  The largest X9.3 solar flare in solar cycle 24 and the preceding X2.2 flare
occurred on September 6, 2017, in the solar active region NOAA 12673. This
study aims to understand the onset mechanism of these flares via analysis of
multiple observational datasets from the Hinode and Solar Dynamics Observatory
and results from a non-linear force-free field extrapolation. The most
noticeable feature is the intrusion of a major negative-polarity region,
appearing similar to a peninsula, oriented northwest into a neighboring
opposite-polarity region. We also observe proxies of magnetic reconnection
caused by related to the intrusion of the negative peninsula: rapid changes of
the magnetic field around the intruding negative peninsula; precursor
brightening at the tip of the negative peninsula, including a cusp-shaped
brightening that shows a transient but significant downflow (~100 km/s) at a
leg of the cusp; a dark tube-like structure that appears to be a magnetic flux
rope that erupted with the X9.3 flare; and coronal brightening along the dark
tube-like structure that appears to represent the electric current generated
under the flux rope. Based on these observational features, we propose that (1)
the intrusion of the negative peninsula was critical in promoting the push-mode
magnetic reconnection that forms and grows a twisted magnetic flux rope that
erupted with the X2.2 flare, (2) the continuing intrusion progressing even
beyond the X2.2 flare is further promoted to disrupt the equilibrium that leads
the reinforcement of the magnetic flux rope that erupted with the X9.3 flare.
