THE MAGNETIC EVOLUTION OF AR-6555 WHICH LED TO 2 IMPULSIVE, RELATIVELY COMPACT, X-TYPE FLARES

We study the evolution of the vector magnetic field and the sunspot mo tions observed in AR 6555 during 1991 March 23-26. This region display s two locations of large magnetic shear that were also sites of flare activity. The first location produced two large (X-class) flares durin g the period covered by our observations. The second location had larg er magnetic shear than the first but produced only small (M- and C-cla ss) flares during our observations. We study the evolution of the phot ospheric magnetic field in relation to the large flares in the first l ocation. These flares occurred around the same included polarity and h ave very similar characteristics (soft X-ray light curves, energies, e tc.). However, the whole active region has changed substantially in th e period between them. We found several characteristics of the region that appear related to the occurrence of these hares: (1) The flares o ccurred near regions of large magnetic ''shear,'' but not at the locat ions of maximum shear or maximum field. (2) Potential field extrapolat ions of the observed field suggest that the topology changed, prior to the first of the two flares, in such a way that a null appeared in th e coarse magnetic field. (3) This null was located close to both X-cla ss flares and remained in that location for a few days while the two f lares were observed. (4) The flaring region has a pattern of vector fi eld and sunspot motions in which material is ''squeezed'' along the po larity inversion line. This pattern is very different from that usuall y associated with shearing arcades, but it is similar to that suggeste d previously by Fontenla and Davis. The vertical electric currents, in ferred from the transverse field, are consistent with this pattern. (5 ) A major reconfiguration of the longitudinal field and the vertical e lectric currents occurred just prior to the first of the two flares. B oth changes imply substantial variations of the magnetic structure of the region. On the basis of the available data we suggest that these c hanges made the flaring possible, and we develop a scenario that can e xplain the origin of the magnetic free-energy that was released in the se flares.