Author name code: kneer
ADS astronomy entries on 2022-09-14
author:"Kneer, Franz J."
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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: 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: 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: 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: On the structure and dynamics of Ellerman bombs. Detailed
study of three events and modelling of Hα
Authors: Bello González, N.; Danilovic, S.; Kneer, F.
Bibcode: 2013A&A...557A.102B
Altcode:
Aims: We study the structure and dynamics of three Ellerman
bombs (EBs) observed in an evolving active region.
Methods: The
active region NOAA 11271 was observed with the Vacuum Tower Telescope
at Observatorio del Teide/Tenerife on August 18, 2011. We used the
two-dimensional Triple Etalon SOlar Spectrometer (TESOS) to obtain time
sequences of the active region and of EBs in Hα at a cadence of 15
s. Simultaneously, we obtained full Stokes profiles with the Tenerife
Infrared Polarimeter (TIP II) in the two magnetically sensitive Fe i
infrared lines (IR) at 1.56 μ, scanning spatial sections of the area
with cadences of 28-46 s. The Hα data were reconstructed with speckle
methods to study the evolution of the atmospheric stratification. Two
methods were used to extract magnetic field information from the IR
Stokes profiles: 1) fitting of the (Q,U,V) profiles by Gaussians; and
2) applying the Milne-Eddington approximation, assuming two separate
magnetic structures in the resolution element and fitting by trial and
error some profiles from the EB areas. Data from SDO-HMI and -AIA were
also used. We performed two-dimensional (2D) non-LTE radiative transfer
calculations of Hα in parameterised models of EBs.
Results:
The three EBs studied in detail occurred in a complex active region near
sunspots. They were very bright with a factor of 1.5-2.8 brighter than
the nearby area. They lived for 1/2 h and longer. They were related to
broadband faculae, but the latter were not the brightest features in the
field of view. The EBs occurred in magnetic field configurations with
opposite polarity close together. One EB was located at the outskirts
of a penumbra of a complex sunspot and showed repeated "flaring" in
SDO-AIA data. Another was close to a strong field patch and moved into
this during the end of its lifetime. The third EB showed clear changes
of field structure during the time it was observed. We obtained from
the 2D modelling that heating and increase in Hα opacity are likely
to occur at heights of 300-800 km. Line shifts and asymmetries can
well be reproduced by velocities at these heights and also at much
larger heights.
Conclusions: The three EBs occurred at sites
with magnetic fields of opposite polarity, which were likely the cause
of the Hα brightening upon reconnection.
Title: The GREGOR Solar Telescope on Tenerife
Authors: Schmidt, W.; von der Lühe, O.; Volkmer, R.; Denker, C.;
Solanki, S. K.; Balthasar, H.; Bello González, N.; Berkefeld, T.;
Collados Vera, M.; Hofmann, A.; Kneer, F.; Lagg, A.; Puschmann, K. G.;
Schmidt, D.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.
Bibcode: 2012ASPC..463..365S
Altcode: 2012arXiv1202.4289S
2011 was a successful year for the GREGOR project. The telescope was
finally completed in May with the installation of the 1.5-meter primary
mirror. The installation of the first-light focal plane instruments was
completed by the end of the year. At the same time, the preparations
for the installation of the high-order adaptive optics were finished,
its integration to the telescope is scheduled for early 2012. This
paper describes the telescope and its instrumentation in their present
first-light configuration, and provides a brief overview of the science
goals of GREGOR.
Title: The GREGOR Solar Telescope
Authors: Denker, C.; Lagg, A.; Puschmann, K. G.; Schmidt, D.; Schmidt,
W.; Sobotka, M.; Soltau, D.; Strassmeier, K. G.; Volkmer, R.; von
der Luehe, O.; Solanki, S. K.; Balthasar, H.; Bello Gonzalez, N.;
Berkefeld, T.; Collados Vera, M.; Hofmann, A.; Kneer, F.
Bibcode: 2012IAUSS...6E.203D
Altcode:
The 1.5-meter GREGOR solar telescope is a new facility for
high-resolution observations of the Sun. The telescope is located at the
Spanish Observatorio del Teide on Tenerife. The telescope incorporates
advanced designs for a foldable-tent dome, an open steel-truss telescope
structure, and active and passive means to minimize telescope and mirror
seeing. Solar fine structure can be observed with a dedicated suite
of instruments: a broad-band imaging system, the "GREGOR Fabry-Perot
Interferometer", and the "Grating Infrared Spectrograph". All post-focus
instruments benefit from a high-order (multi-conjugate) adaptive optics
system, which enables observations close to the diffraction limit of
the telescope. The inclusion of a spectrograph for stellar activity
studies and the search for solar twins expands the scientific usage
of the GREGOR to the nighttime domain. We report on the successful
commissioning of the telescope until the end of 2011 and the first
steps towards science verification in 2012.
Title: A retrospective of the GREGOR solar telescope in scientific
literature
Authors: Denker, C.; von der Lühe, O.; Feller, A.; Arlt, K.;
Balthasar, H.; Bauer, S. -M.; Bello González, N.; Berkefeld, Th.;
Caligari, P.; Collados, M.; Fischer, A.; Granzer, T.; Hahn, T.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Kentischer, T.; Klva{ňa,
M.; Kneer, F.; Lagg, A.; Nicklas, H.; Popow, E.; Puschmann, K. G.;
Rendtel, J.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Solanki, S. K.;
Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.; Waldmann,
T.; Wiehr, E.; Wittmann, A. D.; Woche, M.
Bibcode: 2012AN....333..810D
Altcode: 2012arXiv1210.3167D
In this review, we look back upon the literature, which had the
GREGOR solar telescope project as its subject including science cases,
telescope subsystems, and post-focus instruments. The articles date
back to the year 2000, when the initial concepts for a new solar
telescope on Tenerife were first presented at scientific meetings. This
comprehensive bibliography contains literature until the year 2012,
i.e., the final stages of commissioning and science verification. Taking
stock of the various publications in peer-reviewed journals and
conference proceedings also provides the ``historical'' context
for the reference articles in this special issue of Astronomische
Nachrichten/Astronomical Notes.
Title: Hopes and expectations with GREGOR
Authors: Kneer, F.
Bibcode: 2012AN....333..790K
Altcode:
This contribution to the series of GREGOR inauguration articles
addresses the history of the GREGOR telescope. It was obvious since a
long time that the study of the atmospheric dynamics on the Sun needs
telescopes with a large aperture. So the first plans to replace the
40 years old Gregory-Coudé Telescope, with its 45 cm primary mirror,
by a large, 1.5-meter telescope date back to 1997. After a positive
review of the project by the Deutsche Forschungsgemeinschaft in 2000,
the large financial support started in 2000. Unfortunately, the new
technology of the Cesic mirrors was not yet ripe to produce the large
primary mirror with this light-weight material. So, the project was
much delayed. After recollecting for the reader several dates, I also
go through some properties of GREGOR. I recall the aims of the project
and discuss difficulties and ways to realise the intentions.
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: The GREGOR Fabry-Pérot Interferometer
Authors: Puschmann, K. G.; Denker, C.; Kneer, F.; Al Erdogan, N.;
Balthasar, H.; Bauer, S. M.; Beck, C.; Bello González, N.; Collados,
M.; Hahn, T.; Hirzberger, J.; Hofmann, A.; Louis, R. E.; Nicklas, H.;
Okunev, O.; Martínez Pillet, V.; Popow, E.; Seelemann, T.; Volkmer,
R.; Wittmann, A. D.; Woche, M.
Bibcode: 2012AN....333..880P
Altcode: 2012arXiv1210.2921P
The GREGOR Fabry-Pérot Interferometer (GFPI) is one of three
first-light instruments of the German 1.5-meter GREGOR solar telescope
at the Observatorio del Teide, Tenerife, Spain. The GFPI uses two
tunable etalons in collimated mounting. Thanks to its large-format,
high-cadence CCD detectors with sophisticated computer hard- and
software it is capable of scanning spectral lines with a cadence
that is sufficient to capture the dynamic evolution of the solar
atmosphere. The field-of-view (FOV) of 50 arcsec × 38 arcsec is well
suited for quiet Sun and sunspot observations. However, in the vector
spectropolarimetric mode the FOV reduces to 25 arcsec × 38 arcsec. The
spectral coverage in the spectroscopic mode extends from 530-860 nm
with a theoretical spectral resolution of R ≈ 250,000, whereas in
the vector spectropolarimetric mode the wavelength range is at present
limited to 580-660 nm. The combination of fast narrow-band imaging and
post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales
down to ∼50 km on the solar surface.
Title: Shear and vortex motions in a forming sunspot . Twist
relaxation in magnetic flux ropes
Authors: Bello González, N.; Kneer, F.; Schlichenmaier, R.
Bibcode: 2012A&A...538A..62B
Altcode:
Aims: We measure proper motions of fine structures in a forming
sunspot to infer information about the dynamics of flux emergence at
the sub-photospheric level.
Methods: The active region NOAA
11024 was observed with the Vacuum Tower Telescope at Observatorio del
Teide/Tenerife over several days in July 2009. Here, we concentrate
on a two-hour sequence taken on July 4, when the leading spot was
at an early stage of its evolution. Speckle reconstructions from Ca
ii K images and polarimetric data in Fe i λ6173 allow us to study
proper motions of umbral fine structures.
Results: We detect
three prominent features: (1) A light bridge, divided by a dark lane
along its axis, shows proper motions in opposing directions on its
sides, with velocities of ~100-500 m s-1. The flows are
seen in both the Ca ii K and the broadband time sequences. (2) Umbral
dots in one umbral region outline a vortex with speeds of up to 550
m s-1. The direction of the motion of the umbral dots is
different from that in the light bridge. (3) At one rim of the umbra,
the fine structure of the magnetic field moves horizontally with typical
velocities of 250-300 m s-1, prior to the formation of the
penumbra.
Conclusions: We report on shear and vortex motions in
a forming sunspot and interpret them as tracers of twist relaxation
in magnetic flux ropes. We suggest that the forming sunspot contains
detached magnetic flux ropes that emerge at the surface with different
amounts of twist. As they merge to form a sunspot, they untwist giving
rise to the observed shear and vortex motions.
Title: On acoustic and gravity waves in the solar photosphere and
their energy transport
Authors: Kneer, F.; Bello González, N.
Bibcode: 2011A&A...532A.111K
Altcode:
Aims: We study acoustic and atmospheric gravity waves in the
quiet Sun to estimate their energy transport to the chromosphere.
Methods: A two-dimensional time sequence from quiet Sun disc centre
was analysed with simultaneous spectroscopic observations in Fe i 5576
Å and Fe i 5434 Å (both with Landé factor g = 0). We calculated
response functions of the velocities for the line minimum shifts
and atmospheric transmissions of waves for the two lines. For this,
NLTE line formation in granular and intergranular model atmospheres
from numerical simulations were performed. For the interpretation
of the observed waves and for the estimates of energy fluxes, we
assumed adiabatic propagation of plane waves in an isothermal model
atmosphere. Fourier analyses of intensity and velocity fluctuations
were carried out. They yield power, phase, and coherence as functions of
frequency ν (from temporal Fourier transforms) and in the kh
- ν plane (from three-dimensional transforms). The power spectra,
together with the mass densities at velocity formation heights, give
then the energy fluxes.
Results: The rms velocities found here
in the acoustic and gravity wave domains are lower by a factor ~1.5
as in earlier work. We therefore admit a factor of 2 for an upward
correction of the estimated fluxes. For acoustic waves we find: 1)
upward propagating waves are present on the Sun with frequencies up to
14-15 mHz (periods U ≈ 70 s); 2) the approximation of plane adiabatic
waves in an isothermal atmosphere appears adequate for estimating the
energy fluxes; 3) the acoustic energy fluxes are in the same range
as found in our earlier work from ground-based, two-dimensional
spectroscopy, 1500-3100 W m-2 at an atmospheric height
of ~380 km and 1300-2700 W m-2 at 570 km. The energy flux
carried by gravity waves is difficult to determine. We find: 1) phase
and coherence spectra between continuum and velocity fluctuations
show that convective overshoot and gravity waves are superimposed. We
account for the convective flows using these coherence spectra. 2) At
low frequencies, the vertical wavelength Λz can be short
(≪300 km), yielding large corrections for atmospheric transmissions
(factors > 100). We thus exclude from the flux estimates waves
with |kz| > 20 Mm-1 and with vertical group
velocities υgr,z < 0.3 km s-1. They are
likely to be strongly reduced in amplitude by radiative damping. 3)
With these caveats, the energy fluxes carried by gravity waves are found
in the range of 4000 - 8200 W m-2 at 380 km and 700-1400
W m-2 at 570 km. Gravity waves thus also contribute to the
energy transport into the chromosphere.
Title: Acoustic waves in the solar atmosphere at high spatial
resolution. II. Measurement in the Fe I 5434 Å line
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev,
O.; Shchukina, N.
Bibcode: 2010A&A...522A..31B
Altcode:
Aims: We investigate the energy supply of the solar chromosphere
by acoustic waves.
Methods: A time sequence with high spatial
and temporal resolution from the quiet Sun disc centre in Fe i
5434 Å (Landé factor g = 0) is analysed. We used models from a
numerical simulation of granular convection and apply NLTE spectral
line transfer to determine the height of formation. For estimates of
acoustic energy flux, we adopted wave propagation with inclinations
of the wave vector with respect to the vertical of 0°, 30°, and
45°. For a granular and an intergranular model, the transmissions of
the atmosphere to high-frequency waves were determined for the three
inclination angles. Wavelet and Fourier analyses were performed
and the resulting power spectra were corrected for atmospheric
transmission.
Results: We find waves with periods down to ~40
s. They occur intermittently in space and time. The velocity signal
is formed at a height of 500 km in the granular model and at 620 km
in the intergranule. At periods shorter than the acoustic cutoff
(~190 s), ~40% of the waves occur above granules and ~60% above
intergranules. By adopting vertical propagation, we estimate total
fluxes above granules of 2750-3360 W m-2, and of 910-1 000
W m-2 above intergranules. The weighted average is 1730-2
060 W m-2. The estimates of the total fluxes increase by 15%
when inclined wave propagation of 45° is assumed.
Title: GREGOR telescope: start of commissioning
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvana, M.; Kneer, F.; Lagg, A.; Popow,
E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier, K.
