Forward mapping of solar energetic proton distributions through the geomagnetic field

Abstract

[1] During solar proton events, large ejections of energetic protons spread throughout the interplanetary medium, penetrate the geomagnetic field, and are deposited in the upper polar atmosphere where they play important roles in its physical and chemical processes. We develop a model of direct proton propagation through a static geomagnetic field based on a generalized leapfrog method and validate it through comparison with Stormer theory. We then apply the algorithm to two ideal cases representing the late and early phases of a solar particle event, for proton energies of 10, 100, and 1000 MeV. The late-phase case is represented by an isotropic infinite bath of protons surrounding the magnetosphere; in this case, most protons reaching the polar cap originate within ±20° of the magnetic equatorial plane, with pronounced peaks near ±20°. The early-phase case is represented by broad, monodirectional proton beams; these sources are highly focused by the geomagnetic field, with all particles mapping to small regions in the polar cap only a few hundred kilometers across, and with low-energy particles being focused more than higher-energy ones. Pure dipole and International Geomagnetic Reference Field-11 magnetic field models lead to very similar fluence patterns in the polar cap, though the latter are somewhat less focused.