Twisted Flux Tube Emergence From the Convection Zone to the Corona

Three-dimensional numerical simulations of a horizontal magnetic flux tube emergence with different twist are carried out in a computational domain spanning the upper layers of the convection zone to the lower corona. We use the Oslo Stagger Code to solve the full MHD equations with non-gray, non-LTE radiative transfer and thermal conduction along the magnetic lines. A magnetic flux tube is input at the bottom boundary into a weakly magnetized atmosphere. The photospheric and chromospheric response is described with magnetograms and synthetic continuum as well as Ca II H line images and velocity field distributions. In the photosphere the granular size increases when the flux tube approaches from below, as has been reported previously in the literature. In the convective overshoot region, some 200 km above the photosphere, adiabatic expansion produces cooling, darker regions with the structure of granulation cells. We also find evidence of collapsed granulation at the boundaries of the rising flux tube. Once the flux tube has crossed the photosphere, bright points related to concentrated magnetic field, vorticity, high vertical velocities, and heating by compressed material are found at heights up to 500 km above the photosphere. At greater heights, in the magnetized chromosphere, the rising flux tube produces a large, cool, magnetized bubble that tends to expel the usual chromospheric oscillations. In addition, the rising flux tube dramatically increases the chromospheric scale height, pushing the transition region and corona aside, such that the chromosphere extends up to 6 Mm above the photosphere. We find flux tube emergence through the photosphere to the lower corona to be a relatively slow process, taking of order 1 hr.