Magnetohydrodynamic instabilities of astrophysical jets

The remarkable stability of astrophysical jets is not yet fully understood and requires further investigation. In order to study the effects of an antiparallel magnetic field topology on the linear stage and nonlinear evolution of the Kelvin Helmholtz (KH) instability, we performed direct numerical simulations to solve the ideal magnetohydrodynamic equations in a variety of initial configurations. Single shear layers presented growth rates of the instability higher than in the uniform (parallel) case, and a typical oscillation wave vector Ka 0.4. Vortical motions were observed for Alfv´en Mach numbers Ma > 2. The presence of tearing type magnetic islands, driven by the KH instability, reduced the magnetic field enhancement around the perimeter of the KH vortices proper of the KH instability and, subsequently, reduced the value of the magnetic saturation energy as compared to the uniform field case. The extended domain simulations showed an inverse cascade to bigger scales, more turbulent than in the uniform case. The lower magnetic amplification, due to the islands, moved the threshold for three-dimensional (3D) reorganization to a laminar flow from Ma . 50 (uniform) to Ma . 20 (antiparallel). Two-dimensional (2D) spatial slab-jet simulations showed episodic disruption and revival of the flow due to a magnetic field amplification process, previously believed to be present only in subsonic flows. This result, retrieved also in 3D simulations, is the same for uniform and antiparallel magnetic fields.