Long-Term Changes in the Number and Magnitude of Forbush-Effects

Abstract

The IZMIRAN database of Forbush effects and interplanetary disturbances has been used to study long-term changes in the number and magnitude of Forbush effects in the last six solar cycles (1957–2016) for cosmic rays of rigidity of 10 GV. Solar activity cycles have been shown to be well expressed in data of Forbush effects, especially in large magnitude events that almost disappear in minima. The changes in the distribution of Forbush effects and the decrease in their average values from solar activity maximum to minimum are explained by the predominance of cosmic-ray variations due to the action of coronal holes at low activity. It should be noted that the current cycle involves fewer and generally weaker Forbush effects than in the previous five cycles. For each month, an FD index combining the magnitude and number of Forbush effects and convenient for studying long-term variations has been proposed and calculated.