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Jingxiu Wang   Submitted: 2008-09-09 21:18

Rapid magnetic changes in the course of the X7.1 solar flare on 2005 January 20 at the photosphere in the host active region (AR), NOAA AR 10720, are diagnosed. The data base for this study consists of Huairou vector magnetograms, TRACE white light and UV/EUV images, RHESSI hard X-ray and SOHO EUV observations. For such an event close to but not on the solar west limb (N12 W58), the projection effects in the observed vector magnetograms are untangled by combining an intuitive geometric analysis and a transformation of the magnetograms into the heliographic coordination system. The magnetic changes in the horizontal magnetic fields are emphasized. We find definitive evidence of weakening in the horizontal magnetic fields in a few isolated patches in the outskirts of this delta-sunspot group and strengthening in the horizontal fields in an extended area centralized at the magnetic neutral line between major sunspots of opposite polarities. The rapid magnetic changes take place at the level of 100-300 G, several ten times of the noise level. The identified horizontal field changes are consistent with the darkening of inner penumbrae and weakening of outer penumbrae in this delta-sunspot group. The enhanced horizontal fields are spatially coincided with the TRACE 1,550 Å rope-like structures lying low above the magnetic neutral line. Unexpectedly, during the flare the lower-lying rope-like structures remain in place, though exhibited episodic disturbance and brightening, while the outer EUV loops are impulsively expanding. The rapid magnetic changes manifest an impulsive input of free magnetic energy in the photosphere, resulted from an impulsive growth of a new emerging flux region (EFR) along the magnetic neutral line. The fact of the increasing core fields in magnetic non-potentiality, the continued disturbing of the inner rope-like structures, and the breakout of the outer loops during the major flare can not be interpreted by any single flare model. However, a nature of magneto-hydrodynamical catastrophe is clearly implied for the flare triggering.

Authors: Jingxiu Wang, Meng Zhao, and Guiping Zhou
Projects: None

Publication Status: accepted by ApJ
Last Modified: 2008-09-10 09:24
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