The cosmic ray solar daily variation

Abstract

Measurements of the cosmic ray solar daily variation at a depth of 60 metres water equivalent show that the average anisotropy of primary cosmic rays in the energy range 1011 to 1012 eV con be understood in terms of a drift motion round the Sun in accordance with the theoretical work of Dessler, Ahluwalia, Parker and Axford, provided one also includes a flux gradient perpendicular to the ecliptic plane. During the recent sunspot minimum of 1964/65 the drift motion vanished for primaries with energy > 1011 eV, probably because of the increased disorder of the interplanetary magnetic field at that time. During the sunspot minimum period, the remaining observable anisontropy is ascribed to the aberration effect due to the orbital motion of the Earth together with that resulting from the flux gradient.