Stability Properties of Magnetic Tower Jets

Stability properties of ``magnetic tower'' jets propagating in thegravitationally stratified background have been examined by performingthree-dimensional magnetohydrodynamic simulations. The current-carrying,Poynting flux-dominated magnetic tower jet, which possesses a highly woundhelical magnetic field, is subject to the current-driven instability (CDI). Wefind that, under general physical conditions including small perturbations inthe initial background profiles, the propagating magnetic tower jets developthe non-axisymmetric, $m=1$ kink mode of the CDI. The kink mode grows on thelocal Alfv\'en crossing time scale. In addition, two types of kink modes appearin the system. At the central region where external thermal pressureconfinement is strong, only the internal kink mode is excited and will grow. Alarge distance away from the central region where the external thermal pressurebecomes low, the external kink mode is observed. As a result, the exterior ofmagnetic tower jets will be deformed into a large-scale wiggled structure. Wealso discuss extensively the different physical processes that contribute tothe overall stability properties of the magnetic tower jets. Specifically, whenthe jet propagates in an initially unperturbed background, we find that theycan survive the kink mode beyond the point predicted by the well-knownKruskal-Shafranov (K-S) criterion. The stabilization in this case comes mainlyfrom the dynamical relaxation of magnetic twists during the propagation ofmagnetic towers; the magnetic pitch is reduced and the corresponding K-Scritical wavelength becomes longer as the tower jet proceeds. Furthermore, weshow that the pressure-driven and Kelvin-Helmholtz instabilities do not occurin the magnetic tower jets.Comment: 12 pages, 10 figures, submitted to Ap