  Solar flares are often associated with coronal eruptions, but there are
confined ones without eruption, even for some X-class flares. How such large
flares occurred and why they are confined are still not well understood. Here
we studied a confined X2.2 flare in NOAA 12673 on 2017 September 6. It exhibits
two episodes of flare brightening with rather complex, atypical ribbons. Based
on topology analysis of extrapolated coronal magnetic field, we revealed that
there is a two-step magnetic reconnection process during the flare. Prior to
the flare, there is a magnetic flux rope (MFR) with one leg rooted in a
rotating sunspot. Neighboring to the leg is a magnetic null-point structure.
The sunspot drives the MFR to expand, pushing magnetic flux to the null point,
and reconnection is first triggered there. The disturbance from the null-point
reconnection triggers the second reconnection, i.e., a tether-cutting
reconnection below the rope. However, these two reconnections failed to produce
an eruption, because the rope is firmly held by its strapping flux.
Furthermore, we compared this flare with an eruptive X9.3 flare in the same
region with 2 hours later, which has a similar MFR configuration. The key
difference between them is that, for the confined flare, the MFR is fully below
the threshold of torus instability, while for the eruptive one, the MFR reaches
entirely above the threshold. This study provides a good evidence supporting
that reconnection alone may not be able to trigger eruption, rather, MHD
instability plays a more important role.
