Changes in cloud cover associated with Forbush decreases of galactic cosmic rays

Authors:
  • MTMartin C. Todd
    Martin C. Todd
  • DKDominic R. Kniveton
    Dominic R. Kniveton
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Abstract
The results of a study to quantify the relationship between cloud cover and short‐term Forbush decreases (FD) of galactic cosmic ray flux are presented. Using an extensive record of global satellite‐derived cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, epoch superposition analysis of a sample of FD events is conducted. This analysis is conducted at a range of spatial scales from global, through 5° geomagnetic latitude bands to a global grid with 2.5° resolution. Resulting cloud anomalies are tested for significance using a randomized Monte Carlo experiment. The results indicate a small but significant (at 0.001 probability level) decline in the global proportion of cloud cover (of up to 1.4%) immediately prior to and following FD events. Analysis of data averaged over geomagnetic latitude (φ) bands reveals that significant cloud anomalies are concentrated in the high latitudes. A substantial (small) decline in cloud cover occurs at Southern (Northern) Hemisphere polar latitudes and is accompanied by a small but significant increase near φ = 30°N. The high‐latitude anomalies occur largely in the high‐level cloud and are particularly pronounced (up to −30%) in the uppermost cloud (occurring at 10–180 mbar) over Antarctica. In contrast, analysis using a sample of FD events associated with solar proton events shows no statistically significant cloud anomalies. A discussion of possible explanations of the results is provided.

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