A COMPARATIVE STUDY OF CONFINED AND ERUPTIVE FLARES IN NOAA AR 10720

Abstract

We investigate the distinct properties of two types of flares: eruptive\nflares associated with CMEs and confined flares without CMEs. Our sample of\nstudy includes nine M and X-class flares, all from the same active region (AR),\nsix of which are confined and three others are eruptive. The confined flares\ntend to be more impulsive in the soft X-ray time profiles and show more slender\nshapes in the EIT 195 A images, while the eruptive ones are of long-duration\nevents and show much more extended brightening regions. The location of the\nconfined flares are closer to the center of the AR, while the eruptive flares\nare at the outskirts. This difference is quantified by the displacement\nparameter, the distance between the AR center and the flare location: the\naverage displacement of the six confined flares is 16 Mm, while that of\neruptive ones is as large as 39 Mm. Further, through nonlinear force-free field\nextrapolation, we find that the decay index of the transverse magnetic field in\nthe low corona (~10 Mm) have a larger value for eruptive flares than that for\nconfined one. In addition, the strength of the transverse magnetic field over\nthe eruptive flare sites is weaker than that over the confined ones. These\nresults demonstrate that the strength and the decay index of background\nmagnetic field may determine whether or not a flare be eruptive or confined.\nThe implication of these results on CME models is discussed in the context of\ntorus instability of flux rope.\n