Abstract:
We study the magnetic field evolution in AR 12371, related to its successive eruptive nature. During the disk
transit of seven days, the active region (AR) launched four sequential fast coronal mass ejections (CMEs), which
are 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.’s study. The 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 the PIL, initiating the tether-cutting reconnection of field lines and the onset of
the CME explosion. The successive CMEs are explained by a cyclic process of magnetic energy storage and
release referred to as “sigmoid-to-arcade-to-sigmoid” transformation driven by photospheric flux motions.
Furthermore, the continued shear motions inject helicity flux with a 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, which is initiated by some instability as realized by intermittent
CMEs. This AR is in contrast with the confined AR 12192 with a predominant negative sign and larger helicity
flux, but much weaker (−0.02 turns) normalized coronal helicity content. While predominant signed helicity flux is
a requirement for CME eruption, our study suggests that the magnetic flux normalized helicity flux is a necessary
condition accommodating the role of background flux and appeals to a further study of a large sample of ARs.
