204 research outputs found
The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations
The contrast of granulation is an important quantity characterizing solar
surface convection. We compare the intensity contrast at 630 nm, observed using
the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative
MHD simulations of V{\"o}gler & Sch{\"u}ssler (2007). A synthetic image from
the simulation is degraded using a theoretical point-spread function of the
optical system, and by considering other important effects. The telescope
aperture and the obscuration by the secondary mirror and its attachment spider,
reduce the simulated contrast from 14.4 % to 8.5 %. A slight effective defocus
of the instrument brings the simulated contrast down to 7.5 %, close to the
observed value of 7.0 %. A proper consideration of the effects of the optical
system and a slight defocus, lead to sufficient degradation of the synthetic
image from the MHD simulation, such that the contrast reaches almost the
observed value. The remaining small discrepancy can be ascribed to straylight
and slight imperfections of the instrument, which are difficult to model.
Hence, Hinode SP data are consistent with a granulation contrast which is
predicted by 3D radiation MHD simulations.Comment: 5 pages, 4 figures, to be published in A&
Hydrodynamical model atmospheres and 3D spectral synthesis
We discuss three issues in the context of three-dimensional (3D)
hydrodynamical model atmospheres for late-type stars, related to spectral line
shifts, radiative transfer in metal-poor 3D models, and the solar oxygen
abundance. We include a brief overview about the model construction, taking the
radiation-hydrodynamics code CO5BOLD (COnservative COde for the COmputation of
COmpressible COnvection in a BOx of L Dimensions with L=2,3) and the related
spectral synthesis package Linfor3D as examples.Comment: 6 pages, 2 figures, to appear in the Proceedings of the
ESO/Lisbon/Aveiro Workshop "Precision Spectroscopy in Astrophysics", eds. L.
Pasquini, M. Romaniello, N.C. Santos, and A. Correi
Explanation of the activity sensitivity of Mn I 5394.7 \AA
There is a long-standing controversy concerning the reason why the Mn I
5394.7 A line in the solar irradiance spectrum brightens more at larger
activity than most other photospheric lines. The claim that this activity
sensitivity is caused by spectral interlocking to chromospheric emission in Mg
II h & k is disputed.
Classical one-dimensional modeling is used for demonstration; modern
three-dimensional MHD simulation for verification and analysis.
The Mn I 5394.7 A line thanks its unusual sensitivity to solar activity to
its hyperfine structure. This overrides the thermal and granular Doppler
smearing through which the other, narrower, photospheric lines lose such
sensitivity. We take the nearby Fe I 5395.2 A line as example of the latter and
analyze the formation of both lines in detail to demonstrate and explain
granular Doppler brightening. We show that this affects all narrow lines.
Neither the chromosphere nor Mg II h & k play a role, nor is it correct to
describe the activity sensitivity of Mn I 5394.7 A through plage models with
outward increasing temperature contrast.
The Mn I 5394.7 A line represents a proxy diagnostic of strong-field magnetic
concentrations in the deep solar photosphere comparable to the G band and the
blue wing of H-alpha, but not a better one than these. The Mn I lines are more
promising as diagnostic of weak fields in high-resolution Stokes polarimetry.Comment: 12 pages, 8 figures, accepted by A&
Simulation of a flux emergence event and comparison with observations by Hinode
We study the observational signature of flux emergence in the photosphere
using synthetic data from a 3D MHD simulation of the emergence of a twisted
flux tube. Several stages in the emergence process are considered. At every
stage we compute synthetic Stokes spectra of the two iron lines Fe I 6301.5
{\AA} and Fe I 6302.5 {\AA} and degrade the data to the spatial and spectral
resolution of Hinode's SOT/SP. Then, following observational practice, we apply
Milne-Eddington-type inversions to the synthetic spectra in order to retrieve
various atmospheric parameters and compare the results with recent Hinode
observations. During the emergence sequence, the spectral lines sample
different parts of the rising flux tube, revealing its twisted structure. The
horizontal component of the magnetic field retrieved from the simulations is
close to the observed values. The flattening of the flux tube in the
photosphere is caused by radiative cooling, which slows down the ascent of the
tube to the upper solar atmosphere. Consistent with the observations, the
rising magnetized plasma produces a blue shift of the spectral lines during a
large part of the emergence sequence.Comment: A&A Letter, 3 figure
Decay of a simulated mixed-polarity magnetic field in the solar surface layers
Magnetic flux is continuously being removed and replenished on the solar
surface. To understand the removal process we carried out 3D radiative MHD
simulations of the evolution of patches of photospheric magnetic field with
equal amounts of positive and negative flux. We find that the flux is removed
at a rate corresponding to an effective turbulent diffusivity, of 100--340
km^2/s, depending on the boundary conditions. For average unsigned flux
densities above about 70 Gauss, the percentage of surface magnetic energy
coming from different field strengths is almost invariant. The overall process
is then one where magnetic elements are advected by the horizontal granular
motions and occasionally come into contact with opposite-polarity elements.
