Abstract
We present an investigation on the UV footpoint motions of an M9.1 two-ribbon solar flare that occurred on 2004 July 22, employing the TRACE 1700 A images and a magnetogram observed by SOHO/MDI. In the beginning, we found that the UV footpoints spread a two-dimensional distribution in, especially, the negative magnetic region, rather than a quasi-one-dimensional distribution as expected. Based on an assumption, we extract the information of the footpoints and link them into trajectories. From the results, we find some unexpected motion patterns and features, such as bidirectional elongations of footpoints, large angles between the footpoint trajectories, a merging pattern of the motion, and an apparent prevalence of motions along the iso-Gauss contours over those along the gradient of the magnetic field. We suggest that the quasi-three-dimensional CSHKP model with a uniform arcade may be inapplicable to the flare, in spite of the two-ribbon appearance. Instead, the patchy concept presented by Klimchuk in 1997 may be appropriate. Only a fraction of the magnetic field participates in the energetic reconnection. The elongations may offer crucial restrictions on the excitation of the reconnections. The large angles and the merging process may imply a chaotic geometry of the reconnection subsystems, rather than a symmetrical one. The prevalence may offer an important clue in solar flare studies.
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