SPIRAL FILAMENTS - THE SIGNATURE OF SINGULAR POINTS IN A LARGE DELTA-SPOT

A large delta spot (active region NOAA 6891, October 23-November 4, 19 91) is analysed, and it is found that some spiral filaments across the spot can be regarded as signatures of a singular point entity (SPE) w hich lies near a separator of this complicated magnetic field region. Near such an entity, 86% of the flares in the region were produced, in cluding two white-light flares, one of which, being the largest flare of the region, was accompanied by a powerful mass ejection. In an isla nd delta spot, a SPE could be recognized very close to the usual U-sha ped inversion lines. Together with the other characteristics (bright H alpha emission, highly sheared magnetic field, umbra obscured by H al pha, magnetic flux imbalance in the range 2:1-3:1), the SPE can help u s to predict effectively the sites for great flares to occur (Zirin an d Liggett, 1987).