DEPENDENCE OF OCCURRENCE RATES OF SOLAR FLARES AND CORONAL MASS EJECTIONS ON THE SOLAR CYCLE PHASE AND THE IMPORTANCE OF LARGE-SCALE CONNECTIVITY

We investigate the dependence of the occurrence rates of major solar flares (M- and X-class) and front-side halo coronal mass ejections (FHCMEs), observed from 1996 to 2013, on the solar cycle (SC) phase for six active McIntosh sunspot group classes: Fkc, Ekc, Dkc, Fki, Eki, and Dki. We classify SC phases as follows: (1) ascending phase of SC 23 (1996-1999), (2) maximum phase of SC 23 (2000-2002), (3) descending phase of SC 23 (2003-2008), and (4) ascending phase of SC 24 (2009-2013). We find that the occurrence rates of major flares and FHCMEs during the descending phase are noticeably higher than those during the other phases for most sunspot group classes. For the most active sunspot group class, Fkc, the occurrence rate of FHCMEs during the descending phase of SC 23 is three times as high as that during the ascending phase of SC 23. The potential of each McIntosh sunspot group class to produce major flares or FHCMEs is found to depend on the SC phase. The occurrence rates (R) of major flares and FHCMEs are strongly anti-correlated with the annual average latitude of the sunspot groups (L): R similar to L-2.07 for major flares and R similar to L-2.42 for FHCMEs. This finding indicates the possible role of large-scale coronal connectivity, e.g., trans-equatorial loops, in powerful energy releases. Interestingly, this relationship is very similar to that between the volumetric coronal heating rate and X-ray loop lengths, indicating common energy release mechanisms.