Variations in cosmic-ray spectra during Forbush events in 1966

Abstract

Measurements of cosmic-ray spectra, using the neutron multiplicity technique, were made during 1966 at White Mountain, California. Multiplicity spectra observed during Forbush events beginning on March 23 and August 30, 1966 demonstrated that the modulation mechanism most strongly affected the low-multiplicity (i.e. low-energy) rates. The decreases were about 5.5% and 7.0% in the multiplicity m = 1 rates, gradually diminished with increasing m, and were unobservable around m = 50. During the August 30 Forbush event, intensity increases of a few hours' duration occurred. The increase, which began on September 3 at 1800 U.T. and occurred simultaneously with the largest magnetic storm of the year, exhibited a preferential increase in the low-multiplicity rates. The recovery effect decreased with increasing multiplicity and essentially disappeared at m = 10. The second largest magnetic storm of 1966 occurred on March 14 and again a significant intensity increase was observed at White Mountain. It also displayed a preferential increase in the low-multiplicity rates. A brief discussion, with reference to the storm time increases previously observed on September 13, 1957 and February 11, 1958, of possible modulation mechanisms responsible for these increases is presented.