Continuous Null-point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar AR 12673

Abstract

Two X-class solar flares occurred on 2017 September 6 from active region NOAA 12673: the first one is a confined X2.2 flare, and it is followed only ∼3 hr later by the second one, which is the strongest flare in solar cycle 24, reaching X9.3 class and accompanied by a coronal mass ejection. Why did these two X-class flares occur in the same position with similar magnetic configurations, but one is eruptive while the other is not? Here we track the coronal magnetic field evolution via nonlinear force-free field extrapolations from a time sequence of vector magnetograms with high cadence. A detailed analysis of the magnetic field shows that a magnetic flux rope (MFR) forms and grows gradually before the first flare, and shortly afterward, the MFR’s growth is significantly enhanced with a much faster rise in height, from far below the threshold of torus instability to above it, while the magnetic twist only increases mildly. Combining EUV observations and the magnetic field extrapolation, we found that overlying the MFR is a null-point magnetic topology, where recurrent brightening is seen after the first flare. We thus suggest a scenario to interpret the occurrence of the two flares. The first flare occurred since the MFR reached a high enough height to activate the null point, and its continuous expansion forces the null-point reconnection recurrently. Such reconnection weakens the overlying field, allowing the MFR to rise faster, which eventually crosses the threshold of torus instability and triggers the second, eruptive flare.

Deducing the reliability of relative helicities from nonlinear force-free coronal models

“…In particular, using the twist number method, they pictured a considerable increase in the flux rope twist during the confined flare. Based on the same method, Zou et al (2020) pictured a rather weakly increasing self-helicity during the X2.2 flare (see their Fig. 4c).…”

“…Based on an magneto-hydrodynamic relaxation method, Zou et al (2020) pictured the formation and gradual growing of a magnetic flux rope prior to the confined X2.2 flare, covering the time span 00:00 UT to 11:48 UT on 2017 September 6. The existing magnetic flux rope was found to grow in an accelerated manner after the occurrence of the confined flare, along with a (mild) increase in the flux rope twist (an approximation for its self-helicity).…”

Microwave indicator of potential geoeffectiveness and magnetic flux-rope structure of a solar active region

“…Abramov-Maximov et al [2018] have suggested that there are magnetic flux ropes in the magnetosphere of NOAA 12673. Their real presence and the conditions necessary for their eruption have been discussed in [Yang et al, 2017;Hou et al, 2018;Zou et al, 2019Zou et al, , 2020. Inoue et al [2018] have also numerically simulated the eruption of interconnected magnetic flux ropes, which were revealed in the coronal magnetic field of NOAA 12673 reconstructed from a vector magnetogram.…”

Microwave indicator of potential geoeffectiveness and magnetic flux-rope structure of a solar active region

“…В работе [Абрамов-Максимов и др., 2018] предполагалось наличие магнитных жгутов в магнитосфере NOAA 12673. Их реальное присутствие и условия, необходимые для их эрупции, рассматривались в работах [Yang et al, 2017;Hou et al, 2018;Zou et al, 2019Zou et al, , 2020…”