Galactic cosmic ray modulation and the passage of the heliospheric current sheet at Earth

Abstract

We have examined the effect of heliospheric current sheet (HCS) crossings on cosmic ray intensity (CRI), solar wind speed, interplanetary magnetic field (IMF) and the cosmic ray solar diurnal anisotropy in free space. Data from eight neutron monitors and three muon telescopes, covering a wide energy and latitude range, were separated into two epochs, 1971–1979 (A > 0) and 1981–1990 (A < 0) according to the solar magnetic polarity. The CRI is more strongly affected during the A > 0 than during the A < 0 epoch. The increase in CRI associated with A-T HSC crossings was larger in magnitude than that of T-A crossings, and the magnitude was dependent on the rigidity of the particles. A symmetric negative cosmic ray gradient was observed with respect to the HCS. HCS crossings have no observable effect on the CRI observed by shallow underground detectors. Changes in solar wind speed and IMF magnitude, near HCS crossings, are related to the distance of the Earth from the sheet. We found no consistent changes in the solar diurnal variation related to HCS crossings.