1,406 research outputs found
Dynamo quenching due to shear flow
We provide a theory of dynamo (α effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the α effect is reduced by the shear even in the absence of magnetic field. The α effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantly reduced by shear and magnetic fields, with positive value. These results suggest a crucial effect of shear and magnetic field on dynamo quenching and momentum transport reduction, with important implications for laboratory and astrophysical plasmas, in particular, for the dynamics of the Sun
Gauge vortex dynamics at finite mass of bosonic fields
The simple derivation of the string equation of motion adopted in the
nonrelativistic case is presented, paying the special attention to the effects
of finite masses of bosonic fields of an Abelian Higgs model. The role of the
finite mass effects in the evaluation of various topological characteristics of
the closed strings is discussed. The rate of the dissipationless helicity
change is calculated. It is demonstrated how the conservation of the sum of the
twisting and writhing numbers of the string is recovered despite the changing
helicity.Comment: considerably revised to include errata to journal versio
Unplugging the Universe: the neglected electromagnetic consequence of decoupling
This letter concentrates on the non-equilibrium evolution of magnetic field
structures at the onset of recombination, when the charged particle current
densities decay as neutrals are formed.
We consider the effect that a decaying magnetic flux has on the acceleration
of particles via the transient induced electric field. Since the residual
charged-particle number density is small as a result of decoupling, we shall
consider the magnetic and electric fields essentially to be imposed, neglecting
the feedback from any minority accelerated population.
We find that the electromagnetic treatment of this phase transition can
produce energetic electrons scattered throughout the Universe. Such particles
could have a significant effect on cosmic evolution in several ways: (i) their
presence could delay the effective end of the recombination era; (ii) they
could give rise to plasma concentrations that could enhance early gravitational
collapse of matter by opposing cosmic expansion to a greater degree than
neutral matter could; (iii) they could continue to be accelerated, and become
the seed for reionisation at the later epoch .Comment: 4 pages, no figure
Analytical theory of forced rotating sheared turbulence: The perpendicular case
Rotation and shear flows are ubiquitous features of many astrophysical and geophysical bodies. To understand their origin and effect on turbulent transport in these systems, we consider a forced turbulence and investigate the combined effect of rotation and shear flow on the turbulence properties. Specifically, we study how rotation and flow shear influence the generation of shear flow (e.g., the direction of energy cascade), turbulence level, transport of particles and momentum, and the anisotropy in these quantities. In all the cases considered, turbulence amplitude is always quenched due to strong shear (Ο=Îœky2/AâȘĄ1, where A is the shearing rate, Îœ is the molecular viscosity, and ky is a characteristic wave number of small-scale turbulence), with stronger reduction in the direction of the shear than those in the perpendicular directions. Specifically, in the large rotation limit (ΩâȘąA), they scale as Aâ1 and Aâ1|lnâΟ|, respectively, while in the weak rotation limit (ΩâȘĄA), they scale as Aâ1 and Aâ2/3, respectively. Thus, flow shear always leads to weak turbulence with an effectively stronger turbulence in the plane perpendicular to shear than in the shear direction, regardless of rotation rate. The anisotropy in turbulence amplitude is, however, weaker by a factor of Ο1/3|lnâΟ| (âAâ1/3|lnâΟ|) in the rapid rotation limit (ΩâȘąA) than that in the weak rotation limit (ΩâȘĄA) since rotation favors almost-isotropic turbulence. Compared to turbulence amplitude, particle transport is found to crucially depend on whether rotation is stronger or weaker than flow shear. When rotation is stronger than flow shear (ΩâȘąA), the transport is inhibited by inertial waves, being quenched inversely proportional to the rotation rate (i.e., âΩâ1) while in the opposite case, it is reduced by shearing as Aâ1. Furthermore, the anisotropy is found to be very weak in the strong rotation limit (by a factor of 2) while significant in the strong shear limit. The turbulent viscosity is found to be negative with inverse cascade of energy as long as rotation is sufficiently strong compared to flow shear (ΩâȘąA) while positive in the opposite limit of weak rotation (ΩâȘĄA). Even if the eddy viscosity is negative for strong rotation (ΩâȘąA), flow shear, which transfers energy to small scales, has an interesting effect by slowing down the rate of inverse cascade with the value of negative eddy viscosity decreasing as |ÎœT|âAâ2 for strong shear. Furthermore, the interaction between the shear and the rotation is shown to give rise to a nondiffusive flux of angular momentum (Î effect), even in the absence of external sources of anisotropy. This effect provides a mechanism for the existence of shearing structures in astrophysical and geophysical systems
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
A numerical model of the VKS experiment
We present numerical simulations of the magnetic field generated by the flow
of liquid sodium driven by two counter-rotating impellers (VKS experiment).
