Astron. Astrophys. 355, 818-828 (2000)

1. Introduction

Magnetohydrodynamic (MHD) models are favoured for the formation of jets from active galactic nuclei (AGN) and young stellar objects (YSO). Such outflows are, however, liable to magnetohydrodynamic instabilities, which may affect their propagation, or even prevent them from maintaining their integrity over hundreds of kpc or several pc, respectively. Various observed features such as wiggles, knots, and filamentary structures in extragalactic (e.g. Pearson 1996) and stellar (e.g. Reipurth & Heathcote 1997) jets may on the other hand be manifestations of instabilities of the underlying flow.

Jets of magnetic origin possess a helical field configuration. It is the azimuthal component, , which is equivalent to an axial electric current, that gives rise to concern. The growth of Kelvin-Helmholtz instabilities (KHI) of supermagnetosonic jets has been shown to be reduced by the presence of the azimuthal field (Appl & Camenzind 1992). The long wavelength KH modes are, however, essentially determined by the fast magnetosonic Mach number. Similar investigations extended to the submagnetosonic regime (Appl 1996a) showed that the azimuthal field exhibits a destabilizing behaviour at small velocities. In addition to the Kelvin-Helmholtz instability, such jets are liable to current-driven instabilities (CDI), which are the subject of this paper. Great efforts have been undertaken to understand the onset and evolution of CDI in the context of controlled fusion (e.g. Bateman 1978). The CDI are also a major source of concern for magnetohydrodynamical jet models. Their role in this context has been addressed by Eichler (1993), Istomin & Pariev (1994, 1996), Appl (1996a), Spruit et al. (1997), Begelman (1998), and Lyubarskii (1999).

The situation in controlled thermonuclear reactors (CTR) differs from magnetized jets in several respects. The former are essentially static magnetic configurations, which are separated by a vacuum region from the container wall, while jets propagate at superalfvénic velocities through an ionized ambient medium. Finally, the majority of fusion machines such as the TOKAMAK, are operating in the regime where the longitudinal magnetic field is much stronger than the azimuthal field, whereas in magnetized jets both components are more likely to be at least of the same order. These differences make it difficult to directly carry over the results from fusion plasmas to astrophysical jets. The regime of a dominantly longitudinal field has been studied in detail in the context of fusion research. This is not the case for configurations with large azimuthal fields.

The purpose of this paper is the quantitative study of the linear evolution of global current-driven instabilities of astrophysical jets, to discuss the consequences for the dynamics of the outflows, and to point out the differences to fusion plasmas. To this end we start by discussing simple magnetic models for the propagating jet (Sect. 2). In Sect. 3 the linear stability analysis is described and in the next section some results from the stability of screw pinches are reviewed, to the extent they are relevant for our study. The results for relevant configurations are presented in Sect. 5, followed by a careful analysis of the boundary conditions. This provides the base for the general findings which we summarize and discuss in Sect. 6. We conclude with the consequences for astrophysical jets in Sect. 7.

© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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