Cosmic-ray intensities near the heliospheric current sheet throughout three solar activity cycles

Cosmic-ray data from seven neutron monitors and three muon telescope have been used to examine the effect of a heliospheric current sheet (HCS) crossing the Earth on the cosmic-ray intensities (CRIs) during different phases of the three solar activity cycles (SACs). The median primary rigidity of these detectors covers the range 16-185 GV. We used the superposed epoch analysis to study the average behaviour of the daily (on an hourly basis) cosmic-ray intensity over the period 1965-94. We find a significant difference in the propagation characteristics and energy dependence between the descending and the recovery phases of 11-year variation, as well as large variations in the energy dependence from one solar cycle to another. The magnitude of CRI decrease following the away-toward (A-T) crossings is larger than that of T-A sector crossing during the descending region of the 20th SAC, and it is dependent on the rigidity of the particles. The net CR modulation at low rigidities indicated that the amplitude of the modulation for the 20th cycle is larger than for the 21st SAC, while at higher rigidities no remarkable changes have been obtained as a result of HCS passage. The phase of the solar activity and the state of the IMF direction (A or T) are dominant factors for the rigidity dependence of the particles. Furthermore, for the qA>0 periods, the total rate of CRI variation of toward polarity days is larger in magnitude than those for away days. In addition, the rate of decrease in the region 1981-84 (qA<0) for the A-T field HCS crossing was smaller than those observed in the regions 1972-74 and 1975-76 (qA>0). No significant north-south effect in CRI is observed for the polar stations. (author)