The Contraction and Shearing of Solar Flare Loops

In this paper we analyze two flares, which are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They both have classical two-ribbon features which were observed in TRACE UV (160.0nm) or EUV (17.1nm) images and have long-duration conjugate footpoints with hard X-ray (HXR) emission. Using the "center-of-mass" method, we find the locations of the centroids of the flare ribbons and HXR kernels. The results are as follows. (1) The conjugate ribbons in UV/EUV or kernels in HXR of the two flares show converging motions during the impulsive phase. For the two flares, the converging motions last about 3 and 10 minutes, respectively. (2) The conjugate ribbons and footpoints of the two events exhibit strong shearing motions. The shear angles decrease during the flares. These results show that the magnetic reconnections for the flares occur in highly sheared magnetic fields. Furthermore, the results support the magnetic-field model constructed by Ji (2007), who proposed that the contraction of a flaring loop is the signature of the relaxation of sheared magnetic fields.