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Author name code: bruls
ADS astronomy entries on 2022-09-14
author:"Bruls, Jo H.M.J."
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Title: Calculation of Spectral Darkening and Visibility Functions
for Solar Oscillations
Authors: Nutto, C.; Roth, M.; Zhugzhda, Y.; Bruls, J.; von der
Lühe, O.
2008SoPh..251..179N Altcode: 2008SoPh..tmp...63N; 2008arXiv0803.1228N
Calculations of spectral darkening and visibility functions for
the brightness oscillations of the Sun resulting from global solar
oscillations are presented. This has been done for a broad range of
the visible and infrared continuum spectrum. The procedure for the
calculations of these functions includes the numerical computation of
depth-dependent derivatives of the opacity caused by p modes in the
photosphere. A radiative-transport code was used for this purpose to
get the disturbances of the opacities from temperature and density
fluctuations. The visibility and darkening functions are obtained
for adiabatic oscillations under the assumption that the temperature
disturbances are proportional to the undisturbed temperature of the
photosphere. The latter assumption is the only way to explore any
opacity effects since the eigenfunctions of p-mode oscillations have not
been obtained so far. This investigation reveals that opacity effects
have to be taken into account because they dominate the violet and
infrared part of the spectrum. Because of this dominance, the visibility
functions are negative for those parts of the spectrum. Furthermore,
the darkening functions show a wavelength-dependent change of sign
for some wavelengths owing to these opacity effects. However, the
visibility and darkening functions under the assumptions used contradict
the observations of global p-mode oscillations, but it is beyond doubt
that the opacity effects influence the brightness fluctuations of the
Sun resulting from global oscillations.
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Title: Reversal-free Ca II H Profiles: a Challenge for Solar
Chromosphere Modeling in Quiet Inter-Network
Authors: Rezaei, R.; Bruls, J.; Beck, C.; Schmidt, W.; Kalkofen, W.;
Schlichenmaier, R.
2008ESPM...12.2.13R Altcode:
There is no agreement on the thermal structure of the solar
chromosphere. While results of the CO observations and 3D MHD
simulations suggest very cool structures in the upper atmosphere,
SUMER observations of UV spectral lines is interpreted as signature
of a full-time hot chromosphere. We tried to look for cool structures
in the solar chromosphere. We observed the intensity profile of the
Ca II H line in a quiet Sun region close to the disk center at the
German Vacuum Tower Telescope. We analyze over 10^5 line profiles from
inter-network regions. For comparison with the observed profiles, we
synthesize spectra for a variety of model atmospheres with a non local
thermodynamic equilibrium(NLTE) radiative transfer code. A fraction of
about 25% of the observed Ca II H line profiles do not show a measurable
emission peak in H2v and H2r wavelength bands (reversal-free). All of
the chosen model atmospheres with a temperature rise fail to reproduce
such profiles. On the other hand, the synthetic calcium profile of a
model atmosphere that has a monotonic decline of the temperature with
height shows a reversal-free profile that has much lower intensities
than any observed line profile. The observed reversal-free profiles,
at a spatial resolution of 1 arcs and a temporal resolution of 5 s,
indicate the existence of cool patches in the interior of chromospheric
network cells, at least for short time intervals. Our finding is not
only in conflict with a full-time hot chromosphere (e.g., FALC), but
also with a very cool chromosphere as found in some dynamic simulations.
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Title: Reversal-free Ca II H profiles: a challenge for solar
chromosphere modeling in quiet inter-network
Authors: Rezaei, R.; Bruls, J. H. M. J.; Schmidt, W.; Beck, C.;
Kalkofen, W.; Schlichenmaier, R.
2008A&A...484..503R Altcode: 2008arXiv0804.2325R
Aims: We study chromospheric emission to understand the temperature
stratification in the solar chromosphere. <BR />Methods: We observed
the intensity profile of the Ca II H line in a quiet Sun region close
to the disk center at the German Vacuum Tower Telescope. We analyze
over 10<SUP>5</SUP> line profiles from inter-network regions. For
comparison with the observed profiles, we synthesize spectra for a
variety of model atmospheres with a non local thermodynamic equilibrium
(NLTE) radiative transfer code. <BR />Results: A fraction of about
25% of the observed Ca II H line profiles do not show a measurable
emission peak in H<SUB>2v</SUB> and H<SUB>2r</SUB> wavelength bands
(reversal-free). All of the chosen model atmospheres with a temperature
rise fail to reproduce such profiles. On the other hand, the synthetic
calcium profile of a model atmosphere that has a monotonic decline of
the temperature with height shows a reversal-free profile that has much
lower intensities than any observed line profile. <BR />Conclusions:
The observed reversal-free profiles indicate the existence of cool
patches in the interior of chromospheric network cells, at least
for short time intervals. Our finding is not only in conflict with a
full-time hot chromosphere, but also with a very cool chromosphere as
found in some dynamic simulations.
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Title: The Solar Perspective on Radiative Transfer and Interferometry
Authors: Bruls, J. H. M. J.
2008EAS....28...17B Altcode:
Interferometry with large telescopes such as the VLTI is at present
not a realistic option in solar physics, nor will it be feasible in the
near future. Its proximity, however, allows us to view the Sun in far
more detail than we can reasonably expect for other stars even with
the next generations of interferometric instruments. For that reason
the Sun imposes extremely high requirements on the radiative transfer
computations that serve to explain the observations. In the last
few decades we have seen the picture of the solar photosphere evolve
to a point where we think that we have “understood” most of the
basic structures and their physics. The chromosphere, however, still
presents a challenge for (magneto)hydrodynamic and radiative transfer
modeling. Time-independent gray or multiband radiative transfer assuming
local thermodynamic equilibrium (LTE), which for obvious reasons work
well in the photosphere, are not valid in the chromosphere. In principle
all equilibrium assumptions and time independence should be dropped. The
theoretical formulation of that problem is straightforward, but it leads
to a numerical problem whose solution requires far more computational
resources than currently available. For that reason modelers will have
no other option but to continue reducing the full problem to a number
of tractable simpler problems with different assumptions and trying
to figure out which assumptions are valid.
