Magnetic Flux Changes and Cancellation Associated with X-Class and M-Class Flares
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Magnetic Flux Changes and Cancellation Associated with X-Class and M-Class Flares Olga Burtseva · Gordon Petrie
Received: 9 July 2012 / Accepted: 21 January 2013 / Published online: 8 February 2013 © Springer Science+Business Media Dordrecht 2013
Abstract We perform a statistical study of permanent changes in longitudinal fields associated with solar flares by tracking magnetic features. The YAFTA feature tracking algorithm is applied to GONG++ 1-minute magnetograms for 77 X-class and M-class flares to analyze the evolution and interaction of the magnetic features and to estimate the amount of canceled magnetic flux. We find that significantly more magnetic flux decreases than increases occurred during the flares, consistent with a model of collapsing loop structure for flares. Correlations between both total (unsigned) and net (signed) flux changes and the GOES peak X-ray flux are dominated by X-class flares at limb locations. The flux changes were accompanied in most cases by significant cancellation, most of which occurred during the flares. We find that the field strength and complexity near the polarity inversion line are approximately equally important in the flux cancellation processes that accompany the flares. We do not find a correlation between the flux cancellation events and the stepwise changes in the magnetic flux in the region. Keywords Flares, relation to magnetic field · Magnetic fields, photosphere
1. Introduction It is now known that photospheric magnetic fields change significantly, abruptly, and permanently as a result of major X-class and M-class flares (see, e.g., Cameron and Sammis, 1999; Kosovichev and Zharkova, 2001; Wang et al., 2002, 2012; Sudol and Harvey, 2005; Wang and Liu, 2010; Petrie and Sudol, 2010; Gosain, 2012; Sun et al., 2012; Petrie, 2012b). Detection and study of photospheric magnetic field changes during solar flares can aid in a better understanding of the mechanisms of energy buildup and release in flares. Despite these observations, most models of flares assume that these events are coronal and that no photospheric magnetic field changes occur over the flare duration (Priest and Forbes, 2002). Ideally, coronal field models should be based on boundary conditions observed in the lower O. Burtseva () · G. Petrie National Solar Observatory, 950 North Cherry Ave., Tucson, AZ 85719, USA e-mail: [email protected]
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atmosphere. However, it is not clear how to relate the restructuring of an approximately force-free or current-free coronal field with the changes in the forced photosphere. A few possible mechanisms were offered in past works that could cause the sudden and permanent photospheric field changes observed. The most frequently mentioned processes are the emergence of new magnetic flux and changes in the direction of the field (Spirock, Yurchyshyn, and Wang, 2002; Sudol and Harvey, 2005). Supporting magnetic flux emergence as a possible mechanism, Wang and Tang (1993) observed emergence of new flux in sunspot umbra immediately after an X-c
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