Using a dynamo kinematic code in cylindrical geometry, it is shown that
different magnetic modes can be generated depending on the flow configuration.
While the time averaged axisymmetric mean flow generates an equatorial dipole,
our simulations show that an axial field of either dipolar or quadrupolar
symmetry can be generated by taking into account non-axisymmetric components of
the flow. Moreover, we show that by breaking a symmetry of the flow, the
magnetic field becomes oscillatory. This leads to reversals of the axial dipole
polarity, involving a competition with the quadrupolar component.Comment: 6 pages, 5 figure
Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium
The first direct measurements of effective magnetic diffusivity in turbulent
flow of electro-conductive fluids (the so-called beta-effect) under magnetic
Reynolds number Rm >> 1 are reported. The measurements are performed in a
nonstationary turbulent flow of liquid sodium, generated in a closed toroidal
channel. The peak level of the Reynolds number reached Re \approx 3 10^6, which
corresponds to the magnetic Reynolds number Rm \approx 30. The magnetic
diffusivity of the liquid metal was determined by measuring the phase shift
between the induced and the applied magnetic fields. The maximal deviation of
magnetic diffusivity from its basic (laminar) value reaches about 50% .Comment: 5 pages, 6 figuser, accepted in PR
Expansions for the Bollobas-Riordan polynomial of separable ribbon graphs
We define 2-decompositions of ribbon graphs, which generalise 2-sums and
tensor products of graphs. We give formulae for the Bollobas-Riordan polynomial
of such a 2-decomposition, and derive the classical Brylawski formula for the
Tutte polynomial of a tensor product as a (very) special case. This study was
initially motivated from knot theory, and we include an application of our
formulae to mutation in knot diagrams.Comment: Version 2 has minor changes. To appear in Annals of Combinatoric
The alpha-effect and current helicity for fast sheared rotators
We explore the alpha-effect and the small-scale current helicity, for the
case of weakly compressible magnetically driven turbulence that is subjected to
the differential rotation. No restriction is applied to the amplitude of
angular velocity, i.e., the derivations presented are valid for an arbitrary
Coriolis number, though the differential rotation itself is assumed to be weak.
The expressions obtained are used to explore the possible distributions of
alpha-effect and current helicity in convection zones (CZ) of the solar-type
stars. The implications of the obtained results to the mean-field dynamo models
are discussed.Comment: 20 pages, 6 figure
The alpha-effect in rotating convection: a comparison of numerical simulations
Numerical simulations are an important tool in furthering our understanding
of turbulent dynamo action, a process that occurs in a vast range of
astrophysical bodies. It is important in all computational work that
comparisons are made between different codes and, if non-trivial differences
arise, that these are explained. Kapyla et al (2010: MNRAS 402, 1458) describe
an attempt to reproduce the results of Hughes & Proctor (2009: PRL 102, 044501)
and, by employing a different methodology, they arrive at very different
conclusions concerning the mean electromotive force and the generation of
large-scale fields. Here we describe why the simulations of Kapyla et al (2010)
are simply not suitable for a meaningful comparison, since they solve different
equations, at different parameter values and with different boundary
conditions. Furthermore we describe why the interpretation of Kapyla et al
(2010) of the calculation of the alpha-effect is inappropriate and argue that
the generation of large-scale magnetic fields by turbulent convection remains a
problematic issue.Comment: Submitted to MNRAS. 5 pages, 3 figure
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