  Using the photospheric magnetic and coronal observations of Solar Dynamics
Observatory, we studied the build-up and eruption of coronal non-potential
magnetic structure in emerging active region (AR) 12673. The velocity field
derived from tracked vector-magnetograms indicates persistent shear and
converging motions of flux regions about the polarity inversion line (PIL). A
major helicity injection occurs during rapid flux emergence consistent with the
very fast flux emergence phase. While this helicity flux builds-up the sigmoid
by September 4, the helicity injection by the continued shear and converging
motions in the later evolution contributes to sigmoid sustenance and its core
field twist as a manifestation of the flux rope which erupts after exceeding
critical value of twist. Moreover, the total length of sheared PIL segments
correlates with the non-neutralized current and maintains a higher value in
both the polarity regions as a signature of eruptive capability of the AR
according to the flux rope models. The modelled magnetic field qualitatively
reproduces the sigmoidal structure capturing major features like twisted core
flux as flux rope, and hook-shaped parts connecting at the middle of the PIL.
Study of quasi-separatrix-layers reveals that the sheared arcade, enclosing the
flux rope, is stressed to a critically stable state and its coronal height
becomes doubled from September 4-6. While demonstrating the fast injection of
helicity per unit flux as the crucial factor for severe space-weather events,
this study explains the formation of the flux rope and recurrent eruptive
nature of the AR by the critically stable state of sheared arcade early on
September 6.
