The emergence of a twisted magnetic flux tube into a preexisting coronal arcade

To investigate the dynamic evolution of a coronal magnetic field in response to the emergence of significantly twisted magnetic structures, we perform MHD simulations in the low-β regime of the emergence of a twisted magnetic flux tube into a preexisting coronal potential magnetic arcade. Our simulation of a twisted flux tube, which when fully emerged contains a twist of 1.875 × 2π field-line rotation about the axis between the anchored footpoints, leads to a magnetic structure with substantial writhing of the tube axis (with an apex rotation >90°) as a result of the nonlinear evolution of the kink instability. For an emerging tube with a left-handed twist (which is the preferred sense of twist for active regions in the northern hemisphere), the writhing of the tube is also left-handed, producing a forward-S shape for the tube axis as viewed from the top, which is opposite to the inverse-S-shaped X-ray sigmoid structures preferentially seen in the northern hemisphere. However, we find that the writhing motion of the tube and its interaction with the ambient coronal magnetic field also drive the formation of an intense current layer that displays an inverse-S shape, consistent with the shape of X-ray sigmoids.