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Title: What is Heating the Quiet-Sun Chromosphere?
Authors: Wedemeyer-Böhm, S.; Steiner, O.; Bruls, J.; Rammacher, W.
2007ASPC..368...93W Altcode: 2006astro.ph.12627W
It is widely believed that the heating of the chromosphere in quiet-Sun
internetwork regions is provided by dissipation of acoustic waves
that are excited by the convective motions close to the top of
the convection zone and in the photospheric overshoot layer. This
view lately became challenged by observations suggesting that the
acoustic energy flux into the chromosphere is too low, by a factor
of at least ten. Based on a comparison of TRACE data with synthetic
image sequences for a three-dimensional simulation extending from
the top layers of the convection zone to the middle chromosphere,
we come to the contradicting conclusion that the acoustic flux in the
model provides sufficient energy for heating the solar chromosphere of
internetwork regions. The role of a weak magnetic field and associated
electric current sheets is also discussed.
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Title: Relation between photospheric magnetic field and chromospheric
emission
Authors: Rezaei, R.; Schlichenmaier, R.; Beck, C. A. R.; Bruls,
J. H. M. J.; Schmidt, W.
2007A&A...466.1131R Altcode: 2007astro.ph..1896R
Aims: We investigate the relationship between the photospheric
magnetic field and the emission of the mid chromosphere of the
Sun. <BR />Methods: We simultaneously observed the Stokes parameters
of the photospheric iron line pair at 630.2 nm and the intensity
profile of the chromospheric Ca II H line at 396.8 nm in a quiet
Sun region at a heliocentric angle of 53°. Various line parameters
have been deduced from the Ca II H line profile. The photospheric
magnetic field vector has been reconstructed from an inversion of
the measured Stokes profiles. After alignment of the Ca and Fe maps,
a common mask has been created to define network and inter-network
regions. We perform a statistical analysis of network and inter-network
properties. The H-index is the integrated emission in a 0.1 nm band
around the Ca core. We separate a non-magnetically, H<SUB>non</SUB>,
and a magnetically, H<SUB>mag</SUB>, heated component from a
non-heated component, H<SUB>co</SUB> in the H-index. <BR />Results:
The average network and inter-network H-indices are equal to 12 and
10 pm, respectively. The emission in the network is correlated with
the magnetic flux density, approaching a value of H ≈ 10 pm for
vanishing flux. The inter-network magnetic field is dominated by weak
field strengths with values down to 200 G and has a mean absolute
flux density of about 11 Mx cm<SUP>-2</SUP>. <BR />Conclusions:
We find that a dominant fraction of the calcium emission caused by
the heated atmosphere in the magnetic network has non-magnetic origin
(H<SUB>mag</SUB>≈2 pm, H<SUB>non</SUB>≈3 pm). Considering the effect
of straylight, the contribution from an atmosphere with no temperature
rise to the H-index (H<SUB>co</SUB>≈6 pm) is about half of the
observed H-index in the inter-network. The H-index in the inter-network
is not correlated to any property of the photospheric magnetic field,
suggesting that magnetic flux concentrations have a negligible role
in the chromospheric heating in this region. The height range of the
thermal coupling between the photosphere and low/mid chromosphere
increases in presence of magnetic field. In addition, we demonstrate
that a poor signal-to-noise level in the Stokes profiles leads to a
significant over-estimation of the magnetic field strength.
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Title: A First Three-Dimensional Model for the Carbon Monoxide
Concentration in the Solar Atmosphere
Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.;
Steffen, M.
2006ASPC..354..301W Altcode:
The time-dependent and self-consistent treatment of carbon monoxide
(CO) has been added to the radiation chemo-hydrodynamics code
CO5BOLD. It includes the solution of a chemical reaction network and
the advection of the resulting particle densities with the hydrodynamic
flow field. Here we present a first 3D simulation of the non-magnetic
solar photosphere and low chromosphere, calculated with the upgraded
code. In the resulting model, the highest amount of CO is located in
the cool regions of the reversed granulation pattern in the middle
photosphere. A large fraction of carbon is bound by CO throughout
the chromosphere with exception of hot shock waves where the CO
concentration is strongly reduced. The distribution of carbon monoxide
is very inhomogeneous due to co-existing regions of hot and cool gas
caused by the hydrodynamic flow. High-resolution observations of CO
could thus provide important constraints for the thermal structure of
the solar photosphere and chromosphere.
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Title: Carbon monoxide in the solar atmosphere. I. Numerical method
and two-dimensional models
Authors: Wedemeyer-Böhm, S.; Kamp, I.; Bruls, J.; Freytag, B.
