FLARES AND THE MAGNETIC NONPOTENTIALITY

For a flare-productive region, AR 6233, a complete time sequence of ve ctor magnetograms in the photosphere was obtained at the Huairou Solar Observing Station of Beijing Astronomical Observatory from 1990 Augus t 27 to September 1. Images of angular shear, source field, and vertic al current were deduced from the data on August 30 to quantify the non potentiality of the magnetic field, when the region was closest to the disk center. The relationship between flare occurrence and nonpotenti ality development was examined in detail. The magnetic shear in this r egion has a regularly clockwise sense when looking downward against th e photosphere. Two sets of strong shear zones (or bundles of strong so urce field) are identified. All four flares studied straddle either of these two sets of strong shear zones. However, flare ribbons generall y deviate from areas of strong magnetic shear. Shear evolution in asso ciation to an M1 flare has been imaged for the first time. It is chara cterized by the great enhancement from approximately 3 hours before th e flare to the earliest brightening at H beta wavelength, and the mode rate relaxation to the late phase of the flare. Vertical currents in t his region generally emerge from areas of negative line-of-sight magne tic field and return to areas of positive field. Most flare ribbons ap pear on the edges of the current concentrations above the 3 sigma leve l. Great enhancements of vertical currents preceding the M1 flare are identified. Combining the information comprised of vertical currents a nd source fields, two major systems of atmospheric currents are specul ated upon. One straddling a large fraction of the region seems to play a vigorous role in flare activity.