Modeling Field Line Curvature Scattering Loss of 1-10 MeV Protons During Geomagnetic Storms.

The proton radiation belt contains high fluxes of adiabatically trapped protons varying in energy from ~one to hundreds of megaelectron volts (MeV). At large radial distances, magnetospheric field lines become stretched on the nightside of Earth and exhibit a small radius of curvature R_C near the equator. This leads protons to undergo field line curvature (FLC) scattering, whereby changes to the first adiabatic invariant accumulate as field strength becomes nonuniform across a gyroorbit. The outer boundary of the proton belt at a given energy corresponds to the range of magnetic L shell over which this transition to nonadiabatic motion takes place, and is sensitive to the occurrence of geomagnetic storms. In this work, we first find expressions for nightside equatorial R_C and field strength B_e as functions of Dst and L* to fit the TS04 field model. We then apply the Tu et al. (2014, https://doi.org/10.1002/2014ja019864) condition for nonadiabatic onset to solve the outer boundary L*, and refine our expression for R_C to achieve agreement with Van Allen Probes observations of 1-50 MeV proton flux over the 2014-2018 era. Finally, we implement this nonadiabatic onset condition into the British Antarctic Survey proton belt model (BAS-PRO) to solve the temporal evolution of proton fluxes at L ≤ 4. Compared with observations, BAS-PRO reproduces storm losses due to FLC scattering, but there is a discrepancy in mid-2017 that suggests a ~5 MeV proton source not accounted for. Our work sheds light on outer zone proton belt variability at 1-10 MeV and demonstrates a useful tool for real-time forecasting. [ABSTRACT FROM AUTHOR]