A Data-Constrained MHD Modeling of Huge Solar Flares in September 6 2017: From Initiation to Large Eruption

We performed a data-constrained magnetohydrodynamic (MHD) simulation to reveal the initiation of, and large eruption associated with, an X9.3 flare, which was observed in active region (AR) 12673 in September 2017. This AR produced multiple X-flares, in particular, an X2.2 flare that was observed approximately 3 hours prior to the X9.3 flare. This AR showed not only strong shearing motion in the north-south direction between the main polarities, but also shows the intruding motion of one polarity into its opposite. In order to reveal the initiation processes of the X2.2 and X9.3 flares, as well as the dynamics between the events, we first modeled the three-dimensional magnetic field, based on the observed photospheric magnetic field 20 minutes prior to the X2.2 flare, under the nonlinear force-free field (NLFFF) approximation. Next, we performed an MHD simulation using the NLFFF as the initial condition.

The NLFFF extrapolation shows multiple twisted field lines above the polarity inversion line(PIL). According to the results of the MHD simulation, a first instance of reconnection starts in the strong current region formed by the intruding motion of opposite polarity field. Consequently, long twisted lines are formed and begin to ascend. This breaking of magnetic equilibrium induces tether-cutting reconnection among the remaining twisted field lines in the strong current region but is driven by the shearing motion of the main polarities. Consequently, more highly twisted field lines ( a magnetic flux rope ) are formed which subsequently erupt dramatically. We suggest that the former and later reconnections are responsible for the X2.2 and X9.3 flare respectively, inferred from the observations. In our numerical experiment, if the first reconnection event did not occur, the X9.3 flare also did not occur. This result suggests that the X2.2 flare is required to lead to the X9.3 flare. Furthermore, if the reconnection was allowed, every time, above PIL, we found that the solar eruption could be achieved, at least, 2 days before the X2.2 flare.