected from the tail are followed using fields from MHD simulations driven by upstream solar wind measurements. It is found that rebuilding of electron flux in the high 10-100 s of keV, at geosynchronous orbit, is due entirely to the injected source population from the plasmasheet, with simulated fluxes in general agreement with GOES measurements at magnetopause crossings across all GOES energies (69.4-863 keV). The simulated spectra agree well with measured spectra for both storms.