133 research outputs found
Numerical Simulations of Incompressible MHD Turbulence
The study of incompressible magnetohydrodynamic (MHD) turbulence gives useful
insights on many astrophysical problems. We describe a pseudo-spectral MHD code
suitable for the study of incompressible turbulence. We review our recent works
on direct three-dimensional numerical simulations for MHD turbulence in a
periodic box. In those works, we use a pseudo-spectral code to solve the
incompressible MHD equations. We first discuss the structure and properties of
turbulence as functions of scale. The results are consistent with the scaling
law recently proposed by Goldreich & Sridhar. The scaling law is based on the
concept of scale-dependent isotropy: smaller eddies are more elongated than
larger ones along magnetic field lines. This scaling law substantially changes
our views on MHD turbulence. For example, as noted by Lazarian & Vishniac, the
scaling law can provide a fast reconnection rate. We further discuss how the
study of incompressible MHD turbulence can help us to understand physical
processes in interstellar medium (ISM) by considering imbalanced cascade and
viscous damped turbulence.Comment: 5 pages; 3 figures; Conference proceedin
Fast Diffusion of Magnetic Field in Turbulence and Origin of Cosmic Magnetism
Turbulence is believed to play important roles in the origin of cosmic
magnetism. While it is well known that turbulence can efficiently amplify a
uniform or spatially homogeneous seed magnetic field, it is not clear whether
or not we can draw a similar conclusion for a localized seed magnetic field.
The main uncertainty is the rate of magnetic field diffusion on scales larger
than the outer scale of turbulence. To measure the diffusion rate of magnetic
field on those large scales, we perform a numerical simulation in which the
outer scale of turbulence is much smaller than the size of the system. We
numerically compare diffusion of a localized seed magnetic field and a
localized passive scalar. We find that diffusion of the magnetic field can be
much faster than that of the passive scalar and that turbulence can efficiently
amplify the localized seed magnetic field. Based on the simulation result, we
construct a model for fast diffusion of magnetic field. Our model suggests that
a localized seed magnetic field can fill the whole system in (L_sys/L) times
the large-eddy turnover time and that growth of the magnetic field stops in
max(15, L_sys/L) times the large-eddy turnover time, where L_sys is the size of
the system and is the driving scale. Our finding implies that, regardless
of the shape of the seed field, fast magnetization is possible in turbulent
systems, such as large-scale structure of the universe or galaxies.Comment: 6 pages, 4 figures, published in PR
Polarization of FIR emission from T Tauri Disks
Recent observation of 850 micron sub-mm polarization from T Tauri disks opens
up the possibility of studying magnetic field structure within protostellar
disks. The degree of polarization is around 3 % and the direction of
polarization is perpendicular to the disk. Since thermal emission from dust
grains dominates the spectral energy distribution at the sub-mm/FIR regime,
dust grains are thought to be the cause of the polarization. We discuss grain
alignment by radiation and we explore the efficiency of dust alignment in T
Tauri disks. Calculations show that dust grains located far away from the
Central proto-star are more efficiently aligned. In the presence of a regular
magnetic field, the aligned grains produce polarized emission in sub-mm/FIR
wavelengths. The direction of polarization is perpendicular to the local
magnetic field direction. When we use a recent T Tauri disk model and take a
Mathis-Rumpl-Nordsieck-type distribution with maximum grain size of 500-1000
m, the degree of polarization is around 2-3 % level at wavelengths larger
than m. Our study indicates that multifrequency infrared
polarimetric studies of protostellar disks can provide good insights into the
details of their magnetic structure. We also provide predictions for polarized
emission for disks viewed at different wavelengths and viewing angles.Comment: 8 pages, 8 figures; submitted to RevMexAA (conference series
Imbalanced Relativistic Force-Free Magnetohydrodynamic Turbulence
When magnetic energy density is much larger than that of matter, as in
pulsar/black hole magnetospheres, the medium becomes force-free and we need
relativity to describe it. As in non-relativistic magnetohydrodynamics (MHD),
Alfv\'enic MHD turbulence in the relativistic limit can be described by
interactions of counter-traveling wave packets. In this paper we numerically
study strong imbalanced MHD turbulence in such environments. Here, imbalanced
turbulence means the waves traveling in one direction (dominant waves) have
higher amplitudes than the opposite-traveling waves (sub-dominant waves). We
find that (1) spectrum of the dominant waves is steeper than that of
sub-dominant waves, (2) the anisotropy of the dominant waves is weaker than
that of sub-dominant waves, and (3) the dependence of the ratio of magnetic
energy densities of dominant and sub-dominant waves on the ratio of energy
injection rates is steeper than quadratic (i.e., \$b_+^2/b_-^2 \propto
(\epsilon_+/\epsilon_-)^n \$ with n>2). These results are consistent with those
obtained for imbalanced non-relativistic Alfv\'enic turbulence. This
corresponds well to the earlier reported similarity of the relativistic and
non-relativistic balanced magnetic turbulence.Comment: 6 pages, 3 figure
Statistics of Galactic Synchrotron and Dust Foregrounds: Spectra, PDFs and Higher-Order Moments
We present statistical analysis of diffuse Galactic synchrotron emission and
polarized thermal emission from dust. Both Galactic synchrotron emission and
polarized thermal emission from dust reflect statistics of magnetic field
fluctuations and, therefore, Galactic turbulence. We mainly focus on the
relation between observed angular spectra and underlying turbulence statistics.