These reconnect above the photosphere on a comparatively short time scale after
which the U loops produced rapidly escape through the upper surface while the
downward retraction of inverse-U loops is significantly slower, because of the
higher inertia and lower plasma beta in the deeper layers.Comment: 8 pages, 10 figures accepted in A&
Magneto-convection in a sunspot umbra
Results from a realistic simulation of 3D radiative magneto-convection in a
strong background magnetic field corresponding to the conditions in sunspot
umbrae are shown. The convective energy transport is dominated by narrow upflow
plumes with adjacent downflows, which become almost field-free near the surface
layers. The strong external magnetic field forces the plumes to assume a
cusp-like shape in their top parts, where the upflowing plasma loses its
buoyancy. The resulting bright features in intensity images correspond well (in
terms of brightness, size, and lifetime) to the observed umbral dots in the
central parts of sunspot umbrae. Most of the simulated umbral dots have a
horizontally elongated form with a central dark lane. Above the cusp, most
plumes show narrow upflow jets, which are driven by the pressure of the
piled-up plasma below. The large velocities and low field strengths in the
plumes are effectively screened from spectroscopic observation because the
surfaces of equal optical depth are locally elevated, so that spectral lines
are largely formed above the cusp. Our simulations demonstrate that nearly
field-free upflow plumes and umbral dots are a natural result of convection in
a strong, initially monolithic magnetic field.Comment: Accepted by Astrophysical Journal Letter
Stokes diagnostics of simulated solar magneto-convection
We present results of synthetic spectro-polarimetric diagnostics of radiative
MHD simulations of solar surface convection with magnetic fields. Stokes
profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared
spectral ranges emerging from the simulated atmosphere have been calculated in
order to study their relation to the relevant physical quantities and compare
with observational results. We have analyzed the dependence of the Stokes-I
line strength and width as well as of the Stokes-V signal and asymmetries on
the magnetic field strength. Furthermore, we have evaluated the correspondence
between the actual velocities in the simulation with values determined from the
Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles
(zero-crossing shift). We confirm that the line weakening in strong magnetic
fields results from a higher temperature (at equal optical depth) in the
magnetic flux concentrations. We also confirm that considerable Stokes-V
asymmetries originate in the peripheral parts of strong magnetic flux
concentrations, where the line of sight cuts through the magnetopause of the
expanding flux concentration into the surrounding convective donwflow.Comment: Astronomy & Astrophysics, in pres
A solar surface dynamo
Context: Observations indicate that the `quiet' solar photosphere outside
active regions contains considerable amounts of magnetic energy and magnetic
flux, with mixed polarity on small scales. The origin of this flux is unclear.
Aims: We test whether local dynamo action of the near-surface convection
(granulation) can generate a significant contribution to the observed magnetic
flux. Methods: We have carried out MHD simulations of solar surface convection,
including the effects of strong stratification, compressibility, partial
ionization, radiative transfer, as well as an open lower boundary. Results:
Exponential growth of a weak magnetic seed field (with vanishing net flux
through the computational box) is found in a simulation run with a magnetic
Reynolds number of about 2600. The magnetic energy approaches saturation at a
level of a few percent of the total kinetic energy of the convective motions.
Near the visible solar surface, the (unsigned) magnetic flux density reaches at
least a value of about 25 G. Conclusions: A realistic flow topology of
stratified, compressible, non-helical surface convection without enforced
recirculation is capable of turbulent local dynamo action near the solar
surface.Comment: accepted by Astronomy&Astrophysics (Letter
SIMULATIONS OF SOLAR PORES
ABSTRACT We have used the MURaM code to simulate solar pores. These are magnetic features intermediate in size between the small-scale flux concentrations in the inter-granular lanes and sunspots. They appear only in young active regions and thus their formation seems not to be dominated by processes occurring in the photosphere. We therefore start the simulations with pre-existing pores and study their structure and evolution resulting from the effects of granular convection and radiative cooling. The simulations, unlike observations, allow the full 3-D structure to be examined, and various physical effects to be disentangled. We find a reasonable agreement between the temperature and density stratification obtained in the simulations and those reported by observers. We shall present results concerning the magnetic and thermal structure of the simulated pores, their temporal evolution, and a number of synthetic diagnostics, such as intensity images at different wavelengths and limb distances
3D simulations of M star atmosphere velocities and their influence on molecular FeH lines
We present an investigation of the velocity fields in early to late M-type
star hydrodynamic models, and we simulate their influence on FeH molecular line
shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff
of 2500 K and 4000 K. Our aim is to characterize the Teff- and log g
-dependence of the velocity fields and express them in terms of micro- and
macro-turbulent velocities in the one dimensional sense. We present also a
direct comparison between 3D hydrodynamical velocity fields and 1D turbulent
velocities. The velocity fields strongly affect the line shapes of FeH, and it
is our goal to give a rough estimate for the log g and Teff parameter range in
which 3D spectral synthesis is necessary and where 1D synthesis suffices. In
order to calculate M-star structure models we employ the 3D
radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these
models is performed with the line synthesis code LINFOR3D. We describe the 3D
velocity fields in terms of a Gaussian standard deviation and project them onto
the line of sight to include geometrical and limb-darkening effects. The micro-
and macro-turbulent velocities are determined with the "Curve of Growth" method
and convolution with a Gaussian velocity profile, respectively. To characterize
the log g and Teff dependence of FeH lines, the equivalent width, line width,
and line depth are regarded. The velocity fields in M-stars strongly depend on
log g and Teff. They become stronger with decreasing log g and increasing Teff.Comment: 14 pages, 17 figures, 3 tables, accepted by Astronomy & Astrophysic
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