Roles of Photospheric Motions and Flux Emergence in the Major Solar Eruption on 2017 September 6

Abstract

We study the magnetic field evolution in the active region (AR) 12673 that produced the largest solar flare in the past decade on 2017 September 6. Fast flux emergence is one of the most prominent features of this AR. We calculate the magnetic helicity from photospheric tangential flows that shear and braid field lines (shear helicity) and from normal flows that advect twisted magnetic flux into the corona (emergence helicity), respectively. Our results show that the emergence helicity accumulated in the corona is −1.6 × 10 43 Mx 2 before the major eruption, while the shear helicity accumulated in the corona is −6 × 10 43 Mx 2 , which contributes about 79% of the total helicity. The shear-helicity flux is dominant throughout the overall investigated emergence phase. Our results imply that the emerged fields initially contain relatively low helicity. Much more helicity is built up by shearing and converging flows acting on preexisting and emerging flux. Shearing motions are getting stronger with the flux emergence, and especially on both sides of the polarity inversion line of the core field region. The evolution of the vertical currents shows that most of the intense currents do not appear initially with the emergence of the flux, which implies that most of the emerging flux is probably not strongly current carrying. The helical magnetic fields (flux rope) in the core field region are probably formed by long-term photospheric motions. The shearing and converging motions are continuously generated, driven by the flux emergence. AR 12673 is representative, as photospheric motions contribute most of the nonpotentiality in the AR with vigorous flux emergence.