Forbush Decreases: No Change of Global Cloud Cover

J. Calogovic¹, F. Arnold², L. Desorgher³, E.O. Flueckiger³, J. Beer⁴

¹Hvar Observatory, Faculty of Geodesy, Kaciceva 26, HR-10000 Zagreb, Croatia
²Max Planck Institute for Nuclear Physics, Saupfercheckweg 1, 69117, Heidelberg, Germany
³University of Bern, Physikalisches Institut, CH-3012 Bern, Switzerland
⁴EAWAG, PO Box 611, CH-8600 Duebendorf, Switzerland

Svensmark and Friis-Christensen analyzing one solar cycle reported 1997 that global cloud cover changed in phase with the GCR flux by 2-3%. This initiated heavy debates in the scientific community. However, up to date no really conclusive results have been published. In our approach we use the same cloud data and assumptions as previous authors, but instead of analyzing cosmic ray changes over the 11-year solar cycle we concentrate on rapid changes, so-called Forbush decreases which are comparable in amplitude but last only about a week. For each of the six selected Forbush decreases extensive Monte Carlo simulations were carried out to calculate the global change in ion production rate in the atmosphere. For each grid cell the ion production change was compared with the corresponding ISCCP D1 cloud data change allowing for time lags ranging from 0 to 10 days. Detailed global correlation analysis of these ionization data with the corresponding cloud data does not show any indication for the proposed link between cosmic ray intensity and global cloud cover.