ON THE BRIGHTENING PROPAGATION OF POST-FLARE LOOPS OBSERVED BYTRACE

Abstract

Examining flare data observed by Transition Region and Coronal Explorer satellite from 1998 May to 2006 December, we choose 190 (151 M-class and 39 X-class) flare events which display post-flare loops (PFLs), observed by 171 A and 195 A wavelengths. One-hundred twenty-four of the 190 events exhibit flare ribbons (FRs), observed by 1600 A images. We investigate the propagation of the brightening of these PFLs along the neutral lines and the separation of the FRs perpendicular to the neutral lines. Observations indicate that the footpoints of the initial brightening PFLs are always associated with the change of the photospheric magnetic fields. In most of the cases, the length of the FRs ranges from 20 Mm to 170 Mm. The propagating duration of the brightening is from 10 to 60 minutes, and from 10 to 70 minutes for the separating duration of the FRs. The velocities of the propagation and the separation range from 3 to 39 km s–1 and 3 to 15 km s–1, respectively. Both the propagating velocities and the separating velocities are associated with the flare strength and the length of the FRs. It appears that the propagation and the separation are dynamically coupled; the greater the propagating velocity, the faster the separation. Furthermore, a greater propagating velocity corresponds to a greater deceleration (or acceleration). These PFLs display three types of propagating patterns. In type I propagation, which includes about half of all the events, the brightening begins at the middle part of a set of PFLs, and propagates bidirectionally toward both of its ends. For type II, including 30% of all events, the brightening first appears at one end of a set of PFLs, then propagates to the other end. The remaining events belong to type III propagation, which display the initial brightening taking place at two (or more than two) positions on two (or more than two) sets of PFLs, then propagating bidirectionally along the neutral line. These three types of propagating patterns can be explained by a three-dimensional magnetic reconnection model.