Statistics of flares sweeping across sunspots

Abstract

Flare ribbons are always dynamic and sometimes sweep across sunspots. After examining 588 (513 M-class and 75 X-class) flare events observed by the TRACE satellite and the Hinode/Solar Optical Telescope from 1998 May to 2009 May, we choose the event displaying one of the flare ribbons that completely sweeps across the umbra of a main sunspot of the corresponding active region, and finally obtain 20 (7 X-class and 13 M-class) events as our sample. In each event, we define the main sunspot completely swept across by the flare ribbon as the A-sunspot and its nearby opposite polarity sunspot as the B-sunspot. Observations show that the A-sunspot is a following polarity sunspot in 18 events and displays flux emergence in 13 cases. All of the B-sunspots are relatively simple, exhibiting either one main sunspot or one main sunspot and several small neighboring sunspots (pores). In two days prior to the flare occurrence, the A-sunspot rotates in all the cases, while the B-sunspot rotates in 19 events. The total rotating angle of the A-sunspot and B-sunspot rotates is 193° on average, and the rotating directions are the same in 12 events. In all cases; the A-sunspot and B-sunspot manifest shear motions with an average shearing angle of 285, and in 14 cases, the shearing direction is opposite to the rotating direction of the A-sunspot. We suggest that the emergence, the rotation, and the shear motions of the A-sunspot and B-sunspot result in the phenomenon that flare ribbons sweep across sunspots completely. © 2009 The American Astronomical Society. All rights reserved.