An Analysis of Solar Modulation of Cosmic Rays and Their Significance on Earth's Climate

Abstract

Earth receives markedly higher solar irradiance during periods of high solar activity than during quiet conditions, and this variation has been considered here to study its bearing on galactic cosmic rays (GCRs) and, in turn, on climate. GCRs, being high-energy charged particles, get modulated by the heliospheric magnetic field carried outward by the solar wind, giving rise to the well-known anti-correlation, seen in neutron monitor data, between cosmic ray intensity and solar activity indices such as sunspot number, following an approximately 11-year cycle corresponding to half of the solar magnetic cycle. Modulation processes differ for positively and negatively charged particles and also vary with solar magnetic polarity. Here, the effects of outward and inward heliospheric field configurations, and of both solar magnetic polarities, on cosmic ray modulation have been examined, drawing on a literature-based synthesis of publicly archived solar, cosmic ray, and climate datasets, viz. WDC-SILSO, NMDB, and standard instrumental temperature compilations. Modulation of the terrestrial cosmic ray flux is considered as a possible solar influence on climate, acting through atmospheric ion production and electrical conductivity. Cloud nucleation associated with cosmic ray flux is also discussed, along with the balancing evidence from CERN's CLOUD experiment, which indicates that the amplification mechanism linking microphysical effects to macroscopic climate response is, at best, only partly quantified. Daily and monthly sunspot numbers over approximately two centuries are analysed, together with the open solar magnetic flux and the 6–12-month lag observed between solar and cosmic ray variability. The correlation between sunspot number and global surface temperature (GST) is found to lie in the range r ≈ 0.5-0.7 for the pre-1980 record, weakening markedly thereafter, consistent with a growing dominance of greenhouse-gas forcing. The results, taken together, contribute to a more balanced understanding of the possible role of solar-modulated cosmic rays in climate variability.