Growth and Saturation of the Kelvin-Helmholtz Instability with Parallel and Anti-Parallel Magnetic Fields

We investigate the Kelvin-Helmholtz instability occurring at the interface of a shear flow configuration in 2D compressible magnetohydrodynamics (MHD). The linear growth and the subsequent non-linear saturation of the instability are studied numerically. We consider an initial magnetic field aligned with the shear flow, and analyze the differences between cases where the initial field is unidirectional everywhere (uniform case), and where the field changes sign at the interface (reversed case). We recover and extend known results for pure hydrodynamic and MHD cases with a discussion of the dependence of the non-linear saturation on the wavenumber, the sound Mach number, and the Alfvénic Mach number for the MHD case.

A reversed field acts to destabilize the linear phase of the Kelvin-Helmholtz instability compared to the pure hydrodynamic case, while a uniform field suppresses its growth. In resistive MHD, reconnection events almost instantly accelerate the buildup of a global plasma circulation. They play an important role throughout the further non-linear evolution as well, since the initial current sheet gets amplified by the vortex flow and can become unstable to tearing instabilities forming magnetic islands. As a result, the saturation behaviour and the overall evolution of the density and the magnetic field is markedly different for the uniform versus the reversed field case.