74 research outputs found
Topology and field strength in spherical, anelastic dynamo simulations
Numerical modelling of convection driven dynamos in the Boussinesq
approximation revealed fundamental characteristics of the dynamo-generated
magnetic fields and the fluid flow. Because these results were obtained for an
incompressible fluid, their validity for gas planets and stars remains to be
assessed. A common approach is to take some density stratification into account
with the so-called anelastic approximation. The validity of previous results
obtained in the Boussinesq approximation is tested for anelastic models. We
point out and explain specific differences between both types of models, in
particular with respect to the field geometry and the field strength, but we
also compare scaling laws for the velocity amplitude, the magnetic dissipation
time, and the convective heat flux. Our investigation is based on a systematic
parameter study of spherical dynamo models in the anelastic approximation. We
make use of a recently developed numerical solver and provide results for the
test cases of the anelastic dynamo benchmark. The dichotomy of dipolar and
multipolar dynamos identified in Boussinesq simulations is also present in our
sample of anelastic models. Dipolar models require that the typical length
scale of convection is an order of magnitude larger than the Rossby radius.
However, the distinction between both classes of models is somewhat less
explicit than in previous studies. This is mainly due to two reasons: we found
a number of models with a considerable equatorial dipole contribution and an
intermediate overall dipole field strength. Furthermore, a large density
stratification may hamper the generation of dipole dominated magnetic fields.
Previously proposed scaling laws, such as those for the field strength, are
similarly applicable to anelastic models. It is not clear, however, if this
consistency necessarily implies similar dynamo processes in both settings.Comment: 14 pages, 11 figure
Oscillatory dynamos and their induction mechanisms
Context: Large-scale magnetic fields resulting from hydromagnetic dynamo
action may differ substantially in their time dependence. Cyclic field
variations, characteristic for the solar magnetic field, are often explained by
an important omega-effect, i.e. by the stretching of field lines due to strong
differential rotation. Aims: The dynamo mechanism of a convective, oscillatory
dynamo model is investigated. Methods: We solve the MHD-equations for a
conducting Boussinesq fluid in a rotating spherical shell. For a resulting
oscillatory model, dynamo coefficients have been computed with the help of the
so-called test-field method. Subsequently, these coefficients have been used in
a mean-field calculation in order to explore the underlying dynamo mechanism.
Results: Although the rather strong differential rotation present in this model
influences the magnetic field, the omega-effect alone is not responsible for
its cyclic time variation. If the omega-effect is suppressed, the resulting
alpha^2-dynamo remains oscillatory. Surprisingly, the corresponding alpha-omega
dynamo leads to a non-oscillatory magnetic field. Conclusions: The assumption
of an alpha-omega mechanism does not explain the occurrence of magnetic cycles
satisfactorily
Kinematic alpha effect in isotropic turbulence simulations
Using numerical simulations at moderate magnetic Reynolds numbers up to 220
it is shown that in the kinematic regime, isotropic helical turbulence leads to
an alpha effect and a turbulent diffusivity whose values are independent of the
magnetic Reynolds number, \Rm, provided \Rm exceeds unity. These turbulent
coefficients are also consistent with expectations from the first order
smoothing approximation. For small values of \Rm, alpha and turbulent
diffusivity are proportional to \Rm. Over finite time intervals meaningful
values of alpha and turbulent diffusivity can be obtained even when there is
small-scale dynamo action that produces strong magnetic fluctuations. This
suggests that small-scale dynamo-generated fields do not make a correlated
contribution to the mean electromotive force.Comment: Accepted for publication in MNRAS Letter
Reynolds stresses from hydrodynamic turbulence with shear and rotation
To study the Reynolds stresses which describe turbulent momentum transport
from turbulence affected by large-scale shear and rotation. Three-dimensional
numerical simulations are used to study turbulent transport under the
influences of large-scale shear and rotation in homogeneous, isotropically
forced turbulence. We study three cases: one with only shear, and two others
where in addition to shear, rotation is present. These cases differ by the
angle (0 or 90\degr) the rotation vector makes with respect to the z-direction.
Two subsets of runs are performed with both values of \theta where either
rotation or shear is kept constant. When only shear is present, the
off-diagonal stress can be described by turbulent viscosity whereas if the
system also rotates, nondiffusive contributions (\Lambda-effect) to the stress
can arise. Comparison of the direct simulations are made with analytical
results from a simple closure model. We find that the turbulent viscosity is of
the order of the first order smoothing result in the parameter regime studied
and that for sufficiently large Reynolds numbers the Strouhal number,
describing the ratio of correlation to turnover times, is roughly 1.5. This is
consistent with the closure model based on the minimal tau-approximation which
produces a reasonable fit to the simulation data for similar Strouhal numbers.
In the cases where rotation is present, separating the diffusive and
nondiffusive components of the stress turns out to be challenging but taking
the results at face value, we can obtain nondiffusive contributions of the
order of 0.1 times the turbulent viscosity. We also find that the simple
closure model is able to reproduce most of the qualitative features of the
numerical results provided that the Strouhal number is of the order of unity.Comment: 19 pages, 12 figures, published versio
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
Weak and Strong Field Dynamos: from the Earth to the stars
Observations of magnetism in very low mass stars recently made important
progress, revealing characteristics that are now to be understood in the
framework of dynamo theory. In parallel, there is growing evidence that dynamo
processes in these stars share many similarities with planetary dynamos. We
investigate the extent to which the weak \emph{vs} strong field bistability
predicted for the geodynamo can apply to recent observations of two groups of
very low mass fully-convective stars sharing similar stellar parameters but
generating radically different types of magnetic fields. Our analysis is based
on previously published spectropolarimetric and spectroscopic data. We argue
that these can be interpreted in the framework of weak and strong field
dynamos.Comment: 5 pages, 4 figures, accepted for publication in MNRAS letter
- …