Testing the effect of solar wind parameters and geomagnetic storm indices on Galactic cosmic ray flux variation with automatically-selected Forbush decreases

Abstract

Forbush decrease (FD), discovered by Scott E. Forbush about 80 years ago, is referred to as the non-repetitive short-term depression in Galactic cosmic ray (GCR) flux, presumed to be associated with large-scale perturbations in solar wind and interplanetary magnetic field (IMF). It is the most spectacular variability in the GCR intensity which appears to be the compass for investigators seeking solar-terrestrial relationships. The method of selection and validation of FD events is very important to cosmic ray (CR) scientists. We have deployed new computer software to determine the amplitude and timing of FDs from daily-averaged CR data at Oulu Neutron Monitor station. The code selected 230 FDs between 1998 and 2002. In an attempt to validate the new FD automated catalog, the relationship between the amplitude of FDs, and IMF, solar wind speed (SWS) and geomagnetic storm indices (Dst, kp, ap) is tested here. A two-dimensional regression analysis indicates significant linear relationship between large FDs (CR(%) ≤ –3) and solar wind data and geomagnetic storm indices in the present sample. The implications of the relationship among these parameters are discussed.