COSMIC-RAY TRANSIENT VARIATIONS OBSERVED FROM THE EARTH

Cosmic ray transient variations are signatures of the underlying solar processes which affect the heliomagnetic structure. They can be found in the data from surface systems, such as neutron monitors and muon t elescopes, and from shallow underground muon telescopes. Although some times observable at all latitudes, the global distribution of the effe cts is important in determining the structures and causes. Three trans ient variation types are known, namely Forbush decreases, ground level enhancements, and quasi-periodic fluctuations. The latter category in cludes some variations which are, perhaps, too long-lived to be consid ered truly transient. In this review, the detection techniques and bac kground of cosmic ray research axe followed by a summary of some obser vations in each transient category. The heliomagnetic structures and d ynamics inferred from such transients are discussed together with the possible impact such events can have on human activity.