Formation of a Magnetic Flux Rope in the Early Emergence Phase of NOAA Active Region 12673

Abstract

Abstract In this work, we investigate the formation of a magnetic flux rope (MFR) above the central polarity inversion line (PIL) of NOAA Active Region 12673 during its early emergence phase. Through analyzing the photospheric vector magnetic field, extreme ultraviolet (EUV) and ultraviolet (UV) images, extrapolated three-dimensional (3D) nonlinear force-free fields (NLFFFs), and the photospheric motions, we find that with the successive emergence of different bipoles in the central region, the conjugate polarities separate, resulting in collision between the nonconjugated opposite polarities. Nearly potential loops appear above the PIL at first, then get sheared and merge at the collision locations as evidenced by the appearance of a continuous EUV sigmoid on 2017 September 4, which also indicates the formation of an MFR. The 3D NLFFFs further reveal the gradual buildup of the MFR, accompanied by the appearance of two elongated bald patches (BPs) at the collision locations and a very-low-lying hyperbolic flux tube configuration between the BPs. Finally, the MFR has relatively steady axial flux and average twist number of around 2.1 × 1020 Mx and −1.5, respective. Shearing motions are found developing near the BPs when the collision occurs, with flux cancellation and UV brightenings being observed simultaneously, indicating the development of a process named collisional shearing (first identified by Chintzoglou et al.). The results clearly show that the MFR is formed by collisional shearing, i.e., through shearing and flux cancellation driven by the collision between nonconjugated opposite polarities during their emergence.