Bibcode: 2010SPIE.7733E..0KV
Altcode: 2010SPIE.7733E..18V
With the integration of a 1-meter Cesic primary mirror the GREGOR
telescope pre-commissioning started. This is the first time, that
the entire light path has seen sunlight. The pre-commissioning period
includes testing of the main optics, adaptive optics, cooling system,
and pointing system. This time was also used to install a near-infrared
grating spectro-polarimeter and a 2D-spectropolarimeter for the visible
range as first-light science instruments. As soon as the final 1.5
meter primary mirror is installed, commissioning will be completed,
and an extended phase of science verification will follow. In the near
future, GREGOR will be equipped with a multi-conjugate adaptive optics
system that is presently under development at KIS.
Title: GREGOR solar telescope: Design and status
Authors: Volkmer, R.; von der Lühe, O.; Denker, C.; Solanki, S. K.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Fischer, A.;
Halbgewachs, C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Kneer, F.;
Lagg, A.; Popow, E.; Schmidt, D.; Schmidt, W.; Sobotka, M.; Soltau,
D.; Strassmeier, K. G.
Bibcode: 2010AN....331..624V
Altcode:
The integration and verification phase of the GREGOR telescope
reached an important milestone with the installation of the
interim 1 m SolarLite primary mirror. This was the first time
that the entire light path had seen sunlight. Since then extensive
testing of the telescope and its subsystems has been carried out. The
integration and verification phase will culminate with the delivery and
installation of the final 1.5 m Zerodur primary mirror in the summer of
2010. Observatory level tests and science verification will commence in
the second half of 2010 and in 2011. This phase includes testing of the
main optics, adaptive optics, cooling and pointing systems. In addition,
assuming the viewpoint of a typical user, various observational modes
of the GREGOR Fabry-Pérot Interferometer (GFPI), the Grating Infrared
Spectrograph (GRIS), and high-speed camera systems will be tested to
evaluate if they match the expectations and science requirements. This
ensures that GREGOR will provide high-quality observations with its
combination of (multi-conjugate) adaptive optics and advanced post-focus
instruments. Routine observations are expected for 2012.
Title: On the energy flux in acoustic waves in the solar atmosphere .
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev, O.
Bibcode: 2010MmSAI..81..757B
Altcode:
The energy supply for the radiative losses of the quiet solar
chromosphere is studied. Time sequences from quiet Sun disc centre
were obtained with the ``Göttingen'' Fabry-Pérot spectrometer at
the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the
non-magnetic Fe I 5576 Å line. The data were reconstructed with speckle
methods. The velocities as measured at the line minimum were subjected
to Fourier and wavelet analysis. The energy fluxes were corrected for
the transmission of the solar atmosphere. We find an energy flux of
∼ 3 000 W m-2 at a height of h=250 km. Approximately 2/3
of it is carried by waves in the 5-10 mHz range, and 1/3 in the 10-20
mHz band. The waves occur predominantly above inter-granular areas. We
speculate that the acoustic flux in waves with periods shorter than the
acoustic cutoff period (U≈190 s) can contribute to the basal heating
of the solar chromosphere, in addition to atmospheric gravity waves.
Title: Chromosphere of active regions on the Sun .
Authors: Kneer, F.
Bibcode: 2010MmSAI..81..604K
Altcode:
The first part of this contribution deals with the chromosphere
of sunspots. The models of umbrae give strongly self-reversed Ca
H and K line cores. Much `macro-turbulent' broadening is needed to
fit the modeled line profiles to the observations. Thus, observed
dynamics in sunspot chromospheres are discussed. In the second part,
the chromsphere of plages is treated. We start with the relation
of K emission vs. average magnetic field strength and continue with
chromospheric models. Since chromospheres are most easily observed in
Halpha , various simplifications for the interpretation of this line,
including the cloud model, are discussed and results are given. Dynamic
phenomena in plages, such as moustaches and magneto-atmospheric waves
along fibrils, will be shown. Ideas about heating mechanisms of active
chromspheres will be mentioned.
Title: Faculae at the poles of the Sun revisited: infrared
observations
Authors: Blanco Rodríguez, J.; Kneer, F.
Bibcode: 2010A&A...509A..92B
Altcode:
Aims: This study extends earlier investigations on faculae and
their small-scale magnetic fields near the solar poles (polar faculae
- PFe) to measurements of the magnetically sensitive infrared (IR)
Fe I lines at 1.5 μm, which provide more accurate information about
the magnetic field than lines in the visible spectral range.
Methods: PFe were observed with the Tenerife Infrared Polarimeter
(TIP II) mounted at the Vacuum Tower Telescope/Observatorio del
Teide/Tenerife. Several areas at various heliocentric angles were
scanned. Faculae near the solar equator (equatorial faculae - EFe) were
also observed for comparison with PFe. The full Stokes vector of the
Fe I line pair at 1.5 μm was measured. The magnetic field properties
were determined (1) from the centre of gravity (COG); (2) with the weak
field approximation (WFA); (3) assuming the strong field regime (SFR);
and (4) with inversions under the hypothesis of Milne-Eddington (ME)
atmospheres. Line-of-sight (LOS) velocities were determined from the
COG of I profiles and from the zero-crossing of the V profiles.
Results: The main findings of this work can be divided in five
parts: (1) the detected PFe do not harbour sufficient magnetic flux
to account for the global flux observed with other methods. (2) Near
the solar limb, the apparent, measured transversal field components
are most times stronger than the longitudinal components by factors of
up to 10 for both PFe and EFe, as found from observations with HINODE
SOT. (3) Many PFe indeed harbour kilo-G magnetic fields. Of those, more
than 85% possess the same magnetic polarity as the global field. The
inclinations γ of the strong fields, \vert B\vert≥ 900 G in the SFR,
are compatible with their vertical emergence from the solar surface. (4)
The results for weaker fields, \vert B\vert≤ 600 G from ME inversions,
indicate a random magnetic field orientation. (5) The velocities from
I profiles and V zero-crossings are in average 0.3 km s-1
towards observer, for both PFe and EFe. The zero-crossings of V exhibit
a large velocity dispersion, of up to 3 km s-1.
Title: Acoustic waves in the solar atmosphere at high spatial
resolution
Authors: Bello González, N.; Flores Soriano, M.; Kneer, F.; Okunev, O.
Bibcode: 2009A&A...508..941B
Altcode:
Aims. The energy supply for the radiative losses of the quiet solar
chromosphere is studied. On the basis of high spatial resolution data,
we investigate the amount of energy flux carried by acoustic waves in
the solar photosphere.
Methods: Time sequences from quiet Sun disc
centre were obtained with the “Göttingen” Fabry-Perot spectrometer
at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in
the non-magnetic Fe i 5576 Å line. The data were reconstructed with
speckle methods. The velocity and intensity fluctuations at line minimum
were subjected to Fourier and wavelet analyses. The energy fluxes at
frequencies higher than the acoustic cutoff frequency (period U ≈
190 s) were corrected for the transmission of the solar atmosphere,
which reduces the signal from short-period waves.
Results:
Both Fourier and wavelet analysis give an amount of energy flux of
~3000 W m-2 at a height h = 250 km. Approximately 2/3 of
it is carried by waves in the 5-10 mHz range, and 1/3 in the 10-20
mHz band. Extrapolation of the flux spectra gives an energy flux
of 230-400 W m-2 at frequencies ν > 20 mHz. We find
that the waves occur predominantly above inter-granular areas.
Conclusions: We conclude that the acoustic flux in waves with periods
shorter than the acoustic cutoff period can contribute to the basal
heating of the solar chromosphere, in addition to the atmospheric
gravity waves found recently.
Title: Full-Stokes Polarimetry with Speckle Techniques
Authors: Bello González, N.; Kneer, F.; Okunev, O.
Bibcode: 2009ASPC..405..407B
Altcode:
The new full-Stokes polarimeter implemented in the 2D `Göttingen'
Fabry-Perot spectrometer, based on ferroelectric liquid crystals,
yields magnetograms with minimised seeing induced crosstalk. A
spatial resolution of 0."30--0."35 is achieved after applying speckle
methods. The detection limit for the magnetic field strength is 16 G
(≡ 3σ), yielding a polarimetric sensitivity of 7--8×1015
Mx. Examples of intensity maps, Dopplergrams and magnetograms from
quiet Sun are discussed.
Title: A full-Stokes polarimeter for the GREGOR Fabry-Perot
interferometer
Authors: Balthasar, Horst; Bello González, N.; Collados, M.; Denker,
C.; Hofmann, A.; Kneer, F.; Puschmann, K. G.
Bibcode: 2009IAUS..259..665B
Altcode:
One of the first post-focus instruments of the new solar telescope
GREGOR will be a Fabry-Perot spectrometer, which is an upgrade of the
Göttingen Fabry-Perot interferometer at the Vacuum Tower Telescope
(VTT) on Tenerife. This spectrometer is equipped with a full-Stokes
polarimeter. The modulation is performed with two ferroelectric liquid
crystals, one acting nominally as quarter-wave plate, and the other as
half-wave plate. A modified Savart plate serves as polarimetric beam
splitter. With the present liquid crystals, the optimum wavelength range
of this polarimeter is between 580 and 660 nm. The spectro-polarimeter
will benefit from the capabilities of the new telescope GREGOR which
will provide a spatial resolution of about 0″.1 (75 km on the solar
surface). Thus we will be able to investigate small magnetic features,
and we will study their development with high cadence.
Title: Dynamics of small-scale magnetic fields on the Sun:
observations and numerical simulations
Authors: Bello González, N.; Yelles Chaouche, L.; Okunev, O.;
Kneer, F.
Bibcode: 2009A&A...494.1091B
Altcode:
Context: Small-scale magnetic fields play an important role in the
structure and the dynamics of the solar atmosphere.
Aims: This
study aims at revealing the evolution of magnetic fields, together
with granular convection in the quiet Sun by means of observations
with high spatial, spectral, and temporal resolution and of numerical
MHD simulations.
Methods: Time sequences from quiet Sun disc
centre were obtained with the upgraded “Göttingen” Fabry-Perot
spectropolarimeter at the Vacuum Tower Telescope, Observatorio del
Teide/Tenerife, in the Fe I 6173 Å line. The data were reconstructed
with speckle methods. For comparison with the observations, numerical
simulations of granular magnetoconvection were carried out with the
MURaM code. The intensities and Stokes vectors emerging from the
simulation box were degraded in wavelength, spatial co-ordinates, and
noise to the quality of the observations.
Results: The noise in
the observed magnetograms from the centre-of-gravity method is σ_B≈2
G, yielding a polarimetric sensitivity of 3 × 1015 Mx, at a
cadence of 23 s with 0.33 arcsec spatial resolution in a field of view
of ∼33 arcsec× 25 arcsec. Many of the observed V profiles in network
and internetwork (IN) areas exhibit strong asymmetries that indicate
strong magnetoconvection. The temporal evolutions of IN structures and
of a bright point (BP), as seen in broadband and line-minimum images,
in Dopplergrams, and in magnetograms, are presented. The magnetic
field structure in the numerical MHD simulations is even more complex
than seen in the observations. Correspondingly, the emergent Stokes
profiles are often very abnormal. The degradation yields a reduction
of the intrinsic field strength to the “observed” one by a factor
4-5. The spectral resolution of the spectrometer is adequate, yet
the limitation in spatial resolution and by noise filtering swamps
the details seen in the non-degraded simulations. A BP was not found
in the simulations, presumably because BP's are not so common and
the simulated box is quiet small: only 6 Mm wide in both horizontal
directions.
Conclusions: The combination of high-resolution
observations with numerical simulation is a highly valuable means
for studying small-scale magnetic fields on the Sun. Two-dimensional,
low-noise data with spectral resolution as good as achieved here and
with spatial resolution of 0.1 arcsec and better are needed to better
understand this important part of the solar magnetism. A movie
is only available in electronic form at http://www.aanda.org
Title: Small-scale magnetic field dynamics on the Sun at high spatial
and temporal resolution
Authors: Bello González, N.; Okunev, O.; Kneer, F.
Bibcode: 2008A&A...490L..23B
Altcode:
Aims: We present examples of the time evolution of phenomena
occurring in the quiet Sun photosphere as seen from observations of
high spatial and spectral resolution and with a time cadence of 23
s.
Methods: A time sequence of about 5 min has been taken with the
Göttingen Fabry-Perot spectropolarimeter at the VTT in the Fe I 6173 Å
line. The data were reconstructed with speckle methods.
Results:
With a polarimetric sensitivity of 2 × 10-3~Ic
(σ_B=2 G), the formation of a bright point as seen from broadband and
line-minimum intensities, as well as in magnetograms and Dopplergrams
can be traced in detail. Other events, like a stable mixed-polarity
configuration in the intergranules and the appearance of magnetic
signature above granules in both dark and bright areas are presented.
Title: Evolution of Small-scale Magnetodynamics on the Sun with High
Spatial and Temporal Resolution
Authors: Bello González, N.; Kneer, F.
Bibcode: 2008ESPM...12..2.6B
Altcode:
For the detection and study of small-scale magnetic fields on
the Sun, observations with both high spatial resolution and high
polarimetric sensitivity are required. The combination formed by
the upgraded Göttingen FPI spectrometer (spectral resolution of
25mÅ, at 617.3nm), full Stokes polarimeter, Adaptive Optics (KAOS)
and speckle reconstruction techniques, provide broadband images of
0.25 arcsec spatial resolution and magnetograms of 0.33 arcsec, with
minimised seeing induced signals yielding a polarimetric sensitivity
of ~0.002 Ic. A description of the optical system as well as results
from the analysis of intensity maps, Dopplergrams and magnetograms from
quiet and active regions on the Sun will be presented. The data were
taken in the FeI 6173Å. We select few examples from the huge variety
of magnetodynamic processes seen in the observations, e.g. the fast
evolution of a bright point in broadband and line minimum intensity,
in magnetograms, and in Dopplergrams is shown. Further examples
indicate convective collapse, magnetic field advection, and magnetic
field diffusion.
Title: High-resolution Spectropolarimetry with the Vacuum Tower
Telescope at the Observatorio del Teide/Tenerife: History and Success
of the Göttingen Programme
Authors: Kneer, F.; Bello Gonzalez, N.
Bibcode: 2008ESPM...12.2.30K
Altcode:
The efforts of the Göttingen solar physics group on high spatial
resolution and 2D spectroscopy started in 1986. First results were
obtained with speckle broadband imaging (de Boer 1992) and with 2D
spectroscopy by means of one Fabry-Perot etalon (FPI) and a Universal
Birefringent Filter (UBF). 2D Stokes V polarimetry followed soon. The
UBF was replaced by a 2nd FPI which allowed short exposures also in
the narrow-band channel of the spectrometer. Thus 2D spectropolari-
metry could be combined with speckle reconstruction. The ``Göttingen
FPI'' has been upgraded in 2005 for high efficiency and in 2007 for
full Stokes polarimetry and good spectral resolution. It has become
a fast and versatile instrument that has been, and will be, operated
sucessfully for many scientific programmes on the atmospheric dynamics
of the Sun.
