Compressible magnetohydrodynamic Kelvin–Helmholtz instability with vortex pairing in the two-dimensional transverse configuration

Abstract

For a two-dimensional (2-D) transverse configuration, where the plasma motion occurs in a 2-D plane transverse to the magnetic field, the nonlinear evolution of the magnetohydrodynamic (MHD) Kelvin–Helmholtz (K–H) instability is investigated by means of a 2-D MHD simulation for a convective fast magnetosonic Mach number 0.35, which is defined for the total jump of the flow velocity. The compressibility and the nonzero baroclinic vector are shown to violate the conservation of the enstrophy for the 2-D MHD transverse configuration and for the 2-D fluid motion. After the nonlinear saturation of the linearly fastest growing vortices, the vortices continue to coalesce until no more vortex pairing is allowed, owing to a finite length of the simulation system. The plasma inside the vortex is rarefied strongly by the fast magnetosonic rarefaction and each vortex is associated with an eddy current, which is inertia current in nature. The plasma flow velocity is enhanced at the periphery of the vortex and the net momentum transport and shear relaxation by the instability occur as long as the vortex pairing continues. Anomalous viscosity by the K–H instability increases with the vortex pairing and its increase is due to the growth of subharmonic modes.