Heliomagnetic dipole moment and daily variation of cosmic rays underground

Abstract

Since the discovery of Forbush decrease of cosmic-ray intensity, it has become clear that the intensity and its daily variation are subject to various kinds of solar modulation; these are due to the diffusion-convection^1,2 of cosmic rays in interplanetary magnetic fields, which is controlled by various kinds of solar activity. We now demonstrate that the amplitude of diurnal variation of cosmic-ray intensity is intimately correlated with the magnitude of magnetic dipole moment of the Sun and that, contrary to the conventional concept of the modulation, such a correlation exists only in the cosmic-ray high-rigidity region.