14,537 research outputs found

### Why coronal mass ejections are necessary for the dynamo

Large scale dynamo-generated fields are a combination of interlocked poloidal
and toroidal fields. Such fields possess magnetic helicity that needs to be
regenerated and destroyed during each cycle. A number of numerical experiments
now suggests that stars may do this by shedding magnetic helicity. In addition
to plain bulk motions, a favorite mechanism involves magnetic helicity flux
along lines of constant rotation. We also know that the sun does shed the
required amount of magnetic helicity mostly in the form of coronal mass
ejections. Solar-like stars without cycles do not face such strong constraints
imposed by magnetic helicity evolution and may not display coronal activity to
that same extent. I discuss the evidence leading to this line of argument. In
particular, I discuss simulations showing the generation of strong mean
toroidal fields provided the outer boundary condition is left open so as to
allow magnetic helicity to escape. Control experiments with closed boundaries
do not produce strong mean fields.Comment: 2 pages, 2 figures, to appear in Highlights of Astronomy, ed. K. G.
Strassmeier & A. Kosovichev, Astron. Soc. Pac. Conf. Se

### The dual role of shear in large-scale dynamos

The role of shear in alleviating catastrophic quenching by shedding
small-scale magnetic helicity through fluxes along contours of constant shear
is discussed. The level of quenching of the dynamo effect depends on the
quenched value of the turbulent magnetic diffusivity. Earlier estimates that
might have suffered from the force-free degeneracy of Beltrami fields are now
confirmed for shear flows where this degeneracy is lifted. For a dynamo that is
saturated near equipartition field strength those estimates result in a 5-fold
decrease of the magnetic diffusivity as the magnetic Reynolds number based on
the wavenumber of the energy-carrying eddies is increased from 2 to 600.
Finally, the role of shear in driving turbulence and large-scale fields by the
magneto-rotational instability is emphasized. New simulations are presented and
the 3pi/4 phase shift between poloidal and toroidal fields is confirmed. It is
suggested that this phase shift might be a useful diagnostic tool in
identifying mean-field dynamo action in simulations and to distinguish this
from other scenarios invoking magnetic buoyancy as a means to explain migration
away from the midplane.Comment: 7 pages, 10 figures, proceedings of the workshop on MHD Laboratory
Experiments for Geophysics and Astrophysic

### Equatorial magnetic helicity flux in simulations with different gauges

We use direct numerical simulations of forced MHD turbulence with a forcing
function that produces two different signs of kinetic helicity in the upper and
lower parts of the domain. We show that the mean flux of magnetic helicity from
the small-scale field between the two parts of the domain can be described by a
Fickian diffusion law with a diffusion coefficient that is approximately
independent of the magnetic Reynolds number and about one third of the
estimated turbulent magnetic diffusivity. The data suggest that the turbulent
diffusive magnetic helicity flux can only be expected to alleviate catastrophic
quenching at Reynolds numbers of more than several thousands. We further
calculate the magnetic helicity density and its flux in the domain for three
different gauges. We consider the Weyl gauge, in which the electrostatic
potential vanishes, the pseudo-Lorenz gauge, where the speed of light is
replaced by the sound speed, and the `resistive gauge' in which the Laplacian
of the magnetic vector potential acts as resistive term. We find that, in the
statistically steady state, the time-averaged magnetic helicity density and the
magnetic helicity flux are the same in all three gauges.Comment: 6 pages 5 figure

### New mechanism of generation of large-scale magnetic field in a sheared turbulent plasma

A review of recent studies on a new mechanism of generation of large-scale
magnetic field in a sheared turbulent plasma is presented. This mechanism is
associated with the shear-current effect which is related to the W x J-term in
the mean electromotive force. This effect causes the generation of the
large-scale magnetic field even in a nonrotating and nonhelical homogeneous
sheared turbulent convection whereby the alpha effect vanishes. It is found
that turbulent convection promotes the shear-current dynamo instability, i.e.,
the heat flux causes positive contribution to the shear-current effect.
However, there is no dynamo action due to the shear-current effect for small
hydrodynamic and magnetic Reynolds numbers even in a turbulent convection, if
the spatial scaling for the turbulent correlation time is k^{-2}, where k is
the small-scale wave number. We discuss here also the nonlinear mean-field
dynamo due to the shear-current effect and take into account the transport of
magnetic helicity as a dynamical nonlinearity. The magnetic helicity flux
strongly affects the magnetic field dynamics in the nonlinear stage of the
dynamo action. When the magnetic helicity flux is not small, the saturated
level of the mean magnetic field is of the order of the equipartition field
determined by the turbulent kinetic energy. The obtained results are important
for elucidation of origin of the large-scale magnetic fields in astrophysical
and cosmic sheared turbulent plasma.Comment: 7 pages, Planetory and Space Science, in pres

### Simulations of the anisotropic kinetic and magnetic alpha effects

The validity of a closure called the minimal tau approximation (MTA), is
tested in the context of dynamo theory, wherein triple correlations are assumed
to provide relaxation of the turbulent electromotive force. Under MTA, the
alpha effect in mean field dynamo theory becomes proportional to a relaxation
time scale multiplied by the difference between kinetic and current helicities.
It is shown that the value of the relaxation time is positive and, in units of
the turnover time at the forcing wavenumber, it is of the order of unity. It is
quenched by the magnetic field -- roughly independently of the magnetic
Reynolds number. However, this independence becomes uncertain at large magnetic
Reynolds number. Kinetic and current helicities are shown to be dominated by
large scale properties of the flow.Comment: 6 pages, 6 figures, accepted by Astron. Nach

