GALACTIC COSMIC-RAY MODULATION AND THE PASSAGE OF THE HELIOSPHERIC CURRENT SHEET AT EARTH

We have examined the effect of heliospheric current sheet (HCS) crossi ngs on cosmic ray intensity (CRI), solar wind speed, interplanetary ma gnetic field (IMF) and the cosmic ray solar diurnal anisotropy in free space. Data from eight neutron monitors and three muon telescopes, co vering a wide energy and latitude range, were separated into two epoch s, 1971-1979 (A > 0) and 1981-1990 (A < 0) according to the solar magn etic 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 c rossings was larger in magnitude than that of T-A crossings, and the m agnitude was dependent on the rigidity of the particles. A Symmetric n egative cosmic ray gradient was observed with respect to the HCS. HCS crossings have no observable effect on the CRI observed by shallow und erground detectors. Changes in solar wind speed and IMF magnitude, nea r HCS crossings, are related to the distance of the Earth from the she et. We found no consistent changes in the solar diurnal variation rela ted to HCS crossings. (C) 1998 Elsevier Science Ltd. All rights reserv ed.