Poynting Flux-dominated Jets in Decreasing-Density Atmospheres. I. The Nonrelativistic Current-driven Kink Instability and the Formation of “Wiggled” Structures

Nonrelativistic three-dimensional magnetohydrodynamic (MHD) simulations of Poynting flux-dominated (PFD) jets are presented. Our study focuses on the propagation of strongly magnetized hypersonic but sub-Alfvénic (C ≪ V < V) flow and on the subsequent development of a current-driven (CD) kink instability. This instability may be responsible for the "wiggled" structures seen in subparsec-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 nonaxisymmetric body mode grows on timescales of order 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 active galactic nuclei 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 bulk flow; (3) they distort the body of the jet, not merely its interface with the ambient medium; (4) local plasma flow, then, follows a helical path along the kinked magnetic field backbone. 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.