Arrival of low-energy cosmic rays via the magnetospheric tail

Abstract

The role the external current responsible for the magnetospheric tail plays at quiet times in the penetration, motion, and arrival of low-energy cosmic rays is deduced from the integration of 1- to 500-Mev proton trajectories in a model magnetosphere. Using the Williams and Mead model, we show that protons can reach the midnight side at latitudes between 60° and 69° and the day side at latitudes between 60° and 80° by way of the tail. Protons have direct access to higher latitudes along open field lines. Thus, for latitudes above 60°, the cutoffs are lowered below the internal field cutoff values. No such lowering occurs for latitudes below 60° because the field lines in this region are not appreciably affected by the external current responsible for the tail. A daily cutoff variation, the minimum rigidity trajectories coming from the night side for any local time of arrival, is found. The directions of approach, along which the low-energy cosmic rays cross the magnetopause en route to the station, show a variation with local time. The daily variation in cutoff and approach directions are induced by axial asymmetry of the geomagnetic cavity.