Poynting Flux Dominated Jets in Decreasing Density Atmospheres. I. The Non-relativistic Current-driven Kink Instability and the Formation of "Wiggled" Structures

Non-relativistic three-dimensional magnetohydrodynamical (MHD) simulations of Poynting flux dominated (PFD) jets are presented. Our study focuses on the propagation of strongly magnetized hypersonic, but sub-Alfv\'enic ($C^{2}_{\rm s} \ll V^{2}_{\rm jet} < V^{2}_{\rm A}$) flow and on the subsequent development of a current-driven (CD) kink instability. This instability may be responsible for the ``wiggled'' structures seen in sub-parsec scale (VLBI) 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 the jets can develop CD distortions in the trans-Alfv\'enic flow case, even when the flow itself is still strongly magnetically dominated. An internal non-axisymmetric body mode grows on time scales of order of the Alfv\'en crossing time and distorts the structure and magnetic configuration of the jet. The kink ($m=1$) mode of the CD instability, driven by the radial component of the Lorentz force, grows faster than other higher order modes ($m>1$). In the jet frame the mode grows locally and expands radially at each axial position where the jet is unstable: the instability, therefore, does not propagate as a wave along the jet length. A naturally-occurring, external helically magnetized wind, which is (quasi-) axially current-free, surrounds the well-collimated current-carrying jet and reduces velocity shear between the jet and external medium. This stabilizes the growth of MHD Kelvin-Helmholtz surface modes in the inner jet flow.Comment: 70 pages, 23 figures, 3 tables, Appendix, submitted to Ap

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-Alfv\'enic flow ($C_{\rm s}^2 << V_{\rm jet}^2 < V_{\rm A}^2$) 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-Alfv\'enic 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 Alfv\'en 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.Comment: 6 pages, 3 figures; to appear in Plasmas in the Laboratory and in the Universe, Como, Italy, 16-19 Sep, 200