On the nonlinear evolution of magnetohydrodynamic Kelvin-Helmholtz instabilities

We investigate the physical behavior in the nonlinear regime of Kelvin-Helmholtz (KH) instabilities in a simple conducting shear flow in the presence of magnetic fields, based upon the use of numerical simulations of the ideal magnetofluid equations of motion in two dimensions. The flow is characterized by three principal control parameters: the Mach number M of the shear flow, the ratio a of the Alfven speed to the sound speed, and the effective diffusivity; we investigate how these parameters affect the evolution and saturation of the instability. The key result of our study is that even relatively small magnetic fields (i.e., small compared to the equipartition intensity) affect the way the KH instability saturates with respect to the purely hydrodynamic case. If the magnetic field intensity is not sufficiently strong to suppress the KH instability entirely, then the field itself can still mediate the turbulent decay and diffusion of energy and mass across the layer. We present a detailed study of the various phases of this process for our simple shear layer configuration.