The current-driven kink instability in magnetically dominated relativistic jets

Abstract

We have investigated the development of current-driven (CD) kink instability in relativistic jets, via 3D relativistic magnetohydrodynamic simulations. For this purpose, a static force-free equilibrium helical magnetic configuration is considered in order to study its influence on the linear and nonlinear stages of the instability. We found that this configuration is strongly distorted but not disrupted by the CD kink instability. Both the linear growth and the nonlinear evolution of this in-stability depend moderately on the radial density profile but are strongly sensitive to the magnetic pitch profile. For decreasing magnetic pitch, kink amplitude growth leads, in the nonlinear regime, to a slender helically twisted column wrapped by magnetic field. Differently, for increasing magnetic pitch, the kink amplitude nearly saturates in the nonlinear regime. We have also investigated the influence of velocity shear on the linear and non-linear development of the instability. We found that helically distorted density structures propagate along the jet with a speed and a flow structure that are dependent on the location of the velocity shear relative to the characteristic radius of the helically twisted force-free magnetic field. At small radius, the plasma flows through the kink. The kink propagation speed increases with the velocity shear radius, and the kink becomes more em-bedded in the plasma flow. Larger velocity shear radius leads to slower linear growth, with a later transition to the nonlinear stage and a larger maximum amplitude than in the case of a static plasma column. However, when the velocity shear radius is much greater than the characteristic radius of the helical magnetic field, linear and non-linear developments become more similar to those of a static plasma column.