Our major findings are as follows. First, we find that magnetohydrodynamic
(MHD) turbulence in the Galaxy can indeed explain diffuse synchrotron emission
from high galactic latitude. Our model calculation suggests that either a
one-component extended halo model or a two-component model, an extended halo
component (scale height > 1kpc) plus a local component, can explain the
observed angular spectrum of the synchrotron emission. However, discrete
sources seem to dominate the spectrum for regions near the Galactic plane.
Second, we study how star-light polarization is related with polarized emission
from thermal dust. We also discuss the expected angular spectrum of polarized
emission from thermal dust. Our model calculations suggest that C_l\propto
l^{-11/3} for l > 1000 and a shallower spectrum for l < 1000.Comment: 15 pages, 11 figures; ApJ, submitted; We welcome comments from
foreground experts on the possibility of the filtering procedur
Growth of a localized seed magnetic field in a turbulent medium
Turbulence dynamo deals with amplification of a seed magnetic field in a
turbulent medium and has been studied mostly for uniform or spatially
homogeneous seed magnetic fields. However, some astrophysical processes (e.g.
jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy
clusters) can provide localized seed magnetic fields. In this paper, we
numerically study amplification of localized seed magnetic fields in a
turbulent medium. Throughout the paper, we assume that driving scale of
turbulence is comparable to the size of the system. Our findings are as
follows. First, turbulence can amplify a localized seed magnetic field very
efficiently. The growth rate of magnetic energy density is as high as that for
a uniform seed magnetic field. This result implies that a magnetic field
ejected from an astrophysical object can be a viable source of magnetic field
in a cluster. Second, the localized seed magnetic field disperses and fills the
whole system very fast. If turbulence in a system (e.g. a galaxy cluster or a
filament) is driven at large scales, we expect that it takes a few large-eddy
turnover times for magnetic field to fill the whole system. Third, growth and
turbulence diffusion of a localized seed magnetic field are also fast in high
magnetic Prandtl number turbulence. Fourth, even in decaying turbulence, a
localized seed magnetic field can ultimately fill the whole system. Although
the dispersal rate of magnetic field is not fast in purely decaying turbulence,
it can be enhanced by an additional forcing.Comment: 11 pages, 9 figures, ApJ (accepted
Effects of multiple-scale driving on turbulence statistics
Turbulence is ubiquitous in astrophysical fluids such as the interstellar
medium (ISM) and the intracluster medium (ICM). In turbulence studies, it is
customary to assume that fluid is driven on a single scale. However, in
astrophysical fluids, there can be many different driving mechanisms that act
on different scales. If there are multiple energy-injection scales, the process
of energy cascade and turbulence dynamo will be different compared with the
case of single energy-injection scale. In this work, we perform
three-dimensional incompressible/compressible magnetohydrodynamic (MHD)
turbulence simulations. We drive turbulence in Fourier space in two wavenumber
ranges, 2\$\leq k \leq \sqrt{12}\$ (large-scale) and 15 \$\lesssim k \lesssim
\$ 26 (small-scale). We inject different amount of energy in each range by
changing the amplitude of forcing in the range. We present the time evolution
of the kinetic and magnetic energy densities and discuss the turbulence dynamo
in the presence of energy injections at two scales. We show how kinetic,
magnetic and density spectra are affected by the two-scale energy injections
and we discuss the observational implications. In the case \$\epsilon_L <
\epsilon_S\$, where \$\epsilon_L\$ and \$\epsilon_S\$ are energy-injection
rates at the large and small scales, respectively, our results show that even a
tiny amount of large-scale energy injection can significantly change the
properties of turbulence. On the other hand, when \$\epsilon_L \gtrsim
\epsilon_S\$, the small-scale driving does not influence the turbulence
statistics much unless \$\epsilon_L \sim \epsilon_S\$.Comment: 17 pages, 6 figure
Scaling of ISM Turbulence: Implications for HI
Galactic HI is a gas that is coupled to magnetic field because of its
fractional ionization. Many properties of HI are affected by turbulence.
