Rapid changes of photospheric magnetic fields around flaring magnetic neutral lines

In this paper we study the short-term evolution of magnetic fields associated with five flares in δ-sunspots. We concentrate on the analysis of the magnetic gradient along the flaring neutral lines (NLs). Obvious changes of the magnetic gradient occurred immediately and rapidly following the onset of each flare. A rapid gradient increase was found to be associated with three events, while a decrease was associated with the other two. The changes were permanent, and therefore not likely due to the flare emissions. In addition, we evaluated the mean relative motions between the two magnetic polarities in these δ-regions, in the directions parallel and perpendicular to the flaring NLs. We derived the mean positions of the two magnetic polarities using a center-of-mass (CoM) calculation and found that (1) converging motions correspond to a gradient increase and diverging motions, to a decrease; (2) for all the events, there appeared a sudden release of magnetic shear associated with each flare, signified by a decrease of CoM separation between 500 and 1200 km in the direction parallel to the NLs. Combining the findings presented here with those in previous papers, we propose that these results are evidence of magnetic reconnection at or close to the photosphere. When an active region is away from the solar disk center, the reconnected transverse fields cause an apparent increase of the flux in the polarity toward the limb and a decrease for the polarity closer to the disk center. This observational pattern was indeed found for all 10 available events that have been studied in the literature and in this paper. Finally, we offer some predictions for future observations when high-quality vector magnetogram sequences become available.