A theory of MHD instability of an inhomogeneous plasma jet

TL;DR

This paper develops an analytical and numerical theory of MHD instabilities in inhomogeneous plasma jets, revealing conditions for Kelvin-Helmholtz instability and discovering a new global unstable mode independent of flow velocity.

Contribution

It provides a comprehensive analytical and numerical analysis of MHD instabilities in inhomogeneous plasma jets, including the discovery of a new global unstable mode.

Findings

Kelvin-Helmholtz instability occurs when flow exceeds maximum Alfven speed.

Resonance surfaces facilitate mode conversion and energy absorption.

A new global unstable mode exists regardless of flow velocity.

Abstract

A problem of the instability of an inhomogeneous axisymmetric plasma jet in a parallel magnetic field is solved. The jet boundary becomes, under certain conditions, unstable relative to magnetosonic oscillations (Kelvin-Helmholtz instability) in the presence of a shear flow at the jet boundary. Because of its internal inhomogeneity the plasma jet has resonance surfaces, where conversion takes place between various modes of plasma MHD oscillations. Propagating in inhomogeneous plasma, fast magnetosonic waves drive the Alfven and slow magnetosonic oscillations, tightly localized across the magnetic shells, on the resonance surfaces. MHD oscillation energy is absorbed in the neighbourhood of these resonance surfaces. The resonance surfaces disappear for the eigen-modes of slow magnetosonic waves propagating in the jet waveguide. The stability of the plasma MHD flow is determined by competition between the mechanisms of shear flow instability on the boundary and wave energy dissipation because of resonant MHD-mode coupling. The problem is solved analytically, in the WKB approximation, for the plasma jet with a boundary in the form of a tangential discontinuity over the radial coordinate. The Kelvin-Helmholtz instability develops if plasma flow velocity in the jet exceeds the maximum Alfven speed at the boundary. The stability of the plasma jet with a smooth boundary layer is investigated numerically for the basic modes of MHD oscillations, to which the WKB approximation is inapplicable. A new "global" unstable mode of MHD oscillations has been discovered which, unlike the Kelvin-Helmholtz instability, exists for any, however weak, plasma flow velocities.