157 research outputs found
Turbulent dynamo with advective magnetic helicity flux
Many astrophysical bodies harbor magnetic fields that are thought to be
sustained by a dynamo process. However, it has been argued that the production
of large-scale magnetic fields by mean-field dynamo action is strongly
suppressed at large magnetic Reynolds numbers owing to the conservation of
magnetic helicity. This phenomenon is known as {\it catastrophic quenching}.
Advection of magnetic fields by stellar and galactic winds toward the outer
boundaries and away from the dynamo is expected to alleviate such quenching.
Here we explore the relative roles played by advective and turbulent--diffusive
fluxes of magnetic helicity in the dynamo. In particular, we study how the
dynamo is affected by advection. We do this by performing direct numerical
simulations of a turbulent dynamo of type driven by forced
turbulence in a Cartesian domain in the presence of a flow away from the
equator where helicity changes sign. Our results indicate that in the presence
of advection, the dynamo, otherwise stationary, becomes oscillatory. We confirm
an earlier result for turbulent--diffusive magnetic helicity fluxes that for
small magnetic Reynolds numbers (\Rm\lesssim 100...200, based on the
wavenumber of the energy-carrying eddies) the magnetic helicity flux scales
less strongly with magnetic Reynolds number (\Rm^{-1/2}) than the term
describing magnetic helicity destruction by resistivity (\Rm^{-1}). Our new
results now suggest that for larger \Rm the former becomes approximately
independent of \Rm, while the latter falls off more slowly. We show for the
first time that both for weak and stronger winds, the magnetic helicity flux
term becomes comparable to the resistive term for \Rm\gtrsim 1000, which is
necessary for alleviating catastrophic quenching.Comment: 9 pages, 9 figures, accepted for publication in MNRA
The fratricide of alpha-Omega dynamos by their alpha-squared siblings
Context. Helically forced magneto-hydrodynamic shearing-sheet turbulence can
support different large-scale dynamo modes, although the {\alpha} - {\Omega}
mode is generally expected to dominate because it is the fastest growing. In an
{\alpha} - {\Omega} dynamo, most of the field amplification is produced by the
shear. As differential rotation is an ubiquitous source of shear in
astrophysics, such dynamos are believed to be the source of most astrophysical
large-scale magnetic fields. Aims. We study the stability of oscillatory
migratory {\alpha} - {\Omega} type dynamos in turbulence simulations. Methods.
We use shearing-sheet simulations of hydromagnetic turbulence that is helically
forced at a wavenumber that is about three times larger than the lowest
wavenumber in the domain so that both {\alpha} - {\Omega} and {\alpha}2 dynamo
action is possible. Results. After initial dominance and saturation, the
{\alpha} - {\Omega} mode is found to be destroyed by an orthogonal {\alpha}2
mode sustained by the helical turbulence alone. We show that there are at least
two processes through which this transition can occur. Conclusions. The
fratricide of {\alpha} - {\Omega} dynamos by its {\alpha}2 sibling is discussed
in the context of grand minima of solar and stellar activity. However, the
genesis of {\alpha} - {\Omega} dynamos from an {\alpha}2 dynamo has not yet
been found.Comment: 10 pages, 12 figures, 3 table
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 alpha effect with imposed and dynamo-generated magnetic fields
Estimates for the nonlinear alpha effect in helical turbulence with an
applied magnetic field are presented using two different approaches: the
imposed-field method where the electromotive force owing to the applied field
is used, and the test-field method where separate evolution equations are
solved for a set of different test fields. Both approaches agree for stronger
fields, but there are apparent discrepancies for weaker fields that can be
explained by the influence of dynamo-generated magnetic fields on the scale of
the domain that are referred to as meso-scale magnetic fields. Examples are
discussed where these meso-scale fields can lead to both drastically
overestimated and underestimated values of alpha compared with the kinematic
case. It is demonstrated that the kinematic value can be recovered by resetting
the fluctuating magnetic field to zero in regular time intervals. It is
concluded that this is the preferred technique both for the imposed-field and
the test-field methods.Comment: 10 pages, 8 figures, published versio
Alpha effect due to buoyancy instability of a magnetic layer
A strong toroidal field can exist in form of a magnetic layer in the
overshoot region below the solar convection zone. This motivates a more
detailed study of the magnetic buoyancy instability with rotation. We calculate
the alpha effect due to helical motions caused by a disintegrating magnetic
layer in a rotating density-stratified system with angular velocity Omega
making an angle theta with the vertical. We also study the dependence of the
alpha effect on theta and the strength of the initial magnetic field. We carry
out three-dimensional hydromagnetic simulations in Cartesian geometry. A
turbulent EMF due to the correlations of the small scale velocity and magnetic
field is generated. We use the test-field method to calculate the transport
coefficients of the inhomogeneous turbulence produced by the layer. We show
that the growth rate of the instability and the twist of the magnetic field
vary monotonically with the ratio of thermal conductivity to magnetic
diffusivity. The resulting alpha effect is inhomogeneous and increases with the
strength of the initial magnetic field. It is thus an example of an
"anti-quenched" alpha effect. The alpha effect is nonlocal, requiring around
8--16 Fourier modes to reconstruct the actual EMF based on the actual mean
field.Comment: 14 pages, 19 figures 3 tables (submitted to A & A
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
Dynamo generated field emergence through recurrent plasmoid ejections
Magnetic buoyancy is believed to drive the transport of magnetic flux tubes
from the convection zone to the surface of the Sun. The magnetic fields form
twisted loop-like structures in the solar atmosphere. In this paper we use
helical forcing to produce a large-scale dynamo-generated magnetic field, which
rises even without magnetic buoyancy. A two layer system is used as
computational domain where the upper part represents the solar atmosphere.
