Dynamics and Structure of Three‐dimensional Trans‐Alfvenic Jets. II. The Effect of Density and Winds

Abstract

Two three-dimensional magnetohydrodynamic simulations of stronglymagnetized conical jets, one with a poloidal and one with a helicalmagnetic field, have been performed. In both simulations the jet isprecessed at the origin to break the symmetry. The jets are denser by anorder of magnitude than the jets in a previous set of simulations.Theoretical work accompanying the previous simulations indicated that anincrease in jet density would stabilize the jets beyond theAlfv�n point, and this prediction is verified by the presentsimulations. In the poloidal simulation a significant sheath ofmagnetized moving material developed around the jet and led toadditional stabilization. New theoretical work analyzing the effect of ajet embedded in a magnetized wind shows that the velocity shear,Deltau≡uj-ue, must exceed a ``surface''Alfv�n speed,VAs≡[(rhoj+rhoe)(B2j+B2e)/(4pirhojrhoe)]1/2, before the jet becomes unstableto helical and higher order modes of jet distortion, and this explainsthe enhanced stability found in the poloidal simulation. The fundamentalpinch mode is not similarly affected, and emission knots with spacing~4Rj developed in the poloidal simulation. Thus, we identifya mechanism that can suppress large-scale asymmetric structures whileallowing axisymmetric structures to develop, and astrophysical jetssurrounded by outflowing winds will be more stable than if surrounded bya stationary or backflowing external medium. Knotty structures along astraight protostellar jet like the jet in HH 34 or in the inner part ofthe jet in HH 111 could be triggered by pinching of a low magnetosonicMach number jet surrounded by a suitable wind. Of additional interest isthe development of magnetic field orientation along the line of sightorganized by the toroidal flow field accompanying helical twisting. Onastrophysical jets such structure could lead to a reversal of thedirection of Faraday rotation in adjacent zones along a jet. Thus,Faraday rotation structure like that seen along the 3C 465 jet could beattributed to organized magnetohydrodynamic structures produced by thejet flow.