Study of Three-Dimensional Magnetic Structure and the Successive Eruptive Nature of Active Region 12371

TL;DR

This study models the magnetic field evolution of active region 12371, revealing how flux cancellation and reconnection lead to recurrent eruptions, with the eruptions driven by torus instability at low coronal heights.

Contribution

It provides a detailed NLFFF modeling approach to understand the magnetic structures and eruption mechanisms in active region 12371, highlighting the role of flux cancellation and torus instability.

Findings

Modeled sigmoid structures match observations.

Flux rope formation via flux cancellation and reconnection.

Eruptivity linked to low-height torus instability.

Abstract

We study the magnetic structure of successively erupting sigmoid in active region 12371 by modeling the quasi-static coronal field evolution with non-linear force-free field (NLFFF) equilibria. HMI/SDO vector magnetograms are used as input to the NLFFF model. In all eruption events, the modeled structure resembles the observed pre-eruptive coronal sigmoid and the NLFFF core-field is a combination of double inverse J-shaped and inverse-S field-lines with dips touching the photosphere. Such field-lines are formed by flux-cancellation reconnection of opposite-J field-lines at bald-patch locations. It implies the formation of a weakly twisted flux-rope from large scale sheared arcade field lines. Later on, this flux-rope undergo coronal tether-cutting reconnection until a CME is triggered. The modeled structure captured these major features of sigmoid-to-arcade-to-sigmoid transformation, that is being recurrent under continuous photospheric flux motions. Calculations of the field-line twist reveal a fractional increase followed by a decrease of the number of pixels having a range of twist. This traces the buildup process of a twisted core-field by slow photospheric motions and the relaxation after eruption, respectively. Our study infers that the large eruptivity of this AR is due to a steep decrease of the background coronal field meeting the torus instability criteria at low height ($\approx 40$ Mm) in contrast to non-eruptive ARs.