3-D Simulations of MHD Jets - The Stability Problem

Abstract

Non-relativistic three-dimensional magnetohydrodynamic simulations of Poynting-flux-dominated (PFD) jets are presented. Our study focuses on the propagation of strongly magnetized hypersonic but sub-Alfvénic flow ($C_{\mathrm{s}}^2<<V_{\mathrm{jet}}^2<V_{\mathrm{A}}^2$) and the development of a current-driven (CD) kink instability. This instability may be responsible for the "wiggled" structures seen in VLBI-scale AGN jets. In the present paper we investigate the nonlinear behavior of PFD jets in a variety of external ambient magnetized gas distributions, including those with density, pressure, and temperature gradients. Our numerical results show that PFD jets can develop kink distortions in the trans-Alfvénic flow case, even when the flow itself is still strongly magnetically dominated. In the nonlinear development of the instability, a non-axisymmetric mode grows on time scales of order the Alfvén crossing time (in the jet frame) and proceeds to disrupt the kinematic and magnetic structure of the jet. Because of a large scale poloidal magnetic field in the ambient medium, the growth of surface modes ({\it i.e.}, MHD Kelvin-Helmholtz instabilities) is suppressed. The CD kink mode ($m=1$) grows faster than the other higher order modes ($m>1$), driven in large part by the radial component of the Lorentz force.

6 pages, 3 figures; to appear in Plasmas in the Laboratory and in the Universe, Como, Italy, 16-19 Sep, 2003

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