Analysis of a delta spot

Abstract

δ -groups generally develop in three different ways: eruption of a single complex active region, eruption of large satellite spots near a large older spot, or collision of spots of opposite polarity from different dipoles. In this paper, we present a rare observational result in which a δ -spot forms from rapid coalescence of two opposite magnetic features in a pre-existing δ -configuration. The white-light (WL) observations of this δ -spot in active region NOAA 9077 were performed by TRACE. The time-lapse movie of the co-aligned WL images shows that the whole active region was undergoing rapid disintegration and reconstruction on 11–17 July 2000. The preceding (p ) and following (f ) components of the δ -spot continue to grow in size, while the active region is in the decaying phase. Their proper motions are determined as a function of time using SOHO/MDI full-disk magnetograms. A major flare (3B/X5.7) erupted in the active region on 14 July. Initially, a relative shearing motion is visible between the p and f spots on this day. About two hours before the major flare, the shearing motion suddenly stops and the velocities change significantly. The ribbons of the flare are located just outside of the δ -configuration. An interesting finding is the sign reversal of the helicity of the f spot also just 2 hours before the flare. The δ -spot obviously separates after the flare. Our results clearly demonstrate that helicity reversal in magnetic features of a δ -configuration is likely to destabilize the compact structure, as well as to re-organize the magnetic field configuration, and, hence, is important for the rapid disintegration of a δ -spot during major flares. A model is presented to explain why a spot can change its chirality.