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Author name code: emonet
ADS astronomy entries on 2022-09-14
author:"Emonet, Thierry"
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Title: Simulations of magneto-convection in the solar photosphere.
Equations, methods, and results of the MURaM code
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
Emonet, T.; Linde, T.
2005A&A...429..335V Altcode:
We have developed a 3D magnetohydrodynamics simulation code for
applications in the solar convection zone and photosphere. The code
includes a non-local and non-grey radiative transfer module and takes
into account the effects of partial ionization. Its parallel design
is based on domain decomposition, which makes it suited for use on
parallel computers with distributed memory architecture. We give a
description of the equations and numerical methods and present the
results of the simulation of a solar plage region. Starting with
a uniform vertical field of 200 G, the processes of flux expulsion
and convective field amplification lead to a dichotomy of strong,
mainly vertical fields embedded in the granular downflow network and
weak, randomly oriented fields filling the hot granular upflows. The
strong fields form a magnetic network with thin, sheet-like structures
extending along downflow lanes and micropores with diameters of up to
1000 km which form occasionally at vertices where several downflow
lanes merge. At the visible surface around optical depth unity,
the strong field concentrations are in pressure balance with their
weakly magnetized surroundings and reach field strengths of up to 2
kG, strongly exceeding the values corresponding to equipartition with
the kinetic energy density of the convective motions. As a result of
the channelling of radiation, small flux concentrations stand out as
bright features, while the larger micropores appear dark in brightness
maps owing to the suppression of the convective energy transport. The
overall shape of the magnetic network changes slowly on a timescale
much larger than the convective turnover time, while the magnetic flux
is constantly redistributed within the network leading to continuous
formation and dissolution of flux concentrations. <P />Appendices A-D
are only available in electronic form at http://www.edpsciences.org
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Title: The Generation of Surface Magnetic Fields
Authors: Cattaneo, F.; Emonet, T.
2004cosp...35.4443C Altcode: 2004cosp.meet.4443C
The most readily observable manifestation of convection at the solar
surface is the granulation. Granules with a characteristic size of
1,000 km and a lifetime of 5 min are too small and too short-lived
to be significantly affected by the solar rotation. Thus, the upper
layers of the convective zone are in a state of strongly turbulent,
non-helical convection. Because of the high electrical conductivity
of the solar plasma, the magnetic Reynmolds number of the granulation
is large (> 10^5). These considerations, suggest that the granular
flows can act as local small-scale dynamos, generating disordered
small-scale magnetic fields with lifetimes comparable to that of the
granulation. Numerical simulations support this conclusion showing
that intense highly intermittent fields can readily be generated
provided the magnetic Reynolds number is large enough. One interesting
aspect of the solar small-scale dynamo problem is related to the
extremely small value of the plasma viscosity; much smaller than the
magnetic diffusivity. In this regime (small magnetic Prandtl number)
the velocity has strong fluctuations at the magnetic diffusion scale
with profound consequences for the operation of the dynamo. In this
talk I will address how our ideas of dynamo action, mostly based on
smooth velocity fields, must be modified to account for this fact
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Title: On the Interaction between Convection and Magnetic Fields
Authors: Cattaneo, Fausto; Emonet, Thierry; Weiss, Nigel
2003ApJ...588.1183C Altcode:
Turbulent convection in the solar photosphere can act as a small-scale
dynamo, maintaining a disordered magnetic field that is locally
intense. On the other hand, convection is inhibited in the presence
of a strong, externally imposed magnetic field, as for instance,
in a sunspot. Large-scale, three-dimensional, numerical experiments
on highly nonlinear magnetoconvection in a Boussinesq fluid show
that there is a continuous transition from a dynamo regime through
a convective regime to an oscillatory regime as the strength of the
imposed magnetic field is progressively increased. The patterns found
in these different regimes are described and analyzed.
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Title: Polarization of Photospheric Lines from Turbulent Dynamo
Simulations
Authors: Sánchez Almeida, J.; Emonet, T.; Cattaneo, F.
