A numerical study of magnetohydrodynamic jet collimation and stability.

摘要

We investigate the evolution and stability of extragalactic jets through nonlinear magnetohydrodynamic (MHD) simulation and linear eigenmode analysis. In the simulations of jet evolution, a small-scale equilibrium magnetic arcade is twisted by a differentially rotating accretion disk. These simulations produce a collimated outflow which is unstable to the current driven m=1 kink mode for low rotational velocities of the accretion disk relative to the Alfven speed of the coronal plasma. The growth rate of the kink mode in the jet is shown to be inversely related to the rotation rate of the disk, and the jet is stable for high rotation rates. Linear MHD calculations investigate the effect of rigid rotation on the kink mode in a cylindrical plasma These calculations show that the Coriolis force distorts the m=1 kink eigenmode and stabilizes it at rotation frequencies such that the rotation period is longer than a few Alfven times.;The nonlinear evolution of kink unstable jets is investigated in detail. We find that our choice of initial conditions (uniform mass density and a dipole magnetic field) produces a magnetic tower with a close fitting return magnetic flux and a decreasing magnetic pitch profile. The line-tying effect of the highly conducting accretion disk stabilizes the base of the jet allowing for the formation of a collimated outflow, even though the bulk of the column is unstable. A nonlinear transfer of magnetic energy via the kink mode results in an enhancement of the poloidal magnetic flux at the end of the jet at early times and a large scale helical structure in the bulk of the jet at late times.