4,073 research outputs found
Kinematic alpha effect in isotropic turbulence simulations
Using numerical simulations at moderate magnetic Reynolds numbers up to 220
it is shown that in the kinematic regime, isotropic helical turbulence leads to
an alpha effect and a turbulent diffusivity whose values are independent of the
magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent
coefficients are also consistent with expectations from the first order
smoothing approximation. For small values of \Rm, alpha and turbulent
diffusivity are proportional to \Rm. Over finite time intervals meaningful
values of alpha and turbulent diffusivity can be obtained even when there is
small-scale dynamo action that produces strong magnetic fluctuations. This
suggests that small-scale dynamo-generated fields do not make a correlated
contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter
Turbulent transport in hydromagnetic flows
The predictive power of mean-field theory is emphasized by comparing theory
with simulations under controlled conditions. The recently developed test-field
method is used to extract turbulent transport coefficients both in kinematic as
well as nonlinear and quasi-kinematic cases. A striking example of the
quasi-kinematic method is provided by magnetic buoyancy-driven flows that
produce an alpha effect and turbulent diffusion.Comment: 17 pages, 6 figures, topical issue of Physica Scripta on turbulent
mixing and beyon
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
A numerical model of isotropic homogeneous turbulence with helical forcing is
investigated. The resulting flow, which is essentially the prototype of the
alpha^2 dynamo of mean-field dynamo theory, produces strong dynamo action with
an additional large scale field on the scale of the box (at wavenumber k=1;
forcing is at k=5). This large scale field is nearly force-free and exceeds the
equipartition value. As the magnetic Reynolds number R_m increases, the
saturation field strength and the growth rate of the dynamo increase. However,
the time it takes to built up the large scale field from equipartition to its
final super-equipartition value increases with magnetic Reynolds number. The
large scale field generation can be identified as being due to nonlocal
interactions originating from the forcing scale, which is characteristic of the
alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are
determined simultaneously using numerical experiments where the mean-field is
modified artificially. Both quantities are quenched in a R_m-dependent fashion.
The evolution of the energy of the mean field matches that predicted by an
alpha^2 dynamo model with similar alpha and eta_t quenchings. For this model an
analytic solution is given which matches the results of the simulations. The
simulations are numerically robust in that the shape of the spectrum at large
scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints,
or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity)
from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the
scale separation) makes the inverse cascade effect more pronounced, although it
remains otherwise qualitatively unchanged.Comment: 21 pages, 26 figures, ApJ (accepted
Mean-field transport in stratified and/or rotating turbulence
We investigate the mean electromotive force in the kinematic framework, that
is, ignoring the back-reaction of the magnetic field on the fluid velocity,
under the assumption of axisymmetric turbulence determined by the presence of
either rotation, density stratification, or both. We use an analogous approach
for the mean passive scalar flux. As an alternative to convection, we consider
forced turbulence in an isothermal layer. When using standard ansatzes, the
mean magnetic transport is then determined by nine, and the mean passive scalar
transport by four coefficients. We give results for all these transport
coefficients. We use the test-field method and the test-scalar method, where
transport coefficients are determined by solving sets of equations with
properly chosen mean magnetic fields or mean scalars. These methods are adapted
to mean fields which may depend on all three space coordinates. We find the
anisotropy of turbulent diffusion to be moderate in spite of rapid rotation or
strong density stratification. Contributions to the mean electromotive force
determined by the symmetric part of the gradient tensor of the mean magnetic
field, which were ignored in several earlier investigations, turn out to be
important. In stratified rotating turbulence, the effect is strongly
anisotropic, suppressed along the rotation axis on large length scales, but
strongly enhanced at intermediate length scales. Also the \OO\times\meanJJ
effect is enhanced at intermediate length scales. The turbulent passive scalar
diffusivity is typically almost twice as large as the turbulent magnetic
diffusivity. Both magnetic and passive scalar diffusion are slightly enhanced
along the rotation axis, but decreased if there is gravity.Comment: 12 pages, 8 figures, A&A, publishe