2005A&A...438.1043W Altcode: 2005astro.ph..3496W
The radiation hydrodynamic code CO5BOLD has been supplemented with
the time-dependent treatment of chemical reaction networks. Advection
of particle densities due to the hydrodynamic flow field is also
included. The radiative transfer is treated frequency-independently,
i.e. grey, so far. The upgraded code has been applied to two-dimensional
simulations of carbon monoxide (CO) in the non-magnetic solar
photosphere and low chromosphere. For this purpose a reaction network
has been constructed, taking into account the reactions that are most
important for the formation and dissociation of CO under the physical
conditions of the solar atmosphere. The network has been strongly
reduced to 27 reactions, involving the chemical species H, H2, C, O,
CO, CH, OH and a representative metal. The resulting CO number density
is highest in the cool regions of the reversed granulation pattern
at mid-photospheric heights and decreases strongly above. There, the
CO abundance stays close to a value of 8.3 on the usual logarithmic
abundance scale with [H] = 12 but is reduced in hot shock waves which
are a ubiquitous phenomenon of the model atmosphere. For comparison, the
corresponding equilibrium densities have been calculated, based on the
reaction network but also under the assumption of instantaneous chemical
equilibrium by applying the Rybicki & Hummer (RH) code. Owing to the
short chemical timescales, the assumption holds for a large fraction
of the atmosphere, in particular the photosphere. In contrast, the CO
number density deviates strongly from the corresponding equilibrium
value in the vicinity of chromospheric shock waves. Simulations with
altered reaction networks clearly show that the formation channel via
hydroxide (OH) is the most important one under the conditions of the
solar atmosphere.
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Title: Apparent solar radius variations. The influence of magnetic
network and plage
Authors: Bruls, J. H. M. J.; Solanki, S. K.
2004A&A...427..735B Altcode:
Solar radius measurements, a by-product of the magnetograms recorded
several times daily at Mt. Wilson Observatory over a period of a few
decades, have revealed apparent variations of about 0.4 arcsec that are
correlated with the solar cycle. We note that the radius definition
used for the analysis of those magnetograms automatically converts
intensity variations near the limb into apparent radius variations. A
change in the average temperature structure of the quiet Sun can be
ruled out as the source of these variations, since such a change would
need to be very significant and would lead to other easily measurable
consequences that are not observed. We show that plage emission near
the solar limb associated with the magnetic activity variation during a
solar cycle produces apparent radius changes of the correct sign. The
use of plane-parallel or spherically-symmetric models to describe
the faculae gives apparent radius variations that are a factor of
4-10 too small in magnitude. If the Mt. Wilson results are correct,
then this implies that the small-scale structure of faculae produces
limb extensions that are considerably larger than those returned by
a plane-parallel or spherically-symmetric model.
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Title: Approximations for non-grey radiative transfer in numerical
simulations of the solar photosphere
Authors: Vögler, A.; Bruls, J. H. M. J.; Schüssler, M.
2004A&A...421..741V Altcode:
Realistic simulations of solar (magneto-)convection require an
accurate treatment of the non-grey character of the radiative energy
transport. Owing to the large number of spectral lines in the solar
atmosphere, statistical representations of the line opacities have to
be used in order to keep the problem numerically tractable. We consider
two statistical approaches, the opacity distribution function (ODF)
concept and the multigroup (or opacity binning) method and provide a
quantitative assessment of the errors that arise from the application
of these methods in the context of 2D/3D simulations. In a first step,
the ODF- and multigroup methods are applied to a 1D model-atmosphere
and the resulting radiative heating rates are compared. A number of
4-6 frequency bins is found to warrant a satisfactory modeling of the
radiative energy exchange. Further tests in 2D model-atmospheres show
the applicability of the multigroup method in realistic situations
and underline the importance of a non-grey treatment. Furthermore, we
address the question of an appropriate opacity average in multigroup
calculations and discuss the significance of velocity gradients for
the radiative heating rates.
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Title: The contrast of magnetic elements across the solar spectrum
Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J.
2003AN....324..398S Altcode:
No abstract at ADS
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Title: Radiative properties of magnetic elements. I. Why are vec
G-band bright points bright?
Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J.
2001A&A...372L..13S Altcode:
Photospheric magnetic elements are most conspicuously visible in
high-resolution G-band filtergrams. We show that their enhanced
contrast in the G-band is due to a reduction of the CH abundance by
dissociation in the deep photospheric layers of the flux tube, where
it is hotter than in the surrounding atmosphere. As a consequence, the
CH-lines weaken, allowing more of the continuum to “shine” through
the forest of G-band CH-lines. We suggest that other molecular bands
or atomic lines may exhibit a similar behaviour.
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Title: Photospheric fine structure: An observational challenge. An
analysis of radiative transfer effects on the visibility of
small-scale structures
Authors: Bruls, J. H. M. J.; Lühe, O. v. d.
2001A&A...366..281B Altcode:
The requirement on spatial resolution of a large solar telescope
is frequently based on the assumption that the smallest observable
features in the solar photosphere could not be much different from a
photon mean free path due to smoothing by radiative transfer. Although
hydrodynamic simulations show that much smaller structures should exist,
a resolution of the order of 100 km is typically considered sufficient
for observations in the photosphere. We performed 2D non-LTE radiative
transfer computations for thin flux sheets with widths ranging from
10 to 160 km in the solar photosphere. We demonstrate that such
small structures - should they exist - could be observed as small
scale variations of intensity and polarization. We conclude that the
size limit below which photospheric structure cannot be observed due
to smoothing radiative transfer effects must lie well below 10 km. A
spatial resolution limit for telescopes based on photon mean free path
arguments should therefore be abandoned.
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Title: Why are G-Band Bright Points Bright?
Authors: Steiner, O.; Bruls, J.; Hauschildt, P. H.
2001ASPC..236..453S Altcode: 2001aspt.conf..453S
No abstract at ADS
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Title: Where is the Intrinsic Size Limit of Fine Structure in the
Solar Photosphere? (CD-ROM Directory: contribs/bruls)
Authors: Bruls, J. H. M. J.; von der Lühe, O.
2001ASPC..223..589B Altcode: 2001csss...11..589B
No abstract at ADS
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Title: POLIS: Simultaneous Measurement of Photospheric and
Chromospheric Magnetic Field
Authors: Schmidt, W.; Kentischer, T. J.; Bruls, J.; Lites, B. W.
