EVOLUTION OF VECTOR MAGNETIC-FIELD AND WHITE-LIGHT FLARES IN A SOLAR ACTIVE-REGION (NOAA-6659) IN 1991 JUNE

The active region of 1991 June, NOAA 6659, was the most prolific white -light flare-producing region in recent history. About five white-ligh t flares were observed in the active region. We present here the obser vational results on photospheric vector magnetic and velocity fields o f this region obtained at the Huairou Solar Observing Station of the B eijing Astronomical Observatory and discuss the evolution of the magne tic fields and its relation to the powerful (white-light) flares. The salient characteristic of this active region is that it was a great de lta-group, dominated by spots of preceding (p) polarity. The structure s of following (f) polarity were almost around them, forming some magn etic gulfs and islands. Sites of homologous white-light flares on June 4, 6, and 9 occurred at both sides of the magnetic neutral line and a lmost at the same place in the active region, where the gradient of th e longitudinal field was about 0.3-0.5 G km-1 and the horizontal compo nents of the field, relative to the solar surface, were almost paralle l to the neutral line due to the shear of the magnetic field. After an alysis of a series of monochromatic images of Stokes parameters Q, U, and V with high spatial resolution, the following newly observed pheno mena were found: (1) In this delta-region, some of the magnetic flux s howed inverted polarity, and the longitudinal components of the emergi ng flux were aligned in fibril-like features whose small angles were r elative to the horizontal components of the field on the solar surface , where the lines of force were highly sheared. (2) Before and after p owerful (white-light) flares, the obvious change of vector magnetic fi elds occurred at the magnetic neutral line near the sites of these fla res. The white-light flares probably are caused by the violent motion of the photospheric lines of force.