Causes of Solar Flares: A Comparative Study of Super-active and Low-Active Regions

Solar active regions (ARs), groups of strong magnetic flux of mixed polarities, are the main sources of solar flares. Though it is well known that flare activities vary among ARs, it is unclear what physical processes determine and dominate the flare activity of an AR. In this study we compared the evolution of super-active (highly flare productive; flare index ≥ 4) and low-active (relatively low flare productive but not flareless; flare index < 4) ARs. Flare index is a number, reflecting the average flare productivity, calculated using the number and intensity of flares from an AR. Among all ARs from year 2010–2015, we selected four different types of ARs based on contrasting flare activities and sunspot sizes. Though the magnetic flux emergence is necessary, our study shows that it alone is not sufficient to increase the flare productivity of an AR. The new emergence can lead to either the interaction of like or opposite magnetic fluxes of non-conjugate pairs (magnetic poles not emerging together as a conjugate pair, as in a bipolar configuration). In the former case, the overall magnetic configuration remains simple and the flare productivity of AR does not change with emergence. In the latter case, the convergence of opposite magnetic fluxes of non-conjugate pairs results in a complex magnetic configuration with long polarity inversion line (PIL). Our study of many contrasting ARs shows that the persistent shearing motion and flux cancellation along such PIL results in multiple intense flares. Further, our study suggests that length of PIL is the best magnetic-field parameter to reflect the flare productivity. The implication and usage of these results on flare prediction will be discussed.