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Flares and CMEs
Not every solar flare is accompanied by a coronal mass ejection (CME). Which flares eject material, and which do not, matters both for understanding energy release and for space weather forecasting. Sigmoidal (S-shaped) structures seen in soft X-rays have long been regarded as a signature of the twisted, non-potential field that can erupt.
Does a sigmoid predict a CME?
We investigated the statistical dependences among CMEs, solar flares, and sigmoidal structures in soft X-ray images. The same analysis was applied to every feature so that the correlations reported in earlier studies could be tested for reproducibility. The sample consists of 211 M-class and X-class flares observed between 2006 and 2015 with the X-ray Telescope on board Hinode, LASCO on board SOHO, and GOES. Five kinds of analysis were performed: occurrence rate analysis, linear-correlation analysis, association analysis, the Kolmogorov–Smirnov test, and the Anderson–Darling test.
Three main results emerged. First, for on-disk events, the sigmoidal structure and long-duration events (LDEs) have a stronger dependence on CME occurrence than the X-ray class of the flare. Second, for limb events, a significant dependence exists between LDEs and CME occurrence, and between X-ray class and CME occurrence. Third, 32.4% of on-disk flare events have a sigmoidal structure and are still not accompanied by a CME, whereas the occurrence probability of a CME without a sigmoidal structure is very small, 8.8%.
The first and second results are consistent with previous studies, but the difference between on-disk and limb events is shown here for the first time. The third result, that non-sigmoidal regions produce fewer eruptive events, also differs from earlier work. Sigmoidal structures in soft X-ray images should therefore be a helpful feature for CME prediction in on-disk flare events.
→ Y. Kawabata, Y. Iida, T. Doi, S. Akiyama, S. Yashiro, and T. Shimizu, The Astrophysical Journal, 869, 99, 2018