Ineffectiveness of Narrow CMEs for Cosmic Ray Modulation

Abstract

Monthly coronal mass ejection (CME) counts, – for all CMEs and CMEs with widths > 30^∘, – and monthly averaged speeds for the events in these two groups were compared with both the monthly averaged cosmic ray intensity and the monthly sunspot number. The monthly P _i-index, which is a linear combination of monthly CME count rate and average speed, was also compared with the cosmic ray intensity and sunspot number. The main finding is that narrow CMEs, which were numerous during 2007 – 2009, are ineffective for modulation. A cross-correlation analysis, calculating both the Pearson (r) product–moment correlation coefficient and the Spearman (ρ) rank correlation coefficient, has been used. Between all CMEs and cosmic ray intensity we found correlation coefficients r=− 0.49 and ρ=− 0.46, while between CMEs with widths > 30^∘ and cosmic ray intensity we found r=− 0.75 and ρ=− 0.77, which implies a significant increase. Finally, the best expression for the P _i-index for the examined period was analyzed. The highly anticorrelated behavior among this CME index, the cosmic ray intensity (r=− 0.84 and ρ=− 0.83), and the sunspot number (r=+ 0.82 and ρ=+ 0.89) suggests that the first one is a very useful solar–heliospheric parameter for heliospheric and space weather models in general.