Author name code: sigwarth
ADS astronomy entries on 2022-09-14
author:"Sigwarth, Michael"
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Title: The Polarimetric and Helioseismic Imager on Solar Orbiter
Authors: Solanki, S. K.; del Toro Iniesta, J. C.; Woch, J.; Gandorfer,
A.; Hirzberger, J.; Alvarez-Herrero, A.; Appourchaux, T.; Martínez
Pillet, V.; Pérez-Grande, I.; Sanchis Kilders, E.; Schmidt, W.;
Gómez Cama, J. M.; Michalik, H.; Deutsch, W.; Fernandez-Rico, G.;
Grauf, B.; Gizon, L.; Heerlein, K.; Kolleck, M.; Lagg, A.; Meller, R.;
Müller, R.; Schühle, U.; Staub, J.; Albert, K.; Alvarez Copano, M.;
Beckmann, U.; Bischoff, J.; Busse, D.; Enge, R.; Frahm, S.; Germerott,
D.; Guerrero, L.; Löptien, B.; Meierdierks, T.; Oberdorfer, D.;
Papagiannaki, I.; Ramanath, S.; Schou, J.; Werner, S.; Yang, D.;
Zerr, A.; Bergmann, M.; Bochmann, J.; Heinrichs, J.; Meyer, S.;
Monecke, M.; Müller, M. -F.; Sperling, M.; Álvarez García, D.;
Aparicio, B.; Balaguer Jiménez, M.; Bellot Rubio, L. R.; Cobos
Carracosa, J. P.; Girela, F.; Hernández Expósito, D.; Herranz, M.;
Labrousse, P.; López Jiménez, A.; Orozco Suárez, D.; Ramos, J. L.;
Barandiarán, J.; Bastide, L.; Campuzano, C.; Cebollero, M.; Dávila,
B.; Fernández-Medina, A.; García Parejo, P.; Garranzo-García, D.;
Laguna, H.; Martín, J. A.; Navarro, R.; Núñez Peral, A.; Royo, M.;
Sánchez, A.; Silva-López, M.; Vera, I.; Villanueva, J.; Fourmond,
J. -J.; de Galarreta, C. Ruiz; Bouzit, M.; Hervier, V.; Le Clec'h,
J. C.; Szwec, N.; Chaigneau, M.; Buttice, V.; Dominguez-Tagle, C.;
Philippon, A.; Boumier, P.; Le Cocguen, R.; Baranjuk, G.; Bell,
A.; Berkefeld, Th.; Baumgartner, J.; Heidecke, F.; Maue, T.; Nakai,
E.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.; Volkmer, R.; Blanco
Rodríguez, J.; Domingo, V.; Ferreres Sabater, A.; Gasent Blesa,
J. L.; Rodríguez Martínez, P.; Osorno Caudel, D.; Bosch, J.; Casas,
A.; Carmona, M.; Herms, A.; Roma, D.; Alonso, G.; Gómez-Sanjuan, A.;
Piqueras, J.; Torralbo, I.; Fiethe, B.; Guan, Y.; Lange, T.; Michel,
H.; Bonet, J. A.; Fahmy, S.; Müller, D.; Zouganelis, I.
Bibcode: 2020A&A...642A..11S
Altcode: 2019arXiv190311061S
Aims: This paper describes the Polarimetric and Helioseismic
Imager on the Solar Orbiter mission (SO/PHI), the first magnetograph and
helioseismology instrument to observe the Sun from outside the Sun-Earth
line. It is the key instrument meant to address the top-level science
question: How does the solar dynamo work and drive connections between
the Sun and the heliosphere? SO/PHI will also play an important role
in answering the other top-level science questions of Solar Orbiter,
while hosting the potential of a rich return in further science.
Methods: SO/PHI measures the Zeeman effect and the Doppler shift
in the Fe I 617.3 nm spectral line. To this end, the instrument
carries out narrow-band imaging spectro-polarimetry using a tunable
LiNbO3 Fabry-Perot etalon, while the polarisation modulation
is done with liquid crystal variable retarders. The line and the nearby
continuum are sampled at six wavelength points and the data are recorded
by a 2k × 2k CMOS detector. To save valuable telemetry, the raw data
are reduced on board, including being inverted under the assumption of
a Milne-Eddington atmosphere, although simpler reduction methods are
also available on board. SO/PHI is composed of two telescopes; one,
the Full Disc Telescope, covers the full solar disc at all phases of
the orbit, while the other, the High Resolution Telescope, can resolve
structures as small as 200 km on the Sun at closest perihelion. The high
heat load generated through proximity to the Sun is greatly reduced by
the multilayer-coated entrance windows to the two telescopes that allow
less than 4% of the total sunlight to enter the instrument, most of
it in a narrow wavelength band around the chosen spectral line.
Results: SO/PHI was designed and built by a consortium having partners
in Germany, Spain, and France. The flight model was delivered to
Airbus Defence and Space, Stevenage, and successfully integrated into
the Solar Orbiter spacecraft. A number of innovations were introduced
compared with earlier space-based spectropolarimeters, thus allowing
SO/PHI to fit into the tight mass, volume, power and telemetry budgets
provided by the Solar Orbiter spacecraft and to meet the (e.g. thermal)
challenges posed by the mission's highly elliptical orbit.
Title: Spectropolarimetric Observations of an Arch Filament System
with GREGOR
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kučera, A.; Schwartz, P.; Berkefeld, T.; Collados, M.;
Denker, C.; Feller, A.; Hofmann, A.; Schlichenmaier, R.; Schmidt,
D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau,
D.; Staude, J.; Strassmeier, K. G.; von der Lühe, O.
Bibcode: 2019ASPC..526..217B
Altcode: 2018arXiv180401789B
We observed an arch filament system (AFS) in a sunspot group with the
GREGOR Infrared Spectrograph attached to the GREGOR solar telescope. The
AFS was located between the leading sunspot of negative polarity and
several pores of positive polarity forming the following part of the
sunspot group. We recorded five spectro-polarimetric scans of this
region. The spectral range included the spectral lines Si I 1082.7
nm, He I 1083.0 nm, and Ca I 1083.9 nm. In this work we concentrate
on the silicon line which is formed in the upper photosphere. The
line profiles are inverted with the code 'Stokes Inversion based
on Response functions' to obtain the magnetic field vector. The
line-of-sight velocities are determined independently with a Fourier
phase method. Maximum velocities are found close to the ends of AFS
fibrils. These maximum values amount to 2.4 km s-1 next
to the pores and to 4 km s-1 at the sunspot side. Between
the following pores, we encounter an area of negative polarity that
is decreasing during the five scans. We interpret this by new emerging
positive flux in this area canceling out the negative flux. In summary,
our findings confirm the scenario that rising magnetic flux tubes
cause the AFS.
Title: Photospheric Magnetic Fields of the Trailing Sunspots in
Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2019ASPC..526..291V
Altcode: 2018arXiv180507752V
The solar magnetic field is responsible for all aspects of solar
activity. Sunspots are the main manifestation of the ensuing solar
activity. Combining high-resolution and synoptic observations has
the ambition to provide a comprehensive description of the sunspot
growth and decay processes. Active region NOAA 12396 emerged on 2015
August 3 and was observed three days later with the 1.5-meter GREGOR
solar telescope on 2015 August 6. High-resolution spectropolarimetric
data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the
photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together
with the chromospheric He I λ1083.0 nm triplet. These near-infrared
spectropolarimetric observations were complemented by synoptic
line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO).
Title: High uniformity IBS coatings for the world's largest
Fabry-Perot etalon of the VTF instrument
Authors: Pinard, L.; Michel, C.; Sassolas, B.; Teillon, J.; Cagnoli,
G.; Sigwarth, M.; Kentischer, T.; Schmidt, W.; Reichman, B.
Bibcode: 2018SPIE10706E..1RP
Altcode:
The first large Fabry-Perot etalon (Ø35 cm) of the VTF instrument
was coated successfully using IBS technique. The High Reflective (HR)
coatings need to meet the reflectivity specifications (95 +/- 1%)
over the entire wavelength range 520-870 nm and the entire aperture
(Ø25 cm) and also preserve the plate's flatness and airgap uniformity
between the two platesto be better than 3 nm RMS. The change of the
figure error of the individual faces after HR coating was exceptionally
small: For plate 1 (upper) it changed from 1.7nm RMS before coating
to 2.12 nm after coating, no change at all for plate 2 (lower).
Title: Flows along arch filaments observed in the GRIS `very fast
spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2017IAUS..327...28G
Altcode: 2017arXiv170102206G
A new generation of solar instruments provides improved spectral,
spatial, and temporal resolution, thus facilitating a better
understanding of dynamic processes on the Sun. High-resolution
observations often reveal multiple-component spectral line profiles,
e.g., in the near-infrared He i 10830 Å triplet, which provides
information about the chromospheric velocity and magnetic fine
structure. We observed an emerging flux region, including two small
pores and an arch filament system, on 2015 April 17 with the `very
fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
situated at the 1.5-meter GREGOR solar telescope at Observatorio del
Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
per minute) spectral scans of the solar surface and its potential to
follow dynamic processes on the Sun. We demonstrate the performance
of the `very fast spectroscopic mode' by tracking chromospheric
high-velocity features in the arch filament system.
