  Solar active region 12673 produced two successive X-class flares (X2.2 and
X9.3) approximately 3 hours apart in September 2017. The X9.3 was the recorded
largest solar flare in Solar Cycle 24. In this study we perform a
data-constrained magnetohydrodynamic simulation taking into account the
observed photospheric magnetic field to reveal the initiation and dynamics of
the X2.2 and X9.3 flares. According to our simulation, the X2.2 flare is first
triggered by magnetic reconnection at a local site where at the photosphere the
negative polarity intrudes into the opposite-polarity region. This magnetic
reconnection expels the innermost field lines upward beneath which the magnetic
flux rope is formed through continuous reconnection with external twisted field
lines. Continuous magnetic reconnection after the X2.2 flare enhances the
magnetic flux rope, which is lifted up and eventually erupts via the torus
instability. This gives rise to the X9.3 flare.
