Twisted Coronal Mass Ejection on 4 August 2011

On 4 August 2011, a GOES M9.3 flare with a fast coronal mass ejection was erupted from solar active region NOAA AR11261. After the flare, a rotating structure is observed by STEREO, in which the rotating motion lasted about 4 hours. The magnetic helicity flux associated with the flare was calculated based on the SDO/HMI vector magnetograms by tracking the plasma velocity of the photosphere. It is found that the photospheric shear and twist motion contribute significantly to the magnetic helicity injection before the flare. The coronal magnetic fields were extrapolated using the CESE-MHD-NLFFF code, which shows that twisted magnetic flux rope exists before the flare and disappears after the flare. With these analyses, a scenario for the formation and evolution of this eruption is suggested as follows. The rotation of the foot point on the west side of the eruption filament twisted the magnetic flux rope, and the highly twisted structure became unstable and erupted. During the eruption, it reconnected with open magnetic field lines near one leg, and a long duration of untwisting movement followed the eruption. Interestingly, the observations of WIND satellites at 1AU show characteristics of typical magnetic clouds in the corresponding interplanetary coronal mass ejections of this eruption, which indicates that a magnetic cloud might be a flux rope with one leg anchored on the Sun, and it is not previously considered.