Title: Slipping reconnection in a solar flare observed in high
resolution with the GREGOR solar telescope
Authors: Sobotka, M.; Dudík, J.; Denker, C.; Balthasar, H.; Jurčák,
J.; Liu, W.; Berkefeld, T.; Collados Vera, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Kuckein, C.; Lagg, A.; Louis, R. E.; von der Lühe, O.;
Nicklas, H.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.;
Volkmer, R.; Waldmann, T.
Bibcode: 2016A&A...596A...1S
Altcode: 2016arXiv160500464S
A small flare ribbon above a sunspot umbra in active region 12205 was
observed on November 7, 2014, at 12:00 UT in the blue imaging channel
of the 1.5 m GREGOR telescope, using a 1 Å Ca II H interference
filter. Context observations from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO), the Solar Optical
Telescope (SOT) onboard Hinode, and the Interface Region Imaging
Spectrograph (IRIS) show that this ribbon is part of a larger one
that extends through the neighboring positive polarities and also
participates in several other flares within the active region. We
reconstructed a time series of 140 s of Ca II H images by means of the
multiframe blind deconvolution method, which resulted in spatial and
temporal resolutions of 0.1″ and 1 s. Light curves and horizontal
velocities of small-scale bright knots in the observed flare ribbon
were measured. Some knots are stationary, but three move along the
ribbon with speeds of 7-11 km s-1. Two of them move in the
opposite direction and exhibit highly correlated intensity changes,
which provides evidence of a slipping reconnection at small spatial
scales. Movies associated to Figs. 1 and 2 are available at http://www.aanda.org
Title: Deep probing of the photospheric sunspot penumbra: no evidence
of field-free gaps
Authors: Borrero, J. M.; Asensio Ramos, A.; Collados, M.;
Schlichenmaier, R.; Balthasar, H.; Franz, M.; Rezaei, R.; Kiess, C.;
Orozco Suárez, D.; Pastor Yabar, A.; Berkefeld, T.; von der Lühe,
O.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Soltau, D.; Volkmer, R.;
Waldmann, T.; Denker, C.; Hofmann, A.; Staude, J.; Strassmeier, K. G.;
Feller, A.; Lagg, A.; Solanki, S. K.; Sobotka, M.; Nicklas, H.
Bibcode: 2016A&A...596A...2B
Altcode: 2016arXiv160708165B
Context. Some models for the topology of the magnetic field in
sunspot penumbrae predict regions free of magnetic fields or with
only dynamically weak fields in the deep photosphere.
Aims:
We aim to confirm or refute the existence of weak-field regions in
the deepest photospheric layers of the penumbra.
Methods:
We investigated the magnetic field at log τ5 = 0 is
by inverting spectropolarimetric data of two different sunspots
located very close to disk center with a spatial resolution of
approximately 0.4-0.45''. The data have been recorded using the GRIS
instrument attached to the 1.5-m solar telescope GREGOR at the El
Teide observatory. The data include three Fe I lines around 1565 nm,
whose sensitivity to the magnetic field peaks half a pressure scale
height deeper than the sensitivity of the widely used Fe I spectral
line pair at 630 nm. Before the inversion, the data were corrected
for the effects of scattered light using a deconvolution method with
several point spread functions.
Results: At log τ5
= 0 we find no evidence of regions with dynamically weak (B<
500 Gauss) magnetic fields in sunspot penumbrae. This result is much
more reliable than previous investigations made on Fe I lines at 630
nm. Moreover, the result is independent of the number of nodes employed
in the inversion, is independent of the point spread function used to
deconvolve the data, and does not depend on the amount of stray light
(I.e., wide-angle scattered light) considered.
Title: Spectropolarimetric observations of an arch filament system
with the GREGOR solar telescope
Authors: Balthasar, H.; Gömöry, P.; González Manrique, S. J.;
Kuckein, C.; Kavka, J.; Kučera, A.; Schwartz, P.; Vašková, R.;
Berkefeld, T.; Collados Vera, M.; Denker, C.; Feller, A.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar, A.;
Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1050B
Altcode: 2016arXiv160901514B
Arch filament systems occur in active sunspot groups, where a fibril
structure connects areas of opposite magnetic polarity, in contrast to
active region filaments that follow the polarity inversion line. We
used the GREGOR Infrared Spectrograph (GRIS) to obtain the full
Stokes vector in the spectral lines Si I λ1082.7 nm, He I λ1083.0
nm, and Ca I λ1083.9 nm. We focus on the near-infrared calcium line
to investigate the photospheric magnetic field and velocities, and
use the line core intensities and velocities of the helium line to
study the chromospheric plasma. The individual fibrils of the arch
filament system connect the sunspot with patches of magnetic polarity
opposite to that of the spot. These patches do not necessarily coincide
with pores, where the magnetic field is strongest. Instead, areas are
preferred not far from the polarity inversion line. These areas exhibit
photospheric downflows of moderate velocity, but significantly higher
downflows of up to 30 km s-1 in the chromospheric helium
line. Our findings can be explained with new emerging flux where the
matter flows downward along the field lines of rising flux tubes,
in agreement with earlier results.
Title: Magnetic fields of opposite polarity in sunspot penumbrae
Authors: Franz, M.; Collados, M.; Bethge, C.; Schlichenmaier, R.;
Borrero, J. M.; Schmidt, W.; Lagg, A.; Solanki, S. K.; Berkefeld,
T.; Kiess, C.; Rezaei, R.; Schmidt, D.; Sigwarth, M.; Soltau, D.;
Volkmer, R.; von der Luhe, O.; Waldmann, T.; Orozco, D.; Pastor Yabar,
A.; Denker, C.; Balthasar, H.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Feller, A.; Nicklas, H.; Kneer, F.; Sobotka, M.
Bibcode: 2016A&A...596A...4F
Altcode: 2016arXiv160800513F
Context. A significant part of the penumbral magnetic field returns
below the surface in the very deep photosphere. For lines in the
visible, a large portion of this return field can only be detected
indirectly by studying its imprints on strongly asymmetric and
three-lobed Stokes V profiles. Infrared lines probe a narrow layer
in the very deep photosphere, providing the possibility of directly
measuring the orientation of magnetic fields close to the solar
surface.
Aims: We study the topology of the penumbral magnetic
field in the lower photosphere, focusing on regions where it returns
below the surface.
Methods: We analyzed 71 spectropolarimetric
datasets from Hinode and from the GREGOR infrared spectrograph. We
inferred the quality and polarimetric accuracy of the infrared data
after applying several reduction steps. Techniques of spectral
inversion and forward synthesis were used to test the detection
algorithm. We compared the morphology and the fractional penumbral
area covered by reversed-polarity and three-lobed Stokes V profiles for
sunspots at disk center. We determined the amount of reversed-polarity
and three-lobed Stokes V profiles in visible and infrared data of
sunspots at various heliocentric angles. From the results, we computed
center-to-limb variation curves, which were interpreted in the context
of existing penumbral models.
Results: Observations in visible
and near-infrared spectral lines yield a significant difference in the
penumbral area covered by magnetic fields of opposite polarity. In
the infrared, the number of reversed-polarity Stokes V profiles is
smaller by a factor of two than in the visible. For three-lobed Stokes
V profiles the numbers differ by up to an order of magnitude.
Title: Horizontal flow fields in and around a small active region. The
transition period between flux emergence and decay
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A...3V
Altcode: 2016arXiv160507462V
Context. The solar magnetic field is responsible for all aspects
of solar activity. Thus, emergence of magnetic flux at the surface
is the first manifestation of the ensuing solar activity.
Aims: Combining high-resolution and synoptic observations aims to
provide a comprehensive description of flux emergence at photospheric
level and of the growth process that eventually leads to a mature
active region.
Methods: The small active region NOAA 12118
emerged on 2014 July 17 and was observed one day later with the 1.5-m
GREGOR solar telescope on 2014 July 18. High-resolution time-series
of blue continuum and G-band images acquired in the blue imaging
channel (BIC) of the GREGOR Fabry-Pérot Interferometer (GFPI) were
complemented by synoptic line-of-sight magnetograms and continuum
images obtained with the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO). Horizontal proper motions and
horizontal plasma velocities were computed with local correlation
tracking (LCT) and the differential affine velocity estimator (DAVE),
respectively. Morphological image processing was employed to measure
the photometric and magnetic area, magnetic flux, and the separation
profile of the emerging flux region during its evolution.
Results: The computed growth rates for photometric area, magnetic
area, and magnetic flux are about twice as high as the respective
decay rates. The space-time diagram using HMI magnetograms of five days
provides a comprehensive view of growth and decay. It traces a leaf-like
structure, which is determined by the initial separation of the two
polarities, a rapid expansion phase, a time when the spread stalls,
and a period when the region slowly shrinks again. The separation
rate of 0.26 km s-1 is highest in the initial stage, and
it decreases when the separation comes to a halt. Horizontal plasma
velocities computed at four evolutionary stages indicate a changing
pattern of inflows. In LCT maps we find persistent flow patterns such
as outward motions in the outer part of the two major pores, a diverging
feature near the trailing pore marking the site of upwelling plasma and
flux emergence, and low velocities in the interior of dark pores. We
detected many elongated rapidly expanding granules between the two
major polarities, with dimensions twice as large as the normal granules.
