Diffusion and electromagnetic mechanisms determine the formation of sporadic Forbush decreases. The diffusion mechanism affects the Forbush decrease amplitude in the turbulent layer, and the part of the coronal mass ejection preceding the magnetic cloud, and its efficiency depends on the level of magnetic field turbulence. The electromagnetic mechanism works in a magnetic cloud, and its efficiency depends on the intensity of regular magnetic and electric fields. We analyze solar wind parameters and cosmic ray density, using the superposed epoch analysis. In 1996–2006, 23 strong Forbush decreases (amplitude >5 %) were detected. The average amplitude of 7 % is equally formed by both mechanisms. The events can be divided into 2 groups depending on the contribution of the mechanisms to Forbush decrease amplitude. Group 1 includes the strongest Forbush decreases (amplitude=8.5 %), formed by both diffusion and electromagnetic mechanisms. The diffusion mechanism forms 0.26 amplitude, and the electromagnetic mechanism is responsible for 0.74 one. In group 2, the averege amplitude Forbush decrease =5.7 %, the diffusion mechanism forms 0.79 of amplitude; and the electromagnetic one, 0.21. The spatial distributions of the mean values of the medium parameters in the region of disturbances in the groups differ. This difference can be explained by the fact that Forbush decrease amplitude in groups 1 and 2 are formed in the central and peripheral parts of coronal mass ejection respectively.
cosmic rays, coronal mass ejection, Forbush decrease, solar wind, interplanetary magnetic field, shock, magnetic cloud
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