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

Abstract

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-Alfvenic (C V 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.