53 research outputs found
Large-Scale Magnetic-Field Generation by Randomly Forced Shearing Waves
A rigorous theory for the generation of a large-scale magnetic field by
random non-helically forced motions of a conducting fluid combined with a
linear shear is presented in the analytically tractable limit of low Rm and
weak shear. The dynamo is kinematic and due to fluctuations in the net
(volume-averaged) electromotive force. This is a minimal proof-of-concept
quasilinear calculation aiming to put the shear dynamo, a new effect recently
found in numerical experiments, on a firm theoretical footing. Numerically
observed scalings of the wavenumber and growth rate of the fastest growing
mode, previously not understood, are derived analytically. The simplicity of
the model suggests that shear dynamo action may be a generic property of
sheared magnetohydrodynamic turbulence.Comment: Paper substantially rewritten, results changed (relative to v1).
Revised versio
Magnetic fields of active galactic nuclei and quasars with polarized broad H-alpha lines
We present estimates of magnetic field in a number of AGNs from the
Spectropolarimetric atlas of Smith, Young & Robinson (2002) from the observed
degrees of linear polarization and the positional angles of spectral lines
(H-alpha) (broad line regions of AGNs) and nearby continuum. The observed
polarization is lower than the Milne value in a non-magnetized atmosphere. We
hypothesize that the polarized radiation escapes from optically thick
magnetized accretion discs and is weakened by the Faraday rotation effect. This
effect is able to explain both the value of the polarization and the position
angle. We estimate the required magnetic field in the broad line region by
using simple asymptotic analytical formulas for Milne's problem in magnetized
atmosphere, which take into account the last scattering of radiation before
escaping from the accretion disc. The polarization of a broad spectral line
escaping from disc is described by the same mechanism. The characteristic
features of polarization of a broad line is the minimum of the degree of
polarization in the center of the line and continuous rotation of the position
angle from one wing to another. These effects can be explained by existence of
clouds in the left (velocity is directed to an observer) and the right
(velocity is directed from an observer) parts of the orbit in a rotating
keplerian magnetized accretion disc. The base of explanation is existence of
azimuthal magnetic field in the orbit. The existence of normal component of
magnetic field makes the picture of polarization asymmetric. The existence of
clouds in left and right parts of the orbit with different emissions also give
the contribution in asymmetry effect. Assuming a power-law dependence of the
magnetic field inside the disc, we obtain the estimate of the magnetic field
strength at first stable orbit near the central SMBH for a number of AGNs.Comment: 15 pages, 4 figure
Generation of Magnetic Field by Combined Action of Turbulence and Shear
The feasibility of a mean-field dynamo in nonhelical turbulence with
superimposed linear shear is studied numerically in elongated shearing boxes.
Exponential growth of magnetic field at scales much larger than the outer scale
of the turbulence is found. The charateristic scale of the field is l_B ~
S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly
discovered shear dynamo effect potentially represents a very generic mechanism
for generating large-scale magnetic fields in a broad class of astrophysical
systems with spatially coherent mean flows.Comment: 4 pages, 5 figures; replaced with revised version that matches the
published PR
The polarization effects of radiation from magnetized envelopes and extended accretion structures
The results of numerical calculations of linear polarization from magnetized
spherical optically thick and optically thin envelopes are presented. We give
the methods how to distinguish magnetized optically thin envelopes from
optically thick ones using observed spectral distributions of the polarization
degree and the positional angle. The results of numerical calculations are used
for analysis of polarimetric observations of OB and WR stars, X-ray binaries
with black hole candidates (Cyg X-1, SS 433) and supernovae. The developed
method allows to estimate magnetic field strength for the objects mentioned
above.Comment: 18 pages, 6 figure
Large-scale Dynamo Action Driven by Velocity Shear and Rotating Convection
By incorporating a large-scale shear flow into turbulent rotating convection,
we show that a sufficiently strong shear can promote dynamo action in flows
that in the absence of shear do not act as dynamos. Our results are consistent
with a dynamo driven by either the shear-current effect or by the interaction
between a fluctuating -effect and the velocity shear; they are though
inconsistent with either a classical or mean field
dynamo.Comment: 4 pages, 4 fig
Mean-field dynamo in a turbulence with shear and kinetic helicity fluctuations
We study effects of kinetic helicity fluctuations in a turbulence with
large-scale shear using two different approaches: the spectral
tau-approximation and the second order correlation approximation (or
first-order smoothing approximation). These two approaches demonstrate that
homogeneous kinetic helicity fluctuations alone with zero mean value in a
sheared homogeneous turbulence cannot cause large-scale dynamo. Mean-field
dynamo can be possible when kinetic helicity fluctuations are inhomogeneous
which cause a nonzero mean alpha effect in a sheared turbulence. On the other
hand, shear-current effect can generate large-scale magnetic field even in a
homogeneous nonhelical turbulence with large-scale shear. This effect was
investigated previously for large hydrodynamic and magnetic Reynolds numbers.
