Evolution of Magnetic Field in the Flaring Active Region 11158 Observed by SDO/HMI

Astronomy and Astrophysics – Astronomy

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Scientific paper

We report the evolution of the magnetic field in NOAA AR11158 over 5 days (2011 Feb 12-16) using preliminary vector magnetograms from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). This region consisted of two pairs of major sunspots and displayed a complex quadrupolar field topology. It produced multiple major flares and eruptions, including the first X-class flare of the current solar cycle. Strong shear motion and flux emergence were both present, with apparent emergence preceding each major flare. We reconstruct the coronal field from a series of vector data using a non-linear force-free (NLFF) extrapolation. The estimated free magnetic energy shows a great increase during the early emergence of the current-carrying flux, while a significant, permanent decrease ( 0.5e32 erg, or 20%) is found after the X-class flare despite continuous flux injection. We relate this decrease to a previously reported, sudden change of the photospheric field after the flare. The extrapolated coronal field structure correspondingly becomes more "compact": the low-lying of field appears more sheared and stores more free energy, and higher-altitude field decays faster with height and becomes more potential. The coronal field overall becomes less-energetic.

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