On the Dynamics and Structure of Three-dimensional Trans-Alfvénic Jets

Three-dimensional magnetohydrodynamical simulations of strongly magnetized ``light'' conical jets have been performed. An investigation of the transition from sub-Alfvénic to super-Alfvénic flow has been made for nearly poloidal and for helical magnetic fields. The jets are stable to asymmetric modes of jet distortion provided they are sub-Alfvénic over most of their interior but destabilize rapidly when they become on average super-Alfvénic. The jets are precessed at the origin, and the resulting small-amplitude azimuthal motion is communicated down the jet to the Alfvén point, where it couples to a slowly moving and rapidly growing helical twist. Significant jet rotation can contribute to destabilization via increase in the velocity shear between the jet and the external medium. Destabilization is accompanied by significant mass entrainment, and the jets slow down significantly as denser external material is entrained. Synchrotron intensity images satisfactorily reveal large-scale helical structures but have trouble distinguishing a large-amplitude elliptical jet distortion that appears as an apparent pinching in an intensity image. Smaller scale jet distortions are not clearly revealed in intensity images, largely as a result of the relatively small total pressure variations that accompany destabilization and growing distortions. Fractional polarization is high as a result of the strong ordered magnetic fields except where the intensity image suggests cancellation of polarization vectors by integration through twisted structures.