Title: Active region fine structure observed at 0.08 arcsec resolution
Authors: Schlichenmaier, R.; von der Lühe, O.; Hoch, S.; Soltau, D.;
Berkefeld, T.; Schmidt, D.; Schmidt, W.; Denker, C.; Balthasar, H.;
Hofmann, A.; Strassmeier, K. G.; Staude, J.; Feller, A.; Lagg, A.;
Solanki, S. K.; Collados, M.; Sigwarth, M.; Volkmer, R.; Waldmann,
T.; Kneer, F.; Nicklas, H.; Sobotka, M.
Bibcode: 2016A&A...596A...7S
Altcode: 2016arXiv160707094S
Context. The various mechanisms of magneto-convective energy transport
determine the structure of sunspots and active regions.
Aims: We characterise the appearance of light bridges and other
fine-structure details and elaborate on their magneto-convective
nature.
Methods: We present speckle-reconstructed images taken
with the broad-band imager (BBI) at the 1.5 m GREGOR telescope in the
486 nm and 589 nm bands. We estimate the spatial resolution from the
noise characteristics of the image bursts and obtain 0.08″ at 589
nm. We describe structure details in individual best images as well
as the temporal evolution of selected features.
Results: We
find branched dark lanes extending along thin (≈1″) light bridges
in sunspots at various heliocentric angles. In thick (≳ 2″) light
bridges the branches are disconnected from the central lane and have a Y
shape with a bright grain toward the umbra. The images reveal that light
bridges exist on varying intensity levels and that their small-scale
features evolve on timescales of minutes. Faint light bridges show
dark lanes outlined by the surrounding bright features. Dark lanes are
very common and are also found in the boundary of pores. They have a
characteristic width of 0.1″ or smaller. Intergranular dark lanes of
that width are seen in active region granulation.
Conclusions: We
interpret our images in the context of magneto-convective simulations
and findings: while central dark lanes in thin light bridges are
elevated and associated with a density increase above upflows, the dark
lane branches correspond to locations of downflows and are depressed
relative to the adjacent bright plasma. Thick light bridges with central
dark lanes show no projection effect. They have a flat elevated plateau
that falls off steeply at the umbral boundary. There, Y-shaped filaments
form as they do in the inner penumbra. This indicates the presence of
inclined magnetic fields, meaning that the umbral magnetic field is
wrapped around the convective light bridge.
Title: Probing deep photospheric layers of the quiet Sun with high
magnetic sensitivity
Authors: Lagg, A.; Solanki, S. K.; Doerr, H. -P.; Martínez González,
M. J.; Riethmüller, T.; Collados Vera, M.; Schlichenmaier, R.;
Orozco Suárez, D.; Franz, M.; Feller, A.; Kuckein, C.; Schmidt, W.;
Asensio Ramos, A.; Pastor Yabar, A.; von der Lühe, O.; Denker, C.;
Balthasar, H.; Volkmer, R.; Staude, J.; Hofmann, A.; Strassmeier,
K.; Kneer, F.; Waldmann, T.; Borrero, J. M.; Sobotka, M.; Verma, M.;
Louis, R. E.; Rezaei, R.; Soltau, D.; Berkefeld, T.; Sigwarth, M.;
Schmidt, D.; Kiess, C.; Nicklas, H.
Bibcode: 2016A&A...596A...6L
Altcode: 2016arXiv160506324L
Context. Investigations of the magnetism of the quiet Sun are hindered
by extremely weak polarization signals in Fraunhofer spectral
lines. Photon noise, straylight, and the systematically different
sensitivity of the Zeeman effect to longitudinal and transversal
magnetic fields result in controversial results in terms of the strength
and angular distribution of the magnetic field vector.
Aims:
The information content of Stokes measurements close to the diffraction
limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of
spatial straylight and photon noise into account.
Methods: Highly
sensitive full Stokes measurements of a quiet-Sun region at disk center
in the deep photospheric Fe I lines in the 1.56 μm region were obtained
with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise
statistics and Stokes V asymmetries were analyzed and compared to a
similar data set of the Hinode spectropolarimeter (SOT/SP). Simple
diagnostics based directly on the shape and strength of the profiles
were applied to the GRIS data. We made use of the magnetic line ratio
technique, which was tested against realistic magneto-hydrodynamic
simulations (MURaM).
Results: About 80% of the GRIS spectra
of a very quiet solar region show polarimetric signals above a 3σ
level. Area and amplitude asymmetries agree well with small-scale
surface dynamo-magneto hydrodynamic simulations. The magnetic line ratio
analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss
range with some concentrations of kilo-Gauss fields.
Conclusions:
The GRIS spectropolarimetric data at a spatial resolution of ≈0.̋4
are so far unique in the combination of high spatial resolution scans
and high magnetic field sensitivity. Nevertheless, the unavoidable
effect of spatial straylight and the resulting dilution of the weak
Stokes profiles means that inversion techniques still bear a high risk
of misinterpretating the data.
Title: Flow and magnetic field properties in the trailing sunspots
of active region NOAA 12396
Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer,
C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados,
M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.;
Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei,
R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.;
Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1090V
Altcode:
Improved measurements of the photospheric and chromospheric
three-dimensional magnetic and flow fields are crucial for a precise
determination of the origin and evolution of active regions. We present
an illustrative sample of multi-instrument data acquired during a
two-week coordinated observing campaign in August 2015 involving,
among others, the GREGOR solar telescope (imaging and near-infrared
spectroscopy) and the space missions Solar Dynamics Observatory (SDO)
and Interface Region Imaging Spectrograph (IRIS). The observations
focused on the trailing part of active region NOAA 12396 with complex
polarity inversion lines and strong intrusions of opposite polarity
flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV
spectral profiles in the photospheric Si I λ1082.7 nm line, the
chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I
λ1083.9 nm line. Carefully calibrated GRIS scans of the active region
provided maps of Doppler velocity and magnetic field at different
atmospheric heights. We compare quick-look maps with those obtained
with the ``Stokes Inversions based on Response functions'' (SIR)
code, which furnishes deeper insight into the magnetic properties
of the region. We find supporting evidence that newly emerging flux
and intruding opposite polarity flux are hampering the formation
of penumbrae, i.e., a penumbra fully surrounding a sunspot is only
expected after cessation of flux emergence in proximity to the sunspots.
Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
Bibcode: 2016A&A...596A..59F
Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking.
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay.
Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory.
Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. Movies associated to Figs. 2 and 13
are available at http://www.aanda.org
Title: Inference of magnetic fields in the very quiet Sun
Authors: Martínez González, M. J.; Pastor Yabar, A.; Lagg, A.;
Asensio Ramos, A.; Collados, M.; Solanki, S. K.; Balthasar, H.;
Berkefeld, T.; Denker, C.; Doerr, H. P.; Feller, A.; Franz, M.;
González Manrique, S. J.; Hofmann, A.; Kneer, F.; Kuckein, C.;
Louis, R.; von der Lühe, O.; Nicklas, H.; Orozco, D.; Rezaei, R.;
Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma, M.; Waldman,
T.; Volkmer, R.
Bibcode: 2016A&A...596A...5M
Altcode: 2018arXiv180410089M
Context. Over the past 20 yr, the quietest areas of the solar surface
have revealed a weak but extremely dynamic magnetism occurring
at small scales (<500 km), which may provide an important
contribution to the dynamics and energetics of the outer layers of
the atmosphere. Understanding this magnetism requires the inference
of physical quantities from high-sensitivity spectro-polarimetric
data with high spatio-temporal resolution.
Aims: We present
high-precision spectro-polarimetric data with high spatial resolution
(0.4'') of the very quiet Sun at 1.56 μm obtained with the GREGOR
telescope to shed some light on this complex magnetism.
Methods:
We used inversion techniques in two main approaches. First, we assumed
that the observed profiles can be reproduced with a constant magnetic
field atmosphere embedded in a field-free medium. Second, we assumed
that the resolution element has a substructure with either two constant
magnetic atmospheres or a single magnetic atmosphere with gradients of
the physical quantities along the optical depth, both coexisting with
a global stray-light component.
