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

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. Copyright 2001 by the American Geophysical Union.