Variations in the Cosmic-Ray Nucleonic Intensity

The monthly mean intensity of the nucleonic component of cosmic radiation measured at Mt. Washington, geographic latitude 44.2^∘{}N and elevation 6262 ft, decreased 22% from July, 1954 to December, 1957. The decreased intensity now prevailing came about through a series of sudden decreases from which only partial recovery occurred. During this same period, the amplitude of the day-to-day intensity changes has increased. A study has been made of the largest Forbush-type decreases which have occurred in this interval. Many of these intensity drops have a decrease rate {~}1.5% hr^-1, with a maximum rate of 2.5% hr^-1. In some cases they are preceded by a slight increase of {~}1.5%. These decreases do not have a similar time dependence. There is no apparent 27-day recurrent tendency for these sudden drops. The energy dependency of these events is variable. The restrictions which these decreases impose upon the existing models of solar modulation mechanisms are discussed.