Dynamics and structure of three-dimensional poloidally magnetized supermagnetosonic jets

Abstract

A set of three-dimensional magnetohydrodynamical simulations of supermagnetosonic magnetized jets has been performed. The jets contain an equipartition primarily poloidal magnetic field, and the effect of jet density on jet dynamics and structure is evaluated. The jet is precessed at the origin to break the symmetry and to excite Kelvin-Helmholtz-unstable helical modes. In the linear limit, observed structures are similar in all simulations and can be produced by structures predicted to arise as a result of instability. The amplitude of various unstable modes is evaluated. Most unstable modes do not reach the maximum amplitudes estimated from the linear theory by computing displacement surfaces associated with the modes. Surprisingly, even these large-amplitude distortions are fitted reasonably well by displacement surfaces computed from the linear theory. Large-amplitude helical and elliptical distortions lead to significant differences in the nonlinear development of the jets as a function of the jet density. Jets less dense than the surrounding medium entrain material, lose energy through shock heating, and slow down relatively rapidly once large-amplitude distortions develop as a result of instability. Jets more dense than the surrounding medium lose much less energy as they entrain and accelerate the surrounding medium. The dense jet maintains a high-speed spine that exhibits large-amplitude helical twisting and elliptical distortion over considerable distance without disruption of internal jet structures as happens for the less dense jets. This dense high-speed spine is surrounded by a less dense sheath consisting of slower moving jet fluid and magnetic field mixed with the external medium. Simulated synchrotron intensity and fractional polarization images from these calculations provide 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 similar to structure observed in the Cygnus A jet. These simulations suggest that the extended jets in high-power radio sources propagate to such large distances without disruption by entrainment because they are surrounded by a lobe or cocoon whose density is less than the jet density.