Evolution of magnetic fields and energetics of flares in active region 8210

Abstract

To better understand eruptive events in the solar corona, we combine sequences of multi-wavelength observations and modelling of the coronal magnetic field of NOAA AR 8210, a highly flare-productive active region. From the photosphere to the corona, the observations give us information about the motion of magnetic elements (photospheric magnetograms), the location of flares (e.g., Hα, EUV or soft X-ray brightenings), and the type of events (Hα blueshift events). Assuming that the evolution of the coronal magnetic field above an active region can be described by successive equilibria, we follow in time the magnetic changes of the 3D nonlinear force-free (nlff) fields reconstructed from a time series of photospheric vector magnetograms. We apply this method to AR 8210 observed on May 1, 1998 between 17:00 UT and 21:40 UT. We identify two types of horizontal photospheric motions that can drive an eruption: a clockwise rotation of the sunspot, and a fast motion of an emerging polarity. The reconstructed nlff coronal fields give us a scenario of the confined flares observed in AR 8210: the slow sunspot rotation enables the occurence of flare by a reconnection process close to a separatrix surface whereas the fast motion is associated with small-scale reconnections but no detectable flaring activity. We also study the injection rates of magnetic energy, Poynting flux and relative magnetic helicity through the photosphere and into the corona. The injection of magnetic energy by transverse photospheric motions is found to be correlated with the storage of energy in the corona and then the release by flaring activity. The magnetic helicity derived from the magnetic field and the vector potential of the nlff configuration is computed in the coronal volume. The magnetic helicity evolution shows that AR 8210 is dominated by the mutual helicity between the closed and potential fields and not by the self helicity of the closed field which characterizes the twist of confined flux bundles. We conclude that for AR 8210 the complex topology is a more important factor than the twist in the eruption process.

Nonlinear force-free field extrapolation of the coronal magnetic field using the data obtained by the Hinode satellite

“…For the magnetograms that are not at the central area of the Sun's disk, the projection effect and the 180°ambiguity in the SP magnetic field data also need to be resolved more carefully and accurately [Georgoulis, 2005;Metcalf et al, 2006;Wiegelmann et al, 2008;Martin et al, 2008;Leka et al, 2009]. Besides the comparison between the extrapolated field lines and the coronal loop images, and the preliminary physical analyses used in this paper, topological techniques and more physical measures should be introduced to quantitatively analyze the topological and physical properties of the extrapolated fields [Longcope, 2005;Zhao et al, 2005;DeVore and Antiochos, 2000;Bleybel et al, 2002;Zhang and Low, 2005;Régnier and Canfield, 2006;Démoulin, 2007;Thalmann et al, 2008;Schrijver et al, 2008;DeRosa et al, 2009] and their relationships to the solar eruptive events.…”

“…The first physical measure is the free magnetic energy contained in the extrapolated fields [Bleybel et al, 2002;Régnier and Canfield, 2006;Régnier and Priest, 2007;Thalmann et al, 2008]. The free magnetic energy in a volume W is defined as…”

Physics of Solar Prominences: II—Magnetic Structure and Dynamics

“…This is important because the overlying coronal arcade plays a key role in the equilibrium and stability of the flux rope. For example, Régnier and Priest (2007) construct NLFFF models by "extrapolating" observed photospheric vector fields into the corona (also see Régnier et al, 2002;Régnier and Amari, 2004;Régnier and Canfield, 2006;Canou et al, 2009). They consider two active regions: one a decaying active region with strong currents (AR 8151), the other a newly emerged active region with weak currents (AR 8210).…”

The magnetic field in the solar atmosphere

“…Also, the flare class of the observed events was successively smaller (X-to C-class) for ARs with successively less free energy and relative helicity budgets. Early estimates of the helicity budget of flare and CME-productive ARs, based on force-free field models, range from ≈10 34 Mx 2 to ≈10 43 Mx 2 , with the general tendency that ARs with a higher helicity content are more flare productive (Régnier et al 2002;Régnier and Canfield 2006;Régnier and Priest 2007).…”