We investigated emergence of a twisted flux tube into the solar corona by performing a three-dimensional MHD simulation. After emerging, outer field lines composing the envelope of the flux tube expanded to form a potential field-like (shearless) arcade, while inner field lines underlying an axis of the flux tube formed sigmoidal structure. These underlying inner field lines had footpoints in a chromospheric layer where high current density was distributed, suggesting that a plasma distributed along the inner field lines could be heated via dissipation of intense current and observed as a soft X-ray sigmoid (Magara & Longcope 2001).
Magara, T. & Longcope, D. W. 2001 ApJ, 559, L55
Evolution of magnetic polarity regions in photosphere.
(contours: vertical
magnetic flux density;
colors: vertical flow velocity;
arrows: horizontal flow velocity field)
Snapshot of flow and magnetic structure below photosphere.
(contours: vertical
magnetic flux density;
colors: vertical flow velocity;
arrows: horizontal flow velocity field)