Evolution of the Non-potential Magnetic Field in the Solar Active Region 12673 Based on a Nonlinear Force-free Modeling

TL;DR

This study investigates the evolution of magnetic flux ropes in solar active region 12673 using nonlinear force-free field modeling, linking magnetic twist and null points to flare activity, including the largest flare of solar cycle 24.

Contribution

It provides a detailed analysis of magnetic flux rope formation and evolution, highlighting the role of magnetic twist and null points in flare occurrence, based on time-series nonlinear force-free field extrapolations.

Findings

Three magnetic flux ropes formed, one linked to the X9.3 flare.

Positive twist flux rope correlates with M-class flare activity.

Magnetic null points and reconnection likely triggered the largest flare.

Abstract

Active region (AR) 12673 produced many M-class and several X-class flares, one of which being an X9.3 flare, which is recorded as the largest solar flare in solar cycle 24. We studied the evolution of the three-dimensional flare-productive magnetic field within AR 12673, using a time series of nonlinear force-free field extrapolations of every 12 hours from September 4th 00:00 UT to 6th 00:00 UT. Our analysis found that three magnetic flux ropes (MFRs) are formed by September 4th, one of which produced the X9.3 flare on September 6th. One MFR has positive magnetic twist which is a different sign from other two MFRs. Since the temporal evolution of the magnetic flux of the MFR accumulating the positive twist is consistent with the profile of the GOES X-ray flux well observed from September 4th to 6th, we suggest that the formation of the MFR having the positive twist is closely related to the occurrence of the M-class flares including an M5.5 flare. We further found a magnetic null in the magnetic field surrounding the MFRs, in particular, above the MFR having positive twist before the M5.5 flare which is the largest M-flare observed during this period. By comparing with the AIA 1600 angstrom images, we found that the footpoints of the overlying field lines are anchored to the area where the brightening was initially observed. Therefore, we suggest that reconnection induced by the torus instability of the positively twisted MFR at the null possibly drived the M5.5 flare.