221 research outputs found
On radiation-zone dynamos
It is shown that the magnetic current-driven (`kink-type') instability
produces flow and field patterns with helicity and even with \alpha-effect but
only if the magnetic background field possesses non-vanishing current helicity
\bar{\vec{B}}\cdot curl \bar{\vec{B}} by itself. Fields with positive
large-scale current helicity lead to negative small-scale kinetic helicity. The
resulting \alpha-effect is positive. These results are very strict for
cylindric setups without z/I>-dependence of the background fields. The sign
rules also hold for the more complicated cases in spheres where the toroidal
fields are the result of the action of differential rotation (induced from
fossil poloidal fields) at least for the case that the global rotation is
switched off after the onset of the instability.Comment: 6 pages, 6 figures, submitted to Proceedings of IAU Symp. 274:
Advances in Plasma Astrophysic
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Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations
A mean-field theory of the electrodynamics of a turbulent fluid is formulated under the assumption that the molecular electric conductivity is correlated with the turbulent velocity fluctuation in the (radial) direction, (Formula presented.). It is shown that for such homogeneous fluids a strong turbulence-induced field advection anti-parallel to (Formula presented.) arises almost independently of rotation. For rotating fluids, an extra (Formula presented.) effect appears with the known symmetries and with the expected maximum at the poles. Fast rotation, however, with Coriolis number exceeding unity suppresses this term. Numerical simulations of forced turbulence using the nirvana code demonstrate that the radial advection velocity, (Formula presented.), always dominates the (Formula presented.) term. We show finally with simplified models that (Formula presented.) dynamos are strongly influenced by the radial pumping: for (Formula presented.) the solutions become oscillatory, while for (Formula presented.) they become highly exotic if they exist at all. In conclusion, dynamo models for slow and fast solid-body rotation on the basis of finite conductivity–velocity correlations are unlikely to work, at least for (Formula presented.) dynamos without strong shear
Alpha tensor and dynamo excitation in turbulent fluids with anisotropic conductivity fluctuations
A mean-field theory of the electrodynamics of a turbulent fluid is formulated
under the assumption that the molecular electric conductivity is correlated
with the turbulent velocity fluctuation in the (radial) direction,
. It is shown that for such homogeneous fluids a strong
turbulence-induced field advection anti-parallel to arises almost
independently of rotation. For rotating fluids, an extra effect
appears with the known symmetries and with the expected maximum at the poles.
Fast rotation, however, with Coriolis number exceeding unity suppresses this
term. Numerical simulations of forced turbulence using the NIRVANA code
demonstrate that the radial advection velocity, , always dominates the
term. We show finally with simplified models that dynamos
are strongly influenced by the radial pumping: for the
solutions become oscillatory, while for they become highly
exotic if they exist at all. In conclusion, dynamo models for slow and fast
solid-body rotation on the basis of finite conductivity-velocity correlations
are unlikely to work, at least for dynamos without strong
shear.Comment: 10 pages, 8 figures, to be published in A
Non-axisymmetric Magnetorotational Instabilities in Cylindrical Taylor-Couette Flow
We study the stability of cylindrical Taylor-Couette flow in the presence of
azimuthal magnetic fields, and show that one obtains non-axisymmetric
magnetorotational instabilities, having azimuthal wavenumber m=1. For
Omega_o/Omega_i only slightly greater than the Rayleigh value (r_i/r_o)^2, the
critical Reynolds and Hartmann numbers are Re_c ~ 10^3 and Ha_c ~ 10^2,
independent of the magnetic Prandtl number Pm. These values are sufficiently
small that it should be possible to obtain these instabilities in the PROMISE
experimental facility.Comment: final version as accepted by Phys Rev Let
The Ekman-Hartmann layer in MHD Taylor-Couette flow
We study magnetic effects induced by rigidly rotating plates enclosing a
cylindrical MHD Taylor-Couette flow at the finite aspect ratio . The
fluid confined between the cylinders is assumed to be liquid metal
characterized by small magnetic Prandtl number, the cylinders are perfectly
conducting, an axial magnetic field is imposed \Ha \approx 10, the rotation
rates correspond to \Rey of order . We show that the end-plates
introduce, besides the well known Ekman circulation, similar magnetic effects
which arise for infinite, rotating plates, horizontally unbounded by any walls.
In particular there exists the Hartmann current which penetrates the fluid,
turns into the radial direction and together with the applied magnetic field
gives rise to a force. Consequently the flow can be compared with a Taylor-Dean
flow driven by an azimuthal pressure gradient. We analyze stability of such
flows and show that the currents induced by the plates can give rise to
instability for the considered parameters. When designing an MHD Taylor-Couette
experiment, a special care must be taken concerning the vertical magnetic
boundaries so they do not significantly alter the rotational profile.Comment: 9 pages, 6 figures; accepted to PR
Supernova-driven interstellar turbulence and the galactic dynamo
The fractal shape and multi-component nature of the interstellar medium
together with its vast range of dynamical scales provides one of the great
challenges in theoretical and numerical astrophysics. Here we will review
recent progress in the direct modelling of interstellar hydromagnetic
turbulence, focusing on the role of energy injection by supernova explosions.
The implications for dynamo theory will be discussed in the context of the
mean-field approach. Results obtained with the test field-method are confronted
with analytical predictions and estimates from quasilinear theory. The
simulation results enforce the classical understanding of a turbulent Galactic
dynamo and, more importantly, yield new quantitative insights. The derived
scaling relations enable confident global mean-field modelling.Comment: 7 pages, 2 figures, conference proceedings of the IAU Symposium 274,
Advances in Plasma Astrophysic
New type of magneto-rotational instability in cylindrical Taylor-Couette flow
We study the stability of cylindrical Taylor-Couette flow in the presence of
combined axial and azimuthal magnetic fields, and show that adding an azimuthal
field profoundly alters the previous results for purely axial fields. For small
magnetic Prandtl numbers Pm, the critical Reynolds number Re_c for the onset of
the magneto-rotational instability becomes independent of Pm, whereas for
purely axial fields it scales as Pm^{-1}. For typical liquid metals, Re_c is
then reduced by several orders of magnitude, enough that this new design should
succeed in realizing this instability in the laboratory
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