Explaining sudden losses of outer radiation belt electrons during geomagnetic storms

The Van Allen radiation belts were first discovered in 1958 by the Explorer series of spacecraft(1). The dynamic outer belt consists primarily of relativistic electrons trapped by the Earth's magnetic field. Magnetospheric processes driven by the solar wind(2) cause the electron flux in this belt to fluctuate substantially over timescales ranging from minutes to years(3). The most dramatic of these events are known as flux 'dropouts' and often occur during geomagnetic storms. During such an event the electron flux can drop by several orders of magnitude in just a few hours(4,5) and remain low even after a storm has abated. Various solar wind phenomena, including coronal mass ejections and co-rotating interaction regions(6), can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA-POES spacecraft, we show that the sudden electron depletion observed during a recent storm's main phase is primarily a result of outward transport rather than loss to the atmosphere.