56,977 research outputs found
Velocity statistics from spectral line data: effects of density-velocity correlations, magnetic field, and shear
In a previous work Lazarian and Pogosyan suggested a technique to extract
velocity and density statistics, of interstellar turbulence, by means of
analysing statistics of spectral line data cubes. In this paper we test that
technique, by studying the effect of correlation between velocity and density
fields, providing a systematic analysis of the uncertainties arising from the
numerics, and exploring the effect of a linear shear. We make use of both
compressible MHD simulations and synthetic data to emulate spectroscopic
observations and test the technique. With the same synthetic spectroscopic
data, we also studied anisotropies of the two point statistics and related
those anisotropies with the magnetic field direction. This presents a new
technique for magnetic field studies. The results show that the velocity and
density spectral indices measured are consistent with the analytical
predictions. We identified the dominant source of error with the limited number
of data points along a given line of sight. We decrease this type of noise by
increasing the number of points and by introducing Gaussian smoothing. We argue
that in real observations the number of emitting elements is essentially
infinite and that source of noise vanishes.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
Scaling, Intermittency and Decay of MHD Turbulence
We discuss a few recent developments that are important for understanding of
MHD turbulence.
First, MHD turbulence is not so messy as it is usually believed. In fact, the
notion of strong non-linear coupling of compressible and incompressible motions
along MHD cascade is not tenable. Alfven, slow and fast modes of MHD turbulence
follow their own cascades and exhibit degrees of anisotropy consistent with
theoretical expectations. Second, the fast decay of turbulence is not related
to the compressibility of fluid. Rates of decay of compressible and
incompressible motions are very similar. Third, viscosity by neutrals does not
suppress MHD turbulence in a partially ionized gas. Instead, MHD turbulence
develops magnetic cascade at scales below the scale at which neutrals damp
ordinary hydrodynamic motions. Forth, density statistics does not exhibit the
universality that the velocity and magnetic field do. For instance, at small
Mach numbers the density is anisotropic, but it gets isotropic at high Mach
numbers. Fifth, the intermittency of magnetic field and velocity are different.
Both depend on whether the measurements are done in local system of reference
oriented along the local magnetic field or in the global system of reference
related to the mean magnetic field.Comment: 12 pages, Invited Review, Workshop on Theoretical Plasma Physics,
Trieste, Italy, 5-16 Jul
Magnetic Reconnection and Turbulent Mixing: From ISM to Clusters of Galaxies
Magnetic reconnection, or the ability of the magnetic field lines that are
frozen in plasma to change their topology, is a fundamental problem of
magnetohydrodynamics (MHD). We briefly examine the problem starting with the
well-known Sweet-Parker scheme, discuss effects of tearing modes, anomalous
resistivity and the concept of hyperresistivity. We show that the field
stochasticity by itself provides a way to enable fast reconnection even if, at
the scale of individual turbulent wiggles, the reconnection happens at the slow
Sweet-Parker rate. We show that fast reconnection allows efficient mixing of
magnetic field in the direction perpendicular to the local direction of
magnetic field. While the idea of stochastic reconnection still requires
numerical confirmation, our numerical simulations testify that mixing motions
perpendicular to the local magnetic field are up to high degree hydrodynamical.
This suggests that the turbulent heat transport should be similar to that in
non-magnetized turbulent fluid, namely, should have a diffusion coefficient
\sim LV_L, where V_L is the amplitude of the turbulent velocity and L is the
scale of the turbulent motions. We present numerical simulations which support
this conclusion. The application of this idea to thermal conductivity in
clusters of galaxies shows that this mechanism may dominate the diffusion of
heat and may be efficient enough to prevent cooling flow formation.Comment: 12 pages, 2 figures, invited talk at JENAM2002 - The Unsolved
Universe:Challenges for the Future (v2: minor changes
Density scaling and anisotropy in supersonic MHD turbulence
We study the statistics of density for supersonic turbulence in a medium with
magnetic pressure larger than the gaseous pressure. This study is motivated by
molecular cloud research. Our simulations exhibit clumpy density structures,
which contrast increases with the Mach number. At 10 Machs densities of some
clumps are three orders of magnitude higher than the mean density. These clumps
give rise to flat and approximately isotropic density spectrum corresponding to
the random distribution of clumps in space. We claim that the clumps originate
from our random, isotropic turbulence driving. When the contribution from those
clumps is suppressed by studying logarithm of density, the density statistics
exhibit scale-dependent anisotropy consistent with the models where density
structures arise from shearing by Alfv\'en waves. It is noteworthy that
originally such models were advocated for the case of low-Mach, nearly
incompressible turbulence.Comment: 4 pages, 4 figures, using emulateap
Super Jackstraws and Super Waterwheels
We construct various new BPS states of D-branes preserving 8 supersymmetries.
