Local-time dependence of geomagnetic cutoffs for solar protons, 0.52≤Ep≤ 4 Mev

Low-altitude, high-latitude observations of 0.52≤Ep≤4 Mev protons by the University of Iowa/NASA satellite Injun 4 are presented for three solar proton events having maximum intensities on January 20, May 30, and June 23, 1966. Only weak magnetic storms were associated with these events. A substantial body of new data is given on the magnetic local time (MLT) dependence of the geomagnetic cutoffs and the counting rate versus invariant latitude profiles from 0600 to 1300 hours and 1800 to 2200 hours for low-energy protons moving approximately normal to the local magnetic field. The observed cutoffs consist of two sets, one is independent of MLT at 66.5° invariant latitude and the upper set has marked local-time dependence, going from 68° at 0600 hours MLT to ∼75° at 1000 hours and back to 68° by 1900 hours. Pitch-angle distributions were not actually measured, but an interpretation based on pitch-angle anisotropies is suggested which is consistent with the quasi-trapping suggested by Taylor and others and experimentally observed by Paulikas, Blake, and Freden. The results so interpreted yield information on the extent of the quasi-trapped region over an extensive range of local time. The boundaries are qualitatively in agreement with Taylor in the morning.