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 magnetic gradient along the flaring neutral lines. Obvious changes of magnetic gradient occurred immediately and rapidly following the onset of each flare. Rapid gradient increase was found to be associated with three events, while decrease was found with the other two. The changes were permanent, therefore, not likely due to the effect of flare emissions. In addition, we evaluated the mean relative motions between two magnetic polarities in these δ regions, in the direction parallel and perpendicular to the flaring neutral lines, respectively. We derived the mean positions of two magnetic polarities using the method of Center-of-Mass (CoM) calculation, and found that: (1) Converging motions correspond to the gradient increase; diverging motion, the 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 1,200 km in the direction parallel to the neutral lines. Combining the findings presented here with those in previous papers, we propose that these are evidences of magnetic reconnection at or close to the photosphere. When an active region is away from the solar disk center, the reconnected transverse fields would cause an apparent increase of the flux in the polarity toward the limb, 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 literatures or in this paper. Finally, we offer some predictions for future observations when high-quality vector magnetogram sequences become available