The existence of long-term variations in cosmic-ray intensity negatively correlated with sunspot activity, a discovery due to Forbush, has been explained by Davis in terms of a cavity in the galactic magnetic field centered about the sun. In the present paper, it is shown that a “diamagnetic” interplanetary region, rather than a “superconducting” one, can account for the cut-off in the primary cosmic-ray rigidity spectrum, as well as for the 11 year variations. The suggested diamagnetic region is an oblate spheroid in shape, with semiaxes of roughly 5×1015 cm and 5×1014 cm, and its effective permeability of 10−4 implies a mean interplanetary field of about 10−9 gauss. The 11-year variations and the rigidity cut-off are analyzed in detail on the basis of this model.
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Arthur Beiser (1958) studied this question.