Results: Half of our observed
quiet-Sun region is better explained by magnetic substructure within
the resolution element. However, we cannot distinguish whether this
substructure comes from gradients of the physical parameters along the
line of sight or from horizontal gradients (across the surface). In
these pixels, a model with two magnetic components is preferred, and
we find two distinct magnetic field populations. The population with
the larger filling factor has very weak ( 150 G) horizontal fields
similar to those obtained in previous works. We demonstrate that the
field vector of this population is not constrained by the observations,
given the spatial resolution and polarimetric accuracy of our data. The
topology of the other component with the smaller filling factor is
constrained by the observations for field strengths above 250 G:
we infer hG fields with inclinations and azimuth values compatible
with an isotropic distribution. The filling factors are typically
below 30%. We also find that the flux of the two polarities is not
balanced. From the other half of the observed quiet-Sun area 50% are
two-lobed Stokes V profiles, meaning that 23% of the field of view
can be adequately explained with a single constant magnetic field
embedded in a non-magnetic atmosphere. The magnetic field vector and
filling factor are reliable inferred in only 50% based on the regular
profiles. Therefore, 12% of the field of view harbour hG fields with
filling factors typically below 30%. At our present spatial resolution,
70% of the pixels apparently are non-magnetised.
Title: Fitting peculiar spectral profiles in He I 10830Å absorption
features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016AN....337.1057G
Altcode: 2016arXiv160300679G
The new generation of solar instruments provides better
spectral, spatial, and temporal resolution for a better
understanding of the physical processes that take place on the
Sun. Multiple-component profiles are more commonly observed with these
instruments. Particularly, the He I 10830 Å triplet presents such
peculiar spectral profiles, which give information on the velocity
and magnetic fine structure of the upper chromosphere. The purpose
of this investigation is to describe a technique to efficiently fit
the two blended components of the He I 10830 Å triplet, which are
commonly observed when two atmospheric components are located within
the same resolution element. The observations used in this study were
taken on 2015 April 17 with the very fast spectroscopic mode of the
GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
telescope, located at the Observatorio del Teide, Tenerife, Spain. We
apply a double-Lorentzian fitting technique using Levenberg-Marquardt
least-squares minimization. This technique is very simple and much
faster than inversion codes. Line-of-sight Doppler velocities can
be inferred for a whole map of pixels within just a few minutes. Our
results show sub- and supersonic downflow velocities of up to 32 km
s-1 for the fast component in the vicinity of footpoints of
filamentary structures. The slow component presents velocities close
to rest.
Title: Development of high reflectivity coatings for large format
Fabry-Perot etalons
Authors: Sigwarth, M.; Baumgartner, J.; Bell, A.; Cagnoli, G.; Fischer,
A.; Halbgewachs, C.; Heidecke, F.; Kentischer, Th. J.; Kestner, B.;
Kuschnir, P.; von der Lühe, O.; Pinard, L.; Michel, Ch.; Reichman,
W. J.; Sassolas, B.; Scheiffelen, Th.; Schmidt, W.
Bibcode: 2016SPIE.9908E..4FS
Altcode:
The Visible Tunable Filter (VTF) is a diffraction-limited narrowband
tunable instrument for imaging spectropolarimetry in the wavelength
range between 520 and 860 nm. It is based on large-format Fabry
Perot. The instrument will be one of the first-light instruments of
the 4m aperture Daniel K. Inoue Solar Telescope (DKIST). To provide a
field of view of 1 arcmin and a spectral resolution λ/Δλ of about
100.000, the required free aperture of the Fabry Perot is 250mm. The
high reflectivity coatings for the Etalon plates need to meet the
specifications for the reflectivity over the entire wavelength range
and preserve the plate figure specifications of better λ/300, and
a micro roughness of < 0.4 nm rms. Coated surfaces with similar
specifications have successfully been made for reflecting mirrors on
thick substrates but not for larger format Fabry-Perot systems. Ion Beam
Sputtering (IBS) based coatings provide stable, homogeneous, and smooth
coatings. But IBS coatings also introduce stresses to the substrate
that influence the plate figure in our case at the nm level. In a joint
effort with an industry partner and a French CNRS research laboratory,
we developed and tested processes on small and full size substrates,
to provide coated Etalon plates to the required specifications. Zygo
Extreme Precision Optics, Richmond, CA, USA, is polishing and figuring
the substrates, doing the metrology and FE analysis. LMA (Laboratoire
Matériaux Avancés, Lyon, France) is designing and making the IBS
coatings and investigating the detailed behavior of the coatings and
related processes. Both partners provide experience from manufacturing
coated plane optics for gravitational wave detection experiments
and EUV optics. The Kiepenheuer-Institut für Sonnenphysik, Freiburg,
Germany is designing and building the VTF instrument and is leading the
coating development. We present the characteristics of the coatings
and the substrate processing concept, as well as results from tests
on sample size and from full size substrate processing. We demonstrate
that the tight specifications for a single Etalon can be reached.
Title: End-to-end simulations of the visible tunable filter for the
Daniel K. Inouye Solar Telescope
Authors: Schmidt, Wolfgang; Schubert, Matthias; Ellwarth, Monika;
Baumgartner, Jörg; Bell, Alexander; Fischer, Andreas; Halbgewachs,
Clemens; Heidecke, Frank; Kentischer, Thomas; von der Lühe, Oskar;
Scheiffelen, Thomas; Sigwarth, Michael
Bibcode: 2016SPIE.9908E..4NS
Altcode: 2016arXiv160706767S
The Visible Tunable Filter (VTF) is a narrowband tunable filter system
for imaging spectroscopy and spectropolarimetry based. The instrument
will be one of the first-light instruments of the Daniel K. Inouye Solar
Telescope that is currently under construction on Maui (Hawaii). The
VTF is being developed by the Kiepenheuer Institut fuer Sonnenphysik in
Freiburg as a German contribution to the DKIST. We perform end-to-end
simulations of spectropolarimetric observations with the VTF to
verify the science requirements of the instrument. The instrument
is simulated with two Etalons, and with a single Etalon. The clear
aperture of the Etalons is 250 mm, corresponding to a field of view with
a diameter of 60 arcsec in the sky (42,000 km on the Sun). To model
the large-scale figure errors we employ low-order Zernike polynomials
(power and spherical aberration) with amplitudes of 2.5 nm RMS. We use
an ideal polarization modulator with equal modulation coefficients
of 3-1/2 for the polarization modulation We synthesize
Stokes profiles of two iron lines (630.15 nm and 630.25 nm) and for
the 854.2 nm line of calcium, for a range of magnetic field values
and for several inclination angles. We estimated the photon noise on
the basis of the DKIST and VTF transmission values, the atmospheric
transmission and the spectral flux from the Sun. For the Fe 630.25 nm
line, we obtain a sensitivity of 20 G for the longitudinal component
and for 150 G for the transverse component, in agreement with the
science requirements for the VTF.
Title: Flows in and around Active Region NOAA12118 Observed with
the GREGOR Solar Telescope and SDO/HMI
Authors: Verma, M.; Denker, C.; Balthasar, H.; Kuckein, C.; González
Manrique, S. J.; Sobotka, M.; Bello González, N.; Hoch, S.; Diercke,
A.; Kummerow, P.; Berkefeld, T.; Collados, M.; Feller, A.; Hofmann,
A.; Kneer, F.; Lagg, A.; Löhner-Böttcher, J.; Nicklas, H.; Pastor
Yabar, A.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Schubert,
M.; Sigwarth, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
Bibcode: 2016ASPC..504...29V
Altcode: 2016arXiv160301109V
Accurate measurements of magnetic and velocity fields in and around
solar active regions are key to unlocking the mysteries of the
formation and the decay of sunspots. High spatial resolution images
and spectral sequences with a high cadence obtained with the GREGOR
solar telescope give us an opportunity to scrutinize 3-D flow fields
with local correlation tracking and imaging spectroscopy. We present
GREGOR early science data acquired in 2014 July - August with the GREGOR
Fabry-Pérot Interferometer and the Blue Imaging Channel. Time-series
of blue continuum (λ 450.6 nm) images of the small active region
NOAA 12118 were restored with the speckle masking technique to derive
horizontal proper motions and to track the evolution of morphological
changes. In addition, high-resolution observations are discussed in
the context of synoptic data from the Solar Dynamics Observatory.
Title: DKIST visible tunable filter control software: connecting
the DKIST framework to OPC UA
Authors: Bell, Alexander; Halbgewachs, Clemens; Kentischer, Thomas J.;
Schmidt, Wolfgang; von der Lühe, Oskar; Sigwarth, Michael; Fischer,
Andreas
Bibcode: 2014SPIE.9152E..1DB
Altcode:
The Visible Tunable Filter (VTF) is a narrowband tunable filter system
for imaging spectroscopy and spectropolarimetry based on large-format
Fabry Perot interferometers that is currently built by the Kiepenheuer
Institut fuer Sonnenphysik for the Daniel K. Inouye Solar Telescope
(DKIST). The control software must handle around 30 motorised drives,
3 etalons, a polarizing modulator, a helium neon laser for system
calibration, temperature controllers and a multitude of sensors. The
VTF is foreseen as one of the DKISTs first-light instruments and should
become operational in 2019. In the design of the control software
we strongly separate between the high-level part interfacing to the
DKIST common services framework (CSF) and the low-level control system
software which guarantees real-time performance and synchronization
to precision time protocol (PTP) based observatory time. For the
latter we chose a programmable logic controller (PLC) from Beckhoff
Automation GmbH which supports a wide set of input and output devices
as well as distributed clocks for synchronizing signals down to the
sub-microsecond level. In this paper we present the design of the
required control system software as well as our work on extending the
DKIST CSF to use the OPC Unified Architecture (OPC UA) standard which
provides a cross-platform communication standard for process control
and automation as an interface between the high-level software and
the real-time control system.
