Sigmoid Structure of an Emerging Flux Tube

We present the results from three-dimensional MHD simulations of a magnetic flux tube emerging through the solar photosphere. The simulation is initialized with a straight tube of twisted magnetic field located in the upper convection zone. Buoyancy effects drive an arched segment of the tube upward through the photospheric layer and into the corona. Matter drains from the coronal field, which thereafter undergoes a rapid expansion. The coronal magnetic field formed in this manner exhibits outer poloidal field lines that resemble a potential arcade and inner toroidal field lines that emerge after the tube axis, forming sigmoid structure. The simulations suggest that the neutral-line shear and sigmoidal field arise as a natural by-product of flux emergence.