The nonpotentiality of coronae of solar active regions, the dynamics of   the surface magnetic field, and the potential for large flares
  Flares and eruptions from solar active regions are associated with atmospheric electrical currents accompanying distortions of the coronal field away from a lowest-energy potential state. In order to better understand the origin of these currents and their role in M- and X-class flares, I review all active-region observations made with SDO/HMI and SDO/AIA from 2010/05 through 2014/10 within approximately 40 degrees from disk center. I select the roughly 4% of all regions that display a distinctly nonpotential coronal configuration in loops with a length comparable to the scale of the active region, and all that emit GOES X-class flares. The data for 41 regions confirm, with a single exception, that strong-field, high-gradient polarity inversion lines (SHILs) created during emergence of magnetic flux into, and related displacement within, pre-existing active regions are associated with X-class flares. Obvious nonpotentiality in the active region-scale loops occurs in 6 of 10 selected regions with X-class flares, all with relatively long SHILs along their primary polarity inversion line, or with a long internal filament there. Nonpotentiality can exist in active regions well past the flux-emergence phase, often with reduced or absent flaring. I conclude that the dynamics of the flux involved in the compact SHILs is of preeminent importance for the large-flare potential of active regions within the next day, but that their associated currents may not reveal themselves in active region-scale nonpotentiality. In contrast, active region-scale nonpotentiality, which can persist for many days, may inform us about the eruption potential other than those from SHILs which is almost never associated with X-class flaring. 
C.J. Schrijver
astro-ph.SR