61 research outputs found
Near-surface stellar magneto-convection: simulations for the Sun and a metal-poor solar analog
We present 2D local box simulations of near-surface radiative
magneto-convection with prescribed magnetic flux, carried out with the MHD
version of the CO5BOLD code for the Sun and a solar-like star with a metal-poor
chemical composition (metal abundances reduced by a factor 100, [M/H]=-2). The
resulting magneto-hydrodynamical models can be used to study the influence of
the metallicity on the properties of magnetized stellar atmospheres. A
preliminary analysis indicates that the horizontal magnetic field component
tends to be significantly stronger in the optically thin layers of metal-poor
stellar atmospheres.Comment: Proc. IAU Symposium 259, Cosmic Magnetic Fields: from Planets, to
Stars and Galaxies, K.G. Strassmeier, A.G. Kosovichev and J.E. Beckman, eds.
(2009) p.23
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 Diagnostis of 2D MHD-simulated Solar Magnetogranulation
We study the properties of solar magnetic fields on scales less than the
spatial resolution of solar telescopes. A synthetic infrared
spectropolarimetric diagnostics based on a 2D MHD simulation of
magnetoconvection is used for this. We analyze two time sequences of snapshots
that likely represent two regions of the network fields with their immediate
surrounding on the solar surface with the unsigned magnetic flux density of 300
and 140 G. In the first region we find from probability density functions of
the magnetic field strength that the most probable field strength at logtau_5=0
is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface,
while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic
flux is -28 G and its imbalance is -0.04. In the second region, these
parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10.
We estimate the distribution of line-of-sight velocities on the surface of log
tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region.
The averaged velocity in the granules is -1.2 km/s and in the intergranules is
2.5 km/s. In the second region, the corresponding values of the mean velocities
are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of
synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the
amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude
asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 figure
Magnetic Coupling in the Quiet Solar Atmosphere
Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz force.
First the coupling of the convecting and overshooting fluid in the surface
layers of the Sun with the magnetic field. Here, the plasma motion provides the
dominant force, which shapes the magnetic field and drives the surface dynamo.
Progress in the understanding of the horizontal magnetic field is summarized
and discussed. Second, the coupling between acoustic waves and the magnetic
field, in particular the phenomenon of wave conversion and wave refraction. It
is described how measurements of wave travel times in the atmosphere can
provide information about the topography of the wave conversion zone, i.e., the
surface of equal Alfv\'en and sound speed. In quiet regions, this surface
separates a highly dynamic magnetic field with fast moving magnetosonic waves
and shocks around and above it from the more slowly evolving field of high-beta
plasma below it. Third, the magnetic field also couples to the radiation field,
which leads to radiative flux channeling and increased anisotropy in the
radiation field. It is shown how faculae can be understood in terms of this
effect. The article starts with an introduction to the magnetic field of the
quiet Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field with the
help of the Hanle effect.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Stochastic excitation of acoustic modes in stars
For more than ten years, solar-like oscillations have been detected and
frequencies measured for a growing number of stars with various characteristics
(e.g. different evolutionary stages, effective temperatures, gravities, metal
abundances ...).
Excitation of such oscillations is attributed to turbulent convection and
takes place in the uppermost part of the convective envelope. Since the
pioneering work of Goldreich & Keely (1977), more sophisticated theoretical
models of stochastic excitation were developed, which differ from each other
both by the way turbulent convection is modeled and by the assumed sources of
excitation. We review here these different models and their underlying
approximations and assumptions.
We emphasize how the computed mode excitation rates crucially depend on the
way turbulent convection is described but also on the stratification and the
metal abundance of the upper layers of the star. In turn we will show how the
seismic measurements collected so far allow us to infer properties of turbulent
convection in stars.Comment: Notes associated with a lecture given during the fall school
organized by the CNRS and held in St-Flour (France) 20-24 October 2008 ; 39
pages ; 11 figure
Small-scale solar magnetic fields
As we resolve ever smaller structures in the solar atmosphere, it has become
clear that magnetism is an important component of those small structures.
