3-D Simulations of MHD Jets - The Stability Problem

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-Alfvenic flow (Cs2 << Vjet2 < VA2) 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-Alfvenic 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 Alfven 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 (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