Successive Homologous Coronal Mass Ejections driven by shearing and converging motions in solar active region NOAA 12371

Abstract: We study magnetic field evolution in AR12371 relating its successive eruptive nature. During the disk transit of seven days, the AR launched four sequential fast coronal mass ejections (CMEs) associated with long duration M-class flares. Morphological study delineates a pre-eruptive coronal sigmoid structure above the polarity inversion line (PIL) similar to Moore et al study. Velocity field derived from tracked magnetograms indicates persistent shear and converging motions of polarity regions about the PIL. While these shear motions continue, the crossed arms of two sigmoid elbows are being brought to interaction by converging motions at the middle of PIL, initiating tether-cutting reconnection of field lines and the onset of CME explosion. The successive CMEs are explained by a cyclic process of magnetic energy storage and release terming "sigmoid-to-arcade-to-sigmoid" transformation driven by photospheric flux motions. Further, the continued shear motions inject helicity flux of dominant negative sign, which contributes to core field twist and its energy by building a twisted flux rope (FR). After a limiting value, the excess coronal helicity is expelled by bodily ejection of the FR initiated by some instability as realized by intermittent CMEs. This AR is in contrast to the confined AR12192 with predominant negative signed larger helicity flux but very weaker (-0.02turns) normalised coronal helicity content. While predominant signed helicity flux is a requirement for CME eruption, our study suggests the magnetic flux normalized helicity flux as a necessary condition accommodating the role of background flux and appeals a further study of large sample of ARs.