Statistical Investigation On The Galactic Cosmic Rays And Solar Wind Variation Based On ACE Observations

Galactic cosmic rays(GCRs)are originated from the interstellar space and modulated by the heliospheric magnetic field when they enter the heliosphere.Based on the GCR and plasma observations from ACE spacecraft,we analyzed the relation between the GCR counts and the solar wind parameters during the recent two periods of solar minimum(the years of 2007.0-2009.0 and 2016.5-2019.0)by means of the super epoch analysis method.The results indicate that GCRs are strongly modulated by the corotating interaction regions(CIRs)in solar wind,the locations of 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 is 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 at 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 simply are caused by the enhancement of the diffusion coefficient after the SI crossing.Moreover,heliospheric current sheet(HCS)correlate with GCR counts well,the GCRs drift along the current sheet and then accumulate to a pileup structure,which is physically balanced between their diffusion and drift effects.Finally,based on the observation and Parker transport theory,we discuss the physical mechanism of the GCR variation for the crossings of SIs and HCS,and proposed that the interplay between drift and diffusion determines the GCR distribution and variation at a heliocentric distance of 1 AU.