Model for the uneven illumination of polar caps by solar protons

Abstract

A model for the uneven illumination of polar caps by solar proton fluxes of 5–300 Mev is proposed. It is based on observations and on the assumption that cosmic rays move with the direct mode in the geomagnetic field. Theoretical characteristics of the illumination are derived from extensive computations of cosmic-ray orbits, of points of access and directions of approach at the boundary and the tail, for arrival at all local times in auroral and polar latitudes at satellite heights. The model applies to the anisotropic phase of the proton event. A preferential flow of the anisotropic radiation along the neutral sheet, a preferential flow along the main body of the tail, and reconnection with the interplanetary field are necessary for the model to account for auroral peaks, polar peaks, and north-south asymmetry, respectively, the valleys being illuminated only by the non-anisotropic fraction of the interplanetary flux. The model implies that illumination by the direct impact of the anisotropic flux through the sunlit magnetopause is inhibited.