Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds

Two three-dimensional magnetohydrodynamical simulations of strongly magnetized conical jets, one with a poloidal and one with a helical magnetic field, have been performed. In the poloidal simulation a significant sheath (wind) of magnetized moving material developed and partially stabilized the jet to helical twisting. The fundamental pinch mode was not similarly affected and emission knots developed in the poloidal simulation. Thus, astrophysical jets surrounded by outflowing winds could develop knotty structures along a straight jet triggered by pinching. Where helical twisting dominated the dynamics, magnetic field orientation along the line-of-sight could be 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. Theoretical analysis showed that the different dynamical behavior of the two simulations could be entirely understood as a result of dependence on the velocity shear between jet and wind which must exceed a surface Alfven speed before the jet becomes unstable to helical and higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September

Plasma physics in clusters of galaxies

Clusters of galaxies are the largest self-gravitating structures in the universe. Each cluster is filled with a large-scale plasma atmosphere, in which primordial matter is mixed with matter that has been processed inside stars. This is a wonderful plasma physics laboratory. Our diagnostics are the data we obtain from X-ray and radio telescopes. The thermal plasma is a strong X-ray source; from this we determine its density and temperature. Radio data reveal a relativistic component in the plasma, and first measurements of the intracluster magnetic field have now been made. Energization of the particles and the field must be related to the cosmological evolution of the cluster. The situation is made even richer by the few galaxies in each cluster which host radio jets. In these galaxies, electrodynamics near a massive black hole in the core of the galaxy lead to a collimated plasma beam which propagates from the nucleus out to supergalactic scales. These jets interact with the cluster plasma to form the structures known as radio galaxies. The interaction disturbs and energizes the cluster plasma. This complicates the story but also helps us understand both the radio jets and the cluster plasma.Comment: 12 pages, 6 figures, 3 in color. Invited review, to appear in Physics of Plasmas, May 2003. After publication it can be found at http://ojps.aip.org/po

3C454.3 reveals the structure and physics of its 'blazar zone'

Recent multi-wavelength observations of 3C454.3, in particular during its giant outburst in 2005, put severe constraints on the location of the 'blazar zone', its dissipative nature, and high energy radiation mechanisms. As the optical, X-ray, and millimeter light-curves indicate, significant fraction of the jet energy must be released in the vicinity of the millimeter-photosphere, i.e. at distances where, due to the lateral expansion, the jet becomes transparent at millimeter wavelengths. We conclude that this region is located at ~10 parsecs, the distance coinciding with the location of the hot dust region. This location is consistent with the high amplitude variations observed on ~10 day time scale, provided the Lorentz factor of a jet is ~20. We argue that dissipation is driven by reconfinement shock and demonstrate that X-rays and gamma-rays are likely to be produced via inverse Compton scattering of near/mid IR photons emitted by the hot dust. We also infer that the largest gamma-to-synchrotron luminosity ratio ever recorded in this object - having taken place during its lowest luminosity states - can be simply due to weaker magnetic fields carried by a less powerful jet.Comment: 19 pages, 3 figures, accepted for publication in Ap

On the Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets

Three-dimensional magnetohydrodynamical simulations of strongly magnetized ``light'' conical jets have been performed. An investigation of the transition from sub-Alfv\'enic to super-Alfv\'enic 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-Alfv\'enic over most of their interior but destabilize rapidly when they become on average super-Alfv\'enic. The jets are precessed at the origin and the resulting small amplitude azimuthal motion is communicated down the jet to the Alfv\'en 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.Comment: 27 pages, 11 figures, AASTeX, to appear in Oct 20 issue of ApJ, postscript versions of Figures 5 and 6 are available at this URL http://crux.astr.ua.edu/~rosen/tralf/hr.htm