Title: A two-dimensional spectropolarimeter as a first-light
instrument for the Daniel K. Inouye Solar Telescope
Authors: Schmidt, Wolfgang; Bell, Alexander; Halbgewachs, Clemens;
Heidecke, Frank; Kentischer, Thomas J.; von der Lühe, Oskar;
Scheiffelen, Thomas; Sigwarth, Michael
Bibcode: 2014SPIE.9147E..0ES
Altcode:
The Visible Tunable Filter (VTF) is a narrowband tunable filter
system for imaging spectropolarimetry. The instrument will be one of
the first-light instruments of the Daniel K. Inouye Solar Telescope
(DKIST) that is currently under construction on Maui (Hawaii). The
DKIST has a clear aperture of 4 meters. The VTF is being developed by
the Kiepenheuer Institut für Sonnenphysik in Freiburg, as a German
contribution to the DKIST. The VTF is designed as a diffraction-limited
narrowband tunable instrument for Stokes spectro-polarimetry
in the wavelength range between 520 and 860 nm. The instrument
uses large-format Fabry-Perot interferometers (Etalons) as tunable
monochromators with clear apertures of about 240 mm. To minimize the
influence of gravity on the interferometer plates, the Fabry-Perots
are placed horizontally. This implies a complex optical design and a
three-dimensional support structure instead of a horizontal optical
bench. The VTF has a field of view of one arc minute squared. With
4096x4096 pixel detectors, one pixel corresponds to an angle of 0.014"
on the sky (10 x 10 km on the Sun). The spectral resolution is 6 pm at a
wavelength of 600 nm. One 2Dspectrum with a polarimetric sensitivity of
5E-3 will be recorded within 13 seconds. The wavelength range of the VTF
includes a number of important spectral lines for the measurement flows
and magnetic fields in the atmosphere of the Sun. The VTF uses three
identical large-format detectors, two for the polarimetric measurements,
and one for broadband filtergrams. The main scientific observables
of the VTF are Stokes polarimetric images to retrieve the magnetic
field configuration of the observed area, Doppler images to measure
the line-of-sight flow in the solar photosphere, and monochromatic
intensity filtergrams to study higher layers of the solar atmosphere.
Title: The 1.5 meter solar telescope GREGOR
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.; Berkefeld, Th.;
Collados, M.; Fischer, A.; Halbgewachs, C.; Heidecke, F.; Hofmann,
A.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Schmidt, D.; Sigwarth, M.; Sobotka, M.; Soltau, D.; Staude, J.;
Strassmeier, K. G.; Waldmann , T. A.
Bibcode: 2012AN....333..796S
Altcode:
The 1.5 m telescope GREGOR opens a new window to the understanding
of solar small-scale magnetism. The first light instrumentation
includes the Gregor Fabry Pérot Interferometer (GFPI), a filter
spectro-polarimeter for the visible wavelength range, the GRating
Infrared Spectro-polarimeter (GRIS) and the Broad-Band Imager (BBI). The
excellent performance of the first two instruments has already been
demonstrated at the Vacuum Tower Telescope. GREGOR is Europe's largest
solar telescope and number 3 in the world. Its all-reflective Gregory
design provides a large wavelength coverage from the near UV up to at
least 5 microns. The field of view has a diameter of 150 arcsec. GREGOR
is equipped with a high-order adaptive optics system, with a subaperture
size of 10 cm, and a deformable mirror with 256 actuators. The science
goals are focused on, but not limited to, solar magnetism. GREGOR
allows us to measure the emergence and disappearance of magnetic flux
at the solar surface at spatial scales well below 100 km. Thanks to its
spectro-polarimetric capabilities, GREGOR will measure the interaction
between the plasma flows, different kinds of waves, and the magnetic
field. This will foster our understanding of the processes that heat the
chromosphere and the outer layers of the solar atmosphere. Observations
of the surface magnetic field at very small spatial scales will shed
light on the variability of the solar brightness.
Title: Image stabilisation system of the photospheric and helioseismic
imager
Authors: Volkmer, R.; Bosch, J.; Feger, B.; Gomez, J. M.; Heidecke,
F.; Schmidt, W.; Scheiffelen, T.; Sigwarth, M.; Soltau, D.
Bibcode: 2012SPIE.8442E..4PV
Altcode:
The Photospheric and Helioseismic imager (PHI) on board of the ESA
mission Solar Orbiter, to be launched in 2017, will provide measurements
with high polarimetric accuracy of the photospheric solar magnetic
field at high solar latitudes. The needed pointing precision requires an
image stabilisation (ISS) to compensate for spacecraft jitter. The image
stabilisation system works as a correlation tracker with a high-speed
camera and a fast steerable mirror. The optomechanical and electronic
design of the system will be presented.
Title: The visible tunable filtergraph for the ATST
Authors: Kentischer, T. J.; Schmidt, W.; von der Lühe, O.; Sigwarth,
M.; Bell, A.; Halbgewachs, C.; Fischer, A.
Bibcode: 2012SPIE.8446E..77K
Altcode:
The Kiepenheuer-Institut will develop for the Advanced Technology Solar
Telescope (ATST) a narrowband tunable filter system (Visible Tunable
Filter, VTF) for imaging spectroscopy and spectropolarimetry based
on large-format Fabry Perot interferometers. A major challenge for
the realization of this instrument is the development of large-format
Fabry-Perots with a free aperture of about 250 mm. The instrument will
operate in the spectral range between 500 and 900 nm with access to
a host of magnetically sensitive lines. The instrument is designed to
match the diffraction limit of the 4m-aperture ATST and will be able to
observe processes on the sun at spatial scales of 35 km. Its multi-line
capability, together with a field of view of one arc minute, and the
ability to measure polarization states of the incoming light allow
to probe different layers of the solar atmosphere within a couple of
seconds. The instrument is capable to vary the spectral sampling, the
integration time, and the temporal cadence over a wide range without
changing or compromising the opto-mechanical setup. This versatility
gives unique possibilities to apply different measurement schemes to a
variety of science questions. The ATST is a fully funded US project,
with the VTF as the only non-US contribution, and is ready to start
construction at the Haleakala summit. The VTF is foreseen as one of
the ATST's firstlight instruments and should become operational in 2018.
Title: The Wave-Front Correction System for the Sunrise Balloon-Borne
Solar Observatory
Authors: Berkefeld, T.; Schmidt, W.; Soltau, D.; Bell, A.;
Doerr, H. P.; Feger, B.; Friedlein, R.; Gerber, K.; Heidecke, F.;
Kentischer, T.; v. d. Lühe, O.; Sigwarth, M.; Wälde, E.; Barthol,
P.; Deutsch, W.; Gandorfer, A.; Germerott, D.; Grauf, B.; Meller, R.;
Álvarez-Herrero, A.; Knölker, M.; Martínez Pillet, V.; Solanki,
S. K.; Title, A. M.
Bibcode: 2011SoPh..268..103B
Altcode: 2010SoPh..tmp..236B; 2010arXiv1009.3196B
This paper describes the wave-front correction system developed for
the Sunrise balloon telescope, and it provides information about its
in-flight performance. For the correction of low-order aberrations,
a Correlating Wave-Front Sensor (CWS) was used. It consisted of a
six-element Shack - Hartmann wave-front sensor (WFS), a fast tip-tilt
mirror for the compensation of image motion, and an active telescope
secondary mirror for focus correction. The CWS delivered a stabilized
image with a precision of 0.04 arcsec (rms), whenever the coarse
pointing was better than ± 45 arcsec peak-to-peak. The automatic
focus adjustment maintained a focus stability of 0.01 waves in the
focal plane of the CWS. During the 5.5 day flight, good image quality
and stability were achieved during 33 hours, containing 45 sequences,
which lasted between 10 and 45 min.
Title: Flight control software for the wave-front sensor of SUNRISE
1m balloon telescope
Authors: Bell, Alexander; Barthol, Peter; Berkefeld, Thomas; Feger,
Bernhard; Gandorfer, Achim M.; Heidecke, Frank; Knoelker, Michael;
Martinez Pillet, Valentin; Schmidt, Wolfgang; Sigwarth, Michael;
Solanki, Sami K.; Soltau, Dirk; Title, Alan M.
Bibcode: 2010SPIE.7740E..03B
Altcode: 2010SPIE.7740E...2B
This paper describes the flight control software of the wave-front
correction system that flew on the 2009 science flight of the Sunrise
balloon telescope. The software discussed here allowed fully automated
operations of the wave-front sensor, communications with the adaptive
optics sub-system, the pointing system, the instrument control unit
and the main telescope controller. The software was developed using
modern object oriented analysis and design techniques, and consists
of roughly 13.000 lines of C++ code not counting code written for the
on-board communication layer. The software operated error free during
the 5.5 day flight.
