188 research outputs found
Structure and Evolution of Giant Cells in Global Models of Solar Convection
The global scales of solar convection are studied through three-dimensional
simulations of compressible convection carried out in spherical shells of
rotating fluid which extend from the base of the convection zone to within 15
Mm of the photosphere. Such modelling at the highest spatial resolution to date
allows study of distinctly turbulent convection, revealing that coherent
downflow structures associated with giant cells continue to play a significant
role in maintaining the strong differential rotation that is achieved. These
giant cells at lower latitudes exhibit prograde propagation relative to the
mean zonal flow, or differential rotation, that they establish, and retrograde
propagation of more isotropic structures with vortical character at mid and
high latitudes. The interstices of the downflow networks often possess strong
and compact cyclonic flows. The evolving giant-cell downflow systems can be
partly masked by the intense smaller scales of convection driven closer to the
surface, yet they are likely to be detectable with the helioseismic probing
that is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate out these
signals from those attributed to the faster horizontal flows of
supergranulation. To aid in such detection, we use our simulations to describe
how the properties of giant cells may be expected to vary with depth, how their
patterns evolve in time, and analyze the statistical features of correlations
within these complex flow fields.Comment: 22 pages, 16 figures (color figures are low res), uses emulateapj.cls
Latex class file, Results shown during a Press release at the AAS meeting in
June 2007. Submitted to Ap
Evolution of forced shear flows in polytropic atmospheres: A comparison of forcing methods and energetics
Shear flows are ubiquitous in astrophysical objects including planetary and stellar interiors, where their dynamics can have significant impact on thermo-chemical processes. Investigating the complex dynamics of shear flows requires numerical calculations that provide a long time evolution of the system. To achieve a sufficiently long lifetime in a local numerical model the system has to be forced externally. However, at present, there exist several different forcing methods to sustain large-scale shear flows in local models. In this paper we examine and compare various methods used in the literature in order to resolve their respective applicability and limitations. These techniques are compared during the exponential growth phase of a shear flow instability, such as the Kelvin-Helmholtz (KH) instability, and some are examined during the subsequent non-linear evolution. A linear stability analysis provides reference for the growth rate of the most unstable modes in the system and a detailed analysis of the energetics provides a comprehensive understanding of the energy exchange during the system's evolution. Finally, we discuss the pros and cons of each forcing method and their relation with natural mechanisms generating shear flows
Simulations of turbulent convection in rotating young solar-like stars: Differential rotation and meridional circulation
We present the results of three-dimensional simulations of the deep
convective envelope of a young (10 Myr) one-solar-mass star, obtained with the
Anelastic Spherical Harmonic code. Since young stars are known to be faster
rotators than their main sequence counterparts, we have systematically studied
the impact of the stellar rotation speed, by considering stars spinning up to
five times as fast as the Sun. The aim of these nonlinear models is to
understand the complex interactions between convection and rotation. We discuss
the influence of the turbulence level and of the rotation rate on the intensity
and the topology of the mean flows. For all of the computed models, we find a
solar-type superficial differential rotation, with an equatorial acceleration,
and meridional circulation that exhibits a multicellular structure. Even if the
differential rotation contrast decreases only marginally for high rotation
rates, the meridional circulation intensity clearly weakens according to our
simulations. We have also shown that, for Taylor numbers above a certain
threshold (Ta>10^9), the convection can develop a vacillating behavior. Since
simulations with high turbulence levels and rotation rates exhibit strongly
cylindrical internal rotation profiles, we have considered the influence of
baroclinic effects at the base of the convective envelope of these young Suns,
to see whether such effect can modify the otherwise near cylindrical profiles
to produce more conical, solar-like profiles.Comment: 32 pages, 18 figures, 2 tables, to appear in Ap
Velocity Field Statistics in Star-Forming Regions. I. Centroid Velocity Observations
The probability density functions (pdfs) of molecular line centroid velocity
fluctuations and fluctuation differences at different spatial lags are
estimated for several nearby molecular clouds with active internal star
formation. The data consist of over 75,000 CO line profiles divided
among twelve spatially and/or kinematically distinct regions. Although three
regions (all in Mon R2) appear nearly Gaussian, the others show strong evidence
for non-Gaussian, often nearly exponential, centroid velocity pdfs, possibly
with power law contributions in the far tails. Evidence for nearly exponential
centroid pdfs in the neutral HI component of the ISM is also presented, based
on older optical and radio observations. These results are in contrast to pdfs
found in isotropic incompressible turbulence experiments and simulations.
