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We study magnetic field evolution in AR12371 relating its successive eruptivenature. During the disk transit of seven days, the AR launched four sequentialfast coronal mass ejections (CMEs) associated with long duration M-classflares. Morphological study delineates a pre-eruptive coronal sigmoid structureabove the polarity inversion line (PIL) similar to Moore et al study. Velocityfield derived from tracked magnetograms indicates persistent shear andconverging motions of polarity regions about the PIL. While these shear motionscontinue, the crossed arms of two sigmoid elbows are being brought tointeraction by converging motions at the middle of PIL, initiatingtether-cutting reconnection of field lines and the onset of CME explosion. Thesuccessive CMEs are explained by a cyclic process of magnetic energy storageand release terming "sigmoid-to-arcade-to-sigmoid" transformation driven byphotospheric flux motions. Further, the continued shear motions inject helicityflux of dominant negative sign, which contributes to core field twist and itsenergy by building a twisted flux rope (FR). After a limiting value, the excesscoronal helicity is expelled by bodily ejection of the FR initiated by someinstability as realized by intermittent CMEs. This AR is in contrast to theconfined AR12192 with predominant negative signed larger helicity flux but veryweaker (-0.02turns) normalised coronal helicity content. While predominantsigned helicity flux is a requirement for CME eruption, our study suggests themagnetic flux normalized helicity flux as a necessary condition accommodatingthe role of background flux and appeals a further study of large sample of ARs.

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