Mean-field diffusivities in passive scalar and magnetic transport in irrotational flows
Certain aspects of the mean-field theory of turbulent passive scalar
transport and of mean-field electrodynamics are considered with particular
emphasis on aspects of compressible fluids. It is demonstrated that the total
mean-field diffusivity for passive scalar transport in a compressible flow may
well be smaller than the molecular diffusivity. This is in full analogy to an
old finding regarding the magnetic mean-field diffusivity in an electrically
conducting turbulently moving compressible fluid. These phenomena occur if the
irrotational part of the motion dominates the vortical part, the P\`eclet or
magnetic Reynolds number is not too large, and, in addition, the variation of
the flow pattern is slow. For both the passive scalar and the magnetic cases
several further analytical results on mean-field diffusivities and related
quantities found within the second-order correlation approximation are
presented, as well as numerical results obtained by the test-field method,
which applies independently of this approximation. Particular attention is paid
to non-local and non-instantaneous connections between the turbulence-caused
terms and the mean fields. Two examples of irrotational flows, in which
interesting phenomena in the above sense occur, are investigated in detail. In
particular, it is demonstrated that the decay of a mean scalar in a
compressible fluid under the influence of these flows can be much slower than
without any flow, and can be strongly influenced by the so-called memory
effect, that is, the fact that the relevant mean-field coefficients depend on
the decay rates themselves.Comment: 13 pages, 10 figures, published on PR
Imprints of magnetic power and helicity spectra on radio polarimetry statistics
Statistical properties of turbulent magnetic fields in radio-synchrotron
sources should imprint on the statistics of polarimetric observables. In search
of these imprints, we calculate correlation and cross-correlation functions
from a set of observables containing the total intensity I, the polarized
intensity P and the Faraday depth phi. The correlation functions are evaluated
for all combinations of observables up to fourth order in the magnetic field B.
We derive these as far as possible analytically and from first principles only
using some basic assumptions such as Gaussian statistics of the underlying
magnetic field in the observed region and statistical homogeneity. We further
assume some simplifications to reduce the complexity of the calculations, as
for a start we were interested in a proof of concept. Using this statistical
approach, we show that it is in principle possible to gain information about
the helical part of the magnetic power spectrum, namely via the correlation
functions and . Using this insight, we
construct an easy-to-use test for helicity, called LITMUS (Local Inference Test
for Magnetic fields which Uncovers heliceS). For now, all calculations are
given in a Faraday-free case, but set up in a way so that Faraday rotational
effects could be included later on.Comment: 24 pages, 4 figures; typos corrected; additional explanations in
section 1 and 2; revised and extended derivation in section 5, results
unchange
Dissociation in a polymerization model of homochirality
A fully self-contained model of homochirality is presented that contains the
effects of both polymerization and dissociation. The dissociation fragments are
assumed to replenish the substrate from which new monomers can grow and undergo
new polymerization. The mean length of isotactic polymers is found to grow
slowly with the normalized total number of corresponding building blocks.
Alternatively, if one assumes that the dissociation fragments themselves can
polymerize further, then this corresponds to a strong source of short polymers,
and an unrealistically short average length of only 3. By contrast, without
dissociation, isotactic polymers becomes infinitely long.Comment: 16 pages, 6 figures, submitted to Orig. Life Evol. Biosp
Apparent suppression of turbulent magnetic dynamo action by a dc magnetic field
Numerical studies of the effect of a dc magnetic field on dynamo action
(development of magnetic fields with large spatial scales), due to
helically-driven magnetohydrodynamic turbulence, are reported. The apparent
effect of the dc magnetic field is to suppress the dynamo action, above a
relatively low threshold. However, the possibility that the suppression results
from an improper combination of rectangular triply spatially-periodic boundary
conditions and a uniform dc magnetic field is addressed: heretofore a common
and convenient computational convention in turbulence investigations. Physical
reasons for the observed suppression are suggested. Other geometries and
boundary conditions are offered for which the dynamo action is expected not to
be suppressed by the presence of a dc magnetic field component.Comment: To appear in Physics of Plasma
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
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