Low-energy solar-cosmic-ray cutoffs: Diurnal variations and pitch-angle distributions

Observation of solar protons in the 1–100 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 nearer the noon meridian than 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° 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.