ar X iv : a st ro - p h / 99 10 07 7 v 1 5 O ct 1 99 9 EVOLUTION OF MAGNETIC NONPOTENTIALITY ASSOCIATED WITH AN X - CLASS FLARE IN AR 6919

It is generally believed that the energy released in solar flares is stored in nonpotential magnetic fields. The energy buildup could be a response of the coronal magnetic field to the changes in photospheric magnetic environment caused mostly by sunspot motions and emerging fluxes. Since the measurement of magnetic fields in the corona is not available, the field measurement at the photospheric level has been widely used for the study of magnetic nonpotentiality in flare-producing active regions. In particular, the change of some nonpotentiality parameters during flare activity is regarded as a very important clue for understanding flare mechanisms. The MSFC (Marshal Space Flight Center) group studied the magnetic nonpotentiality associated with solar flares using the MSFC magnetograph (Hagyard et al., 1982; Hagyard et al., 1984). They examined the vertical current density, source field and magnetic shear derived from vector magnetograms of active regions in parallel with flare observations (Hagyard et al., 1984; Gary et al., 1987; Hagyard et al., 1990). On the basis of these studies, they suggested a few important points characterizing flare producing active regions. They are a large