A Study of Connections Between Solar Flares and Subsurface Flow Fields of Active Regions
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A Study of Connections Between Solar Flares and Subsurface Flow Fields of Active Regions Yu Gao · Junwei Zhao · Hongqi Zhang
Received: 16 November 2012 / Accepted: 6 March 2013 © Springer Science+Business Media Dordrecht 2013
Abstract We investigate the connections between the occurrence of major solar flares and subsurface dynamic properties of active regions. For this analysis, we select five active regions that produced a total of 11 flares with peak X-ray flux intensity higher than M5.0. The subsurface velocity fields are obtained from time–distance helioseismology analysis using SDO/HMI (Solar Dynamics Observatory/Helioseismic and Magnetic Imager) Doppler observations, and the X-ray flux intensity is taken from GOES (Geostationary Operational Environmental Satellites). It is found that among the eight amplitude bumps in the evolutionary curves of subsurface kinetic helicity, five (62.5%) of them had a flare stronger than M5.0 occurring within 8 hours, either before or after the bumps. Another subsurface parameter is the Normalized Helicity Gradient Variance (NHGV), reflecting kinetic helicity spread in different depth layers; it also shows bumps near the occurrence of these solar flares. Although there is no one-to-one correspondence between the flare and the subsurface properties, these observational phenomena are worth further studies to better understand the flares’ subsurface roots, and to investigate whether the subsurface properties can be used for major flare forecasts. Keywords Sun: photosphere · Sun: helicity · Sun: flare
Solar Origins of Space Weather and Space Climate Guest Editors: I. González Hernández, R. Komm, and A. Pevtsov Y. Gao () · H. Zhang National Astronomical Observatories of China, Chinese Academy of Sciences, Beijing, P.R. China e-mail: [email protected] H. Zhang e-mail: [email protected] J. Zhao W.W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085, USA e-mail: [email protected]
Y. Gao et al.
1. Introduction Helical features of observed solar magnetic fields have been widely investigated in the previous two decades. Helicity is a physical quantity closely related to the mechanism of solar dynamo, and in this regard, hemispheric preponderance in magnetic (or current) helicity distribution was found and confirmed by different observational groups (Seehafer, 1990; Pevtsov, Canfield, and Metcalf, 1995; Bao and Zhang, 1998; Hagino and Sakurai, 2004; Zhang et al., 2010). On the other hand, the helicity of the magnetic field is supposedly connected with the storage and transfer of energy inside active regions. Pevtsov, Canfield, and Metcalf (1995) and Bao, Ai, and Zhang (2001) found that active regions that did not follow the hemispheric sign rule of helicity tend to be more active than the others in the flare productivity, but this characteristic was not supported by the case study performed by Sakurai and Hagino (2003). It was found that significant helicity accumulation occurred preceding some major flares (Park et al., 2008). During flares, acti
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