Electron Precipitation From the Outer Radiation Belt During the St. Patrick's Day Storm 2015: Observations, Modeling, and Validation

Recently, a model for medium-energy (30-1000 keV) radiation belt-driven electron precipitation (ApEEP) has been put forward for use in decadal to century-long climate model runs as part of the Climate Modelling Intercomparison Project, phase 6 (CMIP6). The ApEEP model is based on directly observed precipitation data spanning 2002-2012 from the constellation of low-Earth-orbiting Polar Operational Environmental Satellites (POES). Here, we test the ApEEP model's ability using its magnetic local time variant, ApEEP_MLT, to accurately represent electron precipitation fluxes from the radiation belts during a large geomagnetic storm that occurred outside of the span of the development data set. In a study of narrowband subionospheric very low frequency (VLF) transmitter data collected during March 2015, continuous phase observations have been analyzed throughout the entire St. Patrick's Day geomagnetic storm period for the first time. Using phase data from the U.K. transmitter, call-sign GVT (22.1 kHz), received in Reykjavik, Iceland, electron precipitation fluxes from L = 2.8 to 5.4 are calculated around magnetic local noon (12 MLT) and magnetic midnight (00 MLT). VLF-inferred >30-keV fluxes are similar to the equivalent directly observed POES fluxes. The ApEEP_MLT >30-keV fluxes for L < 5.5 describe the overall St. Patrick's Day geomagnetic storm-driven flux enhancement well, although they are a factor of 1.7 (1.3) lower than POES (VLF-inferred) fluxes during the recovery phase. Such close agreement in >30-keV flux levels during a large geomagnetic storm, using three different techniques, indicates this flux forcing are appropriate for decadal climate simulations for which the ApEEP model was created.