  The triggering mechanism(s) and critical condition(s) of solar flares are
still not completely clarified, although various studies have attempted to
elucidate them. We have also proposed a theoretical flare-trigger model based
on MHD simulations Kusano et al. 2012, in which two types of small-scale bipole
field, the so-called Opposite Polarity (OP) and Reversed Shear (RS) types of
field, can trigger flares. In this study, we evaluated the applicability of our
flare-trigger model to observation of 32 flares that were observed by the Solar
Dynamics Observatory (SDO), by focusing on geometrical structures. We
classified the events into six types, including the OP and RS types, based on
photospheric magnetic field configuration, presence of precursor brightenings,
and shape of the initial flare ribbons. As a result, we found that
approximately 30% of the flares were consistent with our flare-trigger model,
and the number of RS type triggered flares is larger than that of the OP type.
We found none of the sampled events contradicts our flare model, although we
cannot clearly determine the trigger mechanism of 70% of the flares in this
study. We carefully investigated the applicability of our flare-trigger model
and the possibility that other models can explain the other 70% of the events.
Consequently, we concluded that our flare-trigger model has certainly proposed
important conditions for flare-triggering.
