Numerical Simulations of Axially Symmetric Magnetized Jets - Part Three - Collimation of Underexpanded Jets by Magnetic Fields

Using a finite difference scheme based on the FCT-algorithm, we have performed a set of hydrodynamic and magnetohydrodynamic simulations to investigate the stability of both magnetized and unmagnetized underexpanded (ζ_H_ = 10) jets. We find that unmagnetized jets with a fairly high Mach number (Ma_B_ = 6) are stable; low Mach number jets (Ma_B_ <~ 2), on the other hand, exhibit prominent instabilities. A toroidal magnetic fie1d of approximately equipartition field strength is capable of stabilizing these instabilities due to the pinch effect. However, it changes the morphology of the jet completely by generating a nose cone. The influence of an additional poloidal magnetic field, with a field strength comparable to the fie1d strength of the toroidal component, is small. Its main effect is that it forces part of the nose cone into a sonic rotation.