### Vorticity from irrotationally forced flow

In the interstellar medium the turbulence is believed to be forced mostly
through supernova explosions. In a first approximation these flows can be
written as a gradient of a potential being thus devoid of vorticity. There are
several mechanisms that could lead to vorticity generation, like viscosity and
baroclinic terms, rotation, shear and magnetic fields, but it is not clear how
effective they are, neither is it clear whether the vorticity is essential in
determining the turbulent diffusion acting in the ISM. Here we present a study
of the role of rotation, shear and baroclinicity in the generation of vorticity
in the ISM.Comment: 2 pages, 1 figure, to be published in Proceedings of IAU Symp. 271,
Astrophysical Dynamics: from Stars to Galaxies, ed. N. Brummell and A.S.
Brun, CU

### Magnetic helicity flux in the presence of shear

Magnetic helicity has risen to be a major player in dynamo theory, with the
helicity of the small-scale field being linked to the dynamo saturation process
for the large-scale field. It is a nearly conserved quantity, which allows its
evolution equation to be written in terms of production and flux terms. The
flux term can be decomposed in a variety of fashions. One particular
contribution that has been expected to play a significant role in dynamos in
the presence of mean shear was isolated by Vishniac & Cho (2001, ApJ 550, 752).
Magnetic helicity fluxes are explicitly gauge dependent however, and the
correlations that have come to be called the Vishniac-Cho flux were determined
in the Coulomb gauge, which turns out to be fraught with complications in
shearing systems. While the fluxes of small-scale helicity are explicitly gauge
dependent, their divergences can be gauge independent. We use this property to
investigate magnetic helicity fluxes of small-scale field through direct
numerical simulations in a shearing-box system and find that in a numerically
usable gauge the divergence of the small-scale helicity flux vanishes, while
the divergence of the Vishniac-Cho flux remains finite. We attribute this
seeming contradiction to the existence of horizontal fluxes of small-scale
magnetic helicity with finite divergences even in our shearing-periodic domain.Comment: 8 pages, 5 figures, Accepted, Ap

### Turbulence and its parameterization in accretion discs

Accretion disc turbulence is investigated in the framework of the shearing
box approximation. The turbulence is either driven by the magneto-rotational
instability or, in the non-magnetic case, by an explicit and artificial forcing
term in the momentum equation. Unlike the magnetic case, where most of the
dissipation occurs in the disc corona, in the forced hydrodynamic case most of
the dissipation occurs near the midplane. In the hydrodynamic case evidence is
presented for the stochastic excitation of epicycles. When the vertical and
radial epicyclic frequencies are different (modeling the properties around
rotating black holes), the beat frequency between these two frequencies appear
to show up as a peak in the temporal power spectrum in some cases. Finally, the
full turbulent resistivity tensor is determined and it is found that, if the
turbulence is driven by a forcing term, the signs of its off-diagonal
components are such that this effect would not be capable of dynamo action by
the shear--current effect.Comment: 11 pages, 11 figure

### How can vorticity be produced in irrotationally forced flows?

A spherical hydrodynamical expansion flow can be described as the gradient of
a potential. In that case no vorticity should be produced, but several
additional mechanisms can drive its production. Here we analyze the effects of
baroclinicity, rotation and shear in the case of a viscous fluid. Those flows
resemble what happens in the interstellar medium. In fact in this astrophysical
environment supernovae explosion are the dominant flows and, in a first
approximation, they can be seen as spherical. One of the main difference is
that in our numerical study we examine only weakly supersonic flows, while
supernovae explosions are strongly supersonic.Comment: 3 pages, 3 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche

### Large-scale dynamos at low magnetic Prandtl numbers

Using direct simulations of hydromagnetic turbulence driven by random
polarized waves it is shown that dynamo action is possible over a wide range of
magnetic Prandtl numbers from 10^-3 to 1. Triply periodic boundary conditions
are being used. In the final saturated state the resulting magnetic field has a
large-scale component of Beltrami type. For the kinematic phase, growth rates
have been determined for magnetic Prandtl numbers between 0.01 and 1, but only
the case with the smallest magnetic Prandtl number shows large-scale magnetic
fields. It is less organized than in the nonlinear stage. For small magnetic
Prandtl numbers the growth rates are comparable to those calculated from an
alpha squared mean-field dynamo. In the linear regime the magnetic helicity
spectrum has a short inertial range compatible with a -5/3 power law, while in
the nonlinear regime it is the current helicity whose spectrum may be
compatible with such a law. In the saturated case, the spectral magnetic energy
in the inertial range is in slight excess over the spectral kinetic energy,
although for small magnetic Prandtl numbers the magnetic energy spectrum
reaches its resistive cut off wavenumber more quickly. The viscous energy
dissipation declines with the square root of the magnetic Prandtl number, which
implies that most of the energy is dissipated via Joule heat.Comment: 8 pages, 12 figures, Astrophys. J. (in press

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