2003ApJ...585..536S Altcode: 2002astro.ph.11175S; 2002astro.ph.11175A
We employ the magnetic and velocity fields from turbulent dynamo
simulations to synthesize the polarization of a typical photospheric
line. The synthetic Stokes profiles have properties in common with
those observed in the quiet Sun. The simulated magnetograms present a
level of signal similar to that of the Inter-Network regions. Asymmetric
Stokes V profiles with two, three, and more lobes appear in a natural
way. The intensity profiles are broadened by the magnetic fields
in fair agreement with observational limits. Furthermore, the Hanle
depolarization signals of the Sr I λ4607 Å line turn out to be within
the solar values. Differences between synthetic and observed polarized
spectra can also be found. There is a shortage of Stokes V asymmetries,
which we attribute to a deficit of structuring in the magnetic and
velocity fields from the simulations as compared to the Sun. This
deficit may reflect the fact that the Reynolds numbers of the numerical
data are still far from solar values. We consider the possibility that
intense and tangled magnetic fields, like those in the simulations,
exist in the Sun. This scenario has several important consequences. For
example, less than 10% of the existing unsigned magnetic flux would
be detected in present magnetograms. The existing flux would exceed
by far that carried by active regions during the maximum of the solar
cycle. Detecting these magnetic fields would involve improving the
angular resolution, the techniques to interpret the polarization
signals, and to a lesser extent, the polarimetric sensitivity.
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Title: The Polarized Spectrum Emerging from Fast Dynamo Simulations
Authors: Sánchez Almeida, J.; Emonet, T.; Cattaneo, F.
2003ASPC..307..293S Altcode:
No abstract at ADS
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Title: Simulation of Solar Magnetoconvection
Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.;
Emonet, T.; Linde, T.
2003IAUS..210..157V Altcode:
No abstract at ADS
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Title: Theories and observations of surface dynamos
Authors: Emonet, Thierry
2002ocnd.confE...7E Altcode:
No abstract at ADS
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Title: Small-Scale Photospheric Fields: Observational Evidence and
Numerical Simulations
Authors: Emonet, Thierry; Cattaneo, Fausto
2001ApJ...560L.197E Altcode:
Observations suggest that magnetic fields at the solar photosphere
may be structured below the limit of the present resolution. We
argue that numerical simulations could be used in a complementary
way to observations in order to study the small-scale structure of
photospheric fields. We present a number of illustrative examples.
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Title: The Zigzag Path of Buoyant Magnetic Tubes and the Generation
of Vorticity along Their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
2001ApJ...549.1212E Altcode:
We study the generation of vorticity in the magnetic boundary layer
of buoyant magnetic tubes and its consequences for the trajectory of
magnetic structures rising in the solar convection zone. When the
Reynolds number is well above 1, the wake trailing the tube sheds
vortex rolls, producing a von Kármán vortex street, similar to the
case of flows around rigid cylinders. The shedding of a vortex roll
causes an imbalance of vorticity in the tube. The ensuing vortex force
excites a transverse oscillation of the flux tube as a whole so that
it follows a zigzag upward path instead of rising along a straight
vertical line. In this paper, the physics of vorticity generation in
the boundary layer is discussed and scaling laws for the relevant terms
are presented. We then solve the two-dimensional magnetohydrodynamic
equations numerically, measure the vorticity production, and show the
formation of a vortex street and the consequent sinusoidal path of the
magnetic flux tube. For high values of the plasma beta, the trajectory
of the tubes is found to be independent of β but varying with the
Reynolds number. The Strouhal number, which measures the frequency
of vortex shedding, shows in our rising tubes only a weak dependence
with the Reynolds numbers, a result also obtained in the rigid-tube
laboratory experiments. In fact, the actual values measured in the
latter are also close to those of our numerical calculations. As
the Reynolds numbers are increased, the amplitude of the lift force
grows and the trajectory becomes increasingly complicated. It is
shown how a simple analytical equation (which includes buoyancy,
drag, and vortex forces) can satisfactorily reproduce the computed
trajectories. The different regimes of rise can be best understood in
terms of a dimensionless parameter, χ, which measures the importance
of the vortex force as compared with the buoyancy and drag forces. For
χ<SUP>2</SUP><<1, the rise is drag dominated and the trajectory
is mainly vertical with a small lateral oscillation superposed. When
χ becomes larger than 1, there is a transition toward a drag-free
regime and epicycles are added to the trajectory.
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Title: Effects of Limited Resolution on the Inferred Structure of
Photospheric Magnetic Fields
Authors: Emonet, T.; Cattaneo, F.