Here, the evolution of the magnetic field is solved with the stress--and--relax
method. Below this region a magnetic field is produced by a helical forcing
function in the momentum equation, which leads to dynamo action. We find
twisted magnetic fields emerging frequently to the outer layer, forming
arch-like structures. In addition, recurrent plasmoid ejections can be found by
looking at space--time diagrams of the magnetic field. Recent simulations in
spherical coordinates show similar results.Comment: 4 pages, 8 figures, To appear in the proceedings of the IAU273
"Physics of Sun and Star Spots
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
Shear-driven and diffusive helicity fluxes in alpha-Omega dynamos
We present nonlinear mean-field alpha-Omega dynamo simulations in spherical
geometry with simplified profiles of kinematic alpha effect and shear. We take
magnetic helicity evolution into account by solving a dynamical equation for
the magnetic alpha effect. This gives a consistent description of the quenching
mechanism in mean-field dynamo models. The main goal of this work is to explore
the effects of this quenching mechanism in solar-like geometry, and in
particular to investigate the role of magnetic helicity fluxes, specifically
diffusive and Vishniac-Cho (VC) fluxes, at large magnetic Reynolds numbers
(Rm). For models with negative radial shear or positive latitudinal shear, the
magnetic alpha effect has predominantly negative (positive) sign in the
northern (southern) hemisphere. In the absence of fluxes, we find that the
magnetic energy follows an Rm^-1 dependence, as found in previous works. This
catastrophic quenching is alleviated in models with diffusive magnetic helicity
fluxes resulting in magnetic fields comparable to the equipartition value even
for Rm=10^7. On the other hand, models with a shear-driven Vishniac-Cho flux
show an increase of the amplitude of the magnetic field with respect to models
without fluxes, but only for Rm<10^4. This is mainly a consequence of assuming
a vacuum outside the Sun which cannot support a significant VC flux across the
boundary. However, in contrast with the diffusive flux, the VC flux modifies
the distribution of the magnetic field. In addition, if an ill-determined
scaling factor in the expression for the VC flux is large enough, subcritical
dynamo action is possible that is driven by the action of shear and the
divergence of current helicity flux.Comment: 12 pages, 12 figures. Submitted to MNRA
Magnetic helicity fluxes in an alpha-squared dynamo embedded in a halo
We present the results of simulations of forced turbulence in a slab where
the mean kinetic helicity has a maximum near the mid-plane, generating
gradients of magnetic helicity of both large and small-scale fields. We also
study systems that have poorly conducting buffer zones away from the midplane
in order to assess the effects of boundaries. The dynamical alpha quenching
phenomenology requires that the magnetic helicity in the small-scale fields
approaches a nearly static, gauge independent state. To stress-test this steady
state condition we choose a system with a uniform sign of kinetic helicity, so
that the total magnetic helicity can reach a steady state value only through
fluxes through the boundary, which are themselves suppressed by the velocity
boundary conditions. Even with such a set up, the small-scale magnetic helicity
is found to reach a steady state. In agreement with earlier work, the magnetic
helicity fluxes of small-scale fields are found to be turbulently diffusive. By
comparing results with and without halos, we show that artificial constraints
on magnetic helicity at the boundary do not have a significant impact on the
evolution of the magnetic helicity, except that "softer" (halo) boundary
conditions give a lower energy of the saturated mean magnetic field.Comment: 12 pages, 5 figures, submitted to GAF
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