2001ASPC..236...49S Altcode: 2001aspt.conf...49S
No abstract at ADS
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Title: Properties of the solar granulation obtained from the inversion
of low spatial resolution spectra
Authors: Frutiger, C.; Solanki, S. K.; Fligge, M.; Bruls, J. H. M. J.
2000A&A...358.1109F Altcode:
The spectra of cool stars are rich in information on elemental
abundances, convection and non-thermal heating. Extracting this
information is by no means straightforward, however. Here we demonstrate
that an inversion technique may not only provide the stratification of
the classical parameters describing a model atmosphere, but can also
determine the properties of convection at the stellar surface. The
inversion technique is applied to spectra of photospheric lines, one
recorded at the quiet solar disk center, the other integrated over the
whole disk. We find that a model based on a single plane-parallel
atmosphere gives unsatisfactory fits to the spectral lines and
suffers from considerable uncertainties in the derived temperature
stratification. Also, the elemental abundances returned by the inversion
are not particularly reliable. These problems are greatly reduced if
two atmospheric components, corresponding to granular up- and downflows
are allowed for. The best results are obtained if the line profiles
and bisectors of a neutral and ionized species are fit and the results
are constrained using a simple mass conservation scheme. We find that
inversions based on two- and three-component models of disk-integrated
spectra give similar results to inversions of disk-center observations,
although with somewhat lower accuracy. This similarity is promising
for future applications of line profile inversions to the study of
late-type stars and in particular their convection.
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Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
1999A&A...349..961S Altcode:
Radiative energy transport is of key importance for the dynamics of
slender magnetic flux tubes in the solar atmosphere, particularly
so in connection with the filamentation of the sunspot penumbra. In
investigations using the thin-flux-tube approximation of the MHD
equations, the radiative exchange with the surrounding atmosphere has
hitherto been described by the relaxation-time approach, also called
`Newton's law of cooling'. The strongly nonlinear temperature-dependence
of the radiative absorption coefficient and large temperature
differences between the tube and its environment render this concept
questionable. As a simple model of a bright penumbral filament we
consider the cooling of a hot horizontal flux tube with a longitudinal
flow, embedded in a non-stratified, homogeneous atmosphere at 4 800
K. We compare the results of the relaxation-time approach and of
a nonlinear diffusion approximation with the numerical solution of
the equation of (grey) radiative transfer. We find that the cooling
times given by the relaxation-time method compare well with the
results from radiative transfer as long as the initial temperature
of the tube is below 7 500 K and its lateral optical depth does not
exceed unity. Under these conditions, the tube cools more or less
homogeneously over its cross section. For hotter and optically thick
tubes, the strong temperature-dependence of the absorption coefficient
leads to the formation of a cooling front, which migrates radially
inward at approximately constant speed. Such inhomogeneous cooling
is well represented by the nonlinear diffusion approximation. The
self-similar evolution of the cooling front permits an analytical
estimate of the cooling time, which provides a reasonable approximation
of the result of the radiative transfer calculation. This estimate can
be used to derive an improved radiative cooling term in the framework
of the thin-flux-tube approximation, so that both optically thin and
optically thick flux tubes can be treated adequately. The results of
the radiative transfer calculations are applied to obtain an estimate
of the length and brightness of penumbral bright grains.
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Title: Computing radiative heating on unstructured spatial grids
Authors: Bruls, J. H. M. J.; Vollmöller, P.; Schüssler, M.
1999A&A...348..233B Altcode:
We discuss the basic problems and methods involved in the design of a
radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation
code aimed at applications in cool-star atmospheres. Attention
is focused on the difficulties arising from the unstructured
triangular/tetrahedral grid and the need to minimize the communication
overhead, so that the code runs efficiently on parallel computers. In a
first step, we use the gray approximation and ignore scattering effects,
but even then the computation of the radiative heating rate, required
as a source term in the energy equation, involves several integration
steps that are discussed in detail. In particular, the details of the
short-characteristics solver for the radiative transfer equation,
the influence of the cell size, and the accuracy of the angular
integrations of the specific intensity are considered. Theoretical
estimates of possible errors are in general cumbersome to obtain;
instead we use simple model problems for the accuracy estimates. A
plane-parallel model for the quiet Sun serves as a testground for the
basics while a schematic model of a magnetic flux sheet provides an
acid test for the behavior of the computational methods under typical
circumstances arising during simulations. Two alternative methods to
compute the radiative heating rate are compared and their weaknesses
are identified. The errors are minimized by a hybrid scheme that selects
a method depending on the optical path length within a grid cell.
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Title: Can Chromospheric Activity mimic a Polar Spot?
Authors: Bruls, J. H. M. J.; Schüssler, M.; Solanki, S. K.
1999ASPC..158..182B Altcode: 1999ssa..conf..182B
No abstract at ADS
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Title: Inversion of Stokes profiles
Authors: Frutiger, C.; Solanki, S. K.; Fligge, M.; Bruls, J. H. M. J.
1999ASSL..243..281F Altcode: 1999sopo.conf..281F
No abstract at ADS
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Title: Radiative Transfer On Unstructured Triangular Grids
Authors: Bruls, J.; Vollmöller, P.; Schüssler, M.
1999ASPC..183...44B Altcode: 1999hrsp.conf...44B
No abstract at ADS
---------------------------------------------------------
Title: Radiative transfer for MHD simulations on unstructured grids
Authors: Bruls, J.; Vollmöller, P.; Schüssler, M.