Title: Fast events and waves in an active region of the Sun observed
in Hα with high spatial resolution
Authors: Sánchez-Andrade Nuño, B.; Bello González, N.; Blanco
Rodríguez, J.; Kneer, F.; Puschmann, K. G.
Bibcode: 2008A&A...486..577S
Altcode:
Context: We study the chromosphere of an active region of the Sun in
the Hα line.
Aims: The development of new instrumentation and new
methods of data analysis allows to scrutinize the dynamics of the solar
chromosphere with high spatial, spectral, and temporal resolution. The
observations we present shed light on some magneto-dynamic processes
occurring above an active region in the chromosphere.
Methods:
We took a time series of 55 min in Hα from AR 10875 at θ≈36°. We
used the “Göttingen” Fabry-Perot spectrometer at the Vacuum Tower
Telescope, Observatorio del Teide/Tenerife, to obtain two-dimensional
spectrograms in Hα. Adaptive optics and image reconstruction yielded
a spatial resolution better than 0.5 arcsec throughout the time
sequence. From the wealth of structures, we selected areas of interest
to study further, in detail, some ongoing processes.
Results:
A small straight surge developed aside of a pore with upward phase
speed of 100 km s-1 and line-of-sight (LOS) velocity of 15
km s-1. The surge retreated rapidly with LOS velocity of
45 km s-1 at its mouth. It underwent a rebound and fell
back again. Two sympathetic mini-flares were observed that lasted
only approximately 40 s, but showed strong Hα emission. We found
magnetoacoustic waves in long fibrils as mainly short wave trains,
short packets or pulses, i.e., solitary waves consisting of small
(1´´-2´´) blobs. They start at either end of the fibrils and travel
with phase speeds of 12-14 km s-1, i.e., close to the tube
speed and approximately the sound velocity for sufficiently large
magnetic field strengths. Some waves speed up to reach velocities of
the order of 30 km s-1. This is much lower than the expected
Alfvén velocity of ≥200 km s-1 for reasonable magnetic
field strengths and mass densities. We suggest that slow waves are not
purely longitudinal, but possess gas velocities perpendicular to the
direction of propagation of few km s-1. Also, fast waves
travel along sinuous lines suggesting entangled magnetic fields. They
spread out along the direction of propagation in the course of their
evolution and often vanish. We discuss the implications.
Title: Narrow-band full Stokes polarimetry of small structures on
the Sun with speckle methods
Authors: Bello González, N.; Kneer, F.
Bibcode: 2008A&A...480..265B
Altcode:
Aims:For the detection and the study of small-scale magnetic fields
on the Sun, it is important to obtain observations with both high
spatial resolution and high polarimetric sensitivity.
Methods:
A second narrow-band etalon and a full Stokes polarimeter, based on
ferroelectric liquid crystals, were implemented in the two-dimensional
“Göttingen” Fabry-Perot spectrometer/polarimeter at the Vacuum
Tower Telescope, Observatorio del Teide/Tenerife. First observations
with the Fe I 6173 Å line and their data analysis with speckle
methods are described.
Results: The new polarimeter yields
magnetograms of a field of view of 31 arcsec×52 arcsec with minimised
seeing induced signals and without spurious signals as from the use of
beam-splitting calcites. The achieved spatial and temporal resolution
are 0.30-0.35 arcsec and 36 s, respectively. With a detection limit
for the field strength of 16 G (\cor3σ), a polarimetric sensitivity
of 7-8×1015 Mx is obtained. Examples of intensity maps,
Dopplergrams, and magnetograms from quiet and active regions on the
Sun are discussed. Some of the results on solar magnetism are on 1)
a pore with small-scale structure where we find a region with very
low temperature gradient, 2) polar faculae with strong magnetic field
signals and weaker signals in other areas surrounding them, and 3)
small-scale inter-network magnetic fields with area fillings in the
resolution elements of the order of 0.15.
Title: On the properties of faculae at the poles of the Sun
Authors: Blanco Rodríguez, J.; Okunev, O. V.; Puschmann, K. G.;
Kneer, F.; Sánchez-Andrade Nuño, B.
Bibcode: 2007A&A...474..251B
Altcode:
Context: Faculae at the poles of the Sun, or polar faculae (PFe),
take part in the solar magnetic cycle. Their occurrence maximum is
shifted by 5-6 years with respect to the sunspot cycle. PFe are stable
phenomena, with lifetimes of several hours to days, and harbour magnetic
fields of kilo-Gauss strength. Yet their role for the global magnetic
field at the solar poles is unknown.
Aims: To contribute to the
knowledge of the physical properties of PFe and to the understanding of
their role in the global magnetism of the Sun.
Methods: PFe were
observed on 21-24 August 2005 with the Vacuum Tower Telescope at the
Observatorio del Teide. The “Göttingen” Fabry-Perot spectrometer
was used with a Stokes V polarimeter to scan the Fe i 6173 Å line
(Landé factor g = 2.5) and the Hα line in two-dimensional fields
of view (FOVs). A large observational coverage of the polar caps was
obtained. The data were analysed with speckle methods. Magnetic field
strengths were determined with the weak field approximation, with the
approximation of the strong field regime, and with the centre of gravity
(COG) method. Velocities were measured with the COG method and from
the zero-crossing of the Stokes V profiles.
Results: PFe show
a decrease of the continuum and broadband intensity contrast towards
the disc centre and no decrease of contrast towards the limb, similar
to as faculae in active regions near the equator. Extrapolating from
the observed FOVs to the total areas of PF occurrence around the solar
poles, we find 4 120 PFe in the northern polar cap and, asymmetrically
to this number, 1 250 PFe near the south pole. The total area coverages
by PFe are ~7.6×108 km2 and ~3.4×108
km2 near the solar north and south poles, respectively. Some
of the PFe exhibit magnetic polarities opposite to the global polarity
at the time of observation. The resulting total magnetic fluxes in PFe
fall short by an order of magnitude from those found in the literature
for the fluxes at the polar caps. This also holds if we include magnetic
structures which are not related to brightenings. We conclude that
with the present spatial resolution of 0.4 arcsec-0.5 arcsec (FWHM),
PFe represent the “large-scale” end of a distribution of unipolar
strands near the solar poles. The velocities in PFe show amplitudes of
2 km s-1, with systematic up-flows in the Stokes I profile,
but no average velocity measured in the V zero-crossings.
Title: Spicule emission profiles observed in He i 10 830 Å
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
Trujillo Bueno, J.; Blanco Rodríguez, J.; Kneer, F.
Bibcode: 2007A&A...472L..51S
Altcode: 2007arXiv0707.4421S
Aims:Off-the-limb observations with high spatial and spectral resolution
will help us understand the physical properties of spicules in the
solar chromosphere.
Methods: Spectropolarimetric observations
of spicules in the He i 10 830 Å multiplet were obtained with the
Tenerife Infrared Polarimeter on the German Vacuum Tower Telescope
at the Observatorio del Teide (Tenerife, Spain). The analysis shows
the variation of the off-limb emission profiles as a function of the
distance to the visible solar limb. The ratio between the intensities
of the blue and the red components of this triplet (R=I_blue/I_red)
is an observational signature of the optical thickness along the light
path, which is related to the intensity of the coronal irradiation.
Results: We present observations of the intensity profiles of spicules
above a quiet Sun region. The observable R as a function of the distance
to the visible limb is also given. We have compared our observational
results to the intensity ratio obtained from detailed radiative transfer
calculations in semi-empirical models of the solar atmosphere assuming
spherical geometry. The agreement is purely qualitative. We argue that
future models of the solar chromosphere and transition region should
account for the observational constraints presented here.
Title: Modern Solar Facilities - Advanced Solar Science
Authors: Kneer, Franz; Puschmann, Klaus G.; Wittmann, Axel D.
Bibcode: 2007msfa.conf.....K
Altcode:
An international workshop entitled: Modern Solar Facilities - Advanced
Solar Science was held in Göttingen Sept. 27-29, 2006. The workshop,
which was attended by 88 participants from 24 different countries, gave
a broad overview of the current state of solar research, with emphasis
on telescopes and instrumentation, high-resolution and high-precision
observations, and theory and interpretation. The book collects written
versions of 71 papers presented at the conference.
Title: GREGOR: the New German Solar Telescope
Authors: Balthasar, H.; von der Lühe, O.; Kneer, F.; Staude, J.;
Volkmer, R.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvaňa, M.; Nicklas, H.; Popow, E.;
Puschmann, K.; Schmidt, W.; Sobotka, M.; Soltau, D.; Strassmeier,
K.; Wittmann, A.
Bibcode: 2007ASPC..368..605B
Altcode: 2007arXiv0704.2693B
GREGOR is a new open solar telescope with an aperture of 1.5 m. It
replaces the former 45-cm Gregory Coudé telescope on the Canary island
Tenerife. The optical concept is that of a double Gregory system. The
main and the elliptical mirrors are made from a silicon-carbide material
with high thermal conductivity. This is important to keep the mirrors
on the ambient temperature avoiding local turbulence. GREGOR will be
equipped with an adaptive optics system. The new telescope will be ready
for operation in 2008. Post-focus instruments in the first stage will be
a spectrograph for polarimetry in the near infrared and a 2-dimensional
spectrometer based on Fabry-Pérot interferometers for the visible.
Title: Temporal evolution of intensity, velocity and magnetic field
of sunspots at high spatial resolution
Authors: Bello González, N.; Kneer, F.; Puschmann, K. G.
Bibcode: 2007msfa.conf..217B
Altcode:
We present results of sunspot observations obtained in April 2006
with the new "Göttingen" Fabry-Perot spectrometer. Thanks to the
large field of view (77"x58") of the new optical setup it has been
possible to perform 2D-spectropolarimetric observations of a small
sunspot and ist surroundings at a heliocentric angle [Theta] ~ 40Å. A
long time series of about one hour has been taken scanning along the
magnetic Fe I 6173 Å and the non-magnetic Fe I 5576 Å spectral lines
quasi-simultaneously. Hence, with the help of image reconstruction
techniques, the temporal evolution of the sunspot fine-structure in
intensity as well as in velocity and magnetic field is analysed at
high spatial resolution.
Title: Off-limb spectroscopy of the He I 10830 Å multiplet:
observations vs. modelling
Authors: Sánchez-Andrade Nuño, B.; Centeno, R.; Puschmann, K. G.;
Trujillo Bueno, J.; Kneer, F.
Bibcode: 2007msfa.conf..177S
Altcode:
Spectropolarimetric observations of spicules were carried out with
the new optical setup of the Tenerife Infrared Polarimeter (TIP 2)
at the VTT, showing the variation of the He I 10830Å multiplet
off-limb emission profiles with increasing distance to the solar
visible limb. The ratio between the intensities of the blue and the
red components of this triplet (R = Iblue/Ired) is an observational
signature of the optical thickness along the light path, which, at the
same time, is proportional to the population of the lower (metastable)
level that takes part in these transitions. Our observational results
show a variation of R as a function of the distance to the limb. In
agreement with recent theoretical results we conclude that R could be
used as a diagnostic tool for downward UV coronal irradiance, which is
believed to be responsible for the population of the metastable level of
the He I 10830 Å triplet. We have compared our observational results
with the ratio obtained from detailed radiative transfer calculations
in semi-empirical models of the solar atmosphere (assuming spherical
geometry) finding a fairly poor agreement. We argue that future models
of the solar chromosphere and transition region should account for
the observational constraints presented here.
Title: From the "Göttingen" Fabry-Perot Interferometer to the
GREGOR FPI
Authors: Puschmann, K. G.; Kneer, F.; Nicklas, H.; Wittmann, A. D.
Bibcode: 2007msfa.conf...45P
Altcode:
Fabry-Perot Interferometers (FPIs) have advantages over slit
spectrographs, allowing fast two-dimensional, narrowband imaging
and post factum image reconstruction of the spectropolarimetric data
obtained. The resulting intensity, velocity and magnetic field maps
are a fundamental base for the understanding of the dynamics of
the solar atmosphere and its magnetic fields at smallest spatial
scales. Efforts are undertaken to provide, with the Göttingen
Fabry-Perot interferometer, an up-todate post-focus instrument for
the German 1.5 m GREGOR solar telescope. Therefore a renewal of the
spectrometer has been achieved during the first half of 2005. First
observations at the German Vacuum Tower Telescope (VTT) reveal new
scientific aspects and a very promising outlook for the future at
GREGOR. In this contribution a general description of the upgraded
spectrometer is given. Its final optical design at GREGOR is described
and an optical analysis of the GREGOR FPI is outlined. Latest results
with the new instrument obtained at the VTT are presented.
Title: New high resolution solar telescope GREGOR
Authors: Volkmer, R.; von der Lühe, O.; Kneer, F.; Staude, J.;
Balthasar, H.; Berkefeld, T.; Caligari, P.; Collados, M.; Halbgewachs,
C.; Heidecke, F.; Hofmann, A.; Klvaña, M.; Sobotka, M.; Nicklas, H.;
Popow, E.; Puschmann, K. G.; Schmidt, W.; Soltau, D.; Strassmeier,
K.; Wittmann, A. D.
Bibcode: 2007msfa.conf...39V
Altcode:
The 1.5m solar telescope GREGOR is being constructed at Tenerife,
Spain. Its purpose is to observe with high spatial and spectral
resolution small-scale dynamic magnetic features on the Sun. The
telescope is completely open with retractable dome and actively cooled
primary mirror made of silicon carbide to minimize thermal effects
on the image quality. After completion it will be one of the most
powerful solar telescopes. This paper presents a general overview of
the telescope characteristics and the current status.
Title: Modern solar facilities - advanced solar science
Authors: Kneer, Franz; Puschmann, Klaus G.; Wittmann, Axel D.
Bibcode: 2007msfa.conf....0K
Altcode:
An international workshop entitled: Modern Solar Facilities - Advanced
Solar Science was held in Göttingen Sept. 27-29, 2006. The workshop,
which was attended by 88 participants from 24 different countries, gave
a broad overview of the current state of solar research, with emphasis
on telescopes and instrumentation, high-resolution and high-precision
observations, and theory and interpretation. The book collects written
versions of 71 papers presented at the conference.
Title: Methods in high-resolution solar spectroscopy
Authors: Kneer, F.
