Long-Term Trends in Forbush Decrease Activity over the Last Six Solar Cycles

Abstract

Forbush decrease (FD) parameters (magnitude, rate of decrease, anisotropy, and duration of the main phase) are compared for solar cycles 19–24 and in the maxima of cycles 23 and 24 and the minimum between them. The Forbush Effects and Interplanetary Disturbances database created and maintained at IZMIRAN allows statistical means to be used for data analysis. The results obtained in this work show that the relation between FDs and solar cyclicity is observed for all FD parameters; most FDs in the maximum of cycle 23 were caused by interplanetary disturbances associated with coronal mass ejections, while most of FDs in the minimum between cycles 23 and 24 were caused by high-speed streams from coronal holes.