C. Forsyth

Loss mechanisms act independently or in unison to drive rapid loss of electrons in the radiation belts. Electrons may be lost by precipitation into the Earth's atmosphere, or through the magnetopause into interplanetary space—a process known as magnetopause shadowing. While magnetopause shadowing is known to produce dropouts in electron flux, it is unclear if shadowing continues to remove particles in tandem with electron acceleration processes, limiting the overall flux increase. We investigated the contribution of shadowing to overall radiation belt fluxes throughout a geomagnetic storm starting on the 7 September 2017. We use new, multimission phase space density calculations to decipher electron dynamics during each storm phase and identify features of magnetopause shadowing during both the net‐loss and the net‐acceleration storm phases on sub‐hour time scales. We also highlight two distinct types of shadowing; “direct,” where electrons are lost as their orbit intersects the magnetopause, and “indirect,” where electrons are lost through ULF wave driven radial transport toward the magnetopause boundary.