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We studied a flare onset process in terms of stability of a three-dimensional(3D) magnetic field in active region 12371 producing an eruptive M6.5 flare in2015 June 22. In order to reveal the 3D magnetic structure, we firstextrapolated the 3D coronal magnetic fields based on time series of thephotospheric vector magnetic fields under a nonlinear force-free field (NLFFF)approximation. The NLFFFs nicely reproduced the observed sigmoidal structurewhich is widely considered to be preeruptive magnetic configuration. Inparticular, we found that the sigmoid is composed of two branches of shearedarcade loops. On the basis of the NLFFFs, we investigated the sheared arcadeloops to explore the onset process of the eruptive flare using threerepresentative magnetohydrodynamic instabilities: the kink, torus, and doublearc instabilities (DAI). The DAI, recently proposed by Ishiguro & Kusano, is adouble arc loop that can be more easily destabilized than a torus loop.Consequently, the NLFFFs are found to be quite stable against the kink andtorus instabilities. However, the sheared arcade loops formed prior to theflare possibly become unstable against the DAI. As a possible scenario for theonset process of the M6.5 flare, we suggest a three-step process: (1) doublearc loops are formed by the sheared arcade loops through the tether-cuttingreconnection during an early phase of the flare, (2) the DAI contributes to theexpansion of destabilized double arc loops, and (3) finally, the torusinstability makes the full eruption.

Onset Mechanism of M6.5 Solar Flare Observed in Active Region 12371