The emergence of a twisted Ω-tube into the solar atmosphere

We report the results of a three-dimensional MHD simulation of the dynamic emergence of a twisted, Omega -shaped tube from the top layer of the solar convection zone into the atmosphere and the corona. It is found that the tube segment at the apex of the Omega -tube expands into the stably stratified atmosphere as a result of the magnetic buoyancy instability (or the Parker instability). Our simulation of the emergence of a left-hand-twisted Omega -tube reproduces several major observed features of a newly developing active region described by Strous and coworkers, including the orientation of the arch-filament system, the distribution of the vertical magnetic field on the photosphere, the locations of sunspot formation, and the organized shear flow pattern in the photospheric horizontal velocity field.