Characteristics of recurrent Forbush decreases in Galactic cosmic ray intensity during positive and negative solar magnetic polarities

Abstract

We present a statistical analysis of recurrent Forbush decreases (RFDs) in Galactic cosmic ray intensity in associated with corotating solar wind streams during low solar activity conditions. Selected RFDs recorded at the ground-based neutron monitors with low to high cutoff rigidities ($P_{c}$) have been separately investigated for the periods 2008-2011 ($A < 0$, 70 events) and 2015-2016 ($A > 0$, 35 events). We found that medians of RFD amplitudes are larger in the $A > 0$ than in the $A < 0$ and exhibit solar phase dependence. Correlations of RFD characteristics with magnitude of interplanetary magnetic field (IMF) and with solar wind speed in case of maximum smooth (moving average) values are more relevant than in case of observed maximum values and difference between the maximum and base values, particular in the $A<0$. Correlation of RFD amplitudes with maximun smooth solar wind speed and IMF magnitude is weak and exist only for $A < 0$. Slopes of the graph of RFD amplitude - the solar wind speed/IMF strength for the epoch $A <0$ tend to decrease from low to high $P_{c}$, suggesting a rigidity dependent relation. These results are discussed in the context of the convection-diffusion process including drift. We suggest that drift is an important process in controlling RFD amplitudes during the $A<0$ and $A>0$ epochs.