1999AGAb...15..141B Altcode: 1999AGM....15.P108B
We discuss the basic problems and methods involved in the design of a
radiative transfer module for a 2D/3D (magneto-)hydrodynamics simulation
code aimed at applications in cool-star atmospheres. Attention
is focused on the difficulties arising from the unstructured
triangular/tetrahedral grid and the need to minimize the communication
overhead, so that the code runs efficiently on parallel computers. In a
first step, we use the gray approximation and ignore scattering effects,
but even then the computation of the radiative heating rate, required
as a source term in the energy equation, involves several integration
steps. In particular, the details of the short-characteristics solver
for the radiative transfer equation, the influence of the cell size,
and the accuracy of the angular integrations of the specific intensity
are considered. Theoretical estimates of possible errors are in general
cumbersome to obtain; instead we use simple model problems for the
accuracy estimates. A plane-parallel model for the quiet Sun serves as
a testground for the basics while a schematic model of a magnetic flux
provides an acid test for the behavior of the computational methods
under typical circumstances arising during simulations. Two alternative
methods to compute the radiative heating rate are compared and their
weaknesses are identified. The errors are minimized by a hybrid scheme
that selects a method depending on the optical path length within a grid
cell. We have implemented the radiative transfer model in a 2D Euler
code and have performed test simulations of solar surface convection.
---------------------------------------------------------
Title: Radiative cooling of a hot flux tube in the solar photosphere
Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M.
1999AGAb...15Q..75S Altcode: 1999AGM....15..J16S
Radiative energy transport is of key importance for the dynamics of
slender magnetic flux tubes in the solar atmosphere. In investigations
using the thin-flux-tube approximation of the MHD equations, the
radiative exchange with the surrounding atmosphere has hitherto been
described by the relaxation-time approach, also called `Newton's law of
cooling'. The strongly nonlinear temperature-dependence of the radiative
absorption coefficient and large temperature differences between
the tube and its environment render this concept questionable. As a
simple model of a bright penumbral filament we consider the cooling
of a hot horizontal flux tube with a longitudinal flow, embedded in
a non-stratified, homogeneous atmosphere at 4 800 K. We compare the
results of the relaxation-time approach and of a nonlinear diffusion
approximation with the numerical solution of the equation of (grey)
radiative transfer. We find that the cooling times given by the
relaxation-time method compare well with the results from radiative
transfer as long as the initial temperature of the tube is below 7
500 K and its lateral optical depth does not exceed unity. Under these
conditions, the tube cools homogeneously over its cross section. For
hotter and optically thick tubes, the strong temperature-dependence of
the absorption coefficient leads to the formation of a cooling front,
which migrates radially inward at approximately constant speed. Such
inhomogeneous cooling is well represented by the nonlinear diffusion
approximation. The self-similar evolution of the cooling front permits
an analytical estimate of the cooling time, which provides a reasonable
approximation of the result of the radiative transfer calculation. This
estimate can be used to derive an improved radiative cooling term in
the framework of the thin-flux-tube approximation.
---------------------------------------------------------
Title: Doppler imaging: the polar SPOT controversy
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schuessler, M.
1998A&A...336..231B Altcode:
Doppler imaging studies have revealed that most rapidly rotating cool
stars have high-latitude spots, which in many cases cover the stellar
poles. The spectroscopic signature of polar spots is a filling in
of the cores of spectral lines, which become flat-bottomed and may
show bumps. Although the existence of polar spots is corroborated by
spectroscopic and photometric measurements, and although theoretical
models predict polar spots, they remain controversial. Most notably, it
has been proposed that the line core filling in might also be caused by
chromospheric activity. We present a NLTE radiative transfer analysis
of 14 of the most-used Doppler-imaging lines which demonstrates that
chromospheric activity can produce filling in of the observed line
profiles only in a few of these lines. Moreover, such filling in is in
general not of the type observed in the spectra of active stars. We
are able to produce a flat-bottomed line core by concentrating the
chromospheric activity near the poles, but only for two of the strongest
lines, Fe i 5497 Angstroms and Fe i 6430 Angstroms. In the observations,
however, also the weaker lines have flat-bottomed cores. Therefore, it
is unlikely that polar spots are an artifact due to misinterpretation
of the spectral signature of chromospheric activity. Nevertheless, we
cannot exclude that chromospheric activity provides part of the filling
in of the cores of some stronger lines; we present a diagnostic that may
help to separate the contributions of chromospheric activity and spots.
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Title: A Non-LTE Analysis of Doppler Imaging Lines
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schussler, M.
1998ASPC..154.1959B Altcode: 1998csss...10.1959B
Doppler imaging studies have revealed that most stars with high activity
levels have polar spots. Although their existence is corroborated by
spectroscopic and photometric measurements, and although theoretical
models have been produced that include polar spots, their existence
remains controversial. Based on a NLTE radiative transfer analysis
of the most-used Doppler-imaging lines we reject the claim that
chromospheric activity might be responsible for the features in the
spectral lines that are commonly interpreted as polar spots.
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Title: Simulations of Zeeman-split CA II K-line Stokes profiles with
angle-dependent partial redistribution.
Authors: Bruls, J. H. M. J.; Solanki, S. K.
