Numerical solution of magnetic flux emergence in gravity-stratified solar atmosphere

Abstract

Numerical MHD simulation in Lagrangian scheme is made of the dynamical evolution of the solar magnetic bipolar field in a gravity-stratified atmosphere when a parallel or an antiparallel dipole emerges from the subphotosphere. It is shown that the emergence will lead to converging and descending motion of the plasma, and that a pressure has to be added for the dipole to float. The floating results in the formation of a current sheet in the interface between the old and the new fields. The sheet may bring about explosive phenomena such as the Ellerman bombs. The computation reveals that the gas at the top of the arch is rising with a small velocity while the gas at the feet is falling with a large velocity. Similar to the Brunt-Vaisala oscillation, this peculiar property stems from the nonhomogeniety of the physical parameters in the gravity-stratified atmosphere. Also, we simulate usual solar active region by making a strong magnetic dipole of 1500 G float into a weak background field of 100 G. It is indicated that a diminished pressure should be added on the boundary in order to avoid strong shocks. The emerging process should last at least several hours. After emergence a strong current sheet is formed in the chromosphere which, on arriving in the lower corona, will become a potential source of solar flares, as envisaged by Heyvaerts and Priest. Finally, the results of computation will be used to inpterpret the magnetic emerging flux data obtained at Huairou Station, given in [1] and [2].