MAGNETIC CHANGES IN THE COURSE OF THE X7.1 SOLAR FLARE ON 2005 JANUARY 20

Abstract

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 database for this study consists of Huairou vector magnetograms, Transition Region and Coronal Explorer (TRACE) white light and UV/EUV images, RHESSI hard X-ray, and Solar and Heliospheric Observatory EUV observations. For such an event that is 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 ?-sunspot group and strengthening in the horizontal fields (HFs) 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 factors of ten of the noise level. The identified HF changes are consistent with the darkening of inner penumbrae and weakening of outer penumbrae in this ?-sunspot group. The enhanced HFs spatially coincide with the TRACE 1550 ? rope-like structures lying low above the magnetic neutral line. Unexpectedly, during the flare, the lower lying rope-like structures remain in place, though they exhibit 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, resulting from an impulsive growth of a new emerging flux region (EFR) along the magnetic neutral line. The facts of the increasing core fields in magnetic nonpotentiality, the continued disturbance of the inner rope-like structures, and the breakout of the outer loops during the major flare cannot be interpreted by any single flare model. However, the nature of magnetohydrodynamical catastrophe is clearly implied for the flare triggering.