  The solar magnetic field in a flare-producing active region (AR) is much more
complicated than theoretical models, which assume a very simple magnetic field
structure. The X1.0 flare, which occurred in AR 12192 on 2014 October 25,
showed a complicated three-ribbon structure. To clarify the trigger process of
the flare and to evaluate the applicability of a simple theoretical model, we
analyzed the data from Hinode/Solar Optical Telescope and the Solar Dynamics
Observatory/Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly. We
investigated the spatio-temporal correlation between the magnetic field
structures, especially the non-potentiality of the horizontal field, and the
bright structures in the solar atmosphere. As a result, we determined that the
western side of the positive polarity, which is intruding on a negative
polarity region, is the location where the flare was triggered. This is due to
the fact that the sign of the magnetic shear in that region was opposite that
of the major shear of the AR, and the significant brightenings were observed
over the polarity inversion line (PIL) in that region before flare onset. These
features are consistent with the recently proposed flare-trigger model that
suggests that small reversed shear (RS) magnetic disturbances can trigger solar
flares. Moreover, we found that the RS field was located slightly off the
flaring PIL, contrary to the theoretical prediction. We discuss the possibility
of an extension of the RS model based on an extra numerical simulation. Our
result suggests that the RS field has a certain flexibility for displacement
from a highly sheared PIL, and that the RS field triggers more flares than we
expected.
