Partial loss and significant depletion of radiation belt electrons during the April 4, 2017, geomagnetic storm.

This article focuses on a specular loss event of radiation belt electrons observed during a moderate geomagnetic storm on April 4, 2017, using data from the Van Allen Probes. It identifies a partial electron dropout during the storm’s initial phase caused by the magnetopause shadowing effect due to increased solar wind dynamic pressure, followed by a significant, energetically isotropic depletion of electron flux and phase space density (PSD) at all pitch angles during the main phase. The study finds that known wave–particle interactions and spatial boundary effects cannot adequately explain this local PSD minimum, highlighting a gap in current radiation belt dynamic theories. The authors emphasize the need for further research into such isotropic electron dropouts and their relation to external geomagnetic and solar wind disturbances.