Forbush decreases, solar irradiance variations and anomalous cloud changes

Changes in the galactic cosmic ray (GCR) flux due to variations in solar activity may provide an indirect connection between the Sun's and the Earth's climates. Epoch superpositional (composite) analyses of high-magnitude GCR fluctuations, known as Forbush decrease (FD) events, have been widely used to test this hypothesis, with varied results. This work provides new information regarding the interpretation of this approach, suggesting that FD events do not isolate the impacts of GCR variations from those of solar irradiance changes. On average, irradiance changes of ~0.4 W m-2 outside the atmosphere occur around 2 days in advance of FD-associated GCR decreases. Using this 2 day gap to separate the effects of irradiance variations from GCR variations on cloud cover, we demonstrate small, but statistically significant, anomalous cloud changes occurring only over areas of the Antarctic plateau in association with the irradiance changes, which previous workers had attributed to GCR variations. Further analysis of the sample shows that these cloud anomalies occurred primarily during polar darkness, precluding the possibility of a causal link to a direct total solar irradiance effect. This work suggests that previous FD-based studies may have ineffectively isolated the impacts of GCR variations on the Earth's atmosphere.