Bibcode: 2007msfa.conf...67K
Altcode:
After few historical remarks, the needs in high-resolution spectroscopy
are outlined. They are concerned with temporal, spatial, and wavelength
resolution for a reasonable field of view of supergranulation cell or
larger. At 0."1 spatial resolution ([about] 72.5 km on the Sun) the
measurement for a snapshot should be finished within the transit time
ttr [about] 10 s (size of resolution element/sound speed). Finally, the
performances of grating spectrometers and Fabry-Perot interferometers
are compared. The latter are far superior to grating spectrographs
due to their high resolution-luminosity product.
Title: Observations of a flaring active region in H[alpha]
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Kneer, F.
Bibcode: 2007msfa.conf..273S
Altcode:
A 40 min time series of the active region NOAA AR10875 was obtained
on April 26th, 2006 under very good seeing conditions using the new
"Göttingen" - Fabry-Perot interferometer at the German Vacuum Tower
Telescope, Observatorio del Teide, Tenerife. The Hα line was scanned
giving two-dimensional images at 21 different wavelength positions
with a cadence of ~ 20 s per scan. After speckle reconstruction the
spatial resolution achieved was better than 0".35. The data provide a
unique possibility to study the temporal evolution of a pre-flaring,
flaring and post-flaring solar active region in Hα. From the analysis
of the present spectrometric data set we expect important advances in
the understanding of Alfén waves, dynamics of fibrils and filaments
and the physical nature of the sigmoid structure seen in the core of
Hα. Here we present first results from this data set.
Title: Dynamics of the quiet photosphere and its magnetic field
under high spatial resolution
Authors: Puschmann, K. G.; Kneer, F.; Domínguez Cerdeña, I.
Bibcode: 2007msfa.conf..151P
Altcode:
An excellent 1 h 10 min time series of a quiet solar region near
disk centre was acquired on 4th of May 2006 at the German Vacuum
Tower Telescope. Using the new "Göttingen" Fabry-Perot interferometer
together with the Kiepenheuer Adaptive Optics system spectropolarimetric
images were obtained, scanning the Fe I 5247Å and Fe I 5250Å lines
with a temporal cadence of 17 s each. Thanks to speckle reconstruction
methods the achieved spatial resolution of the obtained intensity,
velocity and magnetic field maps is ~ 0."3. First results on
the dynamics of the quiet photosphere and magnetic fields reveal
strengthening and weakening of the polarisation signal in interaction
to granular convection processes indicating advection and diffusion
of magnetic flux at smallest scales. Field structures, very elongated
along intergranular lanes, show high dynamics, combine and split, and
leave strong fingerprints at high photospheric layers where related
temperatures fluctuate. Recent numerical MHD simulations come close
to our findings. Possible indications of magnetic flux annihilation,
flux tube evacuation or signs of upward-propagating shock fronts
are illustrated.
Title: Study of polar faculae with north pole coverage of the Sun
Authors: Blanco Rodríguez, J.; Okunev, O.; Puschmann, K. G.; Kneer, F.
Bibcode: 2007msfa.conf..181B
Altcode:
We present here a description of the main characteristics of faculae
at the poles of the Sun (polar faculae PFe) observed in a series of
observations covering the north solar pole. Statistics of a group
of these PFe are used to estimate the contrast behaviour from [mμ]
= 0.6 towards the limb as well as their magnetic field. Hα is also
observed to analyse the behaviour of PFe in the chromosphere.
Title: Magnetic Fields of the Quiet Sun: Distribution of Field
Strengths
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2006ASPC..358...48D
Altcode:
The quiet Sun photospheric plasma has a variety of magnetic field
strengths going from zero to 1800 G. The empirical characterization of
these field strengths requires a probability density function (PDF),
i.e., a function P(B) describing the fraction of quiet Sun occupied by
each field strength B. We present a method to combine magnetic field
strength measurements based on the Zeeman effect and the Hanle effect
in order to estimate the true P(B). The application of the method
to real observations renders a set of possible PDFs, which outline
the general characteristics of the quiet Sun magnetic fields. Their
most probable field strength differs from zero. The magnetic energy
density is a significant fraction of the kinetic energy of the granular
motions at the base of the photosphere (larger than 15 or larger than
2×103 erg cm-3). The unsigned flux density
(or mean magnetic field strength) has to be between 130 G and 190
G. A significant part of the unsigned flux (between 10% and 40%) and
of the magnetic energy (between 40% and 80%) are provided by the field
strengths larger than 1 kG which, however, occupy only a small fraction
of the surface (between 1% and 4%). The quiet Sun photosphere has
far more unsigned magnetic flux and magnetic energy than the active
regions and the network together.
Title: Quiet-Sun Magnetic Fields: Simultaneous Inversion of Visible
and IR Spectro-Polarimetric Observations
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2006ASPC..358...88D
Altcode:
We studied the quiet-Sun magnetic fields interpreting
spectro-polarimetric observations of infrared and visible spectral
lines. Magnetic field strengths and filling factors were inferred by the
simultaneous inversion of Stokes profiles of the Fe I lines at 6301.5,
6302.5, 15648, and 15653 Å under the MISMA hypothesis. They cover a
solar intra-network region at disk center. We analyzed Stokes profiles
with signals above noise in both spectral ranges, which correspond
to 40% of the field of view. Most of these 2280 profiles could only
be inverted with a model including 3 magnetic components with very
different field strengths, which indicates the co-existence of kG and
sub-kG fields. We measured a total unsigned magnetic flux density
of 9.6 G over the whole field of view. For half of the pixels the
magnetic field has opposite polarities within the resolution element. We
computed the probability density function of finding each magnetic field
strength. It has an important contribution of kG field strengths, which
concentrates most of the magnetic flux and energy. This kG contribution
has a preferred magnetic polarity while the weak fields are balanced.
Title: Quiet Sun Magnetic Fields from Simultaneous Inversions of
Visible and Infrared Spectropolarimetric Observations
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2006ApJ...646.1421D
Altcode: 2006astro.ph..4381D
We study the quiet Sun magnetic fields using spectropolarimetric
observations of the infrared and visible Fe I lines at 6301.5, 6302.5,
15648, and 15653 Å. Magnetic field strengths and filling factors
are inferred by the simultaneous fit of the observed Stokes profiles
under the MISMA hypothesis. The observations cover an intranetwork
region at the solar disk center. We analyze 2280 Stokes profiles whose
polarization signals are above noise in the two spectral ranges, which
correspond to 40% of the field of view. Most of these profiles can
be reproduced only with a model atmosphere including three magnetic
components with very different field strengths, which indicates the
coexistence of kG and sub-kG fields in our 1.5" resolution elements. We
measure an unsigned magnetic flux density of 9.6 G considering the full
field of view. Half of the pixels present magnetic fields with mixed
polarities in the resolution element. The fraction of mixed polarities
increases as the polarization weakens. We compute the probability
density function of finding each magnetic field strength. It has a
significant contribution of kG field strengths, which concentrates
most of the observed magnetic flux and energy. This kG contribution
has a preferred magnetic polarity, while the polarity of the weak
fields is balanced.
Title: The new 1.5m solar telescope GREGOR: first light and start
of commissioning
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
Heidecke, Frank; Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald;
Wittmann, Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus;
Sobotka, Michal; Klvana, Miroslav; Collados, Manuel
Bibcode: 2006SPIE.6267E..0WV
Altcode: 2006SPIE.6267E..29V
The integration of the three main silicon carbide mirrors into the new
1.5 m solar telescope GREGOR at Izana on Tenerife, Spain is planned
during 2006. We expect first light at the end of 2006. A progress
report about integration of the optics and mechanics and planning of
the commissioning phase of the telescope and post focus instruments
will be presented at the meeting. The GREGOR telescope is build by a
consortium of the Kiepenheuer Institut fur Sonnenphysik in Freiburg,
the Astrophysikalische Institut Potsdam, the Institut fur Astronomie
Gottingen and additional national and international Partners.
Title: The new Göttingen Fabry-Pérot spectrometer for
two-dimensional observations of the Sun
Authors: Puschmann, K. G.; Kneer, F.; Seelemann, T.; Wittmann, A. D.
Bibcode: 2006A&A...451.1151P
Altcode:
Studies of small-scale dynamics and magnetic fields in the solar
atmosphere require spectroscopy and polarimetry with high spatial
resolution. For this purpose, spectrometers based on Fabry-Pérot
interferometers (FPIs) have advantages over slit spectrographs. They
possess a high throughput and allow fast two-dimensional,
narrow-band imaging and image reconstruction of the data. In the
present contribution we describe an upgrade, essentially renewal,
of the Göttingen FPI spectrometer achieved during the first half of
2005. A new etalon from IC Optical Systems Ltd. (formerly Queensgate),
England, with 70 mm free aperture for high spectral resolution has
been mounted. New CCD detectors from LaVision GmbH (Göttingen) with
powerful computer hard- and software were implemented. We consider the
product of signal-to-noise ratio, frame rate, and field of view as a
measure of the efficiency. At low light levels, e.g. in narrow-band
speckle applications, this product has increased by a factor ~60
compared to the old system. In addition, several spectral regions can
now be scanned quasi-simultaneously. We present first results obtained
with the upgraded spectrometer. The efforts are undertaken to provide
an up-to-date post-focus instrument for the new German 1.5 m GREGOR
solar telescope presently under construction at the Observatorio del
Teide on Tenerife.
Title: The Distribution of Quiet Sun Magnetic Field Strengths from
0 to 1800 G
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2006ApJ...636..496D
Altcode: 2005astro.ph..9243D; 2006ApJ...636..496C
The quiet-Sun photospheric plasma has a variety of magnetic field
strengths going from zero to 1800 G. The empirical characterization
of these field strengths requires a probability density function
(PDF), i.e., a function P(B) describing the fraction of quiet Sun
occupied by each field strength B. We show how to combine magnetic
field strength measurements based on the Zeeman effect and the Hanle
effect to estimate an unbiased P(B). The application of the method
to real observations renders a set of possible PDFs, which outline
the general characteristics of the quiet-Sun magnetic fields. Their
most probable field strength differs from zero. The magnetic energy
density is a significant fraction of the kinetic energy of the
granular motions at the base of the photosphere (larger than 15%
or larger than 2×103 ergs cm-3). The unsigned
flux density (or mean magnetic field strength) has to be between 130
and 190 G. A significant part of the unsigned flux (between 10% and
50%) and of the magnetic energy (between 45% and 85%) are provided by
the field strengths larger than 500 G, which, however, occupy only a
small fraction of the surface (between 1% and 10%). The fraction of kG
fields in the quiet Sun is even smaller, but they are important for a
number of reasons. The kG fields still trace a significant fraction of
the total magnetic energy, they reach the high photosphere, and they
appear in unpolarized light images. The quiet-Sun photosphere has
far more unsigned magnetic flux and magnetic energy than the active
regions and the network combined.
Title: Chromospheric Dynamics of a Solar Active Region
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Sánchez
Cuberes, M.; Blanco Rodríguez, J.; Kneer, F.
Bibcode: 2005ESASP.600E..70S
Altcode: 2005dysu.confE..70S; 2005ESPM...11...70S
No abstract at ADS
Title: Study of Polar Faculae
Authors: Blanco Rodríguez, J.; Sánchez-Andrade Nuño, B.; Puschmann,
K. G.; Kneer, F.
Bibcode: 2005ESASP.600E..59B
Altcode: 2005dysu.confE..59B; 2005ESPM...11...59B
No abstract at ADS
Title: Synthesis of Stokes Profiles from a Two Component Penumbral
Model
Authors: Bello González, N.; Okunev, O.; Kneer, F.
Bibcode: 2005ESASP.600E..56B
Altcode: 2005ESPM...11...56B; 2005dysu.confE..56B
No abstract at ADS
Title: Analysis of a Wide Chromospheric Active Region
Authors: Sánchez-Andrade Nuño, B.; Puschmann, K. G.; Sánchez
Cuberes, M.; Blanco Rodríguez, J.; Kneer, F.
Bibcode: 2005ESASP.596E..62S
Altcode: 2005ccmf.confE..62S
No abstract at ADS
Title: Magnetic Structures on the Sun: Osbervations with the New
"GÖTTINGEN" Two-Dimensional Spectrometer on Tenerife
Authors: Kneer, F.; Puschmann, K. G.; Blanco Rodríguez, J.;
Sánchez-Andrade Nuño, B.; Wittmann, A. D.
Bibcode: 2005ESASP.596E..71K
Altcode: 2005ccmf.confE..71K
No abstract at ADS
Title: Study of Polar Faculae
Authors: Blanco Rodríguez, J.; Sánchez-Andrade Nuño, B.; Puschmann,
K. G.; Kneer, F.
Bibcode: 2005ESASP.596E..52B
Altcode: 2005ccmf.confE..52B
No abstract at ADS
Title: Analysis of Polarimetric Sunspot Data from Tesos/vtt/tenerife
Authors: Valdivielso Casas, L.; Bello González, N.; Puschmann, K. G.;
Sánchez-Andrade Nuño, B.; Kneer, F.
Bibcode: 2005ESASP.596E..67V
Altcode: 2005ccmf.confE..67V
No abstract at ADS
Title: Study of Asymmetries of Stokes Profiles from High Spatial
Resolution Spectropolarimetry
Authors: Bello González, N.; Okunev, O.; Kneer, F.
Bibcode: 2005ESASP.596E..50B
Altcode: 2005ccmf.confE..50B
No abstract at ADS
Title: Numerical modeling of solar faculae close to the limb
Authors: Okunev, O. V.; Kneer, F.
Bibcode: 2005A&A...439..323O
Altcode:
A numerical simulation was performed to interpret high-resolution
spectropolarimetric observations of polar faculae on the Sun. A
semi-empirical three-dimensional model of facula structures,
controlled by several free parameters, was constructed. It consists
of two components, the atmosphere within small-scale magnetic flux
tubes and the exterior atmosphere. Multi-ray 1.5D radiative transfer
calculations were performed along oblique rays passing through a highly
inhomogeneous atmosphere of the simulation box. By the comparison of
the properties of the calculated Stokes profiles from the synthetic
faculae with the observed properties, a set of free parameters of
the model (such as: size, number density of flux tubes, internal
temperature stratification and magnetic field strength) was deduced,
which satisfies the observational constraints. The hypothesis about
solar faculae as a conglomerate of small-scale magnetic flux tubes is
verified. The model reproduces all observed properties: the continuum
contrast and its center-to-limb variation, the Stokes I and V profiles
of Fe I and Fe II lines, the apparent magnetic field strengths, and
the displacement towards the limb of the continuum intensity against
the line-of-sight magnetograms.