Title: The control and data concept for the robotic solar telescope
ChroTel
Authors: Halbgewachs, C.; Bethge, Ch.; Caligari, P.; Elmore, D.;
Kentischer, T. J.; Peter, H.; Sigwarth, M.; Schmidt, W.
Bibcode: 2008SPIE.7019E..2TH
Altcode: 2008SPIE.7019E..93H
The solar telescope ChroTel is designed as a robotic telescope so
that no user interaction is necessary for observation. The telescope
will start tracking in the morning as soon as weather conditions are
appropriate and will process a user defined observation routine until
sunset. Weather conditions and system status are continuously monitored
to close the telescope shutter in case of bad weather or to drive to
the stow position in case of an error. The ChroTel control software
was programmed in LabVIEW.
Title: ChroTel: a robotic telescope to observe the chromosphere of
the Sun
Authors: Kentischer, T. J.; Bethge, Ch.; Elmore, D. F.; Friedlein,
R.; Halbgewachs, C.; Knölker, M.; Peter, H.; Schmidt, W.; Sigwarth,
M.; Streander, K.
Bibcode: 2008SPIE.7014E..13K
Altcode: 2008SPIE.7014E..36K
The Chromospheric Telescope (ChroTel) is a 10 cm robotic telescope
to observe the full solar disk with a 2k × 2k CCD at high temporal
cadence. It is located at the Observatorio del Teide, Tenerife, Spain,
next to the 70 cm German Vacuum Tower Telescope (VTT). ChroTel contains
a turret system that relays a stabilized image of the solar disk into
a laboratory within the VTT building. The control design allows a fully
robotic operation. Observations are carried out in three chromospheric
wavelengths (CaK: 393 nm, Ha: 652 nm, HeI 1083 nm).
Title: The Diffraction Limited Spectro-Polarimeter
Authors: Sankarasubramanian, K.; Lites, B.; Gullixson, C.; Elmore,
D.; Hegwer, S.; Streander, K.; Rimmele, T.; Fletcher, S.; Gregory,
S.; Sigwarth, M.
Bibcode: 2006ASPC..358..201S
Altcode:
The Diffraction Limited Spectro-Polarimeter (DLSP) is a
collaboration between the National Solar Observatory (NSO) and the
High Altitude Observatory (HAO) to provide a stable instrument
for precision measurements of solar vector magnetic fields at high
angular resolution. The DLSP is integrated with the new high-order
Adaptive Optics (HOAO) system at the Dunn Solar Telescope (DST)
and provides Stokes spectra of the Fe I 630 nm lines approaching the
0.2 arcs3c diffraction limit of the DST. It is configured as a fixed,
well-calibrated instrument that may be used simultaneously with G-band
(1 nm bandpass) and a Ca K imagers (0.1 nm bandpass). The 2K×2K G-band
imager allows fast frame selection and includes a burst mode for speckle
imaging. The setup of DLSP and its imagers require only about 10 min of
preparation before start of observations. This fixed setup facilitates
standardized data reduction. The DLSP permits observations with 0.09
arcsec sampling in high resolution mode. In wide-field mode, the 0.27
arcsec sampling allows one to map regions about 3 arcmin on a side. The
achieved continuum S/N is 500 (1500) in high resolution (wide-field)
mode for a 4 s integration. It is possible to achieve higher S/N by
integrating longer. Data reduction routines are now available in IDL
for post-observation processing, and parallel analysis routines in
FORTRAN 77 are being developed to allow ``on-the-fly'' data reduction
and inversion.
Title: Auto alignment and image tracking system for the SUNRISE
telescope
Authors: Schmidt, W.; Berkefeld, T.; Feger, B.; Friedlein, R.; Gerber,
K.; Heidecke, F.; Kentischer, T.; Lühe, O. v. d.; Sigwarth, M.;
Soltau, D.; Wälde, E.
Bibcode: 2006SPIE.6274E..0HS
Altcode: 2006SPIE.6274E..14S
The 1m balloon-borne solar telescope Sunrise will be equipped with
a wave-front sensing system for automatic in-flight focusing and
alignment of the telescope and for high-precision image tracking. A
six-element wavefront sensor measures low order aberrations of the
telescope, including defocus and coma. The correction is achieved by
moving the focusing mirror and the telescope secondary, respectively,
in a closed-loop circuit. The same system measures image motion. The
instrument requirements for the tracking are a dynamical range of
about 30 Hz and a precision of about 0.005 arcs in the sky. The image
motion signal feeds a closed-loop control system that drives both the
tip-tilt mirror assembly and the mirrors that are needed for focusing
and alignment. The tip-tilt unit is a dual-stage system, built at the
Kiepenheuer-Insitut, consisting of a slow component with a large range
of about 60 arcs and a fast component with a short range and high
bandwidth. A breadboard-version of the Correlating Wavefront Sensor
has been successfully tested at the German Vacuum Tower Telescope
on Tenerife in summer of 2005. A closed-loop bandwidth of 80 Hz was
measured for the tracking system. The wave-front sensor detected image
aberrations pre-set by the telescope's adaptive optics system with the
required accuracy. Sunrise will be flown in long duration stratospheric
balloon flights, with a first scientific flight in 2009.
Title: First-Light Instrumentation for the Advanced Technology
Solar Telescope
Authors: Rimmele, T.; Balasubramaniam, K.; Berger, T.; Elmore, D.;
Gary, A.; Keller, C.; Kuhn, J.; Lin, H.; Mickey, D.; Pevtsov, A.;
Robinson, B.; Sigwarth, M.; Soccas-Navarro, H.
Bibcode: 2005AGUSMSP34A..03R
Altcode:
The 4m Advanced Technology Solar Telescope (ATST) is the next
generation ground based solar telescope. In this paper we provide
an overview of the ATST post-focus instrumentation. The majority of
ATST instrumentation is located in an instrument Coude lab facility,
where a rotating platform provides image de-rotation. A high order
adaptive optics system delivers a corrected beam to the Coude lab
facility. Alternatively, instruments can be mounted at the Nasmyth
focus. For example, instruments for observing the faint corona
preferably will be mounted at Nasmyth where maximum throughput
is achieved. In addition, the Nasmyth focus has minimum telescope
polarization and minimum stray light. We give an overview of the
initial set of first generation instruments: the Visible-Light
Broadband Imager (VLBI), the Visible Spectro-Polarimeter (ViSP),
the Near-IR Spectro-Polarimeter (NIRSP), which includes a coronal
module, and the Visible Tunable Filter. We also discuss the unique and
efficient approach to the ATST instrumentation, which builds on the use
of common components such as detector systems, polarimetry packages
and various opto-mechanical components. For example, the science
requirement for polarimetric sensitivity (10-5 relative to intensity)
and accuracy (5'10-4 relative to intensity) place strong constraints
on the polarization analysis and calibration units. Consequently,
these systems are provided at the facility level, rather than making
it part of the requirement for each instrument.
Title: High-precision wavefront sensor for the SUNRISE Telescope
Authors: Schmidt, Wolfgang; Berkefeld, Thomas; Friedlein, Ruediger;
Heidecke, Frank; Kentischer, Thomas; von der Lühe, Oskar F.; Sigwarth,
Michael; Soltau, Dirk; Walde, E.
Bibcode: 2004SPIE.5489.1164S
Altcode:
SUNRISE is a 1m solar telescope for the visible and near UV wavelength
range. It will be flown in long duration stratospheric balloon flights
in Antarctica, with a first scientific flight in 2007. In this paper,
we describe the development of a wave-front sensing system that
will be used for the automatic in-flight alignment of the SUNRISE
telescope and for high-precision tracking. The system is based on
the principles of an adaptive optics system. A 19-element wavefront
sensor is used to determine low order aberrations of the telescope,
including defocus and spherical aberrations. The correction is achieved
by controlling the position of the telescope secondary and a focusing
mirror in closed-loop. In addition to these quasi-static aberrations,
the system will also measure image motion with a dynamical range of
at least 30 Hz and with a precision of about 0.005 arcs. To this end,
the image displacement measured in all sub-apertures is averaged and
used as tip-tilt correction signal. This signal will feed a second
closed-loop system that drives the tip-tilt mirror assembly. The
tip-tilt mirror unit is designed as a dual-stage system that consists
of a slow component with a large range of 60 arcs and a fast component
with high bandwidth.
Title: The Diffraction Limited Spectro-Polarimeter: a new instrument
for high-resolution solar polarimetry
Authors: Sankarasubramanian, K.; Gullixson, Craig; Hegwer, Stephen;
Rimmele, Thomas R.; Gregory, Scott; Spence, Tony; Fletcher, Stephen;
Richards, Kit; Rousset, Emilie; Lites, Bruce; Elmore, David; Streander,
Kim; Sigwarth, Michael
Bibcode: 2004SPIE.5171..207S
Altcode:
The National Solar Observatory in collaboration with the High-Altitude
Observatory is developing a new solar polarimeter, the Diffraction
Limited Spectro-Polarimeter. In conjunction with a new high-order
adaptive optics system at the NSO Dunn Solar Telescope, the DLSP
design facilitates very high angular resolution observations of
solar vector magnetic fields. This project is being carried out in two
phases. As a follow-on to the successful completion of the first phase,
the ongoing DLSP Phase II implements a high QE CCD camera system,
a ferro-electric liquid crystal modulator, and a new opto-mechanical
system for polarization calibration. This paper documents in detail the
development of the modulator system and its performance, and presents
preliminary results from an engineering run carried out in combination
with the new NSO high-order AO system.
