359 research outputs found
Small-scale dynamos in simulations of stratified turbulent convection
Small-scale dynamo action is often held responsible for the generation of
quiet-Sun magnetic fields. We aim to determine the excitation conditions and
saturation level of small-scale dynamos in non-rotating turbulent convection at
low magnetic Prandtl numbers. We use high resolution direct numerical
simulations of weakly stratified turbulent convection. We find that the
critical magnetic Reynolds number for dynamo excitation increases as the
magnetic Prandtl number is decreased, which might suggest that small-scale
dynamo action is not automatically evident in bodies with small magnetic
Prandtl numbers as the Sun. As a function of the magnetic Reynolds number
(), the growth rate of the dynamo is consistent with an scaling. No evidence for a logarithmic increase of the growth rate
with is found.Comment: 6 pages, 5 figures, submitted to Astron. Nach
Testing turbulent closure models with convection simulations
We compare simple analytical closure models of homogeneous turbulent
Boussinesq convection for stellar applications with three-dimensional
simulations. We use simple analytical closure models to compute the fluxes of
angular momentum and heat as a function of rotation rate measured by the Taylor
number. We also investigate cases with varying angles between the angular
velocity and gravity vectors, corresponding to locating the computational
domain at different latitudes ranging from the pole to the equator of the star.
We perform three-dimensional numerical simulations in the same parameter
regimes for comparison. The free parameters appearing in the closure models are
calibrated by two fitting methods using simulation data. Unique determination
of the closure parameters is possible only in the non-rotating case or when the
system is placed at the pole. In the other cases the fit procedures yield
somewhat differing results. The quality of the closure is tested by
substituting the resulting coefficients back into the closure model and
comparing with the simulation results. To eliminate the possibilities that the
results obtained depend on the aspect ratio of the simulation domain or suffer
from too small Rayleigh numbers we performed runs varying these parameters. The
simulation data for the Reynolds stress and heat fluxes broadly agree with
previous compressible simulations. The closure works fairly well with slow and
fast rotation but its quality degrades for intermediate rotation rates. We find
that the closure parameters depend not only on rotation rate but also on
latitude. The weak dependence on Rayleigh number and the aspect ratio of the
domain indicates that our results are generally validComment: 21 pages, 9 figures, submitted to Astron. Nach
From convective to stellar dynamos
Volume: 6 Host publication title: Astrophysical Dynamics Host publication sub-title: From Stars to GalaxiesNon peer reviewe
Helical coronal ejections and their role in the solar cycle
The standard theory of the solar cycle in terms of an alpha-Omega dynamo
hinges on a proper understanding of the nonlinear alpha effect. Boundary
conditions play a surprisingly important role in determining the magnitude of
alpha. For closed boundaries, the total magnetic helicity is conserved, and
since the alpha effect produces magnetic helicity of one sign in the large
scale field, it must simultaneously produce magnetic helicity of the opposite
sign. It is this secondary magnetic helicity that suppresses the dynamo in a
potentially catastrophic fashion. Open boundaries allow magnetic helicity to be
lost. Simulations are presented that allow an estimate of alpha in the presence
of open or closed boundaries, either with or without solar-like differential
rotation. In all cases the sign of the magnetic helicity agrees with that
observed at the solar surface (negative in the north, positive in the south),
where significant amounts of magnetic helicity can be ejected via coronal mass
ejections. It is shown that open boundaries tend to alleviate catastrophic
alpha quenching. The importance of looking at current helicity instead of
magnetic helicity is emphasized and the conceptual advantages are discussed.Comment: 8 pages, 7 figs, IAU Symp. 223, In: Multi-Wavelength Investigations
of Solar Activity. Eds: A.V. Stepanov, E.E. Benevolenskaya & A.G. Kosoviche
Lambda-effect from forced turbulence simulations
Aims: We determine the components of the -effect tensor that
quantifies the contributions to the turbulent momentum transport even for
uniform rotation. Methods: Three-dimensional numerical simulations are used to
study turbulent transport in triply periodic cubes under the influence of
rotation and anisotropic forcing. Comparison is made with analytical results
obtained via the so-called minimal tau-approximation. Results: In the case
where the turbulence intensity in the vertical direction dominates, the
vertical stress is always negative. This situation is expected to occur in
stellar convection zones. The horizontal component of the stress is weaker and
exhibits a maximum at latitude 30 degrees - regardless of how rapid the
rotation is. The minimal tau-approximation captures many of the qualitative
features of the numerical results, provided the relaxation time tau is close to
the turnover time, i.e. the Strouhal number is of order unity.Comment: 20 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
Long-term variations of turbulent transport coefficients in a solar-like convective dynamo simulation
The Sun, aside from its eleven year sunspot cycle is additionally subject to
long term variation in its activity. In this work we analyse a solar-like
convective dynamo simulation, containing approximately 60 magnetic cycles,
exhibiting equatorward propagation of the magnetic field, multiple frequencies,
and irregular variability, including a missed cycle and complex parity
transitions between dipolar and quadrupolar modes. We compute the turbulent
transport coefficients, describing the effects of the turbulent velocity field
on the mean magnetic field, using the test-field method. The test-field
analysis provides a plausible explanation of the missing cycle in terms of the
reduction of in advance of the reduced surface activity,
and enhanced downward turbulent pumping during the event to confine some of the
magnetic field at the bottom of the convection zone, where local maximum of
magnetic energy is observed during the event. At the same time, however, a
quenching of the turbulent magnetic diffusivities is observed, albeit
differently distributed in depth compared to the other transport coefficients.
