2,748 research outputs found
Magnetic helicity fluxes in interface and flux transport dynamos
Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &
Extracting scaling laws from numerical dynamo models
Earth's magnetic field is generated by processes in the electrically
conducting, liquid outer core, subsumed under the term `geodynamo'. In the last
decades, great effort has been put into the numerical simulation of core
dynamics following from the magnetohydrodynamic (MHD) equations. However, the
numerical simulations are far from Earth's core in terms of several control
parameters. Different scaling analyses found simple scaling laws for quantities
like heat transport, flow velocity, magnetic field strength and magnetic
dissipation time.
We use an extensive dataset of 116 numerical dynamo models compiled by
Christensen and co-workers to analyse these scalings from a rigorous model
selection point of view. Our method of choice is leave-one-out cross-validation
which rates models according to their predictive abilities. In contrast to
earlier results, we find that diffusive processes are not negligible for the
flow velocity and magnetic field strength in the numerical dynamos. Also the
scaling of the magnetic dissipation time turns out to be more complex than
previously suggested. Assuming that the processes relevant in the numerical
models are the same as in Earth's core, we use this scaling to estimate an
Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with
recent high CMB heat flux scenarios.Comment: 21 pages, 11 figure
The Magnetic Sun: Reversals and Long-Term Variations
A didactic introduction to current thinking on some aspects of the solar
dynamo is given for geophysicists and planetary scientists.Comment: 17 pages, 9 figures; Space Science Rev., in pres
Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
Conversion of gravitational energy into radiation in accretion discs and the
origin of large scale magnetic fields in astrophysical rotators have often been
distinct topics of research. In semi-analytic work on both problems it has been
useful to presume large scale symmetries, necessarily resulting in mean field
theories. MHD turbulence makes the underlying systems locally asymmetric and
nonlinear. Synergy between theory and simulations should aim for the
development of practical mean field models that capture essential physics and
can be used for observational modeling. Mean field dynamo (MFD) theory and
alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century
MFD theory has more nonlinear predictive power compared to 20th century MFD
theory, whereas accretion theory is still in a 20th century state. In fact,
insights from MFD theory are applicable to accretion theory and the two are
artificially separated pieces of what should be a single theory. I discuss
pieces of progress that provide clues toward a unified theory. A key concept is
that large scale magnetic fields can be sustained via local or global magnetic
helicity fluxes or via relaxation of small scale magnetic fluctuations, without
the kinetic helicity driver of 20th century textbooks. These concepts may help
explain the formation of large scale fields that supply non-local angular
momentum transport via coronae and jets in a unified theory of accretion and
dynamos. In diagnosing the role of helicities and helicity fluxes in disk
simulations, each disk hemisphere should be studied separately to avoid being
misled by cancelation that occurs as a result of reflection asymmetry. The
fraction of helical field energy in disks is expected to be small compared to
the total field in each hemisphere as a result of shear, but can still be
essential for large scale dynamo action.Comment: For the Proceedings of the Third International Conference and
Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28
August 2011 at the Abdus Salam International Centre for Theoretical Physics,
Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica
Scripta (corrected small items to match version in print
Polar Field Puzzle: Solutions from Flux-Transport Dynamo and Surface Transport Models
Polar fields in solar cycle 23 were about 50% weaker than those in cycle 22.
The only theoretical models which have addressed this puzzle are surface
transport models and flux-transport dynamo models. Comparing polar fields
obtained from numerical simulations using surface flux transport models and
flux-transport dynamo models, we show that both classes of models can explain
the polar field features within the scope of the physics included in the
respective models. In both models, how polar fields change as a result of
changes in meridional circulation depends on the details of meridional
circulation profile used. Using physical reasoning and schematics as well as
numerical solutions from a flux-transport dynamo model, we demonstrate that
polar fields are determined mostly by the strength of surface poloidal source
provided by the decay of tilted, bipolar active regions. Profile of meridional
flow with latitude and its changes with time have much less effect in
flux-transport dynamo models than in surface transport models.Comment: ApJ (accepted
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