  We studied the developing conditions of sigmoid structure under the influence
of magnetic non-potential characteristics of a rotating sunspot in the active
region (AR) 12158. Vector magnetic field measurements from Helioseismic
Magnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly
reveal that the erupting inverse-S sigmoid had roots in the location of the
rotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend
in the first half followed by a decrease. Time evolution of many non-potential
parameters had a well correspondence with the sunspot rotation. The evolution
of the AR magnetic structure is approximated by a time series of force free
equilibria. The NLFFF magnetic structure around the sunspot manifests the
observed sigmoid structure. Field lines from the sunspot periphery constitute
the body of the sigmoid and those from interior overly the sigmoid similar to a
fluxrope structure. While the sunspot is being rotating, two major CME
eruptions occurred in the AR. During the first (second) event, the coronal
current concentrations enhanced (degraded) consistent with the photospheric net
vertical current, however the magnetic energy is released during both the
cases. The analysis results suggest that the magnetic connections of the
sigmoid are driven by slow motion of sunspot rotation, which transforms to a
highly twisted flux rope structure in a dynamical scenario. An exceeding
critical twist in the flux rope probably leads to the loss of equilibrium and
thus triggering the onset of two eruptions.