The structure of black hole magnetospheres. I. Schwarzschild black holes

We introduce a multipolar scheme for describing the structure of stationary, axisymmetric, force-free black-hole magnetospheres in the ``3+1'' formalism. We focus here on Schwarzschild spacetime, giving a complete classification of the separable solutions of the stream equation. We show a transparent term-by-term analogy of our solutions with the familiar multipoles of flat-space electrodynamics. We discuss electrodynamic processes around disk-fed black holes in which our solutions find natural applications: (a) ``interior'' solutions in studies of the Blandford-Znajek process of extracting the hole's rotational energy, and of the formation of relativistic jets in active galactic nuclei and ``microquasars'', and, (b) ``exterior'' solutions in studies of accretion disk dynamos, disk-driven winds and jets. On the strength of existing numerical studies, we argue that the poloidal field structures found here are also expected to hold with good accuracy for rotating black holes, except for maximum possible rotation rates. We show that the closed-loop exterior solutions found here are not in contradiction with the Macdonald-Thorne theorem, since these solutions, which diverge logarithmically on the hole's horizon $\cal H$, apply only to those regions which exclude $\cal H$.Comment: 6 figures. Accepted for publication by MNRA

The Effect of Expansion on Mass Entrainment and Stability of Super-Alfv\'enic Jets

We extend investigations of mass entrainment by jets, which previously have focused on cylindrical supermagnetosonic jets and expanding trans-Alfv\'enic jets, to a set of expanding supermagnetosonic jets. We precess these jets at the origin to excite the helical mode of the Kelvin-Helmholtz (or KH) instability, in order to compare the results with predictions from linear stability analysis. We analyze this simulation set for the spatial development of magnetized mass, which we interpret as jet plus entrained, initially unmagnetized external mass. As with the previous simulation sets, we find that the growth of magnetized mass is associated with the growth of the KH instability through linear, nonlinear, and saturated stages and with the expansion of magnetized material in simulated observations of the jet. From comparison of measured wavelengths and wave speeds with the predictions from linear stability analysis, we see evidence that the KH instability is the primary cause for mass entrainment in these simulations, and that the expansion reduces the rate of mass entrainment. This reduced rate can be observed as a somewhat greater distance between the two transition points separating the three stages of expansion.Comment: 18 pages, 6 figures, AASTeX, to appear in Nov 1 issue of ApJ (vol 543), postscript versions of Figures 3 and 5 are available at http://crux.astr.ua.edu/~rosen/supcon/rh.htm

Magnetocentrifugal Winds in 3D: Nonaxisymmetric Steady State

Outflows can be loaded and accelerated to high speeds along rapidly rotating, open magnetic field lines by centrifugal forces. Whether such magnetocentrifugally driven winds are stable is a longstanding theoretical problem. As a step towards addressing this problem, we perform the first large-scale 3D MHD simulations that extend to a distance $\sim 10^2$ times beyond the launching region, starting from steady 2D (axisymmetric) solutions. In an attempt to drive the wind unstable, we increase the mass loading on one half of the launching surface by a factor of $\sqrt{10}$, and reduce it by the same factor on the other half. The evolution of the perturbed wind is followed numerically. We find no evidence for any rapidly growing instability that could disrupt the wind during the launching and initial phase of propagation, even when the magnetic field of the magnetocentrifugal wind is toroidally dominated all the way to the launching surface. The strongly perturbed wind settles into a new steady state, with a highly asymmetric mass distribution. The distribution of magnetic field strength is, in contrast, much more symmetric. We discuss possible reasons for the apparent stability, including stabilization by an axial poloidal magnetic field, which is required to bend field lines away from the vertical direction and produce a magnetocentrifugal wind in the first place.Comment: 10 pages, 2 figures, accepted for publication in ApJ

Structure and Stability of Keplerian MHD Jets

MHD jet equilibria that depend on source properties are obtained using a simplified model for stationary, axisymmetric and rotating magnetized outflows. The present rotation laws are more complex than previously considered and include a Keplerian disc. The ensuing jets have a dense, current-carrying central core surrounded by an outer collar with a return current. The intermediate part of the jet is almost current-free and is magnetically dominated. Most of the momentum is located around the axis in the dense core and this region is likely to dominate the dynamics of the jet. We address the linear stability and the non-linear development of instabilities for our models using both analytical and 2.5-D numerical simulation's. The instabilities seen in the simulations develop with a wavelength and growth time that are well matched by the stability analysis. The modes explored in this work may provide a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap