EVOLUTIONARY AND FLARE-ASSOCIATED MAGNETIC SHEAR VARIATIONS OBSERVED IN A COMPLEX, FLARE-PRODUCTIVE ACTIVE-REGION

Evolution of nonpotential structures in a complex, flare-productive active region NOAA AR 6555 has been studied during the period March 23-26, 1991, using a quantitative description of the degree of magnetic shear at both local and regional scares. Distinct shear evolution in four subareas of AR 6555 is inferred from daily variation of their shear indices. Subareas which showed significant evolution in their overall magnetic structure are found to be most active. Ha Bare ribbons mostly formed bordering, and not within, areas of large shear and expanded over areas of lower shear. Magnetic shear changes are also detected during several flares, including a major X4.7/4B hare. Out of the 14 events for which temporally contiguous magnetograms were available, a decrease, or dip, in area-averaged shear index around flare onset time followed by an increase was observed in eight cases. There were two events with increase, three events with continuous decrease, and one event with no change in the shear index during the flare.