456 research outputs found
Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets
A set of 3D MHD simulations of 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 excite Kelvin-Helmholtz unstable helical modes. We extensively
compare the structure in these simulations with linear stability theory. The
jet that is dense with respect to the external medium develops a high speed
core surrounded by a less dense sheath consisting of slower moving jet fluid.
These simulations suggest that extended extragalactic jets propagate to such
large distances because they are surrounded by a lobe or cocoon whose density
is less than the jet density. (Abridged abstract.)Comment: 30 pages, AASTeX, to appear in ApJ, much better versions of Figures
2-5 are available at http://crux.astr.ua.edu/~rosen/hcr/hcr.htm
A Study in Akka-based Distributed Ray-tracing of Large Scenes
This project creates a ray-tracing and geometry distribution framework through an actor model of parallelism, which is then expanded onto a cluster of machines to show effective data distribution across a network. This is shown to be feasible, but due to problems internal to the actor framework, as well as design failures, fails to effectively and consistently increase usable memory and generate larger ray-traces, though generally scaled well. Despite this, it compares several methods of ray organization across the geometry and shows that more complex methods generally scale better with the amount of geometry. A photometric renderer was added with very little modification, showing the generality of the geometry distribution framework, and the performance benefits of alternative serialization methods are shown to outweigh the drawbacks of more difficult implementation
Lentiviral vectors with amplified beta cell-specific gene expression.
An important goal of gene therapy is to be able to deliver genes, so that they express in a pattern that recapitulates the expression of an endogenous cellular gene. Although tissue-specific promoters confer selectivity, in a vector-based system, their activity may be too weak to mediate detectable levels in gene-expression studies. We have used a two-step transcriptional amplification system to amplify gene expression from lentiviral vectors using the human insulin promoter. In this system, the human insulin promoter drives expression of a potent synthetic transcription activator (the yeast GAL4 DNA-binding domain fused to the activation domain of the Herpes simplex virus-1 VP16 activator), which in turn activates a GAL4-responsive promoter, driving the enhanced green fluorescent protein reporter gene. Vectors carrying the human insulin promoter did not express in non-beta-cell lines, but expressed in murine insulinoma cell lines, indicating that the human insulin promoter was capable of conferring cell specificity of expression. The insulin-amplifiable vector was able to amplify gene expression five to nine times over a standard insulin-promoter vector. In primary human islets, gene expression from the insulin-promoted vectors was coincident with insulin staining. These vectors will be useful in gene-expression studies that require a detectable signal and tissue specificity
Particle Acceleration, Magnetic Field Generation, and Associated Emission in Collisionless Relativistic Jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray
bursts (GRBs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations using injected relativistic electron-ion
(electro-positron) jets show that acceleration occurs within the downstream
jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., the Buneman instability,
other two-streaming instability, and the Weibel instability) created in the
shocks are responsible for particle (electron, positron, and ion) acceleration.
The simulation results show that the Weibel instability is responsible for
generating and amplifying highly nonuniform, small-scale magnetic fields. These
magnetic fields contribute to the electron's transverse deflection behind the
jet head. The ``jitter'' radiation from deflected electrons has different
properties than synchrotron radiation which assumes a uniform magnetic field.
This jitter radiation may be important to understanding the complex time
evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and
supernova remnants.Comment: 4 pages, 3 figures, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007.
Fig. 3 is replaced by the correct versio
Stability Properties of Strongly Magnetized Spine Sheath Relativistic Jets
The linearized relativistic magnetohydrodynamic (RMHD) equations describing a
uniform axially magnetized cylindrical relativistic jet spine embedded in a
uniform axially magnetized relativistically moving sheath are derived. The
displacement current is retained in the equations so that effects associated
with Alfven wave propagation near light speed can be studied. A dispersion
relation for the normal modes is obtained. Analytical solutions for the normal
modes in the low and high frequency limits are found and a general stability
condition is determined. A trans-Alfvenic and even a super-Alfvenic
relativistic jet spine can be stable to velocity shear driven Kelvin-Helmholtz
modes. The resonance condition for maximum growth of the normal modes is
obtained in the kinetically and magnetically dominated regimes. Numerical
solution of the dispersion relation verifies the analytical solutions and is
used to study the regime of high sound and Alfven speeds.Comment: 42 pages includes 7 figures, to appear in Ap
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
Pseudospectra and structured pseudospectra
Pseudospectra and structured pseudospectra are subsets of the complex plane which give a geometric representation, via eigenvalues, of the effects of perturbations to a matrix. We survey the historical development of the subject, and the definitions and characterizations of the various sets of pseudospectra. Motivated by the fact that a nonnormal matrix in the 2-norm can become normal in a different norm, we describe a numerical investigation into the question of characterizing which perturbations have the greatest effect on the eigenvalues of the matrix
Resonant Kelvin-Helmholtz modes in sheared relativistic flows
Qualitatively new aspects of the (linear and non-linear) stability of sheared
relativistic (slab) jets are analyzed. The linear problem has been solved for a
wide range of jet models well inside the ultrarelativistic domain (flow Lorentz
factors up to 20; specific internal energies ). As a distinct
feature of our work, we have combined the analytical linear approach with
high-resolution relativistic hydrodynamical simulations, which has allowed us
i) to identify, in the linear regime, resonant modes specific to the
relativistic shear layer ii) to confirm the result of the linear analysis with
numerical simulations and, iii) more interestingly, to follow the instability
development through the non-linear regime. We find that very high-order
reflection modes with dominant growth rates can modify the global, long-term
stability of the relativistic flow. We discuss the dependence of these resonant
modes on the jet flow Lorentz factor and specific internal energy, and on the
shear layer thickness. The results could have potential applications in the
field of extragalactic relativistic jets.Comment: Accepted for publication in Physical Review E. For better quality
images, please check
http://www.mpifr-bonn.mpg.de/staff/mperucho/Research.htm
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