Todd, MC; Kniveton, DR; (2001) Changes in cloud cover associated with Forbush decreases of galactic cosmic rays. Journal of Geophysical Research - Atmospheres , 106 (D23) 32031 - 32042. 10.1029/2001JD000405.

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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) Dl 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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