A Numerical Investigation of Extragalactic Jet Stability
Christopher Carey, University of Wisconsin
Large-scale, highly collimated energetic plasma outflows are observed in some active galactic nuclei. Recent observations of these extragalactic outflows suggest that some of these jets maintain a large scale helical magnetic structure with an asymmetry about their central axis . The kink instability is known to create similar magnetic structures in laboratory plasmas. Thus, extragalactic jets may resemble a screw pinch topology and be susceptible to the current-driven kink instability. We investigate the launching and stability of extragalactic jets through magnetohydrodynamic (MHD) simulations of jet evolution. In these simulations a small-scale equilibrium magnetic corona is twisted by a differentially rotating accretion disk. Two-dimensional calculations show the formation of a collimated outflow. Three-dimensional calculations show that the outflow is unstable to the m=1 kink instability, and that the growth rate of the kink decreases as the rotation rate of the accretion disk increases. Thus, the kink instability is stabilized for high rotation rates of the accretion disk. This stabilization is shown to be a result of the azimuthal rotation of the jet.
The stabilizing effect of azimuthal rotation on the kink instability is investigated through a normal mode analysis of a cylindrical plasma. A MHD equilibrium is considered with general magnetic field, pressure, and mass density profiles, and solid body rotation in the azimuthal direction. Applying the normal mode analysis to this equilibrium results in an eigenvalue problem for the growth rates of the unstable modes. This eigenvalue problem is solved using a shooting method. The eigenvalues and corresponding eigenmodes are examined as the rotation rate of the plasma is varied.
1. D.M. Worrall, M. Birkinshaw, et al., Mon. Not. R. Astron. Soc., May, 2007
Abstract Author(s): Christopher Carey, Carl Sovinec, Sebastian Heinz, John Everett