Active-Region Twist Derived from Magnetic Tongues and Linear Force-Free Extrapolations

M. POISSON; M. LÓPEZ FUENTES; C. H. MANDRINI; P. DÉMOULIN

SOLAR PHYSICS

SPRINGER

Lugar: Berlin; Año: 2015 vol. 290 p. 3279 - 3294

0038-0938

The main aim of this study is to compare the amount of twist present inemerging active regions (ARs) from photospheric and coronal data. We uselinear force-free field models of the observed coronal structure of ARsto determine the global twist. The coronal twist is derived, on onehand, from the force-free parameter [α] of the model and, on theother, from the computed coronal magnetic helicity normalized by themagnetic flux squared. We compare our results, for the same set of ARs,with those of Poisson et al. ( Solar Phys. 290, 727, 2015), in which thetwist was estimated using the so-called magnetic tongues observed inline-of-sight magnetograms during AR emergence. We corroborate theagreement between the photospheric and coronal twist-sign and thepresence of magnetic tongues as an early proxy of the ARnon-potentiality. We find a globally linear relationship between thecoronal twist and the one previously deduced for the emerging AR fluxrope at the photospheric level. The coronal-twist value is typicallylower by a factor of six than the one deduced for the emerging fluxrope. We interpret this result as due to the partial emergence of theflux rope that forms the region.