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Hui Li   Submitted: 2007-09-07 20:49

The aim of this paper is to understand the magnetic configuration and evolution of an active region, which permits to observe an X1.7 flare during the decaying phase of a long duration X1.5 flare on 13 September 2005. We performed a multi-wavelength analysis using data from space-borne (Solar and Heliospheric Observatory (SOHO), Transition Region and Coronal Explorer (TRACE), Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), GOES) and ground-based (the French-Italian THEMIS telescope and the Huairou Video Vector Magnetograph (HVVM)) instruments. We coaligned all the data in order to study the origin of the flare by comparing the observed magnetic field structures with the emissions detected by different instruments. Reconstructed RHESSI images show three hard X-ray (HXR) sources. In TRACE 195 A images, two loops are seen: a short bright loop and a longer one. Five ribbons are identified in Hα images, two of them being remnant ribbons of the previous flare. We propose the following scenario to explain the X1.7 flare. A reconnection occurs between the short loop system and the longer loops (TRACE 195 A). Two X-ray sources could be the footpoints of the short loop, while the third one between the two others is the site of the reconnection. The Hα ribbons are the footprints in the chromosphere of the reconnected loops. During the reconnection, the released energy is principally non thermal according to the RHESSI energy spectrum analysis (two order of magnitude larger than the maximum thermal energy). The proposed scenario is confirmed by a non linear force-free field (NLFF) extrapolation, which shows the presence of short sheared magnetic field lines before the eruption and less sheared after the econnection, and the connectivity of the field lines involved in the flaring activity is modified after the reconnection process. The evolution of the photospheric magnetic field over few days shows the existence of a continuous emergence of a large-scale magnetic flux tube, the tongue-shape of the two main polarities of the active region being the signature of such an emergence. After the previous X1.5 flare, the emergence of the tube continues and favors new magnetic energy storage and the onset of the X1.7 flare.

Authors: H. Li, B. Schmieder, M.T. Song, and V. Bommier
Projects: None

Publication Status: A&A, accepted.
Last Modified: 2007-09-10 09:05
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