2001ASPC..236..355E Altcode: 2001aspt.conf..355E
No abstract at ADS
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Title: The Dynamics of Buoyant Magnetic Ropes and the Generation of
Vorticity in their Periphery
Authors: Emonet, T.; Moreno-Insertis, F.; Rast, M. P.
2000SPD....31.0133E Altcode: 2000BAAS...32..807E
When the Reynolds number is not small, the wake trailing a buoyant
magnetic flux tube sheds vortex rolls therefore producing a Von Karman
vortex street and an imbalance of vorticity in the tube which results
in a transverse oscillation of the tube as a whole. The actual path
followed by the magnetic structure is therefore directly affected by
the amount of vorticity being produced in its boundary. Analytical
expressions for the magnetic generation and viscous dissipation of
vorticity in the boundary layer of buoyant magnetic flux tubes are
obtained. Corresponding scaling laws are deduced and checked using a
full compressible 2D MHD code. Interestingly, the observed trajectories
can be satisfactorily reproduced by a simple analytical equation (which
includes buoyancy, drag and vortex forces). I will conclude with some
comparisons with classical results from the hydrodynamical literature
(Strouhal number), and some comments about the rise time of buoyant
magnetic structures through the solar convection zone.
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Title: The Physics of Twisted Magnetic Tubes Rising in a Stratified
Medium: Two-dimensional Results
Authors: Emonet, T.; Moreno-Insertis, F.
1998ApJ...492..804E Altcode: 1997astro.ph.11043E
The physics of a twisted magnetic flux tube rising in a stratified
medium is studied using a numerical magnetohydrodynamic (MHD)
code. The problem considered is fully compressible (has no Boussinesq
approximation), includes ohmic resistivity, and is two-dimensional,
i.e., there is no variation of the variables in the direction of
the tube axis. We study a high-plasma β-case with a small ratio
of radius to external pressure scale height. The results obtained
will therefore be of relevance to understanding the transport of
magnetic flux across the solar convection zone. <P />We confirm
that a sufficient twist of the field lines around the tube axis can
suppress the conversion of the tube into two vortex rolls. For a tube
with a relative density deficit on the order of 1/β (the classical
Parker buoyancy) and a radius smaller than the pressure scale height
(R<SUP>2</SUP><<H<SUP>2</SUP><SUB>p</SUB>), the minimum amount
of twist necessary corresponds to an average pitch angle on the order
of sin<SUP>-1</SUP> [(R/H<SUB>p</SUB>)<SUP>1/2</SUP>]. The evolution
of a tube with this degree of twist is studied in detail, including
the initial transient phase, the internal torsional oscillations,
and the asymptotic, quasi-stationary phase. During the initial phase,
the outermost, weakly magnetized layers of the tube are torn off its
main body and endowed with vorticity. They yield a trailing magnetized
wake with two vortex rolls. The fraction of the total magnetic flux
that is brought to the wake is a function of the initial degree of
twist. In the weakly twisted case, most of the initial tube is turned
into vortex rolls. With a strong initial twist, the tube rises with
only a small deformation and no substantial loss of magnetic flux. The
formation of the wake and the loss of flux from the main body of the
tube are basically complete after the initial transient phase. <P />A
sharp interface between the tube interior and the external flows is
formed at the tube front and sides; this area has the characteristic
features of a magnetic boundary layer. Its structure is determined as
an equilibrium between ohmic diffusion and field advection through the
external flows. It is the site of vorticity generation via the magnetic
field during the whole tube evolution. <P />From the hydrodynamical
point of view, this problem constitutes an intermediate case between
the rise of air bubbles in water and the motion of a rigid cylinder
in an external medium. As with bubbles, the tube is deformable and the
outcome of the experiment (the shape of the rising object and the wake)
depends on the value of the Weber number. Several structural features
obtained in the present simulation are also observed in rising air
bubbles, such as a central tail, and a skirt enveloping the wake. As
in rigid cylinders, the boundary layer satisfies a no-slip condition
(provided for in the tube by the magnetic field), and secondary rolls
are formed at the lateral edges of the moving object.
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Title: The internal structure of rising twisted magnetic tubes and
the emergence of magnetic flux in the Sun
Authors: Emonet, T.
