Author name code: langhans ADS astronomy entries on 2022-09-14 author:"Langhans, Kai" ------------------------------------------------------------------------ Title: Is the solar spectrum latitude-dependent?. An investigation with SST/TRIPPEL Authors: Kiselman, D.; Pereira, T. M. D.; Gustafsson, B.; Asplund, M.; Meléndez, J.; Langhans, K. Bibcode: 2011A&A...535A..14K Altcode: 2011arXiv1108.4527K Context. In studies of the solar spectrum compared to spectra of solar twin stars, it has been found that the chemical composition of the Sun seems to depart systematically from those of the twins. One possible explanation could be that the effect is caused by the special aspect angle of the Sun when observed from Earth compared with the aspect angles of the twins. This means that a latitude dependence of the solar spectrum, even with the heliocentric angle constant, could lead to the observed effects.
Aims: We explore a possible variation in the strength of certain spectral lines that are used in the comparisons between the composition of the Sun and the twins at loci on the solar disk with different latitudes but at constant heliocentric angle.
Methods: We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope on La Palma to record spectra in five spectral regions to compare different locations on the solar disk at a heliocentric angle of 45°. Equivalent widths and other parameters are measured for fifteen different lines representing nine atomic species. Spectra acquired at different times are used in averaging the line parameters for each line and observing position.
Results: The relative variations in equivalent widths at the equator and at solar latitude ~45° are found to be less than 1.5% for all spectral lines studied. Translated into elemental abundances as they would be measured from a terrestrial and a hypothetical pole-on observer, the difference is estimated to be within 0.005 dex in all cases.
Conclusions: It is very unlikely that latitude effects could cause the reported abundance difference between the Sun and the solar twins. The accuracy obtainable in measurements of small differences in spectral line strengths between different solar disk positions is very high, and can be exploited in studies of, e.g. weak magnetic fields or effects of solar activity on atmospheric structure. Title: Searching for Overturning Convection in Penumbral Filaments: Slit Spectroscopy at 0farcs2 Resolution Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Langhans, K. Bibcode: 2010ApJ...725...11B Altcode: 2010arXiv1009.5650B Recent numerical simulations of sunspots suggest that overturning convection is responsible for the existence of penumbral filaments and the Evershed flow, but there is little observational evidence of this process. Here, we carry out a spectroscopic search for small-scale convective motions in the penumbra of a sunspot located 5° away from the disk center. The position of the spot is very favorable for the detection of overturning downflows at the edges of penumbral filaments. Our analysis is based on measurements of the Fe I 709.0 nm line taken with the Littrow spectrograph of the Swedish 1 m Solar Telescope under excellent seeing conditions. We compute line bisectors at different intensity levels and derive Doppler velocities from them. The velocities are calibrated using a nearby telluric line, with systematic errors smaller than 150 m s-1. Deep in the photosphere, as sampled by the bisectors at the 80%-88% intensity levels, we always observe blueshifts or zero velocities. The maximum blueshifts reach 1.2 km s-1 and tend to be cospatial with bright penumbral filaments. In the line core, we detect blueshifts for the most part, with small velocities not exceeding 300 m s-1. Redshifts also occur, but at the level of 100-150 m s-1, and only occasionally. The fact that they are visible in high layers casts doubts on their convective origin. Overall, we do not find indications of downflows that could be associated with overturning convection at our detection limit of 150 m s-1. Either no downflows exist, or we have been unable to observe them because they occur beneath τ = 1 or the spatial resolution/height resolution of the measurements is still insufficient. Title: Recent High Resolution Observations and Interpretations of Sunspot Fine Structure Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007ASPC..369...71S Altcode: We review analyses made of highly resolved filtergrams, magnetograms and Dopplergrams of sunspots, recorded with the Swedish 1-meter Solar Telescope (SSTSST) on La Palma. Dark cores in penumbral filaments are shown to be directly linked to peripheral umbral dots and to dark lanes in light bridges, suggesting similar or related underlying physics. The visibility of dark cores is found to depend strongly on the azimuth angle already for spots located at small heliocentric distances. It is shown that dark cores are clearly visible close to the center of the Ca II H line, formed approximately 150--200~km above the photosphere. We conclude that the τ = 1 layer of dark-cored filaments outlines a strongly warped surface, consistent with the finding that the magnetic field strength is strongly reduced in dark cores. We show that several properties of dark-cored filaments derived from SSTSST data are consistent with results of inversions of low-resolution Stokes spectra, but also find important discrepancies with the interpretation that penumbral filaments can be identified with flux tubes. Our data are consistent with the model proposed by Spruit & Scharmer (2006), explaining dark cores as signatures of field-free convection occurring just below the visible surface of the penumbra. We discuss recent simulations of light bridges and umbral dots, providing additional support to that model. Title: Observations of dark-cored filaments in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G. Bibcode: 2007A&A...464..763L Altcode: Context: The recent discovery of dark-cored penumbral filaments suggests that we are resolving the building blocks of sunspot penumbrae. Their properties are largely unknown but provide important clues to understanding penumbral fine structure.