Small-scale magnetism holds the key to many poorly understood facets of solar
magnetism on all scales, such as the existence of a local dynamo, chromospheric
heating, and flux emergence, to name a few. Here, we review our knowledge of
small-scale photospheric fields, with particular emphasis on quiet-sun field,
and discuss the implications of several results obtained recently using new
instruments, as well as future prospects in this field of research.Comment: 43 pages, 18 figure
The Imaging Magnetograph eXperiment (IMaX) for the Sunrise balloon-borne solar observatory
The Imaging Magnetograph eXperiment (IMaX) is a spectropolarimeter built by
four institutions in Spain that flew on board the Sunrise balloon-borne
telesocope in June 2009 for almost six days over the Arctic Circle. As a
polarimeter IMaX uses fast polarization modulation (based on the use of two
liquid crystal retarders), real-time image accumulation, and dual beam
polarimetry to reach polarization sensitivities of 0.1%. As a spectrograph, the
instrument uses a LiNbO3 etalon in double pass and a narrow band pre-filter to
achieve a spectral resolution of 85 mAA. IMaX uses the high Zeeman sensitive
line of Fe I at 5250.2 AA and observes all four Stokes parameters at various
points inside the spectral line. This allows vector magnetograms, Dopplergrams,
and intensity frames to be produced that, after reconstruction, reach spatial
resolutions in the 0.15-0.18 arcsec range over a 50x50 arcsec FOV. Time
cadences vary between ten and 33 seconds, although the shortest one only
includes longitudinal polarimetry. The spectral line is sampled in various ways
depending on the applied observing mode, from just two points inside the line
to 11 of them. All observing modes include one extra wavelength point in the
nearby continuum. Gauss equivalent sensitivities are four Gauss for
longitudinal fields and 80 Gauss for transverse fields per wavelength sample.
The LOS velocities are estimated with statistical errors of the order of 5-40
m/s. The design, calibration and integration phases of the instrument, together
with the implemented data reduction scheme are described in some detail.Comment: 17 figure
Horizontal Magnetic Fields in the Solar Photosphere
The results of 2D MHD simulations of solar magnetogranulation are used to
analyze the horizontal magnetic fields and the response of the synthesized
Stokes profiles of the FeI 1564.85 nm line to the magnetic fields. Selected
1.5-h series of the 2D MHD models reproduces a region of the network fields
with their immediate surrounding on the solar surface with the unsigned
magnetic flux density of 192 G. According to the magnetic field distribution
obtained, the most probable absolute strength of the horizontal magnetic field
at an optical depth of tau_5 = 1 (tau_5 denotes tau at lambda = 500 nm) is 50
G, while the mean value is 244 G. On average, the horizontal magnetic fields
are stronger than the vertical fields to heights of about 400 km in the
photosphere due to their higher density and the larger area they occupy. The
maximum factor by which the horizontal fields are greater is 1.5. Strong
horizontal magnetic flux tubes emerge at the surface as spots with field
strengths of more than 500 G. These are smaller than granules in size, and have
lifetimes of 3.6 min. They form in the photosphere due to the expulsion of
magnetic fields by convective flows coming from deep subphotospheric layers.
The data obtained qualitatively agree with observations with the Hinode space
observatory.Comment: 15 pages, 7 figures, published by Astronomicheskii Zhurnal (in
Russian) and Astronomy Reports (in English
Radiative Cooling in MHD Models of the Quiet Sun Convection Zone and Corona
We present a series of numerical simulations of the quiet Sun plasma threaded
by magnetic fields that extend from the upper convection zone into the low
corona. We discuss an efficient, simplified approximation to the physics of
optically thick radiative transport through the surface layers, and investigate
the effects of convective turbulence on the magnetic structure of the Sun's
atmosphere in an initially unipolar (open field) region. We find that the net
Poynting flux below the surface is on average directed toward the interior,
while in the photosphere and chromosphere the net flow of electromagnetic
energy is outward into the solar corona. Overturning convective motions between
these layers driven by rapid radiative cooling appears to be the source of
energy for the oppositely directed fluxes of electromagnetic energy.Comment: 20 pages, 5 figures, Solar Physics, in pres
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