Pressure-driven instabilities in astrophysical jets

TL;DR

This paper reviews the current understanding of pressure-driven instabilities in astrophysical jets, focusing on their origins, criteria for instability, growth rates, and potential stabilization mechanisms, highlighting unresolved issues in jet stability analysis.

Contribution

It provides a comprehensive review of pressure-driven MHD instabilities in astrophysical jets, including derivation of dispersion relations and discussion of stabilization mechanisms.

Findings

Pressure-driven instabilities have short growth times compared to jet propagation.

Magnetic shear and resonances play crucial roles in instability development.

Axial velocity may nonlinear stabilize pressure-driven modes.

Abstract

Astrophysical jets are widely believed to be self-collimated by the hoop-stress due to the azimuthal component of their magnetic field. However this implies that the magnetic field is largely dominated by its azimuthal component in the outer jet region. In the fusion context, it is well-known that such configurations are highly unstable in static columns, leading to plasma disruption. It has long been pointed out that a similar outcome may follow for MHD jets, and the reasons preventing disruption are still not elucidated, although some progress has been accomplished in the recent years. In these notes, I review the present status of this open problem for pressure-driven instabilities, one of the two major sources of ideal MHD instability in static columns (the other one being current-driven instabilities). I first discuss in a heuristic way the origin of these instabilities. Magnetic resonances and magnetic shear are introduced, and their role in pressure-driven instabilities discussed in relation to Suydam's criterion. A dispersion relation is derived for pressure-driven modes in the limit of large azimuthal magnetic fields, which gives back the two criteria derived by Kadomtsev for this instability. The growth rates of these instabilities are expected to be short in comparison with the jet propagation time. What is known about the potential stabilizing role of the axial velocity of jets is then reviewed. In particular, a nonlinear stabilization mechanism recently identified in the fusion literature is discussed. Key words: Ideal MHD: stability, pressure-driven modes; Jets: stability