Two classes of cosmic ray decrease

In a study of galactic cosmic ray intensity variations it had been found that there are two distinct classes of transient intensity decrease, which have been called type I and type II. Type I events are flare-associated Forbush decreases. Type II events appear to be associated with 27-day recurrences. It is suggested, on the basis of several characteristics of the decreases, that type II events are simply the subsequent evolution of type I events or quasi-stationary 'corotating' structures loosely associated with active regions.