MAGNETIC RECONNECTION AND THE TOPOLOGY OF INTERACTING TWISTED FLUX TUBES

The self-consistent evolution of a pair of initially straight and eith er parallel or antiparallel magnetic flux tubes with prescribed bounda ry twist is studied using fully compressible three-dimensional (3-D) r esistive magnetohydrodynamics (MHD). 3-D visualization techniques spec ially designed for divergence free vector fields are employed to inves tigate topological changes in the held lines and current lines associa ted with 3-D reconnection in the system. Four cases are studied, corre sponding to either parallel or antiparallel initial magnetic fields an d to the same or opposite sign of footpoint twist. It is found that in the Case with antiparallel field and opposite twist, so that the curr ents are parallel, the evolution proceeds in two phases. In the first phase, a series of topological changes involving magnetic nulls (where B=0) create an X-type closed held Line. In the second phase, the X-ty pe line serves as the separator for reconnection, allowing field lines from the two tubes to merge and form loops. The magnetic field Lines exhibit spatial chaos and chaotic scattering. The observed reconnectio n involves the X-type closed held line with evident current sheets. La ter in time, the X-type line changes to an O-type closed field line, s urrounded by a ring of toroidal flux surfaces. Reconnection continues until there emerges a final steady state having two reconnected loops and a toroidal ring of flux surfaces in between. The torus of magnetic surfaces has zero current in steady state because it is not connected by held lines to the twist imposed at the boundary. It is discussed h ow it is possible that such a region of zero current density can exist . The other three cases involve breaking of the ideal MHD flux constra int and changes in topology, but without localized current sheets, i.e ., without reconnection. Implications for coronal loop interaction are discussed. (C) 1996 American Institute of Physics.