A model of a piston shock produced by a sharp jump in the velocity of the solar wind with ahelical magnetic field is suggested to explain the origin of the Forbush decrease of cosmic rays with a hardenergy spectrum at solar activity minimum and, in some cases, at solar activity maximum. The contactvelocity, the compression ratio, the contact position, the ratio of the magnetic field near the contact to theseed field, the maximum momentum of the particles subjected to modulation, and the modulation depthfor the neutron and muon cosmic ray components have been calculated. The theoretical calculations arecompared with experimental data.
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