Statistical Investigation on Galactic Cosmic Rays and Solar Wind Variation Based on ACE Observations

Based on the Galactic Cosmic Rays (GCRs) and plasma observations from ACE spacecraft, the relation between GCR counts and solar wind parameters during the two periods of solar minimums (the years of 2007.0-2009.0 and 2016.5-2019.0) was analyzed by means of the Superposed Epoch Analysis (SEA) method. The results indicate that GCRs are strongly modulated by Co-rotating Interaction Regions (CIRs) in solar wind, the Stream Interfaces (SIs) sandwiched between fast and slow solar wind are closely related with the depression of GCR counts. The mechanism of the GCR variation was investigated through the empirical diffusion coefficients. The so-called “snow-plough” effect of GCR variation prior to the SI crossing appears during the first period, then the GCR counts decrease after the crossing, which corresponds to the sudden drop of diffusion coefficient at the SI. However, this effect is not observed for the second period, the decrease of GCR counts may be caused by the enhancement of the diffusion coefficient after the SI crossing. Moreover, Heliospheric Current Sheet (HCS) correlates with GCR counts well, the GCRs drift along the current sheet, and then accumulate to a pileup structure. The interplay between drift and diffusion determines the GCR distribution and variation at a heliocentric distance of 1 AU.