Furthermore, no evidence is found for the scaling of difference pdf kurtosis
with Reynolds number which is seen in incompressible turbulence, and the
spatial distribution of high-amplitude velocity differences shows little
indication of the filamentary appearance predicted by decay simulations
dominated by vortical interactions. The variation with lag of the difference
pdf moments is presented as a constraint on future simulations.Comment: LaTeX, 23 pages, with 15 Figures included separately as gif image
files. Refereed/revised version accepted to the Astrophysical Journal. A
complete (but much larger) postscript version is available from
http://ktaadn.gsfc.nasa.gov/~miesc
Magnetic field confinement by meridional flow and the solar tachocline
We show that the MHD theory that explains the solar tachocline by an effect
of the magnetic field can work with the decay modes of a fossil field in the
solar interior if the meridional flow of the convection zone penetrates
slightly the radiative zone beneath. An equatorward flow of about 10 m/s
penetrating to a maximum depth of 1000 km below the convection zone is able to
generate almost horizontal field lines in the tachocline region so that the
internal field is almost totally confined to the radiative zone. The theory of
differential solar rotation indeed provides meridional flows of about 10 m/s
and a penetration depth of < 1000 km for viscosity values that are
characteristic of a stable tachocline.Comment: 5 pages, 6 figures, submitted to A&
Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models
The Sun is a magnetic star whose cyclic activity is thought to be linked to
internal dynamo mechanisms. A combination of numerical modelling with various
levels of complexity is an efficient and accurate tool to investigate such
intricate dynamical processes. We investigate the role of the magnetic buoyancy
process in 2D Babcock-Leighton dynamo models, by modelling more accurately the
surface source term for poloidal field. Methods. To do so, we reintroduce in
mean-field models the results of full 3D MHD calculations of the non-linear
evolution of a rising flux tube in a convective shell. More specifically, the
Babcock-Leighton source term is modified to take into account the delay
introduced by the rise time of the toroidal structures from the base of the
convection zone to the solar surface. We find that the time delays introduced
in the equations produce large temporal modulation of the cycle amplitude even
when strong and thus rapidly rising flux tubes are considered. Aperiodic
modulations of the solar cycle appear after a sequence of period doubling
bifurcations typical of non-linear systems. The strong effects introduced even
by small delays is found to be due to the dependence of the delays on the
magnetic field strength at the base of the convection zone, the modulation
being much less when time delays remain constant. We do not find any
significant influence on the cycle period except when the delays are made
artificially strong. A possible new origin of the solar cycle variability is
here revealed. This modulated activity and the resulting butterfly diagram are
then more compatible with observations than what the standard Babcock-Leighton
model produces.Comment: 13 pages, 10 figures, accepted for publication in A&
Global-Scale Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope
The operation of the solar global dynamo appears to involve many dynamical
elements. Self-consistent MHD simulations which realistically incorporate all
of these processes are not yet computationally feasible, though some elements
can now be studied with reasonable fidelity. Here we consider the manner in
which turbulent compressible convection within the bulk of the solar convection
zone can generate large-scale magnetic fields through dynamo action. We
accomplish this through a series of three-dimensional numerical simulations of
MHD convection within rotating spherical shells using our ASH code on massively
parallel supercomputers. Since differential rotation is a key ingredient in all
dynamo models, we also examine here the nature of the rotation profiles that
can be sustained within the deep convection zone as strong magnetic fields are
built and maintained. We find that the convection is able to maintain a
solar-like angular velocity profile despite the influence of Maxwell stresses
which tend to oppose Reynolds stresses and thus reduce the latitudinal angular
velocity contrast throughout the convection zone. The dynamo-generated magnetic
fields exhibit a complex structure and evolution, with radial fields
concentrated in downflow lanes and toroidal fields organized into twisted
ribbons which are extended in longitude and which achieve field strengths of up
to 5000 G. The flows and fields exhibit substantial kinetic and magnetic
helicity although systematic hemispherical patterns are only apparent in the
former. Fluctuating fields dominate the magnetic energy and account for most of
the back-reaction on the flow via Lorentz forces. Mean fields are relatively
weak and do not exhibit systematic latitudinal propagation or periodic polarity
reversals as in the sun. This may be attributed to the absence of a tachocline.Comment: 55 pages (ApJ refereeing format), 15 figures (low res), published by
ApJ on October 2004 (abstract slightly reduced in order to fit in 24 lines
limit) see also Browning, Miesch, Brun & Toomre 2006, ApJL, 648, 157
(astro-ph/0609153) for the effect of a tachocline in organizing the mean
field
Differential rotation of main-sequence dwarfs and its dynamo-efficiency
A new version of a numerical model of stellar differential rotation based on
mean-field hydrodynamics is presented and tested by computing the differential
rotation of the Sun. The model is then applied to four individual stars
including two moderate and two fast rotators to reproduce their observed
differential rotation quite closely. A series of models for rapidly rotating
( day) stars of different masses and compositions is generated.
The effective temperature is found convenient to parameterize the differential
rotation: variations with metallicity, that are quite pronounced when the
differential rotation is considered as a function of the stellar mass, almost
disappear in the dependence of differential rotation on temperature. The
differential rotation increases steadily with surface temperature to exceed the
largest differential rotation observed to date for the hottest F-stars we
considered. This strong differential rotation is, however, found not to be
efficient for dynamos when the efficiency is estimated with the standard
-parameter of dynamo models. On the contrary, the small differential
rotation of M-stars is the most dynamo-efficient. The meridional flow near the
bottom of the convection zone is not small compared to the flow at the top in
all our computations. The flow is distributed over the entire convection zone
in slow rotators but retreats to the convection zone boundaries with increasing
rotation rate, to consist of two near-boundary jets in rapid rotators. The
implications of the change of the flow structure for stellar dynamos are
briefly discussed.Comment: 9 pages, 11 figures, submitted to MNRA
Dual Function of the Cytochrome P450 CYP76 Family from Arabidopsis thaliana in the Metabolism of Monoterpenols and Phenylurea Herbicides
Stellar turbulence and mode physics
An overview of selected topical problems on modelling oscillation properties
in solar-like stars is presented. High-quality oscillation data from both
space-borne intensity observations and ground-based spectroscopic measurements
provide first tests of the still-ill-understood, superficial layers in distant
stars. Emphasis will be given to modelling the pulsation dynamics of the
stellar surface layers, the stochastic excitation processes and the associated
dynamics of the turbulent fluxes of heat and momentum.Comment: Proc. HELAS Workshop on 'Synergies between solar and stellar
modelling', eds M. Marconi, D. Cardini, M. P. Di Mauro, Astrophys. Space
Sci., in the pres
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