Spatial stability of the magnetized slab jet

Abstract

A spatial stability analysis of the Kelvin-Helmholtz instability of a magnetized slab jet is performed and compared to results of numerical simulations. Provided the jet is super-Alfvenic the dispersion relation describing the propagation and growth of Fourier components of some initial perturbation admits the same type of solutions as for a purely fluid jet. A growing fundamental sinusoidal mode at long wavelengths can be identified with an Alfven wave-like disturbance of the jet. Growing internal reflection modes at shorter wavelengths can be identified with fast magnetosonic waves reflecting off the jet's boundaries. Supermagnetosonic resonances comparable to the supersonic resonances of a strictly fluid jet exist where resonant wavelengths and growth lengths scale with the magnetosonic Mach number. The resonances disappear when the jet becomes transmagnetosonic but still super-Alfvenic and only the fundamental sinusoidal mode remains. The fundamental mode is stabilized when the jet is sub-Alfvenic, but for jet velocities slightly less than the slow magnetosonic speed reflection modes can grow even on a sub-Alfvenic jet. This behavior is qualitatively similar to the stability properties of a magnetized cylindrical jet. While the simulations reveal significant non-linear effects associated with increasing magnetic tension, the jet is not stabilized by nonlinear effects, and the linear analysis provides a reasonable description of the spatial stability properties of the jet. The results suggest that a jet which is initially sub-Alfvenic and stable to disruption will be doomed to disruption at the Alfven point if it becomes super-Alfvenic as a result of jet expansion.