Aims: Our observations provide new constraints for the different scenarios put forward to explain the structure of sunspot penumbrae.
Methods: We present an analysis of dark-cored penumbral filaments, based on intensity filtergrams (G-band, continuum and Ca II H line wing), magnetograms and Dopplergrams, obtained at heliocentric distances between 15° and 55°.
Results: In general, the visibility of dark cores degrades with increasing heliocentric distance. Based on Ca II H wing images we conclude that this is due to a geometrical 3D-effect and not due to a simple formation height effect. Only in the center-side penumbra are dark-cored filaments visible at all observed heliocentric distances. We observe that dark-cored filaments frequently split in the umbra, forming a Y-shape that disappears after a few minutes, leaving a shortened filamentary structure and a bright dot in the umbra. The dark-cored filaments have life times ≥ 90 min. The dark cores are related to a much weaker and a more horizontal magnetic field than their lateral brightenings. Where the dark-cored filaments appear in the umbra, the magnetic field is inclined by 40° with respect to the solar surface normal for both the dark core and the bright edges. With increasing distance from the umbra, the magnetic field inclination in the dark cores increases rapidly within a few thousand km. Both the magnetic field strength and inclination in the lateral brightenings show very small variations with spot-center radial distance. The velocity field possesses a strong horizontal component within the dark cores. The absolute line-of-sight (LOS) velocity is larger within the dark cores than in their lateral brightenings. The Evershed flow apparently is present primarily in the dark cores. Title: Signatures of Penumbral Magnetic Fields at Very High Spatial Resolution Authors: Langhans, K. Bibcode: 2006ASPC..358....3L Altcode: Full Stokes spectro-polarimetry, together with refined techniques to interpret the measurements and continual modeling efforts, have improved our understanding of sunspot penumbrae in the last years. In spite of this progress, an improvement in the spatial resolution of the observations is clearly needed to establish in a more direct way the fine structure of the penumbra. The discovery of dark penumbral cores by tet{l3 Sc02} suggests that we are starting to resolve the fundamental scales of the penumbra. Spectro-polarimetric measurements that are sensitive to the magnetic field in both the photosphere and higher layers, and obtained at a spatial resolution approaching 0.1 arcsec, may therefore allow us to draw firm conclusions about the fine scale organization of penumbral magnetic fields.

