27 research outputs found
Non-local effects in the mean-field disc dynamo. II. Numerical and asymptotic solutions
The thin-disc global asymptotics are discussed for axisymmetric mean-field
dynamos with vacuum boundary conditions allowing for non-local terms arising
from a finite radial component of the mean magnetic field at the disc surface.
This leads to an integro-differential operator in the equation for the radial
distribution of the mean magnetic field strength, in the disc plane at a
distance from its centre; an asymptotic form of its solution at large
distances from the dynamo active region is obtained. Numerical solutions of the
integro-differential equation confirm that the non-local effects act similarly
to an enhanced magnetic diffusion. This leads to a wider radial distribution of
the eigensolution and faster propagation of magnetic fronts, compared to
solutions with the radial surface field neglected. Another result of non-local
effects is a slowly decaying algebraic tail of the eigenfunctions outside the
dynamo active region, , which is shown to persist in nonlinear
solutions where -quenching is included. The non-local nature of the
solutions can affect the radial profile of the regular magnetic field in spiral
galaxies and accretion discs at large distances from the centre.Comment: Revised version, as accepted; Geophys. Astrophys. Fluid Dyna
Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma
Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization
Small-scale magnetic buoyancy and magnetic pumping effects in a turbulent convection
We determine the nonlinear drift velocities of the mean magnetic field and
nonlinear turbulent magnetic diffusion in a turbulent convection. We show that
the nonlinear drift velocities are caused by the three kinds of the
inhomogeneities, i.e., inhomogeneous turbulence; the nonuniform fluid density
and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in
the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear
drift velocities of the mean magnetic field cause the small-scale magnetic
buoyancy and magnetic pumping effects in the turbulent convection. These
phenomena are different from the large-scale magnetic buoyancy and magnetic
pumping effects which are due to the effect of the mean magnetic field on the
large-scale density stratified fluid flow. The small-scale magnetic buoyancy
and magnetic pumping can be stronger than these large-scale effects when the
mean magnetic field is smaller than the equipartition field. We discuss the
small-scale magnetic buoyancy and magnetic pumping effects in the context of
the solar and stellar turbulent convection. We demonstrate also that the
nonlinear turbulent magnetic diffusion in the turbulent convection is
anisotropic even for a weak mean magnetic field. In particular, it is enhanced
in the radial direction. The magnetic fluctuations due to the small-scale
dynamo increase the turbulent magnetic diffusion of the toroidal component of
the mean magnetic field, while they do not affect the turbulent magnetic
diffusion of the poloidal field.Comment: 13 pages, 4 figure, REVTEX4, Geophysical and Astrophysical Fluid
Dynamics, in pres
On the magnetic fields generated by experimental dynamos
We review the results obtained by three successful fluid dynamo experiments
and discuss what has been learnt from them about the effect of turbulence on
the dynamo threshold and saturation. We then discuss several questions that are
still open and propose experiments that could be performed to answer some of
them.Comment: 40 pages, 13 figure