Spatial simulations of the Kelvin-Helmholtz instability in astrophysical jets

Abstract

The long term magnetohydrodynamic stability of magnetized transonic/supersonic jets is numerically investigated using a spatial approach. We focus on two-dimensional linearly-unstable slab configurations where the jet is embedded in a flow-aligned uniform magnetic field of weak amplitude. We compare our results with previous studies using a temporal approach where longitudinally periodic domains were adopted. The finite-volume based versatile advection code (VAC) is used to solve the full set of ideal compressible MHD equations. We follow the development of Kelvin-Helmholtz modes that are driven by a white noise perturbation continuously introduced at the jet inlet. No noticeable difference is observed in spatial simulations versus analogous temporal ones during the linear and early non-linear evolution of the configuration. However, in the case of transonic flows, a different long-term scenario occurs in our spatial runs. Indeed, after the large-scale disruption of the flow, a sheath region of enhanced magnetic field encompassing the jet core forms along the whole flow. This provides a partial stabilization mechanism leading to enhanced stability for later times, which is almost independent of the initial magnitude of the magnetic field. The implication of this mechanism for the stability of astrophysical jets is discussed.