Contributions from changes in various solar indices in cycles 20–23 and 24 to the modulation of cosmic rays

Abstract

The modulation of cosmic rays (CRs) observed at neutron monitors during the minimum, growth, maximum, and beginning of the decay of solar activity (SA) of cycle 24 (2008–2015) is compared to the modulation of CRs in the same periods of SA of cycles 20–23. After a prolonged minimum of SA between cycles 23 and 24 in 2007–2009 and the maximum of CRs at the end of 2009, when CR intensity exceeded the previous four CR maxima, the SA growth phase displayed weaker modulation of CRs than in the corresponding periods of the previous cycles. The shorter variations in CRs (oscillations with variable periods and activity spikes) were clearer against the background of the reduction in the 11-year variation during SA growth in 2011–2014. The current SA cycle (24) saw a scenario with two SA maxima, and the CRs showed one minimum at the end of 2014 that differed from the maximum of 2009 by ≈12% for particles with a rigidity of 10 GV. CR modulation in the last cycle was the weakest since neutron monitors began operating. At the beginning of 2015, the CRs started to recover. Possible reasons for the unusually weak CR modulation are considered. Quantitative estimates are made of the contribution from different characteristics of the solar magnetic field to CR modulation in the period of cycle 24 under study and in the corresponding periods of solar cycles 20–23, and the estimates are compared.