The role of nonlinear Alfven waves in shear formation during solar magnetic flux emergence

Observations show that magnetic flux is constantly emerging at the solar ph otosphere to expand into the corona. Magnetic buoyancy is essential in brin ging the magnetic field from the solar interior to the surface and beyond. In the simulations of buoyancy instabilities reported here, it has been dis covered that nonlinear waves may play an important and dramatic role in the rise of magnetic flux. For Alfven initial states, our two-and-a-half-dimen sional (2.5D), time-dependent simulations capitalize on the availability of a family of two-dimensional (2D) analytical solutions of isothermal magnet ostatic atmospheres threaded by a layer of sheared undulating magnetic fiel d. The magnetic field supports the weight of the atmosphere in an unstable configuration that sets the stage for the demonstration of magnetic flux em ergence in a stratified atmosphere. When the system is perturbed, magnetic loops buoyantly rise from the flux layer and shearing motions are found to naturally arise in conjunction with mixed-mode (interchange and undulating) instabilities. The shearing motions take the form of large-amplitude shear waves that are driven by a component of the magnetic tension force pointin g in the invariant Alfven direction. The waves are significant in that they transport magnetic flux from the shear layer into the ascending magnetic l oops, causing them to become greatly inflated. The presence of such shear A lfven waves in magnetic loops rising through the photosphere provides an ex planation for the impulsive shearing motions observed in newly emerged bipo lar active regions.