3D Coronal magnetic field from vector magnetograms: non-constant-αforce-free configuration of the active region NOAA 8151

Abstract

Abstract. The Active Region 8151 (AR 8151) observed in February 1998 is the site of an eruptive event associated with a filament and a S-shaped structure, and producing a slow Coronal Mass Ejection (CME). In order to determine how the CME occurs, we compute the 3D coronal magnetic field and we derive some relevant parameters such as the free magnetic energy and the relative magnetic helicity. The 3D magnetic configuration is reconstructed from photospheric magnetic magnetograms (IVM, Mees Solar Observatory) in the case of a non-constant-α force-free (nlff) field model. The reconstruction method is divided into three main steps: the analysis of vector magnetograms (transverse fields, vertical density of electric current, ambiguity of 180• ), the numerical scheme for the nlff magnetic field, the interpretation of the computed magnetic field with respect to the observations. For AR 8151, the nlff field matches the coronal observations from EIT/SOHO and from SXT/Yohkoh. In particular, three characteristic flux tubes are shown: a highly twisted flux tube, a long twisted flux tube and a quasi-potential flux tube. The maximum energy budget is estimated to 2.6 × 10 31 erg and the relative magnetic helicity to 4.7 × 10 34 G 2 cm 4 . From the simple photospheric magnetic distribution and the evidence of highly twisted flux tubes, we argue that the flux rope model is the most likely to describe the initiation mechanism of the eruptive event associated with AR 8151.

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).…”

Testing magnetofrictional extrapolation with the Titov-Démoulin model of solar active regions

“…The solution of the linear problem was given in closed form in Nakagawa & Raadu (1972), Chiu & Hilton (1977), and Seehafer (1978). Linear methods have several intrinsic limitations, the most severe one being that they cannot match most observed magnetograms closely because α is often found to vary strongly across active regions (e.g., Régnier et al 2002). The particular case of α = 0 yields a potential field, which is inadequate for many purposes, since it does not contain any free energy to power coronal activity.…”