In this paper I will discuss recent polarization measurements at very high spatial resolution, trying to reconcile the different scenarios put forward to explain the structure of the penumbra. Title: Multi-line spectroscopy of dark-cored penumbral filaments Authors: Bellot Rubio, L. R.; Langhans, K.; Schlichenmaier, R. Bibcode: 2005A&A...443L...7B Altcode: Dark-cored filaments could be the basic building blocks of sunspot penumbrae. Yet, their nature and physical conditions are unknown. In an attempt to improve this situation, we present the first high-resolution spectra of dark-cored penumbral filaments. Several such filaments were observed near the umbra/penumbra boundary of a sunspot located at heliocentric angles of 5° and 20°. Our data reveal (a) significantly larger Doppler shifts in the dark cores as compared to their lateral brightenings; (b) Doppler shifts that increase with depth in the photosphere, up to 1.5 km s-1; and (c) Doppler shifts that increase with increasing heliocentric distance. The Doppler velocities measured in the dark cores are almost certainly produced by upflows. In addition, dark-cored penumbral filaments exhibit weaker fields than their surroundings (by 100-300 G). These results provide new constraints for models of dark-cored penumbral filaments. Title: Inclination of magnetic fields and flows in sunspot penumbrae Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Berger, T. E. Bibcode: 2005A&A...436.1087L Altcode: An observational study of the inclination of magnetic fields and flows in sunspot penumbrae at a spatial resolution of 0.2 arcsec is presented. The analysis is based on longitudinal magnetograms and Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma using the Lockheed Solar Optical Universal Polarimeter birefringent filter. Data from two sunspots observed at several heliocentric angles between 12 ° and 39 ° were analyzed. We find that the magnetic field at the level of the formation of the Fe i-line wing (630.25 nm) is in the form of coherent structures that extend radially over nearly the entire penumbra giving the impression of vertical sheet-like structures. The inclination of the field varies up to 45 ° over azimuthal distances close to the resolution limit of the magnetograms. Dark penumbral cores, and their extensions into the outer penumbra, are prominent features associated with the more horizontal component of the magnetic field. The inclination of this dark penumbral component - designated B - increases outwards from approximately 40 ° in the inner penumbra such that the field lines are nearly horizontal or even return to the solar surface already in the middle penumbra. The bright component of filaments - designated A - is associated with the more vertical component of the magnetic field and has an inclination with respect to the normal of about 35 ° in the inner penumbra, increasing to about 60 ° towards the outer boundary. The magnetogram signal is lower in the dark component B regions than in the bright component A regions of the penumbral filaments. The measured rapid azimuthal variation of the magnetogram signal is interpreted as being caused by combined fluctuations of inclination and magnetic field strength. The Dopplergrams show that the velocity field associated with penumbral component B is roughly aligned with the magnetic field while component A flows are more horizontal than the magnetic field. The observations give general support to fluted and uncombed models of the penumbra. The long-lived nature of the dark-cored filaments makes it difficult to interpret these as evidence for convective exchange of flux tubes. Our observations are in broad agreement with the two component model of Bellot Rubi et al. (2003), but do not rule out the embedded flux tube model of Solanki & Montavon (1993). Title: Diagnostic spectroscopy of G-band brightenings in the photosphere of the sun Authors: Langhans, K.; Schmidt, W.; Rimmele, T. Bibcode: 2004A&A...423.1147L Altcode: A detailed analysis of high-resolution spectra obtained in three different wavelength regions (at 430 nm, 526 nm and 569 nm) of G-band bright points in the solar photosphere is presented. They show an average intensity contrast of 11% with respect to the ``quiet'' sun reference. The CH lines are weakened in the bright point interior. The atomic lines, too, e.g. the Fe I line at 569.15 nm, weaken in the bright point interior. In contrast thereto, the absorption line of single ionized iron at 526.48 nm remains almost constant between bright point interior and the immediate surroundings. Line-of-sight velocities show a stronger downflow within bright points than in the close environment. A net downflow relative to the intergranular surroundings of around 80 m/s is measured. Filling factors are calculated from a comparison with synthesized spectra for different flux tube models and are used to estimate the ``true'' velocity in bright points with respect to their immediate surroundings. We obtain up- and downflows in the order of one km s-1, in agreement with the magneto-convective picture of the formation and dispersal of magnetic flux tubes. From the different behavior of the metallic lines and the CH lines we conclude that the line-weakening process that leads to the G-band bright points is mainly due to hot-wall radiation. This confirms that these bright points are indeed magnetic flux elements. Title: Two-dimensional Spectroscopy of G-band Bright Structures in