Dynamics and Structure of Three-dimensional Trans-Alfvénic Jets. II. The Effect of Density and Winds

Two three-dimensional magnetohydrodynamic simulations of strongly magnetized conical jets, one with a poloidal and one with a helical magnetic field, have been performed. In both simulations the jet is precessed at the origin to break the symmetry. The jets are denser by an order of magnitude than the jets in a previous set of simulations. Theoretical work accompanying the previous simulations indicated that an increase in jet density would stabilize the jets beyond the Alfvén point, and this prediction is verified by the present simulations. In the poloidal simulation a significant sheath of magnetized moving material developed around the jet and led to additional stabilization. New theoretical work analyzing the effect of a jet embedded in a magnetized wind shows that the velocity shear, Δu≡u_j-u_e, must exceed a ``surface'' Alfvén speed, V_As≡[(ρ_j+ρ_e)(B²_j+B²_e)/(4πρ_jρ_e)]^1/2, before the jet becomes unstable to helical and higher order modes of jet distortion, and this explains the enhanced stability found in the poloidal simulation. The fundamental pinch mode is not similarly affected, and emission knots with spacing ~4R_j developed in the poloidal simulation. Thus, we identify a mechanism that can suppress large-scale asymmetric structures while allowing axisymmetric structures to develop, and astrophysical jets surrounded by outflowing winds will be more stable than if surrounded by a stationary or backflowing external medium. Knotty structures along a straight protostellar jet like the jet in HH 34 or in the inner part of the jet in HH 111 could be triggered by pinching of a low magnetosonic Mach number jet surrounded by a suitable wind. Of additional interest is the development of magnetic field orientation along the line of sight organized by the toroidal flow field accompanying helical twisting. On astrophysical jets such structure could lead to a reversal of the direction of Faraday rotation in adjacent zones along a jet. Thus, Faraday rotation structure like that seen along the 3C 465 jet could be attributed to organized magnetohydrodynamic structures produced by the jet flow.