Title: The new 1.5 solar telescope GREGOR: progress report and
results of performance tests
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkefeld, Thomas; Caligari, Peter; Halbgewachs, Clemens;
Schmidt, Wolfgang; Soltau, Dirk; Nicklas, Harald; Wittmann, Axel;
Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka, Michal;
Klvana, Miroslav; Collados, Manuel
Bibcode: 2005SPIE.5901...75V
Altcode:
The telescope structure including control system and the complete
retractable dome of the new 1.5 m solar telescope GREGOR were
assembled during 2004 at Izana on Tenerife, Spain. The GREGOR
telescope is build by a consortium of the Kiepenheuer Institut fuer
Sonnenphysik, the Astrophysikalische Institut Potsdam, the Institut
fuer Astrophysik Goettingen and additional national and international
Partners. Pointing, tracking and thermal tests were made to verify
the proposed performance. The results of these tests and a progress
report of the project will be presented.
Title: On super-resolution in astronomical imaging
Authors: Puschmann, K. G.; Kneer, F.
Bibcode: 2005A&A...436..373P
Altcode:
Our work is an attempt to fulfil one of the aims of astronomical
imaging, that is, to obtain information at high angular
resolution. Stars as point sources can be represented by
Dirac δ-functions whose Fourier transforms contain information
about the position and the (integrated) intensity at all angular
frequencies. Thus, we can deconvolve unresolved images of star fields
even at angular distances smaller than the diffraction limit of
the telescope with which the observations are performed. We give an
example of reconstruction of the image of two stars with an angular
separation of a factor 2.5 less than α_Ra=1.22×λ/D. However, we
find that super-resolution is feasible only for point sources. For
extended objects the information about intensity fluctuations at angular
frequencies u>u_max=D/λ=1/α_min is irretrievably lost. We discuss
the impossibility of super-resolution for the Sun using a numerically
simulated image of solar granulation. However, one can enhance the
contrast of solar images, though without increasing angular resolution
beyond the diffraction limit.
Title: Polarimetry of sunspot penumbrae with high spatial resolution
Authors: Bello González, N.; Okunev, O. V.; Domínguez Cerdeña,
I.; Kneer, F.; Puschmann, K. G.
Bibcode: 2005A&A...434..317B
Altcode:
We present two-dimensional high-spatial-resolution spectropolarimetric
observations of sunspot penumbrae. They were obtained in April 2002 and
May 2003 with the "Göttingen" Fabry-Pérot spectrometer at the Vacuum
Tower Telescope at the Observatorio del Teide (Tenerife). Speckle
methods were applied for image reconstruction which resulted
in a spatial resolution of 0.5 arcsec in the magnetograms of the
penumbrae. We analysed Stokes I and V profiles of the Fe II 6149 Å
line, which exhibits no instrumental Stokes Q/U→ V crosstalk, and of
the Fe I line pair at 6302 Å. The main results are the following: 1)
on scales larger than 0.5 arcsec, the intensity pattern of penumbrae
stays the same in the continuum and core images of the 6301.5 Å line,
which stem from 0 km and 300 km (above τ_5=1), respectively. Yet
at scales of 0.5 arcsec and smaller the pattern in the two spectral
features is clearly different. 2) On the limb side of sunspots the
Evershed flow is carried by dark filaments and on the centre side by
bright features and their somewhat weakened tails. We explain this with
a picture in which the velocity of hot rising gas is best seen on the
centre side, while on the limb side the horizontal outward and possibly
downward flows are seen when the gas has cooled down. 3) The un-combed
structure of the magnetic field is confirmed. On the limb side, the more
horizontal fields coincide with dark fibrils or with diffuse intensity
structures. Generally, the more horizontal fields are located at the
positions of strong outflows. 4) Strong line-of-sight components of
the magnetic fields are not found in bright filaments but in dark
structures, somewhat displaced from the darkest parts. Their positions
do not coincide with those of the strongest velocity fields. In general,
our results are compatible with the picture of low lying flow channels
coincident with the horizontal magnetic field, or possibly emerging and
diving down into sub-photospheric layers, like a "sea serpent". Some
further dynamic phenomena are discussed, which demonstrate the richness
of processes in penumbrae, and reveal unexpected properties.
Title: Quiet sun magnetic fields vs. polar faculae - local vs. global
dynamo?
Authors: Okunev, O. V.; Domínguez Cerdeña, I.; Puschmann, K. G.;
Kneer, F.; Sánchez Almeida, J.
Bibcode: 2005AN....326..205O
Altcode:
Quiet Sun magnetic fields in the internetwork are almost
ubiquitous. Simultaneous observations in infra-red and visible lines
and high spatial resolution (< 0.5'') data in visible
lines show that their field strengths range from below few hundred
Gauss to kilo-Gauss. Most of the flux is contained in small-scale,
strong-field features located mainly in intergranular lanes. The
average unsigned flux density exceeds 20 Gauss. The new detections
are confirmed by recent quiet Sun observations in the G band. The
generation of the strong fields in the internetwork, which may be due
to a local dynamo, poses a challenging problem. - Polar faculae (PFe)
are small-scale magnetic features at the polar caps of the Sun. They
take part in the solar cycle and are thus likely to be rooted deeply
in the solar interior. They are the result of the global dynamo at
the solar poles. PFe also possess kilo-Gauss magnetic fields which
have the same polarity as the global magnetic field. The rôle of
quiet Sun magnetic field structures and of PFe for the dynamics of
the corona and for the solar wind are addressed.
Title: Spectropolarimetry in a Sunspot Penumbra at High Spatial
Resolution
Authors: Gonzalez, N. Bello; Ounev, O.; Kneer, F.
Bibcode: 2005ASSL..320..183G
Altcode:
No abstract at ADS
Title: Spectropolarimetry in a sunspot penumbra at high spatial
resolution
Authors: Bello González, N.; Okunev, O.; Kneer, F.
Bibcode: 2005ASSL..320..183B
Altcode: 2005smp..conf..183B
We present results from polarimetric data on sunspot penumbrae obtained
with the Vacuum Tower Telescope at the Observatorio del Teide, Tenerife,
using the `Göttingen' Fabry-Pérot Interferometer. Speckle image
reconstruction has been performed giving us a spatial resolution of
0.6''. The observations were taken in the Fe II 6149.2 Å line which,
given its particular Zeeman splitting, has no instrumental (Q,U) to V
crosstalk and provides us measurements of I and V Stokes profiles to
analyse velocities and magnetic fields in penumbrae.
Title: Simultaneous Visible and IR spectropolarimetry of the quiet Sun
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2005ASSL..320..175D
Altcode: 2005ASSL..320..175C; 2005smp..conf..175D
This work presents the first quiet Sun spectropolarimetric observations
carried out in the visible and the infrared (IR) simultaneously. The
Fe I lines at 6301.5, 6302.5, 15648, and 15652 Å were observed
co-spatially, and at the same time (with a time lag of only 1 minute),
with high sensitive spectropolarimeters operated in two different
telescopes (VTT and THEMIS at the Observatorio del Teide). We find
Stokes V profiles above noise in 30% of the observed area, showing
intrinsic magnetic fields of kG (traced by visible lines) co-existing
with sub-kG fields (traced by infrared lines). We also found V profiles
with opposite polarity in the visible and the IR in 25% of the pixels
under study (8% of the area).
Title: On the structure of polar faculae on the Sun
Authors: Okunev, O. V.; Kneer, F.
Bibcode: 2004A&A...425..321O
Altcode:
Faculae on the polar caps of the Sun, in short polar faculae (PFe),
are investigated. They take part in the magnetic solar cycle. Here,
we study the fine structures of PFe, their magnetic fields and their
dynamics on short time scales. The observations stem from several
periods in 2001 and 2002. They consist of spectropolarimetric data
(Stokes I and V) taken in the Fe I 6301.5 and 6302.5 Å and Fe II
6149.3 Å lines with the Gregory-Coudé Telescope (GCT) and the Vacuum
Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. At the
VTT, the ``Göttingen'' two-dimensional Fabry-Perot spectrometer was
used. It allows image reconstruction with speckle methods resulting in
spatial resolution of approximately 0.25 arcsec for broadband images
and 0.5 arcsec for magnetograms. The application of singular value
decomposition yielded a polarimetric detection limit of | V|≈2 ×
10-3Ic. We find that PFe, of size of 1 arcsec
or larger, possess substantial fine structure of both brightness and
magnetic fields. The brightness and the location of polar facular
points change noticeably within 50 s. The facular points have strong,
kilo-Gauss magnetic fields, they are unipolar with the same polarity
as the global, poloidal magnetic field. The ambient areas, however,
exhibit weak flux features of both polarities, as in the quiet Sun
near disk center. Strong upflows of 1 km s-1 are detected
in the intensity profiles of PFe. The zero-crossings of the Stokes
V profiles yield an average velocity of the magnetized plasma of
vFT≈-0.4 km s-1 (towards the observer).
Title: Progress report of the 1.5 m solar telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar F.; Kneer, Franz;
Staude, Jürgen; Berkefeld, Thomas; Caligari, Peter; Schmidt,
Wolfgang; Soltau, Dirk; Nicklas, Harald; Wiehr, Eberhardt; Wittmann,
Axel; Balthasar, Horst; Hofmann, Axel; Strassmeier, Klaus; Sobotka,
Michal; Klvana, Miroslav; Collados, Manuel
Bibcode: 2004SPIE.5489..693V
Altcode:
GREGOR is the new 1.5 m solar telescope assembled on Tenerife, Spain, by
the German consortium of the Kiepenheuer Institut fur Sonnenphysik, the
Astronomischen Institut Potsdam, the Universitats-Sternwarte Gottingen
and other national and international Partners. The refurbishment of
the building is almost finished. The manufacturing of the telescope
structure and the optics is still in progress. After the integration of
the new complete retractable dome in July 2004 the telescope structure,
optic and post focus instruments will be assembled during the rest of
the year. First light is planned during May 2005.
Title: Dynamics of an enhanced network region observed in Hα
Authors: Al, N.; Bendlin, C.; Hirzberger, J.; Kneer, F.; Trujillo
Bueno, J.
Bibcode: 2004A&A...418.1131A
Altcode:
This investigation is based on Hα observations of high spatial
resolution. They stem from an enhanced network region near disk centre
of the sun and consist of broad-band and narrow-band images taken with
the two-dimensional ``Göttingen'' Fabry-Perot spectrometer mounted in
the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The
``lambdameter method'' was applied to derive intensity and velocity
maps over the two-dimensional field of view reflecting the behaviour
of these parameters in the solar chromosphere. The determination of
the source function, the line-of-sight (LOS) velocity, the Doppler
width and the optical depth was based on Beckers' cloud model
(Beckers \cite{beckers}). From the LOS velocity distribution along
the Hα structures we conclude that ballistic motions are unlikely to
prevail. Especially the bright Hα features cannot be explained by the
cloud model. It is shown how, instead, two-dimensional non-LTE radiative
transfer calculations of embedded chromospheric structures lead to
reasonable agreement with the observed line profiles from these bright
features. It is found that many of the bright fibril-like structures
near dark fibrils can be explained by radiation which is blocked by
absorbing material at large heights and escapes through less opaque
regions. We estimate the number densities and the temperature. With
these and with the measured velocities, the kinetic energy flux and
the enthalpy flux related to the motions of the fine structures can
be calculated. Both fall short by at least an order of magnitude of
covering energy losses by radiation of active chromospheric regions.
Title: Polarimetry in sunspot penumbrae at high spatial resolution.
Authors: Bello Gonzalez, N.; Okunev, O.; Dominguez Cerdena, I.;
Kneer, F.
Bibcode: 2004ANS...325...79B
Altcode: 2004ANS...325..P03B; 2004ANS...325a..79B
No abstract at ADS
Title: Short period waves in the solar atmosphere.
Authors: Andic, A.; Kneer, F.
Bibcode: 2004ANS...325...96A
Altcode: 2004ANS...325a..96A; 2004ANS...325..P26A
No abstract at ADS
Title: Analysis of simultaneous visible and infrared
spectropolarimetric observations of quiet Sun.
Authors: Dominguez Cerdena, I.; Sanchez Almeida, J.; Kneer, F.
Bibcode: 2004ANS...325...94D
Altcode: 2004ANS...325a..94D; 2004ANS...325..P23D
No abstract at ADS
Title: Adaptive optics transfer function estimation for solar
observations.
Authors: Sailer, M.; von der Luehe, O.; Kneer, F.
Bibcode: 2004ANS...325..130S
Altcode: 2004ANS...325..P65S; 2004ANS...325a.130S
No abstract at ADS
Title: Structure and short-time evolution of polar faculae on the sun.
Authors: Okunev, O. V.; Kneer, F.
Bibcode: 2004ANS...325...80O
Altcode: 2004ANS...325..P04O; 2004ANS...325a..80O
No abstract at ADS
Title: Simultaneous Visible and Infrared Spectropolarimetry of a
Solar Internetwork Region
Authors: Sánchez Almeida, J.; Domínguez Cerdeña, I.; Kneer, F.
Bibcode: 2003ApJ...597L.177S
Altcode: 2003astro.ph..9727A; 2003astro.ph..9727S
We present the first simultaneous infrared (IR) and visible
spectropolarimetric observations of a solar internetwork region. The
Fe I lines at 6301.5, 6302.5, 15648, and 15652 Å were observed, with
a lag of only 1 minute, using highly sensitive spectropolarimeters
operated in two different telescopes (Vacuum Tower Telescope and THEMIS
at the Observatorio del Teide). Some 30% of the observed region shows
IR and visible Stokes V signals above noise. These polarization signals
indicate the presence of kilogauss (kG) magnetic field strengths (traced
by the visible lines) coexisting with sub-kG fields (traced by the
infrared lines). In addition, one-quarter of the pixels with signal have
visible and IR Stokes V profiles with opposite polarity. We estimate
the probability density function of finding each longitudinal magnetic
field strength in the region. It has a tail of kG field strengths that
accounts for most of the (unsigned) magnetic flux of the region.
Title: On the fractal dimension of small-scale magnetic structures
in the Sun
Authors: Janßen, K.; Vögler, A.; Kneer, F.
