Flares, Forbush decreases, and geomagnetic storms

Abstract

The authors hypothesize that individual flares are not the cause of either Forbush decreases or geomagnetic storms. Evidence supporting this hypothesis is here presented showing that geomagnetic storms and Forbush decreases can be accounted for entirely by the effects of interplanetary ‘streams.’ Major cosmic-ray variations begin near the time that a flare-producing region on the sun passes central meridian. This relationship and a similar relationship between geomagnetic storms and flare-producing regions is shown to exist even during the intervals that include the energetic flares that have produced ground-level increases in neutron monitor counts. Streams, closely associated with these flare-producing regions, are usefully described by the structure in the interplanetary magnetic field since fluctuations in the interplanetary field are highly correlated with the Kp index and the interplanetary field strength is inversely correlated with the cosmic-ray intensity. Such stream structure is used to account for the observed ‘center limb effect.’ It is also shown that when flares of importance 3— or greater occur within a few days of very high geomagnetic activity and at longitudes greater than 25° west, the major geomagnetic activity precedes the flare. This result is not easily explained by the hypothesis that periods of intense geomagnetic activity are due to individual flare effects.