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We report a detailed event analysis on the M6.6-class flare in the activeregion (AR) NOAA 11158 on 2011 February 13. AR 11158, which consisted of twomajor emerging bipoles, showed prominent activities including one X- andseveral M-class flares. In order to investigate the magnetic structures relatedto the M6.6 event, particularly the formation process of a flare-triggeringmagnetic region, we analyzed multiple spacecraft observations and numericalresults of a flare simulation. We observed that, in the center of thisquadrupolar AR, a highly sheared polarity inversion line (PIL) was formedthrough proper motions of the major magnetic elements, which built a shearedcoronal arcade lying over the PIL. The observations lend support to theinterpretation that the target flare was triggered by a localized magneticregion that had an intrusive structure, namely a positive polarity penetratinginto a negative counterpart. The geometrical relationship between the shearedcoronal arcade and the triggering region was consistent with the theoreticalflare model based on the previous numerical study. We found that the formationof the trigger region was due to a continuous accumulation of the small-scalemagnetic patches. A few hours before the flare occurrence, the series ofemerged/advected patches reconnected with a preexisting fields. Finally, theabrupt flare eruption of the M6.6 event started around 17:30 UT. Our analysissuggests that, in a triggering process of a flare activity, all magneticsystems of multiple scales, not only the entire AR evolution but also the finemagnetic elements, are altogether involved.

Magnetic Systems Triggering the M6.6-class Solar Flare in NOAA Active Region 11158