200 research outputs found
Sustained magneto-shear instabilities in the solar tachocline
We present nonlinear three-dimensional simulations of the stably-stratified
portion of the solar tachocline in which the rotational shear is maintained by
mechanical forcing. When a broad toroidal field profile is specified as an
initial condition, a clam-shell instability ensues which is similar to the
freely-evolving cases studied previously. After the initial nonlinear
saturation, the residual mean fields are apparently too weak to sustain the
instability indefinitely. However, when a mean poloidal field is imposed in
addition to the rotational shear, a statistically-steady state is achieved in
which the clam-shell instability is operating continually. This state is
characterized by a quasi-periodic exchange of energy between the mean toroidal
field and the instability mode with a longitudinal wavenumber m=1. This
quasi-periodic behavior has a timescale of several years and may have
implications for tachocline dynamics and field emergence patterns throughout
the solar activity cycle.Comment: 5 pages, 3 figures (eps format). Fig. 3 also in jpg format. Submitted
to Astrophysical Journal Letter
Solar Dynamics, Rotation, Convection and Overshoot
We discuss recent observational, theoretical and modeling progress made in
understanding the Sun's internal dynamics, including its rotation, meridional
flow, convection and overshoot. Over the past few decades, substantial
theoretical and observational effort has gone into appreciating these aspects
of solar dynamics. A review of these observations, related helioseismic
methodology and inference and computational results in relation to these
problems is undertaken here.Comment: 31 pages, 10 figures, Space Science Review
Three-Dimensional Simulations of Solar and Stellar Dynamos: The Influence of a Tachocline
We review recent advances in modeling global-scale convection and dynamo
processes with the Anelastic Spherical Harmonic (ASH) code. In particular, we
have recently achieved the first global-scale solar convection simulations that
exhibit turbulent pumping of magnetic flux into a simulated tachocline and the
subsequent organization and amplification of toroidal field structures by
rotational shear. The presence of a tachocline not only promotes the generation
of mean toroidal flux, but it also enhances and stabilizes the mean poloidal
field throughout the convection zone, promoting dipolar structure with less
frequent polarity reversals. The magnetic field generated by a convective
dynamo with a tachocline and overshoot region is also more helical overall,
with a sign reversal in the northern and southern hemispheres. Toroidal
tachocline fields exhibit little indication of magnetic buoyancy instabilities
but may be undergoing magneto-shear instabilities.Comment: 14 pages, 5 color figures, to appear in Proc. GONG 2008/SOHO XXI
Meeting on Solar-Stellar Dynamos as Revealed by Helio and Asteroseismology,
held August 15-18, 2008, Boulder, CO, Astronomical Soc. Pac. Conf. Series,
volume TB
New insights about meridional circulation dynamics from 3D MHD global simulations of solar convection and dynamo action
The solar meridional circulation is a "slow", large scale flow that transports magnetic field and plasma throughout the convection zone in the (r, theta) plane and plays a crucial role in controlling the magnetic cycle solutions presented by flux transport dynamo models. Observations indicate that this flow speed varies in anti-phase with the solar cycle at the solar surface. A possible explanation for the source of this variation is based on the fact that inflows into active regions alter the global surface pattern of the meridional circulation. In this work we examine the meridional circulation profile that emerges from a 3D global simulation of the solar convection zone, and its associated dynamics. We find that at the bottom of the convection zone, in the region where the toroidal magnetic field accumulates, the meridional circulation is highly modulated through the action of a magnetic torques and thus provides evidence for a new mechanism to explain the observed cyclic variations
Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields
We present the first results from three-dimensional spherical shell
simulations of magnetic dynamo action realized by turbulent convection
penetrating downward into a tachocline of rotational shear. This permits us to
assess several dynamical elements believed to be crucial to the operation of
the solar global dynamo, variously involving differential rotation resulting
from convection, magnetic pumping, and amplification of fields by stretching
within the tachocline. The simulations reveal that strong axisymmetric toroidal
magnetic fields (about 3000 G in strength) are realized within the lower stable
layer, unlike in the convection zone where fluctuating fields are predominant.
The toroidal fields in the stable layer possess a striking persistent
antisymmetric parity, with fields in the northern hemisphere largely of
opposite polarity to those in the southern hemisphere. The associated mean
poloidal magnetic fields there have a clear dipolar geometry, but we have not
yet observed any distinctive reversals or latitudinal propagation. The presence
of these deep magnetic fields appears to stabilize the sense of mean fields
produced by vigorous dynamo action in the bulk of the convection zone.Comment: 4 pages, 3 color figures (compressed), in press at ApJ
Is the solar convection zone in strict thermal wind balance?
Context: The solar rotation profile is conical rather than cylindrical as one
could expect from classical rotating fluid dynamics (e.g. Taylor-Proudman
theorem). Thermal coupling to the tachocline, baroclinic effects and
latitudinal transport of heat have been advocated to explain this peculiar
state of rotation. Aims: To test the validity of thermal wind balance in the
solar convection zone using helioseismic inversions for both the angular
velocity and fluctuations in entropy and temperature. Methods: Entropy and
temperature fluctuations obtained from 3-D hydrodynamical numerical simulations
of the solar convection zone are compared with solar profiles obtained from
helioseismic inversions. Results: The temperature and entropy fluctuations in
3-D numerical simulations have smaller amplitude in the bulk of the solar
convection zone than those found from seismic inversions. Seismic inversion
find variations of temperature from about 1 K at the surface up to 100 K at the
base of the convection zone while in 3-D simulations they are of order 10 K
throughout the convection zone up to 0.96 . In 3-D simulations,
baroclinic effects are found to be important to tilt the isocontours of
away from a cylindrical profile in most of the convection zone helped
by Reynolds and viscous stresses at some locations. By contrast the baroclinic
effect inverted by helioseismology are much larger than what is required to
yield the observed angular velocity profile. Conclusion: The solar convection
does not appear to be in strict thermal wind balance, Reynolds stresses must
play a dominant role in setting not only the equatorial acceleration but also
the observed conical angular velocity profile.Comment: 8 pages, 6 figures (low resolution), Accepted by Astronomy and
Astrophysics - Affiliation: (1) AIM, CEA/DSM-CNRS-Univ. Paris Diderot,
IRFU/SAp, France & (2) LUTH, Observatoire de Paris, CNRS-Univ. Paris Diderot,
France ; (3) Tata Institute of Fundamental Research, India; (4) Centre for
Basic Sciences, University of Mumbai, Indi
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