Master thesis defense by Patrick Bülow

Title: The effect of Forbush Decreases on the atmosphere

Abstract: We study the effects of galactic cosmic radiation (GCR) on Earth's cloud cover. We investigate Forbush decreases (FD), where the GCR are suppressed, which may result in a decrease in cloud cover. Using a bootstrap, we generate FD based on our observational data from satellites, reanalyses and reforecasts (1990-2017). The generated FD is averaged over a total of 24 FDs. Applying Monte Carlo, we compute random FDs, which includes the entire time series, for a comparison to the generated FD. We account for weather noise by using two detrending methods (linear and Fourier detrend) to normalize each FD-event. From our analysis, we find a significant decrease in cloud cover during a FD-event. However, this is not certainly caused by a FD, since the reforecast does not predict the reanalysis well before a FD-event. Additionally, we find the shortage of observable FD-events to be insufficient for a conclusion. Our results suggest that the decrease in cloud cover might be due to external forces.