Title: Diffraction limited spectro-polarimeter - Phase I
Authors: Sankarasubramanian, Kasiviswanathan; Elmore, David F.; Lites,
Bruce W.; Sigwarth, Michael; Rimmele, Thomas R.; Hegwer, Steven L.;
Gregory, Scott; Streander, Kim V.; Wilkins, Lawrence M.; Richards,
K.; Berst, C.
Bibcode: 2003SPIE.4843..414S
Altcode:
A diffraction limited spectro-polarimeter is under construction at the
National Solar Observatory in collaboration with the High Altitude
Observatory. The scientific objective of the project is to measure
the magnetic fields on the Sun up to the diffraction limit of the Dunn
Solar Telescope. The same instrument would also measure the magnetic
field of large sunspots or sunspot groups with reasonable spatial
resolution. This requires a flexible image scale which cannot be
obtained with the current Advanced Stokes Polarimeter (ASP) without
loosing 50% of the light. The new spectro-polarimeter is designed
in such a way that the image scale can be changed without loosing
much light. It can work either in high-spatial resolution mode (0.09
arcsec per pixel) with a small field of view (FOV: 65 arcsec) or in
large FOV mode (163 arcsec) with low-spatial resolution (0.25 arcsec
per pixel). The phase-I of this project is to design and build the
spectrograph with flexible image scale. Using the existing modulation,
calibration optics of the ASP and the ASP control and data acquisition
system with ASP-CHILL camera, the spectrograph was tested for its
performance. This paper will concentrate on the performance of the
spectrograph and will discuss some preliminary results obtained with
the test runs.
Title: Multiple Etalon Systems for the Advanced Technology Solar
Telescope
Authors: Gary, G. A.; Balasubramaniam, K. S.; Sigwarth, Michael
Bibcode: 2003SPIE.4853..252G
Altcode:
Multiple etalon systems are discussed that meet the science requirements
for a narrow-passband imaging system for the 4-meter National Solar
Observatory (NSO)/Advance Technology Solar Telescope (ATST). A
multiple etalon system can provide an imaging interferometer that
works in four distinct modes: as a spectro-polarimeter, a filter-vector
magnetograph, an intermediate-band imager, and broadband high-resolution
imager. Specific dual and triple etalon configurations are described
that provide a spectrographic passband of 2.0-3.5 pm and reduce
parasitic light levels to 10-4 as required for precise
polarization measurement, e.g., Zeeman measurements of magnetic
sensitive lines. A TESOS-like (Telecentric Etalon SOlar Spectrometer)
triple etalon system provides a spectral purity of 10-5. The
triple designs have the advantage of reducing the finesse requirement
on each etalon; allow the use of more stable blocking filters, and have
very high spectral purity. A dual-etalon double-pass (Cavallini-like)
system can provide a competing configuration. Such a dual-etalon design
can provide high contrast. The selection of the final focal plane
instrument will depend on a trade-off between an ideal instrument and
practical reality. The trade study will include the number of etalons,
their aperture sizes, complexities of the optical train, number of
blocking filters, configuration of the electronic control system,
computer interfaces, temperature controllers, etalon controllers,
and their associated feedback electronics. The heritage of single and
multiple etalon systems comes from their use in several observatories,
including the Marshall Space Flight Center (MSFC) Solar Observatory,
Sacramento Peak Observatory (NSO), and Kiepenheuer-Institut für
Sonnenphysik (KIS, Germany), Mees Solar Observatory (University of
Hawaii), and Arcetri Astrophysical Observatory (Italy). The design
of the ATST multiple etalon system will benefit from the experience
gained at these observatories.
Title: First Results from the HAO/NSO Diffraction-Limited
Spectro-Polarimeter
Authors: Lites, B. W.; Elmore, D. F.; Streander, K. V.;
Sankarasubramanian, K.; Rimmele, T. R.; Sigwarth, M.
Bibcode: 2003ASPC..307..324L
Altcode:
No abstract at ADS
Title: Temporal and spatial variations of the magnetic field vector
in sunspots
Authors: Settele, A.; Sigwarth, M.; Muglach, K.
Bibcode: 2002A&A...392.1095S
Altcode:
In order to look for magnetic field vector oscillations in sunspots we
used data measured with the Advanced Stokes Polarimeter at the Dunn
Solar Telescope. We analyzed two time series of 65 and 110 min that
were taken by scanning repeatedly a 6" 24 x 75" field of view, while
obtaining the full Stokes vector in the lines FeI 630.15 nm and 630.25
nm. An inversion was carried out and a Fast Fourier Transform (FFT)
analysis was used to find oscillatory phenomena. We discuss possible
unwanted effects that lead to apparent magnetic field oscillations and
find an average amplitude of (B, psi ) 5.8 G/0.23o rms by
excluding these effects, which also means that only 6% and 22% of the
two sunspot umbrae respectively remained for analysis. If we smooth
the power spectra over 2x2 pixels, all significant power disappears.
Title: Properties and Origin of Asymmetric and Unusual Stokes V
Profiles Observed in Solar Magnetic Fields
Authors: Sigwarth, M.
Bibcode: 2001ApJ...563.1031S
Altcode:
Between 5% and 10% of Stokes V profiles observed outside of sunspots
with the High Altitude Observatory/National Solar Observatory
Advanced Stokes Polarimeter are of unusual shape. We categorize them as
follows. Normal profiles have two wings of opposite sign that typically
show an asymmetry between both lobes. Single-lobed profiles have
an amplitude above the noise level only for one wing. Single-lobed
profiles occur because of a strong asymmetry between the red and
blue wings in combination with low signal amplitude and the limited
sensitivity of the measurement. The strongest verifiable amplitude
asymmetry found is 90%. Mixed-polarity profiles are the most common
type of unusual profiles. These profiles can occur because of the
unresolved mixture of polarities in the solar atmosphere on spatial
scales <=600 km or because of the spreading of V signals due to
seeing and diffraction. Single-lobed profiles and mixed polarities
are mainly found outside high magnetic flux areas. Dynamic profiles
consist of two or more superposed components with large Doppler shifts
relative to each other. They are mainly observed in an emerging
flux region. The measured relative Doppler shifts range up to 18
km s-1, indicating upward and downward moving shocks in
magnetic fields. We propose two possible configurations for network
and plage fields: (1) a mixture of intense kilogauss flux tubes and a
more turbulent subkilogauss field and (2) return flux in the form of
weak fields surrounding isolated flux tubes and at the edges of flux
tube clusters. Both configurations are supported by the analysis of V
profiles in the vicinity of an isolated kilogauss flux tube. We find
that the use of low-order adaptive optics reduces the uncertainties
in interpreting Stokes data.
Title: Small Scale Dynamics in an Emerging Flux Region
Authors: Sigwarth, M.; Balasubramaniam, K.
Bibcode: 2001AGUSM..SP41B05S
Altcode:
Unusual Stokes spectra measured with the HAO/NSO Advanced Stokes
Polarimeter ASP in a young active region with ongoing flux emergence
indicate upward and downward moving shock fronts in the photosphere. The
data cover the evolution of the region over 1.25 hours. The found flows
are mainly located in magnetic fields only and occur on very small
spatial scales. We present results from the analysis of individual
Stokes profiles and from the inversion of the full Stokes vector in
order to investigate the magnetic field topology and the dynamics that
leads to the observed profiles. First results indicate that rising
flux tubes with the onset of shocked down drafts can explain some of
the observed spectra.
Title: Structure and Dynamics of a Sunspot Penumbra using Imaging
Spectroscopy
Authors: Balasubramaniam, K. S.; Sigwarth, M.
Bibcode: 2001AGUSM..SP41C07B
Altcode:
We present high angular resolution measurements and analysis of a
sunspot penumbra using imaging spectroscopy with the NSO Dual-FP system
(DPF), simultaneous Hα and G-Band measurements. These measurements
were made using the NSO Low-order Adaptive Optics system at the Richard
B. Dunn Solar Telescope. The DFP was used to acquire the spectral images
using the FeI 5576 Å Zeeman insensitive spectral line to map the upper
photospheric layers. The dynamic signatures of the penumbra are analyzed
using the Doppler maps of the sunspot. We present FLOW-LESS maps of the
sunspot to examine the thermal structure of the sunspot. Using spectral
line asymmetries, we probe the dynamical structure of the penumbrae
at different heights. We also examine the structural properties of
the penumbra using high-resolution high-speed images in Hα , and the
G-band. We discuss the impacts of these measurements on the current
models of sunspot penumbrae.
