Nonlinear Force-Free Extrapolation of Emerging Flux with a Global Twist and Serpentine Fine Structures

Abstract

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, while 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, the knowledge of the coronal connectivity confirms the identity of the magnetic tongues as seen in their photospheric signatures, and exemplifies how the twisted flux, that 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.

Evidence of Twisted Flux-Tube Emergence in Active Regions

“…At the beginning, only small portions of the original flux-rope emerge in the form of thin sea-serpent tubes (Strous et al, 1996;Pariat et al, 2004). Recently, Valori et al (2012) studied the emergence of an AR and found that the first manifestations of the emergence occur in the form of small bipoles, which are consistent with the sea-serpent scenario. The characteristic size of the bipoles can be associated with the instability wavelength (see also Bernasconi et al, 2002;Otsuji et al, 2011;Vargas Domínguez, van Driel-Gesztelyi, and Bellot Rubio, 2012).…”

Evolution of Active Regions

“…A study by Valori et al (2012) brought together non-linear force-free field modelling and local correlation tracking of the photospheric flow patterns with high resolution coronal imaging and spectroscopic observations of the emergence of AR 11024. The flow patterns in this AR revealed converging motions of opposite magnetic polarities in the U-loop sections followed by flux cancellation, indicative of magnetic reconnection.…”

The magnetic field in the solar atmosphere

“…So far, these models have not been applied to real solar cases, but have been tested by the use of semianalytic force-free magnetic field solutions. Only Valori et al (2012b) investigated the helicity content of an AR during flux emergence and found a relative helicity of ≈10 42 Mx 2 prior to background, i.e., B-level flaring activity. The hesitation to apply the developed methods certainly arises from the strong influence the choice of a particular gauge as well as its numerical implementation may have on the model outcome, especially when applied to real data.…”