In this study we examine the threshold required for the shear-current dynamo
versus Reynolds number. We demonstrate that there is no need for a developed
inertial range in order to maintain the shear-current dynamo (e.g., the
threshold in the Reynolds number is of the order of 1).Comment: 12 pages, 3 Figures, small corrections to match the final published
version, Physical Review E, in pres
Current status of turbulent dynamo theory: From large-scale to small-scale dynamos
Several recent advances in turbulent dynamo theory are reviewed. High
resolution simulations of small-scale and large-scale dynamo action in periodic
domains are compared with each other and contrasted with similar results at low
magnetic Prandtl numbers. It is argued that all the different cases show
similarities at intermediate length scales. On the other hand, in the presence
of helicity of the turbulence, power develops on large scales, which is not
present in non-helical small-scale turbulent dynamos. At small length scales,
differences occur in connection with the dissipation cutoff scales associated
with the respective value of the magnetic Prandtl number. These differences are
found to be independent of whether or not there is large-scale dynamo action.
However, large-scale dynamos in homogeneous systems are shown to suffer from
resistive slow-down even at intermediate length scales. The results from
simulations are connected to mean field theory and its applications. Recent
work on helicity fluxes to alleviate large-scale dynamo quenching, shear
dynamos, nonlocal effects and magnetic structures from strong density
stratification are highlighted. Several insights which arise from analytic
considerations of small-scale dynamos are discussed.Comment: 36 pages, 11 figures, Spa. Sci. Rev., submitted to the special issue
"Magnetism in the Universe" (ed. A. Balogh
Matter in Strong Magnetic Fields
The properties of matter are significantly modified by strong magnetic
fields, Gauss (), as are typically
found on the surfaces of neutron stars. In such strong magnetic fields, the
Coulomb force on an electron acts as a small perturbation compared to the
magnetic force. The strong field condition can also be mimicked in laboratory
semiconductors. Because of the strong magnetic confinement of electrons
perpendicular to the field, atoms attain a much greater binding energy compared
to the zero-field case, and various other bound states become possible,
including molecular chains and three-dimensional condensed matter. This article
reviews the electronic structure of atoms, molecules and bulk matter, as well
as the thermodynamic properties of dense plasma, in strong magnetic fields,
. The focus is on the basic physical pictures and
approximate scaling relations, although various theoretical approaches and
numerical results are also discussed. For the neutron star surface composed of
light elements such as hydrogen or helium, the outermost layer constitutes a
nondegenerate, partially ionized Coulomb plasma if , and may be in
the form of a condensed liquid if the magnetic field is stronger (and
temperature K). For the iron surface, the outermost layer of the
neutron star can be in a gaseous or a condensed phase depending on the cohesive
property of the iron condensate.Comment: 45 pages with 9 figures. Many small additions/changes. Accepted for
publication in Rev. Mod. Phy
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