Author name code: bruls ADS astronomy entries on 2022-09-14 author:"Bruls, Jo H.M.J." ------------------------------------------------------------------------ Title: Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations Authors: Nutto, C.; Roth, M.; Zhugzhda, Y.; Bruls, J.; von der Lühe, O. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 2008A&A...484..503R Altcode: 2008arXiv0804.2325R Aims: We study chromospheric emission to understand the temperature stratification in the solar chromosphere.
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 105 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.
Results: 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.
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. Title: The Solar Perspective on Radiative Transfer and Interferometry Authors: Bruls, J. H. M. J. Bibcode: 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. Title: What is Heating the Quiet-Sun Chromosphere? Authors: Wedemeyer-Böhm, S.; Steiner, O.; Bruls, J.; Rammacher, W. Bibcode: 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. 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. Bibcode: 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.
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, Hnon, and a magnetically, Hmag, heated component from a non-heated component, Hco in the H-index.
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-2.
Conclusions: We find that a dominant fraction of the calcium emission caused by the heated atmosphere in the magnetic network has non-magnetic origin (Hmag≈2 pm, Hnon≈3 pm). Considering the effect of straylight, the contribution from an atmosphere with no temperature rise to the H-index (Hco≈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. 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. Bibcode: 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. Title: Carbon monoxide in the solar atmosphere. I. Numerical method and two-dimensional models Authors: Wedemeyer-Böhm, S.; Kamp, I.; Bruls, J.; Freytag, B. Bibcode: 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. Title: Apparent solar radius variations. The influence of magnetic network and plage Authors: Bruls, J. H. M. J.; Solanki, S. K. Bibcode: 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. Title: Approximations for non-grey radiative transfer in numerical simulations of the solar photosphere Authors: Vögler, A.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 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. Title: The contrast of magnetic elements across the solar spectrum Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J. Bibcode: 2003AN....324..398S Altcode: No abstract at ADS Title: Radiative properties of magnetic elements. I. Why are vec G-band bright points bright? Authors: Steiner, O.; Hauschildt, P. H.; Bruls, J. Bibcode: 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. 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. Bibcode: 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. Title: Why are G-Band Bright Points Bright? Authors: Steiner, O.; Bruls, J.; Hauschildt, P. H. Bibcode: 2001ASPC..236..453S Altcode: 2001aspt.conf..453S No abstract at ADS 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. Bibcode: 2001ASPC..223..589B Altcode: 2001csss...11..589B No abstract at ADS Title: POLIS: Simultaneous Measurement of Photospheric and Chromospheric Magnetic Field Authors: Schmidt, W.; Kentischer, T. J.; Bruls, J.; Lites, B. W. Bibcode: 2001ASPC..236...49S Altcode: 2001aspt.conf...49S No abstract at ADS 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. Bibcode: 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. Title: Radiative cooling of a hot flux tube in the solar photosphere Authors: Schlichenmaier, R.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 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. Title: Computing radiative heating on unstructured spatial grids Authors: Bruls, J. H. M. J.; Vollmöller, P.; Schüssler, M. Bibcode: 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. Title: Can Chromospheric Activity mimic a Polar Spot? Authors: Bruls, J. H. M. J.; Schüssler, M.; Solanki, S. K. Bibcode: 1999ASPC..158..182B Altcode: 1999ssa..conf..182B No abstract at ADS Title: Inversion of Stokes profiles Authors: Frutiger, C.; Solanki, S. K.; Fligge, M.; Bruls, J. H. M. J. Bibcode: 1999ASSL..243..281F Altcode: 1999sopo.conf..281F No abstract at ADS Title: Radiative Transfer On Unstructured Triangular Grids Authors: Bruls, J.; Vollmöller, P.; Schüssler, M. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Title: A Non-LTE Analysis of Doppler Imaging Lines Authors: Bruls, J. H. M. J.; Solanki, S. K.; Schussler, M. Bibcode: 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. 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1992A&A...265..268U Altcode: This paper investigates the formation of the Na I D2, 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 D2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.