1997A&A...325.1179B Altcode:
The formulation of the polarized radiative transfer equations
for Zeeman-split spectral lines is still incomplete for cases
with frequency-dependent line source function, e.g. when partial
frequency distribution (PRD) of line photons occurs (strong resonance
lines). Under the well-founded assumption that the field-free
approximation works equally well for lines with partial redistribution
as for lines with complete redistribution (CRD), we decouple the
computation of the level populations (accounting for PRD effects) from
the formal Stokes vector solution. The level populations are obtained
with the Hubeny & Lites (1995ApJ...455..376H) version of Carlsson's
(1986UppOR..33....1C) MULTI code, and the formal solution follows
by means of a modified version of the Murphy & Rees (1990, NCAR
Technical Note NCAR/TN-348+IA) SPSR code. Due to lack of the appropriate
equations concerning the combination of partial redistribution and
Zeeman splitting of spectral lines, an ad hoc partial redistribution
approach (basically allowing for wavelength dependence of the line
source function) is implemented in the SPSR code. This combination
of codes is used to study the relevance of partial redistribution
to the Caii K-line diagnostics of solar plage regions using grids of
flux-tube models. In addition to magnetic fields, velocity fields play
an important role in the formation of the K-line profiles, but these
invalidate the commonly-used angle-averaged PRD formalism. We therefore
extended the Hubeny & Lites (1995ApJ...455..376H) angle-averaged
PRD version of MULTI code to the angle-dependent case, which allows
line profiles to be computed under PRD conditions for arbitrary (but
not too large) velocity fields. This code is subsequently used to
perform the non-magnetic non-LTE statistical equilibrium computations
for a few schematic velocity structures in plage flux tubes and in the
surrounding non-magnetic atmosphere. The SPSR code is again used to
obtain the K-line Stokes profiles. We investigate in particular the
asymmetry of the K line intensity and circularly polarized profiles
produced by the velocity fields.
---------------------------------------------------------
Title: Modeling LiI and KI sensitivity to Pleiades activity.
Authors: Stuik, R.; Bruls, J. H. M. J.; Rutten, R. J.
1997A&A...322..911S Altcode:
We compare schematic modeling of spots and plage on the surface of
cool dwarfs with Pleiades data to assess effects of magnetic activity
on the strengths of the LiI and KI resonance lines in Pleiades
spectra. Comprehensive LiI and KI NLTE line formation computation is
combined with comparatively well-established empirical solar spot and
plage stratifications for solar-like stars. For other stars, we use
theoretical constructs to model spots and plage that portray recipes
commonly applied in stellar activity analyses. We find that - up to
B-V=~1.1 - neither the LiI 670.8nm nor the KI 769.9nm line is sensitive
to the presence of a chromosphere, in contrast to what is often
supposed. Instead, both lines respond to the effects of activity on the
stratification in the deep photosphere. They do so in similar fashion,
making the KI line a valid proxy to study LiI line formation without
spread from abundance variations. The computed effects of activity on
line strength are opposite between plage and spots, differ noticeably
between the empirical and theoretical solar-like stratifications, and
considerably affect stellar broad-band colors. Our results indicate that
one can neither easily establish, nor easily exclude, magnetic activity
as major provider of KI line strength variation in the Pleiades. Since
LiI line formation follows KI line formation closely, the same holds
for LiI and the apparent lithium abundance.
---------------------------------------------------------
Title: The Polarization-Free Approximation Applied to Multi-Level
Non-LTE Radiative Transfer
Authors: Bruls, J. H. M. J.; Trujillo Bueno, J.
1996SoPh..164..155B Altcode:
The polarization-free (POF) approximation (Trujillo Bueno and Landi
Degl'Innocenti, 1996) is capable of accounting for the approximate
influence of the magnetic field on the statistical equilibrium,
without actually solving the full Stokes vector radiative transfer
equation. The method introduces the Zeeman splitting or broadening
of the line absorption profile φI in the scalar radiative transfer
equation, but the coupling between Stokes I and the other Stokes
parameters is neglected. The expected influence of the magnetic field
is largest for strongly-split strong lines and the effect is greatly
enhanced by gradients in the magnetic field strength. Formally the
interaction with the other Stokes parameters may not be neglected for
strongly-split strong lines, but it turns out that the error in Stokes
I obtained through the POF approximation to a large extent cancels the
neglect of interaction with the other Stokes parameters, so that the
resulting line source functions and line opacities are more accurate
than those obtained with the field-free approach. Although its merits
have so far only been tested for a two-level atom, we apply the POF
approximation to multi-level non-LTE radiative transfer problems on
the premise that there is no essential difference between these two
cases. Final verification of its validity in multi-level cases still
awaits the completion of a non-LTE Stokes vector transfer code.
---------------------------------------------------------
Title: PRD vs. CRD CaII K Stokes profiles from solar plage
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Uitenbroek, H.
1996ASPC..109..113B Altcode: 1996csss....9..113B
No abstract at ADS
---------------------------------------------------------
Title: Is the optical depth of stellar coronae really negligible?
Authors: Schrijver, C. J.; Mewe, R.; Kaastra, J. S.; van den Oord,
G. H. J.; Bruls, J. H. M. J.
1996ASPC..109..289S Altcode: 1996csss....9..289S
No abstract at ADS
---------------------------------------------------------
Title: Infrared lines as probes of solar magnetic features. IX. MgI
12μm diagnostics of solar plage.
Authors: Bruls, J. H. M. J.; Solanki, S. K.
1995A&A...293..240B Altcode:
The MgI 12.32μm line is a prime diagnostic for studying upper
photospheric magnetic fields on the Sun. We study for the first time
its behavior using flux-tube models with magnetic field strengths and
filling factors characteristic of solar plage in order to establish
the line's capabilities for measuring plage magnetic fields. We
show that this line is only moderately sensitive to the range of
temperatures expected for solar plages, including the chromospheric
temperature rise. This low sensitivity considerably enhances this line
as a magnetic field diagnostic for the upper photosphere of plages. We
find that besides being highly sensitive to the field strength in flux
tubes, the shape and Zeeman splitting of this line are almost equally
sensitive to the magnetic filling factor at the flux-tube base. This
combined sensitivity leads to the large range of profile shapes observed
in plages. Not only can the 12μm emission lines be used to determine
the sub-spatial-resolution distribution of flux-tube field strengths,
but also the sub-spatial-resolution distribution of filling factors
(i.e. they can provide an estimate of the "clumpiness" of the flux-tube
distribution on a small scale). We also provide evidence that simple
two-component modeling of magnetic flux tubes, without taking into
account the height-dependence of the flux-tube size, may lead to
erroneous conclusions in the case of the MgI 12μm lines, and that at
least 1.5-D computations are required to adequately model these lines
in magnetic flux tubes. Computations also show that standard flux-tube
models simultaneously reproduce the observed splitting of the g=3 FeI
lines at 1.5648μm (formed in the low photosphere) and at 525.02nm
(middle photosphere), as well as of the 12.32μm emission line (upper
photosphere). Our computations thus support the currently standard
view that solar plages are composed of flux tubes with kGauss fields
in the lower and mid photosphere, and that the thin-tube approximation
is an adequate representation of the magnetic stratification in these
flux tubes.
