Determination of the Equatorial Electron Differential Flux From Observations Taken at Low Earth Orbit for Use as a Low Energy Boundary Condition in the BAS Radiation Belt Model

Variations in the high energy relativistic electron flux of the radiation belts depend on transport, acceleration, and loss processes, and importantly on the lower energy seed population. However, data on the seed population is limited to a few satellite missions. Here we present a new method that utilizes data from the Medium Energy Proton/Electron Detector (MEPED) on board the low altitude Polar Operational Environmental Satellites (POES) to retrieve the seed population at a pitch angle of 90^o. The POES MEPED measurements were initially converted to omnidirectional values before being used to calculate the differential electron flux for energies in the 100 - 600 keV range. Finally, the differential flux was converted to an equatorial pitch angle of 90^o. We validate the resulting 90^o flux for 100 - 600 keV electrons against measurements from the Van Allen Probes MagEIS instrument and show a good average agreement, within a factor of 4 for L* > 3.7. The final data set offers a high time resolution, across multiple magnetic local time planes, and may be used to formulate event-specific low energy boundary conditions for radiation belt models. Results from the BAS Radiation Belt Model (BAS-RBM), using the developed method to provide the low energy boundary condition, are shown and compared to BAS-RBM outputs when either a statistical model or MagEIS observations were instead used on the low energy boundary. Comparison of the three model runs demonstrates that the level of the seed population, and the timescale for variations, have important consequences for the >1 MeV electron flux.