An MHD Modeling of the initiation and Dynamics of the X9.3 Flare Observed in September 2017

We performed a magnetohydrodynamic (MHD) simulation to reveal the initiation and dynamics of the X9.3 flare, which was observed in active region (AR) 12673 in September 2017. This AR produced X-flares multiple times, in particular, an X2.2 flare that was observed approximately 3 hours prior 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 process and the dynamics of the X9.3 flare, we first modeled the three-dimensional magnetic field, which is 20 minutes prior the X2.2 flare, based on the photospheric magnetic field under the nonlinear force-free field (NLFFF) approximation. Next we performed an MHD simulation using the NLFFF as the initial condition. As a result, we found multiple twisted field lines above the polarity inversion line. According to the results of the MHD simulation, reconnection starts at the strong current region formed by intruding motion of opposite polarity field. Consequently, long twisted lines are formed and begin to erupt. Soon after, tether-cutting reconnection among the remaining twisted lines starts at the strong current region but is driven by the shearing motion of the main polarities. Consequently, more highly twisted lines (magnetic flux rope) are formed. This then results in a dramatic eruption and also writhing motion even though the initial magnetic field is stable to the kink instability. We suggest that the former and later reconnection are responsible for the X2.2 and X9.3 flare, respectively. In our numerical experiment, if the first reconnection even did not occur, the X9.3 flare subsequently did not occur. This result suggests that the X2.2 flare is required to lead to the X9.3 flare.