the Solar Photosphere Authors: Langhans, K.; Schmidt, W.; Tritschler, A. Bibcode: 2003ANS...324...54L Altcode: 2003ANS...324..P06L; 2003ANS...324b..54L No abstract at ADS Title: Observations of G-band bright structures with TESOS Authors: Langhans, K.; Schmidt, W.; Tritschler, A. Bibcode: 2003AN....324..354L Altcode: No abstract at ADS Title: High-resolution solar spectroscopy with TESOS - Upgrade from a double to a triple system Authors: Tritschler, A.; Schmidt, W.; Langhans, K.; Kentischer, T. Bibcode: 2002SoPh..211...17T Altcode: We present the characteristics and demonstrate the performance of the Triple Etalon SOlar Spectrometer (TESOS) operated at the German Vacuum Tower Telescope (VTT) on Tenerife. The Fabry-Pérot interferometer TESOS is ideally suited for precise measurements of photospheric and chromospheric motion. Installed in 1997 and equipped with two etalons, TESOS has recently been completed with a third etalon and upgraded with two high-speed, backside-illuminated CCD cameras. The image scale of 0.089 arc sec pixel−1 is adapted to the resolution of the telescope. The improved system enables frame rates up to 5 frames per second. The spectral resolution of 300 000 allows for spectral diagnostics of weak photospheric lines, including individual CH-lines within the G-band at 430.6 nm. Title: 2D-spectroscopic observations of G-band bright structures Authors: Langhans, K.; Schmidt, W.; Tritschler, A. Bibcode: 2002ESASP.506..455L Altcode: 2002ESPM...10..455L; 2002svco.conf..455L We took two-dimensional spectra with the filter spectrometer TESOS at the German Vacuum Tower Telescope, Tenerife, of an absorption line of the CH-molecule and a Fe II-line in the G-band at 430.3 nm. We observed a region of granulation near a pore, close to disk center that showed many structures with enhanced G-band intensity. We introduce a Bright Point Index (BPI) defined by the ratio of the normalized line depressions of the Fe II and the CH-line. The BPI allows to characterize the bright structures by a quantity based on their spectroscopic signature. Bright structures, caused by significant weakening (up to 40% less absorption) of the absorption lines of the CH-molecule, have high BPI values and are accompanied by downflows. The remaining G-band bright structures, only caused by an enhanced continuum intensity, have low BPI and are related to granules. Title: Spectroscopic observations of G-band bright points Authors: Langhans, K.; Schmidt, W.; Rimmele, T. Bibcode: 2002ESASP.506..397L Altcode: 2002ESPM...10..397L; 2002svco.conf..397L Imaging in the G-band is commonly used to investigate the morphological properties of small magnetic field concentrations in the solar photosphere. However the physical background, explaining the brightness of these structures, has remained unclear. Our observations add the spectroscopic point of view to this discussion. The observations at high spatial resolution are obtained with the horizontal spectrograph at the Dunn Solar Telescope of the National Solar Observatory, USA. We scanned across a pore within an active region and took spectra in three different wavelength regions (430 nm, 526 nm, 569 nm) simultaneously to G-band and broadband continuum images. We present the results and discuss the spectroscopic bright point properties in comparison to former observations and synthetic data. Title: 2D-spectroscopic observations of vec G-band bright structures in the solar photosphere Authors: Langhans, K.; Schmidt, W.; Tritschler, A. Bibcode: 2002A&A...394.1069L Altcode: We took two-dimensional spectra with the filter spectrometer TESOS at the German Vacuum Tower Telescope, Tenerife, of an absorption line of the CH molecule and a Fe II-line in the G-band at 430.3 nm. We observed a region, close to disk center of the Sun, that showed a lot of structures with enhanced G-band intensity (up to 1.3 times the mean intensity of normal granulation). Our spectroscopic investigation of these structures suggests two classes which differ in their spectroscopic signature: (a) Bright structures caused by significant (up to 40\%) weakening of absorption lines of the CH molecule; (b) bright structures only caused by an enhanced continuum intensity. In order to distinguish between those two classes we introduce a Bright Point Index (BPI) defined by the ratio of the normalized line depressions of the Fe~II and the CH-line. The bright structures caused by weakening of the CH-lines have high BPI values and are accompanied by downflows. The remaining G-band bright structures have low BPI and are related to granules. Title: Center-to-limb-variation of the G-band lines at 430.5 nm Authors: Langhans, K.; Schmidt, W. Bibcode: 2002A&A...382..312L Altcode: We measure the center-to-limb-variation of the CH lines in the wavelength range from 430.40 to 430.61 nm (G-band) for both quiet sun and for the cooler atmophere of sunspot umbrae at different heliographic positions. The limb effect of the CH lines is about 650 ms-1 at mu =0.1 with a slope similar to weak Fe lines. We investigate the behaviour of the CH abundance in the range of optical depth (tau =-0.05 to tau =-0.25) that is accessible by center-to-limb observations. The CH abundance decreases with height in accordance with recent numerical models. Title: Spectroscopic Observation of G-Band Bright Points Authors: Langhans, K.; Schmidt, W.; Rimmele, T.; Sigwarth, M. Bibcode: 2001ASPC..236..439L Altcode: 2001aspt.conf..439L No abstract at ADS