1997PhDT.........8E Altcode:
No abstract at ADS
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Title: The Rise of Magnetic Flux Tubes across a Stratified Medium:
Effects of the Twist
Authors: Emonet, T.; Moreno-Insertis, F.
1997ASPC..118...71E Altcode: 1997fasp.conf...71E
The results of a 2D numerical simulation of the rise of twisted
magnetic flux tubes are sketched. The theoretical criterion for the
minimum twist necessary to prevent the conversion of the tube into
a pair of strong vortices is shown to be correct. The transition
from a low-twist to a high-twist regime is exemplified. There is a
sharp transition between the tube interior and the outside medium. A
well-developed wake is formed, which however contains only a fraction
of the original magnetic flux of the tube.
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Title: The Rise of Twisted Magnetic Tubes in a Stratified Medium
Authors: Moreno-Insertis, F.; Emonet, T.
1996ApJ...472L..53M Altcode:
First results from a two-dimensional numerical study of the buoyant
rise of twisted magnetic flux tubes in the solar convection zone
are presented. We show in detail the process by which the transverse
component of the field can suppress the splitting of the rising tube
into two vortex filaments. For the suppression to be effective, the
pitch angle of the twisted field lines has to be above a threshold
given by the condition that the magnetic equivalent of the Weber number
(see § 2.2) be below 1. The shape obtained for the tube and wake is
strongly reminiscent of laboratory experiments with air bubbles rising
in liquids. The magnetized region outside an equipartition boundary is
peeled away from the tube: two sidelobes are formed, which lag behind
the tube and contain only a fraction of the initial magnetic flux. This
is similar to the formation of a skirt in the fluid dynamical case. The
velocities of rise predicted by the thin flux tube approximation are
compared with those obtained here.
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Title: Equilibrium of Twisted Horizontal Magnetic Flux Tubes
Authors: Emonet, T.; Moreno-Insertis, F.
1996ApJ...458..783E Altcode:
The equilibrium of non-force-free twisted horizontal magnetic flux tubes
is studied including gravity and an arbitrary pressure perturbation on
the tube boundary. To solve this free-boundary problem, we use general
nonorthogonal flux coordinates and consider the two-dimensional case
in which there is no variation of the physical quantities along the
tube axis. For the applications in the convection zone and corona,
we consider the case of weak external stratification by assuming that
the radius of the tube is smaller than the external pressure scale
height. This allows us to introduce a perturbation scheme which is much
less restrictive than the customary slender flux-tube approximation. In
particular, it has the advantage of not imposing any limitation on
the strength of the azimuthal field as compared to the longitudinal
field. Within this scheme, one retains to zero order all the functional
degrees of freedom of a general axisymmetric magnetostatic equilibrium;
the geometry of the perturbed azimuthal field lines is then obtained
from the equilibrium equations as a consequence of the zero-order
density (or rather buoyancy) distribution in the tube and of the
circular wavenumber of the external pressure perturbation. We show
that, as a result of the presence of gravity, the field lines are no
longer concentric, although they continue being circular. The resulting
changes in magnetic pressure and tension of the azimuthal field exactly
counteract the differences in buoyancy in the tube cross section. On
the other hand, external pressure fluctuations of circular wavenumber
higher than one can only be countered by bending the azimuthal field
lines. In general terms, the present scheme allows one to study in
detail the mutual dependence of the (differential) buoyancy in the tube,
the intensity and field line geometry of the azimuthal magnetic field,
and the gas pressure and longitudinal magnetic field distributions. <P
/>The main application of the equations and results of this paper is
to study the transverse structure of magnetic flux rings embedded in
a stratified medium with a flow around the tube that causes pressure
fluctuations on its surface. This includes tubes in the deep convection
zone, e.g., in its subadiabatic lower part, or those kept in place by a
meridional flow. It also applies to flux rings moving in a quasi-static
regime in which the drag force of the relative motion with respect
to the external medium exactly compensates the total buoyancy of the
tube. In this way, this study can complement the numerical simulations
of the rise of magnetized tubes and bubbles toward the surface.
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Title: MHS-Equilibrium of Twisted Magnetic Tubes
Authors: Emonet, T.; Moreno-Insertis, F.
1996ApL&C..34....9E Altcode:
No abstract at ADS
