Flares and the Magnetic Nonpotentiality

For a flare-productive region, AR 6233, a complete time sequence of vector magnetograms in the photosphere was obtained at the Huairou Solar Observing Station of Beijing Astronomical Observatory from 1990 August 27 to September 1. Images of angular shear, source field, and vertical current were deduced from the data on August 30 to quantify the nonpotentiality of the magnetic field, when the region was closest to the disk center. The relationship between flare occurrence and nonpotentiality development was examined in detail.

The magnetic shear in this region has a regularly clockwise sense when looking downward against the photosphere. Two sets of strong shear zones (or bundles of strong source field) are identified. All four flares studied straddle either of these two sets of strong shear zones. However, flare ribbons generally deviate from areas of strong magnetic shear. Shear evolution in association to an M1 flare has been imaged for the first time. It is characterized by the great enhancement from approximately 3 hours before the flare to the earliest brightening at Hβ wavelength, and the moderate relaxation to the late phase of the flare.

Vertical currents in this region generally emerge from areas of negative line-of-sight magnetic field and return to areas of positive field. Most flare ribbons appear on the edges of the current concentrations above the 3 σ level. Great enhancements of vertical currents preceding the M1 flare are identified. Combining the information comprised of vertical currents and source fields, two major systems of atmospheric currents are speculated upon. One straddling σ large fraction of the region seems to play a vigorous role in flare activity.