Magnetic Evolution of an Active Region Producing Successive Flares and Confined Eruptions

Abstract

We analyze the magnetic evolution of solar active region (AR) NOAA 11476 that, betweenMay 9 and 10, 2012, produced a series of surge-type eruptions accompanied by GOES X-ray class-M flares. Using force-free models of the AR coronal structure and observationsat several wavelengths, in previous works we studied the detailed evolution of those erup-tions, relating them to the characteristic magnetic topology of the AR and reconstructingthe involved reconnection scheme. We found that the eruptions were due to the ejection ofminifilaments, which were recurrently ejected and reformed at the polarity-inversion lineof a bipole that emerged in the middle of the positive main AR magnetic polarity. Thebipole was observed to rotate for several tens of hours before the events. In this articlewe analyze, for the full AR and the rotating bipole, the evolution of a series of magneticparameters computed using the Helioseismic and Magnetic Imager (HMI) vector magne-tograms. We combine this analysis with estimations of the injection of magnetic energy andhelicity obtained using the Differential Affine Velocity Estimator for Vector Magnetograms(DAVE4VM) method that determines, from vector magnetograms, the affine velocity fieldconstrained by the induction equation. From our results, we conclude that the bipole rotationwas the main driver that provided the magnetic energy and helicity involved in the minifil-ament destabilizations and ejections. The results also suggest that the observed rotation isprobably due to the emergence of a kinked magnetic flux rope with negative writhe helicity.