DYNAMICS AND STRUCTURE OF 3-DIMENSIONAL POLOIDALLY MAGNETIZED SUPERMAGNETOSONIC JETS

A set of three-dimensional magnetohydrodynamical simulations of superm agnetosonic magnetized jets has been performed. The jets contain an eq uipartition primarily poloidal magnetic field, and the effect of jet d ensity on jet dynamics and structure is evaluated. The jet is precesse d at the origin to break the symmetry and to excite Kelvin-Helmholtz-u nstable helical modes. In the linear limit, observed structures are si milar in all simulations and can be produced by structures predicted t o arise as a result of instability. The amplitude of various unstable modes is evaluated. Most unstable modes do not reach the maximum ampli tudes estimated from the linear theory by computing displacement surfa ces associated with the modes. Surprisingly, even these large-amplitud e distortions are fitted reasonably well by displacement surfaces comp uted from the linear theory. Large-amplitude helical and elliptical di stortions lead to significant differences in the nonlinear development of the jets as a function of the jet density. Jets less dense than th e surrounding medium entrain material, lose energy through shock heati ng, and slow down relatively rapidly once large-amplitude distortions develop as a result of instability. Jets more dense than the surroundi ng medium lose much less energy as they entrain and accelerate the sur rounding medium. The dense jet maintains a high-speed spine that exhib its large-amplitude helical twisting and elliptical distortion over co nsiderable distance without disruption of internal jet structures as h appens for the less dense jets. This dense high-speed spine is surroun ded by a less dense sheath consisting of slower moving jet fluid and m agnetic field mixed with the external medium. Simulated synchrotron in tensity and fractional polarization images from these calculations pro vide a considerably improved connection between simulation results and jet observations than do images made using the fluid variables alone. Intensity structure in the dense jet simulation appears remarkably si milar to structure observed in the Cygnus A jet. These simulations sug gest that the extended jets in high-power radio sources propagate to s uch large distances without disruption by entrainment because they are surrounded by a lobe or cocoon whose density is less than the jet den sity.