Bibcode: 2003A&A...409.1127J
Altcode:
We compare, by means of fractal analyses, the shapes of observed
small-scale magnetic structures on the Sun with those of magnetic
features resulting from numerical simulations of magnetoconvection. The
observations were obtained with the ``Göttingen'' Fabry-Perot
spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide
on Tenerife. Magnetograms with 0\farcs4-0\farcs5 spatial resolution were
obtained from two-dimensional Stokes V polarimetry in the Fe I 6302.5 Å
line and by image reconstruction with speckle methods. The simulations
of magnetoconvection was performed with the MURAM code. It solves the
time-dependent MHD equations for a compressible, partly ionized plasma
including radiative transfer in LTE. To determine the fractal dimensions
the perimeter-area relation is used. We discuss the influence of seeing
and noise in the fractal dimension D of the observed magnetograms. A
dependence of D on the distance from disk center could not be found. The
observations give D=1.21+/-0.05 for a pixel size corresponding to
0\farcs105, while for the numerical simulations D=1.38+/-0.07 for a
pixel size of 20.83 km. If we use a yardstick adapted to the spatial
resolution the observations give the dimension D=1.41+/-0.05 in close
agreement with the simulations. This agreement is remarkable since
the pixel sizes and spatial resolutions of the simulations and of the
observations differ by a factor of 15. The finding supports the view
of self-similarity of solar magnetic structures over a large range of
scales. In addition, it demonstrates the realism of the simulations
and suggests that all important physical processes are included. We
discuss our results in comparison with other investigations.
Title: Inter-network magnetic fields observed with sub-arcsec
resolution
Authors: Domínguez Cerdeña, I.; Sánchez Almeida, J.; Kneer, F.
Bibcode: 2003A&A...407..741D
Altcode: 2003astro.ph..6329C; 2003astro.ph..6329D
We analyze a time sequence of Inter-Network (IN) magnetograms
observed at the solar disk center. Speckle reconstruction techniques
provide a good spatial resolution (0\farcs5 cutoff frequency) yet
maintaining a fair sensitivity (some 20 G). Patches with signal above
noise cover 60% of the observed area, most of which corresponds to
intergranular lanes. The large surface covered by signal renders a mean
unsigned magnetic flux density between 17 G and 21 G (1 G equiv 1 Mx
cm-2). The difference depends on the spectral line used to
generate the magnetograms (łinetwo or łineone ). Such systematic
difference can be understood if the magnetic structures producing
the polarization have intrinsic field strengths exceeding 1 kG, and
consequently, occupying only a very small fraction of the surface
(some 2%). We observe both, magnetic signals changing in time scales
smaller than 1 min, and a persistent pattern lasting longer than the
duration of the sequence (17 min). The pattern resembles a network
with a spatial scale between 5 and 10 arcsec, which we identify as the
mesogranulation. The strong dependence of the polarization signals
on spatial resolution and sensitivity suggests that much quiet Sun
magnetic flux still remains undetected.
Title: Current Status of the 1.5m Solar Telescope GREGOR
Authors: Volkmer, Reiner; von der Lühe, Oskar; Kneer, Franz; Staude,
Jürgen; Berkfeld, Thomas; Schmidt, Wolfgang; Soltau, Dirk; Nicklas,
Harald; Wiehr, Eberhardt; Wittman, Axel; Hofmann, Axel; Sobotka,
Michal; Klvana, Miroslav
Bibcode: 2003ANS...324..112V
Altcode: 2003ANS...324..P19V
No abstract at ADS
Title: Transfer Function Calibration for Speckle Reconstruction
Authors: Sailer, Markus; von der Lühe, Oskar; Kneer, Franz
Bibcode: 2003ANS...324...23S
Altcode: 2003ANS...324..C04S
No abstract at ADS
Title: Velocity and Magnetic Fields in Sunspot Penumbrae at Hight
Spatial and Spectral Resolution
Authors: Bello González, Nazaret; Kneer, Franz
Bibcode: 2003ANS...324...28B
Altcode: 2003ANS...324..D03B
No abstract at ADS
Title: Hausdorff-Dimension of Magnetic Structures
Authors: Janssen, Katja; Vögler, Alexander; Kneer, Franz
Bibcode: 2003ANS...324...30J
Altcode: 2003ANS...324..D06J
No abstract at ADS
Title: Dynamics of Solar Chromospheric Finestructures in H-alpha
observed with High Spatial Resolution
Authors: Al, Nurol; Kneer, Franz; Hirzberger, Johann
Bibcode: 2003ANS...324..111A
Altcode: 2003ANS...324..P17A
No abstract at ADS
Title: GREGOR: the new 1.5m solar telescope on Tenerife
Authors: Volkmer, Reiner; von der Luehe, Oskar; Kneer, Franz; Staude,
Juergen; Hofmann, Axel; Schmidt, Wolfgang; Sobotka, Michal; Soltau,
Dirk; Wiehr, Eberhardt; Wittmann, Axel; Berkefeld, Thomas
Bibcode: 2003SPIE.4853..360V
Altcode:
The new 1.5 m high resolution telescope will be build up on the reused
solar tower of the German 45 cm Gregory Coudé Telescope at the Teide
Observatory, Izaña, Tenerife. The new telescope is a Gregory type
with open telescope structure, alt-azimuth mount, complete retractable
dome, and a pool of well established and new developed post focus
instruments. An adaptive optics system provides the capability for
diffraction limited observations at visible wavelengths and the
polarimetry device in the secondary focus reduces the perturbation
due to instrumental polarization in an efficient way. We describe the
main optical characteristics and the focal plane instrumentation with
respect to the latest status of the project.
Title: Quiet-Sun Magnetic Fields at High Spatial Resolution
Authors: Domínguez Cerdeña, I.; Kneer, F.; Sánchez Almeida, J.
Bibcode: 2003ApJ...582L..55D
Altcode: 2002astro.ph.11454C; 2002astro.ph.11454D
We present spectropolarimetric observations of internetwork magnetic
fields at the solar disk center. A Fabry-Pérot spectrometer was used to
scan the two Fe I lines at 6301.5 and 6302.5 Å. High spatial resolution
(0.5") magnetograms were obtained after speckle reconstruction. The
patches with magnetic fields above noise cover approximately 45%
of the observed area. Such large coverage renders a mean unsigned
magnetic flux density of some 20 G (or 20 Mx cm-2), which
exceeds all previous measurements. Magnetic signals occur predominantly
in intergranular spaces. The systematic difference between the flux
densities measured in the two iron lines leads to the conclusion that,
typically, we detect structures with intrinsic field strengths larger
than 1 kG occupying only 2% of the surface.
Title: High Resolution 2D Spectro-polarimetric Observations of Polar
Faculae and Quiet Sun in Two Iron Lines
Authors: Domínguez Cerdeña, I.; Okunev, O.; Kneer, F.; Sánchez
Almeida, J.
Bibcode: 2003ASPC..307..370D
Altcode:
No abstract at ADS
Title: Two-dimensional speckle spectroscopy of Hα features
Authors: Al, N.; Hirzberger, J.; Kneer, F.
Bibcode: 2003AN....324..364A
Altcode:
In May 2002, the solar chromosphere was observed with a two-dimensional
spectrometer which is mounted in the German Vacuum Tower Telescope (VTT)
at the Observatorio del Teide/Tenerife. The aim of this observation was
to investigate the fine structure of the solar chromosphere seen in Hα
. We took narrow-band filtergrams (Delta lambda ~ 72 mÅ) by scanning
through this line. Broad-band images taken strictly simultaneously
with the narrow-band filtergrams were restored by speckle methods. The
instantaneous optical transfer function from this restoration procedure
was used for the reconstruction of the narrow-band images. Some results
of this high spatial resolution observation are presented below.
Title: From the Gregory-Coudé Telescope to GREGOR: a development
from past to future. Summary of workshop held in Göttingen, July
24-26, 2002
Authors: Kneer, F.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2003AN....324..283K
Altcode:
No abstract at ADS
Title: Study of polar faculae by means of spectro-polarimetric
observations and radiative transfer calculations
Authors: Okunev, O.; Kneer, F.
Bibcode: 2003AN....324..330O
Altcode:
Polar faculae are of special interest for solar physics because of
their close relationship to the global magnetic field of the Sun and to
solar activity, and because of the recently found kilogauss magnetic
fields, which are very unusual for the structures outside active
regions at high latitudes of the Sun. The idea is that polar faculae
can be represented by bundles of unresolved small-scale magnetic flux
tubes, which are characterized by sizes of about 100 km and strong
magnetic fields. High resolution spectro-polarimetric observations
of the considered structures were performed and complemented by the
radiation transfer calculations with oblique rays passing through an
inhomogeneous magnetic medium. The recent results of observations and
numerical calculations are presented.
Title: Speckle spectro-polarimetry of magnetic structures
Authors: Janssen, K.; Kneer, F.
Bibcode: 2003AN....324..328J
Altcode:
Two-dimensional spectrograms were obtained with the Vacuum Tower
Telescope, Tenerife, in order to study small-scale structures and
faculae on the Sun. Using the speckle reconstruction method, we obtain
high-resolution images and wavelength scans. Magnetic fields can be
studied from Stokes V profiles, and velocity maps are gained by the
Doppler shift of the center of gravity of Stokes I. Here some results
about small-scale structures and their magnetic fields are shown.
Title: High resolution 2D spectro-polarimetric observations of the
quiet Sun in two iron lines
Authors: Domínguez Cerdeña, I.; Kneer, F.; Sánchez Almeida, J.
Bibcode: 2003AN....324..327D
Altcode:
No abstract at ADS
Title: A Fabry-Perot spectrometer for high-resolution observation
of the Sun
Authors: Kneer, F.; Al, N.; Hirzberger, J.; Nicklas, H.; Puschmann,
K. G.
Bibcode: 2003AN....324..302K
Altcode:
No abstract at ADS
Title: Short-period waves in the solar atmosphere
Authors: Wunnenberg, M.; Andjić, A.; Kneer, F.
Bibcode: 2003AN....324..356W
Altcode:
No abstract at ADS
Title: Evidence for short-period acoustic waves in the solar
atmosphere
Authors: Wunnenberg, M.; Kneer, F.; Hirzberger, J.
Bibcode: 2002A&A...395L..51W
Altcode:
Short-period acoustic waves are thought to supply the energy for the
radiative losses of the non-magnetic chromosphere of the Sun and,
in general, of late-type stars. Here, we present evidence for the
existence of waves in the solar atmosphere with periods in the range
of 50 s <P< 100 s. Two-dimensional time sequences with a cadence
of 25 s were obtained from quiet Sun disk center in Fe I 5434 Å. The
observations were performed with the ``Göttingen'' Fabry-Perot
spectrometer in the Vacuum Tower Telescope at the Observatorio del
Teide/Tenerife. They are subjected to speckle reconstruction and to a
wavelet analysis. The atmospheric ranges forming the velocity signals
are narrowed by linear combinations of Doppler maps from wavelengths
near line center. The power in the short-period range is concentrated
above intergranular spaces. We estimate an acoustic flux into the
chromosphere of approximately 3*E6 erg cm-2
s-1, as needed for the chromospheric radiative losses.
Title: Formation of granular intensity fluctuations on the Sun
Authors: Pérez Rodríguez, E.; Kneer, F.
Bibcode: 2002A&A...395..279P
Altcode:
We deal with granular intensity fluctuations in the Sun and how they are
caused by temperature fluctuations. We calculate temperature response
functions RFT(lambda ,z) in the solar photosphere. We
apply the Diagonal Element Lambda Operator (DELO) method to
calculate the response functions for the continuum (lambda lambda =
380 nm-2.5 mu ). LTE is assumed throughout. It is demonstrated that,
within the framework of the linear approximation, the H-
continuum absorption plays an important role for the formation of the
fluctuations in continuum images. It counteracts substantially the
intensity fluctuations caused by variations of the Planck function. The
temperature response functions are used to calculate, for three models
of temperature fluctuations, the continuum intensity contrasts and
their formation height for the above wavelength range.
Title: GREGOR, a 1.5 m Gregory-type telescope for solar observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Soltau, W. Schmidt
D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2002NCimC..25..689K
Altcode:
No abstract at ADS
Title: Weak magnetic flux features on the Sun
Authors: Hartj, B.; Kneer, F.
Bibcode: 2002A&A...385..264H
Altcode:
We observed weak magnetic flux in enhanced Ca K network and in its
surroundings at various positions on the solar disc (cos theta
=1.0dots0 .1). The Gregory Coudé Telescope at the Observatorio
del Teide on Tenerife was used together with an image scanner and a
Stokes V polarimeter. 1/2(Ilambda +/- Vlambda
) spectrograms in the 6302 Å region were taken. A second order flat
fielding and smoothing brought the polarimetric sensitivity to a
detection limit of 0.001 Ic (with Ic = continuum
intensity) for exposure times of 300 ms in pixels corresponding to
0.5''x0.19'' (widehat = slit width x pixel size of detector). We present
correlations between continuum intensity, line center intensity, and
magnetic flux and discuss Stokes V profiles. The V amplitude asymmetry
from areas with strong flux decreases towards the limb, a change of
sign is not identified. The magnetic fields are ordered around strong
flux centers with decreasing mean field strength outwards from the
centers. In the range of 1x 1016 Mx to 5x 1016
Mx (in one pixel of 5x 1014 cm2) the probability
distribution of the magnetic flux is ~ |Phi |-q with q~3,
different from a Gaussian distribution. From this we derive tentativily
a radial dependence from the flux centers of the form Phi (r) ~
1/r. For the probability distributions a significant center-to-limb
variation is not detected. Together with the center-to-limb behaviour
of the asymmetries, we interpret this tentatively as an isotropic
orientation of the weak magnetic flux elements and of the gas flows. A
vertical orientation of the magnetic fields is also compatible with
the measurements, though.
Title: Dynamics of small features in the solar chromosphere
Authors: Al, N.; Bendlin, C.; Kneer, F.
Bibcode: 2002A&A...383..283A
Altcode:
A spatially highly resolved time series of a quiet region at disk
centre taken in the Na D_2 line with the ``Göttingen'' spectrometer
in the Vacuum Tower Telescope/Tenerife was used to study the dynamic
behaviour of different chromospheric features. Two CCDs were employed
for simultaneous exposures of narrow-band and ``white-light'' images
forming 128 repetitive ``scans'' of 30 images each in nearly two
hours. The data set yielded line profiles over the two-dimensional field
of view which served to create various maps indicating, e.g., vertical
velocities or minimum line intensities. The analysis of the features of
interest was further based on maps displaying the horizontal velocity
field derived from white-light images. The occurrence of various
bright points was monitored, showing quite different properties. Some
persistent intra-network bright points follow the horizontal flows of
the underlying photosphere. During the time series, a ``K grain'' is
seen to occur several times at the same location. The power spectrum
of the fluctuation of the minimum intensity of its corresponding Na
D_2 mean line profile exhibits a pronounced absolute maximum for a
period of about 24 min. The behaviour of several peculiar intra-network
bright points still lacks an unequivocal interpretation. Possibly, the
signature of gravity waves has been detected. Further investigation
revealed that the power spectra of the fluctuation of Doppler shift
and minimum intensity calculated for one of these bright points both
show an absolute maximum for a period of 4.3 min. In the course of
these fluctuations, time spans of regular oscillations were noticed
for all such points lasting from about 15 to 25 min. Power spectra
calculated only over these times exhibit pronounced absolute maxima
in a rather narrow frequency range.
