Low Energy Cosmic Ray Cutoffs: Diurnal Variations and Pitch Angle Distributions

摘要

Observation of solar protons in the 1 to 100 MeV interval during the solar proton event of September 2, 1966, shows that: (a) an important day-night asymmetry in the geomagnetic cutoffs exists, with the flux starting to decrease from the polar plateau value at higher invariant latitudes near the noon meridian than near the midnight meridian; (b) the asymmetry is more pronounced for the lower energy particles. Direct and indirect measurements of pitch angle distributions show that a quasi-trapped flux of protons is established at high altitudes on the dayside of the polar cap. The nonisotropic pitch angle distribution at invariant latitudes less than 75 deg near local noon is a manifestation of limited access of solar protons to some local times and is associated with the midday absorption recoveries observed with riometers. (Author)