163 research outputs found
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
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
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
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
The Propagation of Magneto-Centrifugally Launched Jets: I
We present simulations of the propagation of magnetized jets. This work
differs from previous studies in that the cross-sectional distributions of the
jets's state variables are derived from analytical models for
magneto-centrifugal launching. The source is a magnetized rotator whose
properties are specfied as boundary conditions. The jets in these simulations
are considerably more complex than the ``top-hat''constant density etc.
profiles used in previous work. We find that density and magnetic field
stratification (with radius) in the jet leads to new behavior including the
separation of an inner jet core from a low density collar. We find this {\it
jet within a jet} structure, along with the magnetic stresses, leads to
propagation behaviors not observed in previous simulation studies. Our
methodology allows us to compare MHD jets from different types of sources whose
properties could ultimately be derived from the behavior of the propagating
jets.Comment: 42 pages, accepted by the Ap
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 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 , 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
Sheared Flow As A Stabilizing Mechanism In Astrophysical Jets
It has been hypothesized that the sustained narrowness observed in the
asymptotic cylindrical region of bipolar outflows from Young Stellar Objects
(YSO) indicates that these jets are magnetically collimated. The j cross B
force observed in z-pinch plasmas is a possible explanation for these
observations. However, z-pinch plasmas are subject to current driven
instabilities (CDI). The interest in using z-pinches for controlled nuclear
fusion has lead to an extensive theory of the stability of magnetically
confined plasmas. Analytical, numerical, and experimental evidence from this
field suggest that sheared flow in magnetized plasmas can reduce the growth
rates of the sausage and kink instabilities. Here we propose the hypothesis
that sheared helical flow can exert a similar stabilizing influence on CDI in
YSO jets.Comment: 13 pages, 2 figure
Cosine and Sine Operators Related with Orthogonal Polynomial Sets on the Intervall [-1,1]
The quantization of phase is still an open problem. In the approach of
Susskind and Glogower so called cosine and sine operators play a fundamental
role. Their eigenstates in the Fock representation are related with the
Chebyshev polynomials of the second kind. Here we introduce more general cosine
and sine operators whose eigenfunctions in the Fock basis are related in a
similar way with arbitrary orthogonal polynomial sets on the intervall [-1,1].
To each polynomial set defined in terms of a weight function there corresponds
a pair of cosine and sine operators. Depending on the symmetry of the weight
function we distinguish generalized or extended operators. Their eigenstates
are used to define cosine and sine representations and probability
distributions. We consider also the inverse arccosine and arcsine operators and
use their eigenstates to define cosine-phase and sine-phase distributions,
respectively. Specific, numerical and graphical results are given for the
classical orthogonal polynomials and for particular Fock and coherent states.Comment: 1 tex-file (24 pages), 11 figure
3D Relativistic Magnetohydrodynamic Simulations of Magnetized Spine-Sheath Relativistic Jets
Numerical simulations of weakly magnetized and strongly magnetized
relativistic jets embedded in a weakly magnetized and strongly magnetized
stationary or weakly relativistic (v = c/2) sheath have been performed. A
magnetic field parallel to the flow is used in these simulations performed by
the new GRMHD numerical code RAISHIN used in its RMHD configuration. In the
numerical simulations the Lorentz factor jet is precessed to
break the initial equilibrium configuration. In the simulations sound speeds
are in the weakly magnetized simulations and in the strongly magnetized simulations. The Alfven wave speed is
in the weakly magnetized simulations and in
the strongly magnetized simulations. The results of the numerical simulations
are compared to theoretical predictions from a normal mode analysis of the
linearized relativistic magnetohydrodynamic (RMHD) equations capable of
describing a uniform axially magnetized cylindrical relativistic jet embedded
in a uniform axially magnetized relativistically moving sheath. The theoretical
dispersion relation allows investigation of effects associated with maximum
possible sound speeds, Alfven wave speeds near light speed and relativistic
sheath speeds. The prediction of increased stability of the weakly magnetized
system resulting from c/2 sheath speeds and the stabilization of the strongly
magnetized system resulting from c/2 sheath speeds is verified by the numerical
simulation results.Comment: 31 pages, 8 figures, accepted for publicatin in ApJ. A paper with
high resolution figures available at
http://gammaray.nsstc.nasa.gov/~mizuno/research_new.htm
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