Title: GREGOR: a 1.5 m telescope for solar research
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Berkefeld, Th.;
Kneer, F.; Staude, J.
Bibcode: 2001AN....322..353V
Altcode:
GREGOR will be a new 1.5 meter solar telescope at the Teide Observatory,
Izaña, Tenerife. It will provide observations with high polarimetric
precision at visible and infra-red wavelengths from Earth in the
solar photosphere and chromosphere with a resolution of 70 km on the
Sun. GREGOR replaces the 45 cm Gregory Coudé telescope.
Title: Speckle spectro-polarimetry of magnetic structures
Authors: Janssen, K.; Kneer, F.
Bibcode: 2001AN....322..383J
Altcode:
High spatial resolution images with spectral and polarimetric
information were obtained with the Vacuum Tower Telescope (VTT),
Tenerife, in order to study dynamic small scale structures on
the Sun. With the Göttingen Fabry-Perot interferometer (FPI)
we observed faculae at different ( cos , theta ) positions on the
Sun. Simultaneously with these narrow-band images, 150 broad-band
images were taken from which high resolution images are calculated
with the spectral ratio and the speckle masking methods. With the aid
of the transfer functions, gained with the speckle recontruction,
the narrow-band FPI images can be reconstructed as well. Here our
first results are presented.
Title: Post-focus instrumentation for GREGOR
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2001AN....322..361K
Altcode:
GREGOR is a high-resolution solar telescope with an aperture of 1.5
m. It will be equipped with an Adaptive Optics system and is designed
for high-precision measurements of magnetic fields and plasma motions
in the solar photosphere and chromosphere with a resolution of 70
km on the Sun. GREGOR will replace the Gregory Coudé Telescope at
the Observatorio del Teide on Tenerife. In concert with the other
solar telescopes at Teide Observatory it will be useful for studying
the dynamics of the solar atmosphere and the underlying physical
processes. GREGOR will also serve as a test bed for next generation
solar telescopes. We discuss briefly the postfocus instrumentation
of GREGOR.
Title: A Fabry-Perot spectrometer for high-resolution observation
of the Sun
Authors: Kneer, F.; Hirzberger, H.
Bibcode: 2001AN....322..375K
Altcode:
Fabry-Perot interferometers (FPIs) are powerful instruments for
spectro-polarimetry of the Sun with high spatial resolution. They
allow easy image reconstruction of two-dimensional fields of view. Some
examples of high quality results obtained with the ``Göttingen'' FPI
spectrometer, mounted in the Vacuum Tower Telescope at the Observatorio
del Teide/Tenerife, are presented in a poster to this workshop. We thus
concentrate on the design of a new instrument for the 1.5 m GREGOR solar
telescope. We discuss the pros and cons of telecentric and collimated
mounting and describe the expected performance, especially the spectral
resolution, of our design.
Title: 2D-spectroscopy of the Evershed flow in sunspots
Authors: Hirzberger, J.; Kneer, F.
Bibcode: 2001A&A...378.1078H
Altcode:
The radial variation of the Evershed flow in two small sunspots (NOAA
8737 and NOAA 9145) is studied by means of two-dimensional spectrograms
of high spatial resolution. We find a systematic decrease of the
flow velocity with photospheric height and a shift of the velocity
maximum towards larger penumbral radii in higher layers but no clear
correlation between flow velocity and continuum intensity. At the
outer penumbral boundaries the Evershed flow ceases abruptly and even
downward directed flow velocities in the deepest probed photospheric
layers were found. Furthermore, granules adjacent to the penumbral
boundary show a systematic redshift of their spot-side parts which is
attributed to fast, eventually supersonic, downflows between them and
the penumbral boundary.
Title: GREGOR: high resolution solar observations from 1 AU
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.;
Staude, J.; Pailer, N.
Bibcode: 2001ESASP.493..417V
Altcode: 2001sefs.work..417V
No abstract at ADS
Title: High resolution 2D-spectroscopy of granular dynamics
Authors: Hirzberger, J.; Koschinsky, M.; Kneer, F.; Ritter, C.
Bibcode: 2001A&A...367.1011H
Altcode:
Spectroscopic data with high spatial resolution are used to study the
granular dynamics of the Sun. The observations were obtained with the
``Göttingen'' two-dimensional (2D) Fabry-Perot interferometer in the
Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. Time
sequences of spectral scans across the non-magnetic Fe I 5576 Å
line were taken from disc center. The 2D spectroscopic images were
reconstructed with speckle methods, from which a spatial resolution
of 0\farcs4-0\farcs5 was achieved. A power and coherence analysis of
intensity and velocity maps from different photospheric heights has been
carried out. The coherence between intensity and velocity fluctuations
stays high for structural scales >0\farcs5, which underlines the
high spatial resolution of the data. Furthermore, the vertical flow
field and its time evolution within exploding granules have been
analyzed. We find fast downflows in the dark centers of exploding
granules with velocities up to 1.2 km s-1. Additionally, we
estimated the flow velocities of so-called ``dark dots''. We discuss
indications that these structures represent a new type of downflow
within the centers of bright granules.
Title: A Fabry-Perot Spectrometer for High-Resolution Observation
of the Sun
Authors: Kneer, F.; Hirzberger, J.
Bibcode: 2001AGM....18S1005K
Altcode:
Fabry-Perot interferometers (FPIs) are powerful instruments for
spectro-polarimetry of the Sun with high spatial resolution. They
allow easy image reconstruction of two-dimensional narrow-band fields
of view. Some examples of high quality results obtained with the
``Göttingen'' FPI spectrometer, mounted in the Vacuum Tower Telescope
at the Observatorio del Teide/Tenerife, are presented in a poster to
this workshop. We thus concentrate on the design of a new instrument
for the 1.5 m GREGOR solar telescope. We discuss the pros and cons
of telecentric and collimated mounting and describe the expected
performance, especially the spectral resolution, of our design.
Title: Speckle Spectro-Polarimetry of Small-Scale Magnetic Structures
Authors: Janßen, K.; Koschinsky, M.; Kneer, F.
Bibcode: 2001ASPC..236..407J
Altcode: 2001aspt.conf..407J
No abstract at ADS
Title: GREGOR, a 1.5 m Gregory-type Telescope for Solar Observation
Authors: Kneer, F.; Hofmann, A.; von der Lühe, O.; Schmidt, W.;
Soltau, D.; Staude, J.; Wiehr, E.; Wittmann, A. D.
Bibcode: 2001AGM....18.P223K
Altcode:
GREGOR is a high-resolution solar telescope with an aperture of 1.5
m. It will be equipped with an Adaptive Optics system and is designed
for high-precision measurements of magnetic fields and plasma motions
in the solar atmosphere and chromosphere with a resolution of 70 km
on the Sun. GREGOR will replace the Gregory Coudé Teleskope at the
Observatorio del Teide on Tenerife. We describe the optical design
and the focal plane instrumentation. In concert with the other solar
telescopes at Teide Observatory it will be useful for studying the
dynamics of the solar atmosphere and the underlying processes. GREGOR
will also serve as a test bed for next generation solar telescopes.
Title: Speckle spectro-polarimetry of solar magnetic structures
Authors: Koschinsky, M.; Kneer, F.; Hirzberger, J.
Bibcode: 2001A&A...365..588K
Altcode:
We present speckle observations of small-scale magnetic structures
on the Sun. They were obtained with the ``Göttingen'' Fabry-Perot
interferometer (FPI) in the Vacuum Tower Telescope at the Observatorio
del Teide, Tenerife, from quiet and active regions close to disc
center. A Stokes V polarimeter was added to the FPI to measure
V profiles in the Fe I 6302.5 Å line. The setup allows image
reconstruction with speckle methods. The achieved spatial resolution
in the magnetograms is 0farcs4 -0farcs5 . We describe the observational
technique and the data reduction. The results from small-scale magnetic
flux elements in a very quiet region, in an active region with pores
and abnormal granulation, and in a sunspot with its surroundings
are discussed. In the quiet Sun, granular dynamics dominate the
time evolution of the magnetic elements. Flux occurs in both bright
intergranular points and in dark intergranular spaces. Likewise,
with the present spatial resolution, no preference of magnetic flux in
abnormal granulation in an active region can be found. Flux occurs in
both bright (abnormal) small-scale granules and in the darker spaces
in between them. The small sunspot studied has very little magnetic
flux in its ambient quiet regions, especially no strong, conspicuous
concentrations of returned flux, i.e. of flux with polarity opposite
to that in the sunspot.
Title: Speckle Spectro-Polarimetry of Magnetic Structures
Authors: Janßen, K.; Kneer, F.
Bibcode: 2001AGM....18.P222J
Altcode:
High spatial resolution images with spectral and polarimetric
information were obtained with the Vacuum Tower Telescope (VTT),
Tenerife, in order to study dynamic small scale structures on the
Sun. With the Göttingen Fabry-Perot interferometers (FPIs) we observed
faculae at different cos θ positions on the Sun. Simultaneously with
these narrow-band images, 150 broad-band images were taken from which
high resolution images can be calculated with the Spectral Ratio
and the Speckle Masking methods. Once these high resolution images
are completed, the narrow-band FPI images can be reconstructed as
well. Here we present our first results.
Title: Granular velocities of the Sun from speckle interferometry
Authors: Krieg, J.; Kneer, F.; Koschinsky, M.; Ritter, C.
Bibcode: 2000A&A...360.1157K
Altcode:
We present observations of granular velocities and their relation to
the granular intensity pattern from the disc center of the Sun. They
were obtained in June 1997 with the Vacuum Tower Telescope at the
Observatorio del Teide on Tenerife. High spatial resolution, 0''4-0''5
for the velocities, was achieved with speckle methods applied to
two-dimensional narrow- band images in Na D2 from the
"Göttingen" Fabry- Perot interferometer. The velocity observations
refer to a geometric height of 50-200 km (above τ5 =
1). Velocity amplitudes of ±2.2 km s-1 are seen. The high
velocity regions are small-scale and the upflows coincide frequently
with the bright borders of granules or with small-scale brightenings. A
statistical analysis reveals only a rough consistency to the -5/3 or
-17/3 law of log(power) vs. log(wavenumber) expected for isotropic
turbulence (cf. Espagnet et al. 1993). We consider an agreement with
such power laws as accidental since the intensity and velocity power
spectra found here decrease smoothly from flat ones at low wavenumbers
to steep ones at high wavenumbers. The coherence of velocity and
intensity fluctuation stays above 0.5 up to horizontal wavenumbers
kh ≍ 11 Mm-1 0.8 " and the phase difference
between intensity and velocity stays stable down to structures of
0''5. While the intensity pattern exhibits a clearly non-Gaussian
distribution, the velocity distribution can be represented by a Gaussian
with a "macroturbulent" velocity of 0.825 km s-1.
Title: Die Rotation der Sonne
Authors: Janßen, Katja; Kneer, Franz
Bibcode: 2000A&R....58...40J
Altcode:
Sonnenspektren hoher spektraler Auflösung sind vielfach begehrt. Sie
werden im Unterricht, in Arbeitsgemeinschaften und in Vorlesungen
verwendet. Anhand solcher Spektren lässt sich vieles aus dem wichtigen
Gebiet der astronomischen Spektroskopie erklären und lernen.
Title: On weak magnetic flux structures of the Sun
Authors: Stolpe, F.; Kneer, F.
Bibcode: 2000A&A...353.1094S
Altcode:
The micro-image-scanner of the Gregory Coudé Telescope at the
Observatorio del Teide/Tenerife is used for observations of structures
with weak magnetic flux in the solar atmosphere. The methods for
the data reduction are chosen carefully to achieve the highest
possible performance, also with respect to the needs of handling
large amounts of data. A lower detection limit of magnetic flux on
the Sun of 2 108 Wb on an area of 0\farcs67x0\farcs67 was
achieved. Magnetic structures of weak magnetic flux are observed using
different spectral lines in network and intra-network regions at disk
center of the Sun. The present study deals mainly with a high spatial
resolution observation in the magnetically sensitive Fe I 630.25
nm line. We find that weak flux may appear almost everywhere in the
granular pattern, with a 65 %\ preference for intergranular spaces. Some
elements have diameters of 2\arcsec-3\arcsec with substructure and
distorted shape. Many elements are very small, <1\arcsec. From the
ubiquitous appearance of weak magnetic flux structures in the granular
pattern, in regions of both upward and downward flow, and in regions
of line weakening and line enhancement, we argue that their magnetic
pressure may not exceed the granular flow pressure. This gives an
upper limit of the amplitude of the magnetic flux density of |B|=5.5
10-2 T in agreement with the result by Keller et al. (1994)
1 Wb = 1 V s = 108 Mx 1 T = 1 V s m-2
= 104 Gauss.
Title: Universitäts-Sternwarte. Jahresbericht für 1999.
Authors: Kneer, F.
Bibcode: 2000MitAG..83..339K
Altcode:
No abstract at ADS
Title: GREGOR: A 1.5m Telescope for Solar Research
Authors: von der Lühe, O.; Schmidt, Wolfgang; Soltau, Dirk; Kneer,
Franz; Staude, Jurgen
Bibcode: 2000ESASP.463..629V
Altcode: 2000sctc.proc..629V
No abstract at ADS
Title: High Resolution 2D-Spectroscopy of the Sun
Authors: Hirzberger, J.; Kneer, F.
Bibcode: 2000HvaOB..24...89H
Altcode:
With the "Goettingen" Fabry-Perot interferometer which is installed
at the Vacuum Tower Telescope in Izana, Tenerife the non-magnetic
FeI 5576 A line was scanned in a field of quiet granulation at solar
disk center. From the simultaneously obtained broad band images an
instantaneous optical transfer function (OTF) of the Earth's atmosphere
was computed with speckle interferometric methods. Restoring the narrow
band data from this OTF the spatial resolution of the two-dimensional
spectra was successfully enhanced by a factor of two.
