A comparison of the characteristics of corotating and flare-initiated Forbush decreases

Abstract

Data acquired by the deep space probes Pioneer VI and Pioneer VII which differ by [Formula: see text] in heliographic longitude are presented to illustrate the basic similarities and differences between the classes of Forbush decrease resulting from corotating shock fronts and radially propagating blast waves. An example is presented to establish directly the existence of the corotating Forbush decrease, and from the intensity–time profile of the event it is concluded that the detailed time structure of the Forbush decrease is invariant over a time scale of about 4 days. An example of the Forbush decrease resulting from a solar-induced blast wave is also presented wherein an azimuthal gradient of about a factor of 4 per 60° of azimuth is observed.