Interaction of magnetic field systems leading to an X1.7 flare due to large-scale flux tube emergence

¹ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China e-mail: lihui@mail.pmo.ac.cn
² LESIA, Observatoire de Paris, Section de Meudon, 92195, Meudon Principal Cedex, France
³ LERMA, Observatoire de Paris, Section de Meudon, 92195, Meudon Principal Cedex, France

Received: 18 March 2007
Accepted: 3 September 2007

Abstract

Aims.The aim of this paper is to understand the magnetic configuration and evolution of an active region, which permitted an X1.7 flare to be observed during the decaying phase of a long-duration X1.5 flare on 2005 September 13.

Methods.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.

Results.Reconstructed RHESSI images show three hard X-ray (HXR) sources. In TRACE 195 Å images, two loops are seen: a short bright loop and a longer one. Five ribbons are identified in Hα images, with two of them 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 Å). 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 nonthermal according to the RHESSI energy spectrum analysis (two orders of magnitude higher than the maximum thermal energy). The proposed scenario is confirmed by a nonlinear force-free field (NLFFF) extrapolation, which shows the presence of short sheared magnetic field lines before the eruption and less sheared ones after the reconnection, 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 a few days shows the 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.