What determines the magnitude of forbush decreases? Original Research Article

Cosmic ray Forbush effects (FEs) have been studied for more than 60 years, but even today this phenomenon has no universally accepted definition, which leads to misunderstanding among researchers. There are no complete and convincing answers to the following questions: What are the distinctive features of FEs? What determines the magnitude of FEs and their variety? How are FEs associated with disturbances of the interplanetary medium, coronal mass ejections and high speed flows of the solar wind? What relation exists between FEs and geomagnetic storms? In order to answer these and other questions a data base of transient effects in cosmic rays and in the interplanetary medium is constructed and is being upgraded continuously. It combines variations in cosmic ray densities and anisotropies obtained from data of the world-wide neutron monitor (NM) network, characteristics of solar wind disturbances, solar and geomagnetic data. This data base presently contains more than 1400 events, observed during 1978–1996. The preliminary analysis allowed us to get the main characteristics of FEs, to determine what distinguishes this phenomenon among other cosmic ray variations, and propose a definition of the Forbush effect. In addition, an interplanetary disturbance parameter was derived, which is most closely associated with the magnitude of a FE. Using this large observational database, a relation between FEs and geomagnetic activity was analyzed statistically as a dependence of the FE magnitude on the maximum Kp-index measured during the associated magnetic storm.