Abstract

We find a magnetic flux rope before the M1.1 flare in active region 10767 on 2005 May 27 by a nonlinear force-free field extrapolation. TRACE observations of the filament eruption show that the erupting structure performed a writhing deformation and stopped rising at a certain height, suggesting that the flux rope converted some of its twist into writhe and was confined in the corona. After calculating the twist of the flux rope, we find that it was comparable to thresholds of the helical kink instability found in numerical simulations. We conclude that the activation and rise of the flux rope were triggered and initially driven by the kink instability. The decay index of the external magnetic field stays below the threshold for the torus instability within a long height range. The confinement of the eruption could be explained by the failure of the torus instability. Hard X-ray sources at the peak of the M1.1 flare coincided with the footpoints of the erupting helical structure, which indicates a high possibility that hard X-ray sources were produced more efficiently in the flux rope.