These include super Jackstraws (a bunch of scattered D- or (p,q)-strings
preserving supersymmetries), and super waterwheels (a number of D2-branes
intersecting at generic angles on parallel lines while preserving
supersymmetries). Super D-Jackstraws are scattered in various dimensions but
are dynamical with all their intersections following a common null direction.
Meanwhile, super (p,q)-Jackstraws form a planar static configuration. We show
that the SO(2) subgroup of SL(2,R), the group of classical S-duality
transformations in IIB theory, can be used to generate this latter
configuration of variously charged (p,q)-strings intersecting at various
angles. The waterwheel configuration of D2-branes preserves 8 supersymmetries
as long as the `critical' Born-Infeld electric fields are along the common
direction.Comment: 23 pages, 10 figure
Growth of Magnetic Fields Induced by Turbulent Motions
We present numerical simulations of driven magnetohydrodynamic (MHD)
turbulence with weak/moderate imposed magnetic fields. The main goal is to
clarify dynamics of magnetic field growth. We also investigate the effects of
the imposed magnetic fields on the MHD turbulence, including, as a limit, the
case of zero external field. Our findings are as follows. First, when we start
off simulations with weak mean magnetic field only (or with small scale random
field with zero imposed field), we observe that there is a stage at which
magnetic energy density grows linearly with time. Runs with different numerical
resolutions and/or different simulation parameters show consistent results for
the growth rate at the linear stage. Second, we find that, when the strength of
the external field increases, the equilibrium kinetic energy density drops by
roughly the product of the rms velocity and the strength of the external field.
The equilibrium magnetic energy density rises by roughly the same amount.
Third, when the external magnetic field is not very strong (say, less than ~0.2
times the rms velocity when measured in the units of Alfven speed), the
turbulence at large scales remains statistically isotropic, i.e. there is no
apparent global anisotropy of order B_0/v. We discuss implications of our
results on astrophysical fluids.Comment: 16 pages, 18 figures; ApJ, accepte
Schwinger Effect in Non-parallel D1-branes: A Path Integral Approach
We study the Schwinger effect in a system of non-parallel D1-branes for the
bosonic strings using the path integral formalism. We drive the string pair
creation rate by calculating the one loop vacuum amplitude of the setup in
presence of the background electric filed defined along one of the D1-branes.
We find an angle dependent minimum value for the background field and show that
the decaying of vacuum into string pairs takes place for the field above this
value. It is shown that in limit the vacuum
becomes stable and thus no pair creation occurs
Transverse Bragg-reflector injection lasers
A GaAs-GaAlAs injection laser has been tested that confines light in the lateral dimension (normal to junction plane) by a multilayer Bragg reflector. In the past, light has been confined as a result of the higher-index guiding region and resulting evanescent fields
Compressible Sub-Alfvenic MHD turbulence in Low-beta Plasmas
We present a model for compressible sub-Alfvenic isothermal
magnetohydrodynamic (MHD) turbulence in low-beta plasmas and numerically test
it. We separate MHD fluctuations into 3 distinct families - Alfven, slow, and
fast modes. We find that, production of slow and fast modes by Alfvenic
turbulence is suppressed. As a result, Alfven modes in compressible regime
exhibit scalings and anisotropy similar to those in incompressible regime. Slow
modes passively mimic Alfven modes. However, fast modes show isotropy and a
scaling similar to acoustic turbulence.Comment: 4 pages, 8 figures, Phys. Rev. Lett., in pres
- âŠ