The magnetohydrodynamics of supersonic gas clouds: MHD cosmic bullets and wind-swept clumps

We report an extensive set of two-dimensional MHD simulations exploring the role and evolution of magnetic fields in the dynamics of supersonic plasma clumps. We examine the influence of both ambient field strength and orientation on the problem. Of those two characteristics, field orientation is far more important in the cases we have considered with beta(0) = p(g)/p(b) greater than or equal to 1. That is due to the geometry-sensitivity of field stretching/amplification from large-scale shearing motions around the bullet. When the ambient magnetic field is transverse to the bullet motion, even a very modest field, well below equipartition strength, can be amplified by field line stretching around the bullet within a couple of bullet crushing times so that Maxwell stresses become comparable to the ram pressure associated with the bullet motion. The possibility is discussed that those situations might lead to large, induced electric potentials capable of accelerating charged particles. When the ambient field is aligned to the bullet motion, on the other hand, reconnection-prone topologies develop that shorten the stretched field and release much of the excess energy it contains. In this geometry, the Maxwell stresses on the bullet never approach the ram pressure level. In both cases, however, the presence of a field with even moderate initial strength acts to help the flow realign itself around the bullet into a smoother, more laminar form. That reduces bullet fragmentation tendencies caused by destructive instabilities. Eddies seem less effective at held amplification than flows around the bullet, because fields within eddies tend to be expelled to the eddy perimeters. Similar effects cause the magnetic field within the bullet itself to be reduced below its initial value over time. For oblique fields, we expect that the transverse field cases modeled here are more generally relevant. What counts is whether field lines threading the face of the bullet are swept around it in a fashion that folds them (leading to reconnection) or that keeps them unidirectional one each side of the bullet. In the second instance, behaviors should resemble those of the transverse field cases. We estimate that this quasi-transverse behavior is appropriate whenever the angle, theta, between the motion and the field satisfies tan theta greater than or similar to 1/M, where M is the bullet Mach number. From these simulations, we find support in either held geometry for the conclusions reached in previous studies that nonthermal radio emission associated with supersonic clumps is likely to be controlled largely by the generation of strong magnetic fields around the perimeters of the clumps, rather than local particle acceleration and field compression within the bow shock. In addition, since the magnetic pressure on the nose of the bullet likely becomes comparable to the ram pressure and hence the total pressure behind the bow shock, the gas pressure there could be substantially lower than that in a gasdynamical bullet. That means, as well, that the temperature in the region on the nose of the bullet would be lower than that predicted in the gasdynamical case. That detail could alter expectations of the thermal emission, including X-rays and UV-IR linesopen554

3D MHD simulations of radio galaxies including non-thermal electron transport

We report on an effort to study the connections between dynamics in simulated radio galaxy plasma flows and the properties of non-thermal electron populations carried in those flows. To do this we have introduced a new numerical scheme for electron transport that allows a much more detailed look at this problem than has been possible before. Especially when the dynamics axe fully three dimensional the flows are generally chaotic in the cocoon, and the jet itself can flail about violently. The bending jet can pinch itself off and redirect itself to enhance its penetration of the ambient medium. These behaviours often eliminate the presence of a strong jet termination shock, which is assumed present in all modern cartoon models of the radio galaxy phenomenon. Instead a much more complex "shock web" forms near the end of the jet that leads to a far less predictable pattern of particle acceleration. Similarly, the magnetic fields in these flows are highly filamented, as well as spatially and temporally intermittent. This leads to a very localized and complex pattern of synchrotron aging for relativistic electron populations, which makes it difficult to use properties of the electron spectrum to infer the local rate of aging

Computation of relativistic electron acceleration, transport and emissions in complex astrophysical flows

Relativistic hadrons and leptons are ubiquitous constituents in astrophysical plasmas. Their presence reflects the breakdown of thermodynamic behaviors in collisionless plasmas, particularly at shocks. They can carry substantial energy, and their electromagnetic emissions often provide most of the information available about conditions in such environments. We have developed efficient and practical numerical approaches to computational simulations of the generation and transport of these so-called cosmic-rays. Here we report on a new method and its application for simulating relativistic electrons in supersonic MHD flows, particularly as it applies to radio galaxies. Additionally, we have developed tools that enable us to synthesize astronomical observations of our simulated objects that can be used to study them in ways identical to those of observational astronomers, thus enabling more meaningful comparisons of dynamical simulations with observations than has been possible before. (C) 2002 Elsevier Science B.V. All rights reservedclose0

Nonthermal emission in radio galaxies from simulated relativistic electron transport in 3D MHD flows

We perform a series of so-called "synthetic observations" on a set of 3D MHD jet simulations which explicitly include energy-dependent transport of relativistic electrons, as described in the companion paper by Jones, Tregillis and Ryu. Analyzing them in light of the complex source dynamics and energetic particle distributions described in that paper, we find that the standard model for radiative aging in radio galaxies does not always adequately reflect the detailed source structure