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M. Nakamura, D. L. Meier (Jet Propulsion Laboratory)
The detailed numerical analysis of nonlinear Poynting flux dominated (PFD) jets, powered by the torsional Alfvén waves (TAWs) propagating into extended stratified atmospheres, has been examined in 3-D MHD simulations. We revealed that effects such as MHD shocks or the re-distribution of the axial current profile in the post-shocked region can strongly affect the excitation of the current-driven (CD) kink instability (m=1). This may be responsible for the ``wiggled'' structures seen in sub-parsec scale (VLBI) AGN jets and pulsar jets.
Due to an inertial effect, rotating jets can be stabilized against the CD kink mode beyond the point predicted by the classical Kruskal--Shafranov criterion. The kink destabilization is caused by a sudden decrease of jet rotation (a loss of kinetic angular momentum to the magnetic field), along with a concentration poloidal magnetic flux toward the central axis via nonlinear processes that occur at a {\em reverse slow-mode MHD shock wave}. The CD kink distortion {\em does not propagate} in the jet rest frame. This behavior is quite different from that of the KH instability. The kinked patterns can be advected by the jet bulk flow even after saturation of that instability has occurred (the CD instability does not destroy the interior of the jet). As a result, the local bulk flow itself will appear to travel in a {\em true helical pattern} as it follows the magnetic backbone of the helix.
The CD instabilities of the PFD jets have a number of features that make them an attractive explanation for the helical jet structure observed in AGN and pulsars. However, the CD instability properties have not been given much importance in the astrophysical jet community for the past several decades. Reaching this level of understanding in the non-relativistic regime is a major milestone for us before our beginning to explore the CD instabilities of the relativistic PFD jets. M.N. is supported by a NRC Resident Research Associateship, sponsored by NASA.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.