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The solar magnetic field in a flare-producing active region (AR) is much morecomplicated than theoretical models, which assume a very simple magnetic fieldstructure. The X1.0 flare, which occurred in AR 12192 on 2014 October 25,showed a complicated three-ribbon structure. To clarify the trigger process ofthe flare and to evaluate the applicability of a simple theoretical model, weanalyzed the data from Hinode/Solar Optical Telescope and the Solar DynamicsObservatory/Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly. Weinvestigated the spatio-temporal correlation between the magnetic fieldstructures, especially the non-potentiality of the horizontal field, and thebright structures in the solar atmosphere. As a result, we determined that thewestern side of the positive polarity, which is intruding on a negativepolarity region, is the location where the flare was triggered. This is due tothe fact that the sign of the magnetic shear in that region was opposite thatof the major shear of the AR, and the significant brightenings were observedover the polarity inversion line (PIL) in that region before flare onset. Thesefeatures are consistent with the recently proposed flare-trigger model thatsuggests that small reversed shear (RS) magnetic disturbances can trigger solarflares. Moreover, we found that the RS field was located slightly off theflaring PIL, contrary to the theoretical prediction. We discuss the possibilityof an extension of the RS model based on an extra numerical simulation. Ourresult suggests that the RS field has a certain flexibility for displacementfrom a highly sheared PIL, and that the RS field triggers more flares than weexpected.

Triggering Process of the X1.0 Three-ribbon Flare in the Great Active Region NOAA 12192