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Author name code: langhans
ADS astronomy entries on 2022-09-14
author:"Langhans, Kai"
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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.
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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
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.
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Title: Searching for Overturning Convection in Penumbral Filaments:
Slit Spectroscopy at 0farcs2 Resolution
Authors: Bellot Rubio, L. R.; Schlichenmaier, R.; Langhans, K.
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<SUP>-1</SUP>. 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<SUP>-1</SUP>
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<SUP>-1</SUP>. Redshifts also occur, but at the
level of 100-150 m s<SUP>-1</SUP>, 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<SUP>-1</SUP>. 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.
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Title: Recent High Resolution Observations and Interpretations of
Sunspot Fine Structure
Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G.
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.
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Title: Observations of dark-cored filaments in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.
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. <BR />Aims: Our observations
provide new constraints for the different scenarios put forward to
explain the structure of sunspot penumbrae. <BR />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°. <BR />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.
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Title: Signatures of Penumbral Magnetic Fields at Very High Spatial
Resolution
Authors: Langhans, K.
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. <P />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.
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Title: Multi-line spectroscopy of dark-cored penumbral filaments
Authors: Bellot Rubio, L. R.; Langhans, K.; Schlichenmaier, R.
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<SUP>-1</SUP>; 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.
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Title: Inclination of magnetic fields and flows in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.;
Berger, T. E.
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).
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Title: Diagnostic spectroscopy of G-band brightenings in the
photosphere of the sun
Authors: Langhans, K.; Schmidt, W.; Rimmele, T.
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<SUP>-1</SUP>, 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.
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Title: Two-dimensional Spectroscopy of G-band Bright Structures in
the Solar Photosphere
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
2003ANS...324...54L Altcode: 2003ANS...324..P06L; 2003ANS...324b..54L
No abstract at ADS
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Title: Observations of G-band bright structures with TESOS
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
2003AN....324..354L Altcode:
No abstract at ADS
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Title: High-resolution solar spectroscopy with TESOS - Upgrade from
a double to a triple system
Authors: Tritschler, A.; Schmidt, W.; Langhans, K.; Kentischer, T.
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<SUP>−1</SUP> 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.
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Title: 2D-spectroscopic observations of G-band bright structures
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
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.
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Title: Spectroscopic observations of G-band bright points
Authors: Langhans, K.; Schmidt, W.; Rimmele, T.
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.
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Title: 2D-spectroscopic observations of vec G-band bright structures
in the solar photosphere
Authors: Langhans, K.; Schmidt, W.; Tritschler, A.
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.
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Title: Center-to-limb-variation of the G-band lines at 430.5 nm
Authors: Langhans, K.; Schmidt, W.
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<SUP>-1</SUP> 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.
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Title: Spectroscopic Observation of G-Band Bright Points
Authors: Langhans, K.; Schmidt, W.; Rimmele, T.; Sigwarth, M.
2001ASPC..236..439L Altcode: 2001aspt.conf..439L
No abstract at ADS
