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We present a clear case study on the occurrence of two successive X-classflares including a decade-class flare (X9.3) and two coronal mass ejections(CMEs) triggered by shearing motion and sunspot rotation in active region NOAA12673 on 2017 September 6. A shearing motion between the main sunspots withopposite polarities started on September 5 and even lasted after the secondX-class flare on September 6. Moreover, the main sunspot with negative polarityrotated around its umbral center and another main sunspot with positivepolarity also exhibited a slow rotation. The sunspot with negative polarity atthe northwest of active region also began to rotate counter-clockwise beforethe onset of the first X-class flare. The successive formation and eruption oftwo S-shaped structures were closely related to the counter-clockwise rotationof three sunspots. It is also found that the rotation of sunspots is fasterduring four hours prior to the onset of the flares than the period before. Theexistence of a flux rope is found prior to the onset of two flares by usingnon-linear force free field extrapolation based on the vector magnetogramsobserved by SDO/HMI. These results suggest that shearing motion and sunspotrotation play an important role in the buildup of the free energy and theformation of flux ropes in the corona which produces solar flares and CMEs.

Successive X-class flares and coronal mass ejections driven by shearing motion and sunspot rotation in active region NOAA 12673