Cosmic Ray Modulation with the Maximum CME Speed Index During Solar Cycles 23 and 24

We analyzed modulation of cosmic-ray intensities (CRIs) during Solar Cycles 23 and 24 by using the international sunspot numbers (ISSN) and the maximum CME speed index (MCMESI) as proxies for solar activity. Temporal variations, cross-correlations, and hysteresis patterns of CRI, MCMESI, and ISSN data were investigated. As a result, we concluded that the MCMESI better describes solar modulation of the CRI as compared to the ISSN. This is mainly because the correlation between CRI and ISSN is caused by the general cyclic trend of solar activity, while the correlation between the CRI and the MCMESI is mainly due to short-term fluctuations related to Forbush decreases. In contrast to the ISSN, there is no time lag between the CRI and the MCMESI variations.