---------------------------------------------------------
Title: Infrared lines as probes of solar magnetic features. VIII. MgI
12μm diagnostics of sunspots.
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson,
M.
1995A&A...293..225B Altcode:
Due to their large Zeeman sensitivity, the MgI lines at 12μm are
important diagnostics of solar magnetism. The formation of their
central emission features is now understood, enabling quantitative
modeling and diagnostic application of these lines. We supply the
first systematic analysis of solar MgI 12μm Stokes profiles employing
detailed line-profile synthesis. We compute Stokes profiles of MgI
12.32μm for the quiet Sun, for sunspot penumbrae and for the extended
("superpenumbral") magnetic canopies surrounding sunspots. We use these
computations to analyze recent MgI 12μm observations by Hewagama
et al. (1993). Our results are the following: (1) -Saha-Boltzmann
temperature sensitivity explains that the emission peaks are stronger in
penumbrae than in the quiet Sun, and that they disappear in umbrae. (2)
-The formation heights of the emission features are approximately the
same in penumbrae and in the quiet Sun, namely τ_500_=~10^-3^. (3)
-The simple Seares formula allows relatively accurate determinations
of field strength and magnetic inclination. (4) -The observed excess
broadening of the σ-component peaks compared with the π component
in penumbrae is well explained by primarily horizontal, smooth radial
variation of the magnetic field strength. Additional small-scale
variations are less than {DELTA}B =~200G. (5) -The vertical field
gradients dB/dz in penumbrae range from 0.7G/km to 3G/km; the larger
gradients occur near the umbra, the smaller ones near the outer edge of
the penumbra. (6) -The MgI 12μm lines are well-suited to measure the
base heights of superpenumbral magnetic canopies. These heights range
between 300km and 500km above τ_500_=1 out to twice the sunspot radius,
in excellent agreement with determinations from other infrared lines.
---------------------------------------------------------
Title: The non-LTE formation of Li I lines in cool stars
Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina,
N. G.
1994A&A...288..860C Altcode:
We study the non-LTE (non local thermodynamic equilibrium)
formation of Li I lines in the spectra of cool stars for a grid of
radiative-equilibrium model atmospheres with variation in effective
temperature, gravity, metallicity and lithium abundance. We analyze
the mechanisms by which departures from LTE (local thermodynamic
equilibrium) arise for Li I lines, first for the young sun (prior to
its lithium depletion) and then across the cool-star grid. There are
various mechanisms which compete in their effects on emergent Li I
line strengths. Their neglect produces errors in lithium abundance
determinations that vary in sign as well as size, both across the
stellar grid and between different Li I lines (Figs). The errors are
appreciable for all cooler stars and largest for cool lithium-rich
metal-poor giants. They reverse sign between lithium-rich stars and
lithium-poor stars for the λ=670.8nm resonance line, but not for the
λ=610.4nm subordinate line. The non-LTE corrections are large enough
that they should be taken into account in ongoing debates on lithium
synthesis and depletion. We provide convenient numerical approximations
of our results (Table 1) to this purpose. We end the paper with some
examples in which non-LTE corrections change the slope of published
relationships.
---------------------------------------------------------
Title: New contribution functions for Zeeman split spectral lines
Authors: Solanki, S. K.; Bruls, J. H. M. J.
1994A&A...286..269S Altcode:
The recently introduced emergent line radiation contribution function
(Achmad + de Jager + Nieuwenhuijzen 1991; Gurtovenko + Sheminova +
Sarychev 1991) is extended to the Stokes vector of Zeeman split spectral
lines. This contribution function is compared to the line depresssion
contribution function on the basis of an analytical relation and of
numerical NLTE computations of a set of spectral lines. In general the
two contribution functions give very similar results, although in some
cases the one or the other of them is more suitable.
---------------------------------------------------------
Title: Magnetic Field Strength and Filling Factor Sensitivity of
the MgI 12 MU M Infrared Lines in Solar Plage
Authors: Bruls, J. H. M. J.; Solanki, S. K.
1994ASPC...68..294B Altcode: 1994sare.conf..294B
No abstract at ADS
---------------------------------------------------------
Title: The Non-LTE Formation of Li I Lines from Cool Stars
Authors: Carlsson, M.; Rutten, R. J.; Bruls, J. H. M. J.; Shchukina,
N. G.
1994ASPC...64..270C Altcode: 1994csss....8..270C
No abstract at ADS
---------------------------------------------------------
Title: The upper photosphere and lower chromosphere of small-scale
magnetic features
Authors: Solanki, S. K.; Bruls, J. H. M. J.; Steiner, O.; Ayres, T.;
Livingston, W.; Uitenbroek, H.
1994ASIC..433...91S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: MgI 12 μm diagnostics of sunspot penumbrae
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Rutten, R. J.; Carlsson,
M.
1994smf..conf..191B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The chromospheric temperature rise in solar magnetic flux tubes
Authors: Bruls, J. H. M. J.; Solanki, S. K.