Title: Advanced Solar Polarimetry-Theory, Observation, and
Instrumentation: The 20th NSO/Sacramento Peak Summer Workshop
Authors: Sigwarth, Michael
Bibcode: 2001PASP..113..260S
Altcode:
Conference was held in Sunspot, New Mexico, in 2000
September. Proceedings will be edited by Michael Sigwarth and published
in the ASP Conference Series.
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
Title: Advanced Solar Polarimetry -- Theory, Observation, and
Instrumentation -- 20TH NSO/Sac Summer Workshop
Authors: Sigwarth, Michael
Bibcode: 2001ASPC..236.....S
Altcode: 2001aspt.conf.....S
No abstract at ADS
Title: A new Stokes Polarimeter for the Dunn Solar Telescope
Authors: Sigwarth, M.; Berst, C.; Gregory, S.; Hegwer, S.; Richards,
K.; Rimmele, T.; Wilkins, L.; Lites, B. W.; Elmore, D. F.; Streander,
K. V.
Bibcode: 2001ASPC..236...57S
Altcode: 2001aspt.conf...57S
No abstract at ADS
Title: High resolution spectroscopy of active regions with adaptive
optic
Authors: Sigwarth, M.; Rimmele, T. R.
Bibcode: 2000SPD....31.0304S
Altcode: 2000BAAS...32R.834S
With the NSO low-order adaptive optic system at the NSO Dunn Solar
Telescope it is now possible to perform spectroscopic measurements
at high angular resolution and high signal-to-noise level. The
performance of the AO system for spectroscopic investigations will be
demonstrated. By using a dual Fabry-Perot spectrometer we obtained
spectral line scans of active regions with a spatial resolution
of 0.3 arcsec at 557nm. We use these data to adress questions of
magnetoconvection in active regions. This work is supported by NSO/AURA
Title: Strong Stokes V asymmetries of photospheric spectral lines:
What can they tell us about the magnetic field structure?
Authors: Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M.;
Steiner, O.
Bibcode: 2000A&A...357..351G
Altcode:
In an attempt to identify the mechanism responsible for the extremely
asymmetric Stokes V profiles which were recently observed we analyzed
several simple atmospheric configurations with separated layers of mass
flow and magnetic field. We found that under appropriate conditions
the models are capable of producing the observed one-lobe profiles.
Title: Dynamics of Solar Magnetic Fields -- A Spectroscopic
Investigation
Authors: Sigwarth, Michael
Bibcode: 2000RvMA...13...45S
Altcode:
No abstract at ADS
Title: Dynamics of solar magnetic elements
Authors: Sigwarth, M.; Balasubramaniam, K. S.; Knölker, M.;
Schmidt, W.
Bibcode: 1999A&A...349..941S
Altcode:
We present observational results that demonstrate a strong increase in
the dynamic behavior of magnetic elements at the solar photosphere,
when observed at high spatial and temporal resolution. The HAO/NSO
Advanced Stokes Polarimeter was used to obtain an extensive set of
high resolution Stokes-V spectra from network, intranetwork and active
region magnetic fields at a low noise level. We performed a statistical
analysis of Doppler shifts and asymmetries of the V spectra of FeI
630.15 and 630.25 nm to obtain information on the dynamics of magnetic
elements of different sizes. The spatial resolution of 0.8-1 arcsec in
combination with high polarimetric precision allowed us to investigate
Stokes-V spectra of magnetic elements down to a size of ~ 150 km. The
Doppler velocity within magnetic elements as well as the amplitude and
area asymmetries of the Stokes-V profiles show a strong dependence on
the size of the magnetic elements as well as on the granular velocity
in their vicinity. Applying an absolute velocity calibration we find
that the smallest magnetic features have velocities of up to 5 km s(-1)
in both up- and downflows whereas for larger elements or clusters of
several flux tubes the velocities become smaller and more uniform. The
V-profile asymmetries are larger (both positive and negative) for
small fill fraction than for higher fill fraction within the resolution
element. Averaged over all individual profiles, there remains a positive
amplitude and area asymmetry and a downflow exceeding 0.5 km s(-1)
. The properties of spatially and temporally integrated V profiles are
consistent with results from FTS-observations. Our observations are
in qualitative agreement with results from numerical MHD simulations.
Title: Dynamics of the solar granulation. VI. Time variation of the
granular shear flow
Authors: Nesis, A.; Hammer, R.; Kiefer, M.; Schleicher, H.; Sigwarth,
M.; Staiger, J.
Bibcode: 1999A&A...345..265N
Altcode:
Excellent spectrograms can yield observational insight in the dynamics
of the solar surface not yet accessible to numerical simulations. We
present results of the elaboration of a series of spectrograms taken at
the center of the solar disk. Each of the spectrograms includes more
than 250 granules, while the series covers a time of 12 min. Our main
emphasis is to study the dynamics of the visible solar layers not only
as a function of height but also as a function of time. We investigated
the temporal and spatial behavior of the turbulent concentration at the
granular borders and its spreading-out into the intergranular space. In
the deep photosphere, enhanced turbulence is concentrated predominantly
near granular borders, while at higher layers the turbulence spreads
out over the entire intergranular space. Remarkable is the decay of the
turbulence with the height in the photosphere. There was no significant
variation of the turbulence over the 12 min. We also determined the rms
turbulent pressure at the granulation layers near tau_ {5000}=1. The
average ratio of turbulent to gas pressure is of the order of 0.1;
values of this size are also discussed in recent theoretical works. In
order to take the intermittency into account, we traced the peak to
peak variations of the turbulent velocity, which turn out to be ~
4 km sec(-1) . The corresponding ratio of turbulent to gas pressure
may thus reach locally significant values up to about 0.3. We did not
find either a correlation or an anticorrelation between turbulence
and convective flow, although the turbulence is presumably generated
by granular shear flow. We suggest that the intermittent turbulence
in the visible layers and the convective flow constitutes a dynamical
system. This turbulence-granulation-dynamical system exhibits a cyclic
behavior corresponding to the dynamical time of the granules, i.e. the
growth and decay of their velocity profile. The power spectra of the
turbulent and granular velocity show a two-component character, which
presumably reflects the action of two different processes determining
the dynamics of the solar convective boundary layers and above.
Title: High Resolution Observations of the Dynamics of Magnetic
Elements
Authors: Sigwarth, M.; Balasubramaniam, K.; Knölker, M.
Bibcode: 1999ASPC..183...36S
Altcode: 1999hrsp.conf...36S
No abstract at ADS
Title: The formation of extremely asymmetric Stokes V profiles
Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Sigwarth,
M.
Bibcode: 1999AGAb...15R..10S
Altcode: 1999AGM....15..A11S
Recent polarimetric observations at high spatial resolution (<
1 arcsec) and with high polarimetric accuracy (noise of Stokes V/I_c
< 3 cdot 10^{-4}) have revealed that about 10% of all Stokes V
profiles of a quiet Sun region have an abnormal shape that strongly
deviates from the more common, nearly antisymmetric profiles. 35% of the
abnormal Stokes V profiles are of one-wing type, where the profile shows
essentially one lobe only. We show, that a strongly asymmetric Stokes
V profile can be obtained when the atmosphere, in which the profile
is formed, is divided into two or more layers of different magnetic,
flow, and thermal properties. A simple configuration of that kind
(sometimes referred to as magnetic canopy) consists of a layer of
plasma at rest with a magnetic field component parallel to the line
of sight, located on top of a field-free layer with downdraft. The
Stokes V asymmetry in this case sensitively depends on the position
of the layers interface and may assume values up to about 80%. An
asymmetry of nearly 100% (corresponding to a one-wing profile) is
obtained when the plasma below the magnetic canopy is relatively cool,
giving rise to the formation of a strong, redshifted spectral line in
the field-free plasma beneath the canopy. If the core of this line is
formed at lower temperature than the temperature of the canopy layer,
the red wing of the resulting Stokes V profile is driven into emission,
leading to a ``pathological'' V profile.
Title: TESOS, a double Fabry-Perot instrument for solar spectroscopy
Authors: Kentischer, T. J.; Schmidt, W.; Sigwarth, M.; Uexkuell, M. V.
Bibcode: 1998A&A...340..569K
Altcode:
A double Fabry-Perot spectrometer in a telecentric configuration
(TESOS: Telecentric Etalon SOlar Spectrometer) has been developed and
installed in the Vacuum Tower Telescope at the Observatorio del Teide,
Tenerife. The control system allows to switch from one wavelength band
to another within 2 seconds. The telecentric configuration provides a
constant wavelength passband across the field of view. Servo-stabilized
etalons and a stable thermal environment provide a wavelength stability
of <0.3 picometer per hour. We present initial results obtained
with this instrument.
Title: Upwelling in a young sunspot
Authors: Sigwarth, M.; Schmidt, W.; Schuessler, M.
Bibcode: 1998A&A...339L..53S
Altcode:
An upflow of with a velocity exceeding 0.5 km*s(-1) is found in the
umbra of the preceding spot in a young active region with ongoing
flux emergence. A weak downflow is indicated in the corresponding
spot of follower polarity. Such a flow pattern is consistent with the
counter-rotation flow along a rising magnetic flux loop driven by the
Coriolis force as predicted by numerical simulations.