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Flare-induced Signals in Polarization Measurements during the X2.6 Flare on January 15, 2005  

Meng Zhao   Submitted: 2009-01-19 20:07

Flare-induced signals in polarization measurements manifested as apparent polarity reversal in magnetograms has been reported since 1981. We are motivated to further quantify the phenomenon by two questions: can we distinguish the flare-induced signals from real magnetic changes during flares and what we can learn about flare energy release from the flare-induced signals? We select the X2.6 flare that occurred on 2005 January 15 for further study. The flare took place in NOAA active region (AR) 10720 at approximately central meridian, which makes the interpretation of vector magnetograms to be less ambiguous. We have identified that flare-induced signals during this flare appeared in six zones. The zones are located with an average distance of 5 Mm from their weight center to the main magnetic neutral line, have an average size of (0.6pm0.4) imes 1017cm^2, duration of 13pm4 minutes, and flux density change of 181pm125 G in the area of reversed polarity. The following new facts have been revealed by this study: (1) the flare-induced signal is also seen in the transverse magnetograms but with smaller magnitude, e.g., about 50G; (2) the flare-induced signal mainly manifests itself as apparent polarity reversal, but the signal starts and ends as weakening of flux density; (3) The flare-induced signals appear in phase with the peaks of hard X-ray emission as observed by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and trace in position mostly with the RHESSI hard X-ray footpoint sources. (4) in four zones it takes place co-temporally with real magnetic changes which persist after the flare. Only for the other two zones, does the flux density recover to pre-flare level immediately after the flare. The physical implications of the flare-induced signal are discussed in the view of its relevance to the non-thermal electron precipitation and primary energy release in the flare.

Authors: Meng Zhao, Jingxiu Wang, Sarah Matthews, Mingde Ding, Hui Zhao and Chunlan Jin

Publication Status: RAA in press
Last Modified: 2009-01-20 08:22
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Magnetic Changes in the course of the X7.1 Solar Flare on 2005 January 20  

Meng Zhao   Submitted: 2008-11-25 20:52

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. 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 (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 delta-sunspot group. The enhanced HFs are spatially coincided with the TRACE 1550 angstrom 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 fact 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. Howeve, the nature of magneto-hydrodynamical catastrophe is clearly implied for the flare triggering.

Authors: Jingxiu Wang, Meng Zhao, and Guiping Zhou

Publication Status: ApJ in press
Last Modified: 2008-11-26 10:23
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Flare-induced Signals in Polarization Measurements during the X2.6 Flare on January 15, 2005
Magnetic Changes in the course of the X7.1 Solar Flare on 2005 January 20

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