1993A&A...273..293B Altcode:
We set constraints on the location and steepness of the chromospheric
temperature rise in solar magnetic flux tubes by synthesizing Stokes
V profiles in non-local thermodynamic equilibrium (NLTE) of a set of
"clean" Fe I and Fe II lines of different strengths. We compute profiles
for several flux-tube models of the magnetic features within solar
network and plage. Comparison with observed V profiles indicates that
both in network and plage the chromospheric temperature rise starts
at larger optical depth within the flux tubes than in the surrounding
non-magnetic atmosphere. The exact onset cannot be determined with the
present set of lines because they cannot easily distinguish between
the location and initial steepness of the temperature rise. Assuming
a similar T(τ) gradient as in quiet-Sun models, the chromosphere
sets in 200 - 300 km deeper in flux tubes than in the quiet Sun. The
similarity between the plage and network results suggests that in the
low chromosphere the heating per flux tube is almost independent of
magnetic filling factor.
---------------------------------------------------------
Title: The formation of helioseismology lines. IV - The NI I 676.8
NM intercombination line
Authors: Bruls, J. H. M. J.
1993A&A...269..509B Altcode:
The Ni I 676.8 nm intercombination line will be used as velocity
diagnostic in helioseismology with the instruments of the Global
Oscillations Network Group and the Michelson Doppler Imager onboard
SOHO. In this analysis, I employ the VAL3-C plane-parallel model of
the quiet solar atmosphere and a snapshot of a Nordlund and Stein
solar granulation simulation as input for studying the NLTE behavior
and the sensitivity to granulation of this line. The nickel atom is
ironlike in its structure and complexity, but more regular and easier
to model. The NLTE mechanisms involved are ultraviolet overionization,
infrared overrecombination, optical line pumping, and line photon
losses. I assess their relative importance and the effects imposed
by the solar granulation and confirm that the line is a good choice
for helioseismology.
---------------------------------------------------------
Title: The formation of helioseismology lines. III. Partial
redistribution effects in weak solar resonance lines.
Authors: Uitenbroek, H.; Bruls, J. H. M. J.
1992A&A...265..268U Altcode:
This paper investigates the formation of the Na I D<SUB>2</SUB>,
K I λ769.9 nm and Ba II λ455.4 nm resonance lines in the solar
atmosphere allowing for partial frequency redistribution (PRD). The
authors show that the influence of PRD is negligible on the solar
disk in all three lines for a model of the average quiet-Sun, and
even for the more extreme case of a model with a steeper photospheric
temperature decline. At the solar limb PRD does affect the intensity
profile of the Na I and Ba II resonance lines, but not that of the K
I line. Finally, it is found that effects of PRD are more pronounced
at the limb in the Na I D<SUB>2</SUB> and Ba II λ455.4 nm lines when
the photospheric temperature gradient is steep.
---------------------------------------------------------
Title: The formation of helioseismology lines. I. NLTE effects in
alkali spectra.
Authors: Bruls, J. H. M. J.; Rutten, R. J.; Shchukina, N. G.
1992A&A...265..237B Altcode:
The authors study the NLTE formation of the solar K I and Na I resonance
lines employed in helioseismology. They combine standard modeling of
the solar atmosphere with comprehensive alkali model atoms, complete
up to the Rydberg regime near the continuum, to study various NLTE
mechanisms which interact to make the alkali population balances more
complex than is the case for other minority species. In particular,
they discuss a "photon suction" process which produces overpopulation
of the neutral stage by driving a population flow from the reservoir in
the singly ionized stage. They isolate this and other mechanisms with
specifically tailored model atoms and provide a choice of simplified
model atoms, trading precision against size, which are appropriate
for future use in numerical simulations of the solar atmosphere.
---------------------------------------------------------
Title: The formation of helioseismology lines. II. Modeling of alkali
resonance lines with granulation.
Authors: Bruls, J. H. M. J.; Rutten, R. J.
1992A&A...265..257B Altcode:
The authors model the NLTE formation of the solar Na I and
K I resonance lines for an array of one-dimensional atmospheric
models taken from a numerical simulation of the solar granulation by
Nordlund and Stein. They discuss the nature of alkali-line sensitivity
to granulation using hot and cool extremes from the simulation and
study the granular modulation of diagnostics such as line bisectors and
helioseismological resonance-cell response. They also show that granular
structuring produces apparent spatially-averaged line broadening of
similar magnitude as the ad hoc microturbulent and damping broadening
invoked in traditional plane-parallel modeling.
---------------------------------------------------------
Title: The Chromospheric Temperature Rise in Fluxtubes
Authors: Bruls, J. H. M. J.; Solanki, S. K.
1992ASPC...26..512B Altcode: 1992csss....7..512B
No abstract at ADS
---------------------------------------------------------
Title: The Granulation Sensitivity of Neutral Metal Lines
Authors: Bruls, J. H. M. J.; Uitenbroek, H.; Rutten, R. J.
1989ASIC..263..311B Altcode: 1989ssg..conf..311B
No abstract at ADS
---------------------------------------------------------
Title: The granulation sensitivity of helioseismology lines.
Authors: Rutten, R. J.; Bruls, J. H. M. J.; Gomez, M. T.; Severino, G.
1988ESASP.286..251R Altcode: 1988ssls.rept..251R
The authors address the sensitivity of the Ni I 676.78 nm GONG line and
the K I 769.9 nm resonance line to the temperature fluctuations present
in the solar granulation. The temperature contrasts due to granulation
are probably small in the upper photosphere where the cores of these
two helioseismology lines are formed. However, the cores are sensitive
also to the granulation temperature contrasts in the deep photosphere,
through non-local NLTE effects in their formation. The largest effects
are due to the ultraviolet radiation field, which is strongly modulated
by the granulation in the deep layers where it escapes and carries these
contrasts upwards to the line formation height. The authors discuss
the resulting NLTE mechanisms and their influence on the two lines.