Therefore, dedicated mean-field modelling is required for verification.Comment: 11 pages, 12 figures, accepted by AN for 14th Potsdam Thinksho
Star-in-a-box simulations of fully convective stars
(abridged) Context: Main-sequence late-type stars with masses less than are fully convective. Aims: The goal is to study convection,
differential rotation, and dynamos as functions of rotation in fully convective
stars. Methods: Three-dimensional hydrodynamic and magnetohydrodynamic
numerical simulations with a star-in-a-box model, where a spherical star is
immersed inside of a Cartesian cube, are used. The model corresponds to a
M5 dwarf. Rotation periods () between 4.3 and 430
days are explored. Results: The slowly rotating model with
days produces anti-solar differential rotation with a slow equator and fast
poles, along with predominantly axisymmetric quasi-steady large-scale magnetic
fields. For intermediate rotation ( and days)
differential rotation is solar-like (fast equator, slow poles) and large-scale
magnetic fields are mostly axisymmetric and either quasi-stationary or cyclic.
The latter occurs in a similar parameter regime as in other numerical studies
in spherical shells, and the cycle period is similar to observed cycles in
fully convective stars with comparable . In the rapid rotation
regime the differential rotation is weak and the large-scale magnetic fields
are increasingly non-axisymmetric with a dominating mode. This
large-scale non-axisymmetric field also exhibits azimuthal dynamo waves.
Conclusions: The results of the star-in-a-box models agree with simulations of
partially convective late-type stars in spherical shells in that the
transitions in differential rotation and dynamo regimes occur at similar
rotational regimes in terms of the Coriolis (inverse Rossby) number. This
similarity between partially and fully convective stars suggests that the
processes generating differential rotation and large-scale magnetism are
insensitive to the geometry of the star.Comment: 17 pages, 11 figures, submitted to Astron. Astrophys, revised as per
referee repor
Effects of Rotation and Input Energy Flux on Convective Overshooting
We study convective overshooting by means of local 3D convection
calculations. Using a mixing length model of the solar convection zone (CZ) as
a guide, we determine the Coriolis number (Co), which is the inverse of the
Rossby number, to be of the order of ten or larger at the base of the solar CZ.
Therefore we perform convection calculations in the range Co = 0...10 and
interpret the value of Co realised in the calculation to represent a depth in
the solar CZ. In order to study the dependence on rotation, we compute the
mixing length parameters alpha_T and alpha_u relating the temperature and
velocity fluctuations, respectively, to the mean thermal stratification. We
find that the mixing length parameters for the rapid rotation case,
corresponding to the base of the solar CZ, are 3-5 times smaller than in the
nonrotating case. Introducing such depth-dependent alpha into a solar structure
model employing a non-local mixing length formalism results in overshooting
which is approximately proportional to alpha at the base of the CZ. Although
overshooting is reduced due to the reduced alpha, a discrepancy with
helioseismology remains due to the steep transition to the radiative
temperature gradient. In comparison to the mixing length models the transition
at the base of the CZ is much gentler in the 3D models. It was suggested
recently (Rempel 2004) that this discrepancy is due to the significantly larger
(up to seven orders of magnitude) input energy flux in the 3D models in
comparison to the Sun and solar models, and that the 3D calculations should be
able to approach the mixing length regime if the input energy flux is decreased
by a moderate amount. We present results from local convection calculations
which support this conjecture.Comment: 6 pages, 3 figures, to appear in Convection in Astrophysics, Proc.
IAUS 239, edited by F. Kupka, I.W. Roxburgh, K.L. Cha
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