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We carried out three-dimensional nonlinear MHD simulations of the evolution of twisted flux tubes in gravitationally stratified atmosphere. The initial magnetic field is assumed to be concentrated in a uniformly twisted, force-free horizontal flux tube in convectively unstable layer. The origin of the magnetic twist is either by the convective motion or by the Coriolis force. The nonlinear evolution of this system has been simulated by the MHD code based on the modified Lax-Wendroff scheme with artificial viscosity in rectangular Cartesian coordinate. The number of grid points for typical model is $64 \times 64 \times 150$. When the magnetic twist exceeds a threshold, the flux tube forms itself into a helical structure by the kink instability. After this stage, the particular portion of the helical structure rises by buoyancy, and forms a sequence of sheared magnetic loops. Emergence of such helical flux tube may account for the global structure of the kinked series of active regions prominent in the soft X-ray image of the solar corona as observed by the Yohkoh satellite. Furthermore, numerical results reproduce the separation and the drifting motion of f- and p-spots.
We also studied the interaction between twisted flux tubes through three-dimensional resistive MHD simulation. We found (1) the reconnection time scale for counter-helicity flux tubes is much shorter than that for co-helicity flux tubes, (2) reconnection jets with speed comparable to the Alfv\'en speed are ejected along the magnetic loops. Numerical results will be compared with the observations of interacting magnetic loops in the solar corona.
