  We report a detailed event analysis on the M6.6-class flare in the active
region (AR) NOAA 11158 on 2011 February 13. AR 11158, which consisted of two
major emerging bipoles, showed prominent activities including one X- and
several M-class flares. In order to investigate the magnetic structures related
to the M6.6 event, particularly the formation process of a flare-triggering
magnetic region, we analyzed multiple spacecraft observations and numerical
results of a flare simulation. We observed that, in the center of this
quadrupolar AR, a highly sheared polarity inversion line (PIL) was formed
through proper motions of the major magnetic elements, which built a sheared
coronal arcade lying over the PIL. The observations lend support to the
interpretation that the target flare was triggered by a localized magnetic
region that had an intrusive structure, namely a positive polarity penetrating
into a negative counterpart. The geometrical relationship between the sheared
coronal arcade and the triggering region was consistent with the theoretical
flare model based on the previous numerical study. We found that the formation
of the trigger region was due to a continuous accumulation of the small-scale
magnetic patches. A few hours before the flare occurrence, the series of
emerged/advected patches reconnected with a preexisting fields. Finally, the
abrupt flare eruption of the M6.6 event started around 17:30 UT. Our analysis
suggests that, in a triggering process of a flare activity, all magnetic
systems of multiple scales, not only the entire AR evolution but also the fine
magnetic elements, are altogether involved.
