TY - JOUR A1 - Valori, Gherardo A1 - Green, Lucie M. A1 - Demoulin, P. A1 - Vargas Dominguez, S. A1 - van Driel-Gesztelyi, L. A1 - Wallace, A. A1 - Baker, Daniel N. A1 - Fuhrmann, Marcel T1 - Nonlinear force-free extrapolation of emerging flux with a global twist and serpentine fine structures JF - Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics N2 - We study the flux emergence process in NOAA active region 11024, between 29 June and 7 July 2009, by means of multi-wavelength observations and nonlinear force-free extrapolation. The main aim is to extend previous investigations by combining, as much as possible, high spatial resolution observations to test our present understanding of small-scale (undulatory) flux emergence, whilst putting these small-scale events in the context of the global evolution of the active region. The combination of these techniques allows us to follow the whole process, from the first appearance of the bipolar axial field on the east limb, until the buoyancy instability could set in and raise the main body of the twisted flux tube through the photosphere, forming magnetic tongues and signatures of serpentine field, until the simplification of the magnetic structure into a main bipole by the time the active region reaches the west limb. At the crucial time of the main emergence phase high spatial resolution spectropolarimetric measurements of the photospheric field are employed to reconstruct the three-dimensional structure of the nonlinear force-free coronal field, which is then used to test the current understanding of flux emergence processes. In particular, knowledge of the coronal connectivity confirms the identity of the magnetic tongues as seen in their photospheric signatures, and it exemplifies how the twisted flux, which is emerging on small scales in the form of a sea-serpent, is subsequently rearranged by reconnection into the large-scale field of the active region. In this way, the multi-wavelength observations combined with a nonlinear force-free extrapolation provide a coherent picture of the emergence process of small-scale magnetic bipoles, which subsequently reconnect to form a large-scale structure in the corona. KW - Active regions, magnetic fields KW - Magnetic field, photosphere, corona Y1 - 2012 U6 - https://doi.org/10.1007/s11207-011-9865-8 SN - 0038-0938 VL - 278 IS - 1 SP - 73 EP - 97 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Green, Luci M. A1 - Kliem, Bernhard A1 - Wallace, A. J. T1 - Photospheric flux cancellation and associated flux rope formation and eruption JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. We study an evolving bipolar active region that exhibits flux cancellation at the internal polarity inversion line, the formation of a soft X-ray sigmoid along the inversion line and a coronal mass ejection. The aim is to investigate the quantity of flux cancellation that is involved in flux rope formation in the time period leading up to the eruption. Methods. The active region is studied using its extreme ultraviolet and soft X-ray emissions as it evolves from a sheared arcade to flux rope configuration. The evolution of the photospheric magnetic field is described and used to estimate how much flux is reconnected into the flux rope. Results. About one third of the active region flux cancels at the internal polarity inversion line in the 2.5 days leading up to the eruption. In this period, the coronal structure evolves from a weakly to a highly sheared arcade and then to a sigmoid that crosses the inversion line in the inverse direction. These properties suggest that a flux rope has formed prior to the eruption. The amount of cancellation implies that up to 60% of the active region flux could be in the body of the flux rope. We point out that only part of the cancellation contributes to the flux in the rope if the arcade is only weakly sheared, as in the first part of the evolution. This reduces the estimated flux in the rope to similar to 30% or less of the active region flux. We suggest that the remaining discrepancy between our estimate and the limiting value of similar to 10% of the active region flux, obtained previously by the flux rope insertion method, results from the incomplete coherence of the flux rope, due to nonuniform cancellation along the polarity inversion line. A hot linear feature is observed in the active region which rises as part of the eruption and then likely traces out the field lines close to the axis of the flux rope. The flux cancellation and changing magnetic connections at one end of this feature suggest that the flux rope reaches coherence by reconnection immediately before and early in the impulsive phase of the associated flare. The sigmoid is destroyed in the eruption but reforms quickly, with the amount of cancellation involved being much smaller than in the course of its original formation. KW - Sun: activity KW - Sun: coronal mass ejections (CMEs) KW - magnetic fields KW - magnetic reconnection KW - Sun: photosphere KW - Sun: magnetic topology Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201015146 SN - 0004-6361 VL - 526 IS - 2 PB - EDP Sciences CY - Les Ulis ER -