On the access of solar protons to the synchronous altitude region

Observations of 10- to 90-MeV solar protons were made near the synchronous altitude around the time of a sudden commencement at 0037 UT on August 9, 1972, by using sensors aboard spacecraft 1970-93A and 1972-10A. The observations indicated that the solar protons had direct access to the synchronous altitude region through the dawn flank of the magnetosphere and were transiently trapped with an e-folding lifetime of a few drift periods. A weaker dependence of the cutoff energy upon local time was observed than was predicted by the calculations of Gall et al. (1971).