Recently, there has been a significant breakthrough on the theory of
magnetohydrodynamic (MHD) turbulence. For the first time in the history of the
subject we have a scaling model that is supported by numerical simulations. We
review recent progress in studies of both incompressible and compressible
turbulence. We also discuss the new regime of MHD turbulence that happens below
the scale at which conventional turbulent motions get damped by viscosity. The
viscosity in the case of HI is being produced by neutrals and truncates the
turbulent cascade at parsec scales. We show that below this scale magnetic
fluctuations with a shallow spectrum persist and point out to a possibility of
the resumption of the MHD cascade after ions and neutrals decouple. We discuss
the implications of the new insight into MHD turbulence for cosmic ray
transport and grain dynamics.Comment: 12pages, 12 figures, to appear in "Seeing Through the Dust: The
Detection of HI and the Exploration of the ISM in Galaxies", R. Taylor, T.
Landecker, & A. Willis (eds.), ASP Conference Serie
Origin of Magnetic Field in the Intracluster Medium: Primordial or Astrophysical?
The origin of magnetic fields in clusters of galaxies is still an unsolved
problem, which is largely due to our poor understanding of initial seed
magnetic fields. If the seed magnetic fields have primordial origins, it is
likely that large-scale pervasive magnetic fields were present before the
formation of the large-scale structure. On the other hand, if they were ejected
from astrophysical bodies, they were highly localized in space at the time of
injection. In this paper, using turbulence dynamo models for high magnetic
Prandtl number fluids, we find constraints on the seed magnetic fields. The
hydrodynamic Reynolds number based on the Spitzer viscosity in the intracluster
medium (ICM) is believed to be less than , while the magnetic Reynolds
number can be much larger than that. In this case, if the seed magnetic fields
have primordial origins, they should be stronger than G, which is
very close to the upper limit of G set by the cosmic microwave
background (CMB) observations. On the other hand, if the seed magnetic fields
were ejected from astrophysical bodies, any seed magnetic fields stronger than
G can safely magnetize the intracluster medium. Therefore, it is
less likely that primordial magnetic fields are the direct origin of
present-day magnetic fields in the ICM.Comment: 11 pages, 10 figures, Accepted for publication in the Ap
MHD Turbulence: Scaling Laws and Astrophysical Implications
Turbulence is the most common state of astrophysical flows. In typical
astrophysical fluids, turbulence is accompanied by strong magnetic fields,
which has a large impact on the dynamics of the turbulent cascade. Recently,
there has been a significant breakthrough on the theory of magnetohydrodynamic
(MHD) turbulence. For the first time we have a scaling model that is supported
by both observations and numerical simulations. We review recent progress in
studies of both incompressible and compressible turbulence. We compare
Iroshnikov-Kraichnan and Goldreich-Sridhar models, and discuss scalings of
Alfv\'en, slow, and fast waves. We also discuss the completely new regime of
MHD turbulence that happens below the scale at which hydrodynamic turbulent
motions are damped by viscosity. In the case of the partially ionized diffuse
interstellar gas the viscosity is due to neutrals and truncates the turbulent
cascade at parsec scales. We show that below this scale magnetic
fluctuations with a shallow spectrum persist and discuss the possibility of a
resumption of the MHD cascade after ions and neutrals decouple. We discuss the
implications of this new insight into MHD turbulence for cosmic ray transport,
grain dynamics, etc., and how to test theoretical predictions against
observations.Comment: review, 43 pages, 28 figures, submitted to "Simulations of
magnetohydrodynamic turbulence in astrophysics" Eds. T. Passot & E. Falgarone
(Springer Lecture Notes in Physics
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