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-Alfvénic (C2 s ≪ V 2 jet < V 2 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-Alfvénic 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 Alfvén 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. CD instabilities have a number of features that make them an attractive explanation for the helical jet structure observed in AGN and pulsars: 1) because the magnetic field remains strong, CD instabilities do not develop into full MHD turbulence; 2) the helical structures saturate and advect with the 1NRC Research Fellow