The Role of Nonlinear Alfven Waves in Shear Formation during Solar Magnetic Flux Emergence

Abstract

Observations show that magnetic flux is constantly emerging at the solar photosphere to expand into the corona. Magnetic buoyancy is essential in bringing the magnetic field from the solar interior to the surface and beyond. In the simulations of buoyancy instabilities reported here, it has been discovered that nonlinear Alfven waves may play an important and dramatic role in the rise of magnetic flux. For initial states, our two-and-a-half-dimensional (2.5D), time-dependent simulations capitalize on the availability of a family of two-dimensional (2D) analytical solutions of isothermal magnetostatic atmospheres threaded by a layer of sheared undulating magnetic field. The magnetic field supports the weight of the atmosphere in an unstable configuration that sets the stage for the demonstration of magnetic flux emergence 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 Alfven waves that are driven by a component of the magnetic tension force pointing in the invariant direction. The waves are significant in that they transport magnetic flux from the shear layer into the ascending magnetic loops, causing them to become greatly inflated. The presence of such shear Alfven waves in magnetic loops rising through the photosphere provides an explanation for the impulsive shearing motions observed in newly emerged bipolar active regions.