  On SOL2017-09-06 solar active region 12673 produced an X9.3 flare which is
regarded as largest to occur in solar cycle 24. In this work we have preformed
a magnetohydrodynamic (MHD) simulation in order to reveal the three-dimensional
(3D) dynamics of the magnetic fields associated with the X9.3 solar flare. We
first performed an extrapolation of the 3D magnetic field based on the observed
photospheric magnetic field prior to the flare and then used it as the initial
condition for an MHD simulation. Consequently, the simulation showed a dramatic
eruption. In particular, we found that a large coherent flux rope composed of
highly twisted magnetic field lines is formed during the eruption. A series of
small flux ropes are found to lie along a magnetic polarity inversion line
prior to the flare. Reconnection occurring between each small flux rope during
the early stages of the eruption forms the large and highly twisted flux
rope.Furthermore, we found a writhing motion of the erupting flux rope. The
understanding of these dynamics is important in increasing the accuracy of
space weather forecasting. We report on the detailed dynamics of the 3D
eruptive flux rope and discuss the possible mechanisms of the writhing motion.