Title: Coordinated Observation of a Small Magnetic Region with SUMER
and GCT
Authors: Wilken, V.; Kneer, F.; Curdt, W.
Bibcode: 1999ESASP.446..711W
Altcode: 1999soho....8..711W
No abstract at ADS
Title: Coordinated Observation Of A Small Magnetic Region With GCT
And SUMER
Authors: Wilken, V.; Kneer, F.; Curdt, W.
Bibcode: 1999ESASP.446..707W
Altcode: 1999soho....8..707W
The SUMER instrument and the Gregory-Coude-Telescope (GCT) at the
Observatorio del Teide/Tenerife have been used for observations
of a small magnetic region in the solar atmosphere. With the GCT
2-dimensional spectro-polarimetry was performed and later analysed
by applying the "centre of gravity method" (Semel, 1967) to measure
the longitudinal magnetic flux. In this way we obtained seven
magnetograms of the target area during the three hours campaign,
while SUMER performed every five minutes EUV-scans in two spectral
lines. The magnetograms show two cells with diameters of approximately
20000 km and 26000 km. Inside the cells the measured flux within the
resolution element of 0.76 arcsec*0.76 arcsec. was ~1*109 Wb
(corresponding to 33 Gauss flux density) on average. On the other hand
the boundary regions have fluxes up to 1*1010 Wb. SUMER's
intensity images of Si II (Chromosphere) and C IV (Transition region)
show strong brightenings which very well resemble the magnetic flux
pattern in the Photosphere. The intensities of Ne VIII (lower Corona)
correlate with the magnetic flux as well, but far less than the former
two lines. During the 3h observing run the coarse magnetic structure
remains essentially unchanged. However, on scales smaller than 3
arcsec., the temporal intensity fluctuations of the EUV-lines show
little similarity, if any. Possibly, at small scales the different
EUV-lines originate from different structures which are not excited
simultaneously.
Title: Sonnenspektren für den Unterricht.
Authors: Janßen, K.; Kneer, F.
Bibcode: 1999S&W....38..454J
Altcode:
No abstract at ADS
Title: Height variation of the solar granulation
Authors: Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky, M.;
Ritter, C.
Bibcode: 1999A&A...343..983K
Altcode:
We analyze spectral scans of narrow-band images across the Na
D_2 line. They were obtained from disc centre of the Sun with our
Fabry-Perot interferometer (FPI) in the Vacuum Tower Telescope at
the Observatorio del Teide. The FPI was set to a bandwidth of 200
mAngstroms FWHM and the spectral stepwidth was 100 mAngstroms. Our aim
is to study the variation of the granulation pattern with height in
the atmosphere. To achieve high spatial resolution the simultaneously
taken broad-band images are restored with speckle methods. With the
knowledge of these reconstructed images we were also able to restore
the narrow-band images. The formation heights are found from temperature
response functions RF_T. In the wings of NaD_2, the response functions
for different wavelengths exhibit a substantial overlap in atmospheric
heights. Therefore we use linear combinations which allow a better
height discrimination. Applying the same combinations to the D_2 images
we can visualize the height variation of the granular pattern. The
granular intensity fluctuations are a matter of the deep photosphere
alone. They disappear at heights of about 100 km (above tau_ {5000}=1).
Title: Erratum: Two-dimensional spectroscopic observations of
chromospheric oscillations
Authors: Al, Nurol; Bendlin, Cornelia; Kneer, Franz
Bibcode: 1999A&A...342..311A
Altcode:
For technical reasons, Figs. ref{f8} and ref{f9} were printed with
incorrect colour separation. To rectify this unfortunate error, the
correct Figs. ref{f8} and ref{f9} with the captions are reprinted below.
Title: Solar Observation with High Spatial and Spectral Resolution
Authors: Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky, M.; Ritter,
C.; von der Lühe, O.
Bibcode: 1999AGAb...15...88K
Altcode: 1999AGM....15..P03K
The solar atmosphere is, like stellar atmospheres in general,
highly dynamic on small scales. To understand, e.g., the generation
of turbulent motion and waves or chromospheric and coronal heating
processes, observations with high resolution are needed. We achieve
both high spatial and spectral resolution with a double Fabry-Perot
interferometer which allows image restoration with speckle methods. Our
new results include the height variation of the granular intensity
pattern and velocity maps of unprecedented spatial resolution.
Title: Universitäts-Sternwarte. Jahresbericht für 1998.
Authors: Kneer, F.
Bibcode: 1999MitAG..82..329K
Altcode:
No abstract at ADS
Title: Spectral and Spatial High Resolution Observations of the Sun
with Image Reconstruction and Deconvolution Methods
Authors: Koschinsky, M.; Krieg, J.; Kneer, F.; Ritter, C.; Wunnenberg,
M.
Bibcode: 1999ASPC..183..349K
Altcode: 1999hrsp.conf..349K
No abstract at ADS
Title: GREGOR, a 1.5 M Solar Telescope
Authors: von der Lühe, O.; Schmidt, W.; Soltau, D.; Kneer, F.;
Staude, J.
Bibcode: 1999AGAb...15....5V
Altcode: 1999AGM....15..A01V
We present the design of a new large solar telescope which is going to
be installed at the Observatorio del Teide, in the tower that presently
houses the 45cm Gregory-Coudé-Telescope. The new telescope has an
aperture of 1.5 meters and its optical design is basically a Gregorian
configuration. It will be an open telescope in an azimuthal mount. An
adaptive optics system is incorporated in the optical design as well as
a polarimetry package. The feasibility of lightweight optics for the
primary mirror has been investigated in an industrial pre-study. The
focal plane instrumentation will include a high resolution filter
spectrometer similar to the existing TESOS instrument at the VTT and
a new spectro-polarimeter for the visible and the near UV. The latter
instrument is presently being developed jointly by the KIS and the
High Altitude Observatory in Boulder, USA. Budget permitting, detailed
(Phase-B) planning will start in 2000, and the telescope will be
developed and built in 2002 and 2003 with first light in spring of 2004.
Title: Granular Intensities and Velocities with High Spatial
Resolution
Authors: Ritter, C.; Krieg, J.; Wunnenberg, M.; Kneer, F.; Koschinsky,
M.
Bibcode: 1999AGAb...15....7R
Altcode: 1999AGM....15..A04R
We present results obtained with our two-dimensional spectrometer in the
solar Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. The
spectrometer is based on Fabry-Perot interferometers which scan in
wavelength across spectral lines. The observations were taken in the
Na D_2 line from a quiet region at the disc centre of the Sun. The
data analysis uses speckle methods for image restoration. We find
the heights in the solar atmosphere where the measured intensity
and velocity fluctuations originate from Response Functions. The
granular intensity pattern disappears at 100 - 150 km (above tau_5 =
1). The granular velocities are high with amplitudes of 2.5 km s^{-1},
while the rms velocity is 0.6 km s^{-1}. Thus, the velocity field is
highly intermittent.
Title: Coordinated Observation of a Small Magnetic Region with SUMER
and GCT
Authors: Wilken, V.; Kneer, F.; Curdt, W.
Bibcode: 1999AGAb...15...91W
Altcode: 1999AGM....15..P08W
The SUMER instrument and the Gregory-Coudé-Telescope (GCT) at the
Observatorio del Teide/Tenerife have been used for observations of
a small magnetic region in the solar atmosphere. The GCT measured
the longitudinal magnetic flux, while SUMER performed EUV-scans. The
magnetograms show two cells with diameters of approximately 23 000
km. Inside the cells the flux within the resolution element of 0.''76 x
0.''76 was 1 ṡ 10^9 Wb (corresponding to 33 Gauss flux density). The
boundary regions have fluxes up to 1 ṡ 1010 Wb. SUMER's
intensity images of Si II (Chromosphere) and C IV (Transition Region)
show strong brightenings which very well resemble the magnetic flux
pattern in the Photosphere. The intensities of Ne VIII (lower Corona)
correlate with the magnetic flux far less than the former two lines. On
small scales the temporal intensity fluctuations of the EUV lines show
only little similarity among each other. Possibly, at small scales the
different EUV lines originate from different structures which are not
excited simultaneously.
Title: Solar Observation with high spectral resolution and image
restoration.
Authors: Koschinsky, M.; Kneer, F.; Wunnenberg, M.; Krieg, J.;
Ritter, C.
Bibcode: 1999AGAb...15....6K
Altcode: 1999AGM....15..A02K
Observations of the Sun with both, high spectral and high spatial
resolution are needed for an understanding of the structure, the
dynamics and the temporal evolution of small scale features in the
solar photosphere. To achieve this, high technical requirements on
the optical setup have to be satisfied, and also extensive methods
of data reduction and image restoration are needed. We present our
approach to obtain high resolution observational data by using a
double Fabry-Perot interferometer at the Vacuum Tower Telescope (VTT)
on Tenerife. The spectral resolution is about 30 mÅ. Different methods
of image reconstruction to remove distortions by seeing and telescopic
aberrations to a high extent are presented as well.
Title: Fine structure of polar faculae.
Authors: Makarov, V. I.; Okunev, O. V.; Pravdjuk, L. M.; Kneer, F.
Bibcode: 1999joso.proc..151M
Altcode:
Using photoheliograms with high spatial resolution (SSSO) it was found
that polar faculae consists of a few fine structure elements with the
sizes ranging from 200 km to 400 km (the telescopic resolution limit
is about 180 km). Presumably, polar faculae are caused by magnetic
flux concentrations and their shape reminds of the set of flux tubes.
Title: Some remarkable bright points seen in the Na D2
line.
Authors: Al, N.; Bendlin, C.; Kneer, F.
Bibcode: 1999joso.proc..133A
Altcode:
A spatially highly resolved time series of the solar chromosphere
was taken with a two-dimensional spectrometer mounted in the German
Vacuum Tower Telescope (VTT) on Tenerife. The authors obtained
narrow-band filtergrams (Δλ = 30 mÅ) and white-light images strictly
simultaneously with two CCDs. Repetitive tuning of a Fabry-Perot
interferometer allowed to scan through the Na D2 line (5890
Å) in short intervals. In this contribution, the authors focus on
bright points in the intra-network using different diagnostic tools
to study their temporal and oscillatory behaviour.
Title: Diagnostics and Dynamics of the Solar Chromosphere
Authors: Kneer, F.; von Uexküll, M.
Bibcode: 1999ASSL..239...99K
Altcode: 1999msa..proc...99K
Research of the chromosphere of the Sun is exciting, as it has been over
more than a century. The present contribution can only give glimpses
into the wealth of chromospheric structures and dynamics. Likewise, in
view of the limited space, it is not possible to present in due balance
the published work on the solar chromosphere. The reader is referred to
the monographs by Bray and Loughhead (1974, with a historical account
and many references to early work) and by Athay (1976) for the state
of knowledge two decades ago. Among the conferences dealing meanwhile
with the chromosphere we mention the proceedings edited by Bonnet and
Delache (1976) and by Ulmschneider et al. (1991). Withbroe and Noyes
(1977) treated the mass and energy flow in the chromosphere and corona;
a throughout account of the Ca II K2v cell grains is given
by Rutten and Uitenbroek (1991); the review by Narain and Ulmschneider
(1996) deals with chromospheric and coronal heating mechanisms. Last
but not least, the book ``Le Soleil'' by Secchi (1877) is historically
precious and full of still viable ideas.
Title: Two-dimensional spectroscopic observations of chromospheric
oscillations
Authors: Al, Nurol; Bendlin, Cornelia; Kneer, Franz
Bibcode: 1998A&A...336..743A
Altcode:
The oscillatory behaviour of the solar chromosphere was studied
from observations of a quiet region at disk centre using various
diagnostic tools. The two-dimensional spectrometer in the Vacuum Tower
Telescope/Tenerife (Spain) served to obtain a spatially highly resolved
time series of ``white-light'' images and narrow-band filtergrams in
the Na D_2 line. With a tunable Fabry-Perot interferometer, this line
was scanned taking 30 images (i.e. a ``scan'') around the line core
with wavelength steps of 30 m Angstroms and a spectral resolution
of about 200 000. From these images, line profiles were derived for
every pixel in the field of view. With each such narrow-band scan, a
scan of ``white-light'' images was taken strictly simultaneously. The
whole time series comprises (2x) 128 scans. Every 56 s, a new pair of
scans was started with two CCDs, thus the observation covers nearly
two hours. Finally, after correlation and other reduction procedures,
a field size of 69\farcs4 x 50\farcs4 remained with 0\farcs2/pixel
on the CCD-chips. In the data reduction, new images were created
representing the minimum intensity (I) of each line profile in the
field of view, and also velocity (V) maps (derived from the Doppler
shifts of the line profiles) for all 128 scans. >From these images,
power spectra and diagnostic diagrams were computed. In the subsequent
analysis, a distinction between network and intra-network regions was
made where this seemed appropriate. One- and two-dimensional (V-I) phase
and coherence spectra were analysed with regard to oscillations and to
the nature of the waves leaving their marks in these diagrams. Several
noteworthy results also raised the question of the actual line formation
height of Na D_2, among them being the non-detection of a chromospheric
eigenmode. While an explanation for a conspicuous 70(deg) plateau in a
small region of the phase spectra already exists, the suspected reason
behind the decreasing phase difference from about -60(deg) for the
f-mode down to ~ -120(deg) for higher modes is still subject to some
speculation. Moreover, the data gave evidence of gravity waves, probably
discovered for the first time in a V-I phase spectrum of Na D_2.
Title: MISC, an instrument for multi-dimensional spectroscopy
Authors: Stolpe, F.; Kneer, F.
Bibcode: 1998A&AS..131..181S
Altcode:
A new instrument for multi-dimensional spectroscopy at the Gregory
CoudeTelescope (GCT) at the Observatorio del Teide/Tenerife is
presented. The system, consisting of a scanning device, a Stokes-V
polarimeter and a suitable CCD system, performs fast spatial
scanning. It preserves the good polarization properties of the
telescope by using a Bowen compensator. We describe the instrument,
the different modes of operation, and how it is controlled and give
some performance data.
Title: Oscillations of the Sun's chromosphere. VIII. Horizontal
motions of CA II K bright points
Authors: Wellstein, S.; Kneer, F.; von Uexkuell, M.
Bibcode: 1998A&A...335..323W
Altcode:
We present a re-analysis of a time series of solar disc centre Ca ii
K2v filtergrams taken with the Vacuum Tower Telescope at the
Observatorio del Teide/Tenerife. We concentrate on the measurements of
proper motions of K grains in the internetwork regions and of bright
points in the chromospheric network. For the K grains we find horizontal
velocities of 2-15 km s(-1) , values much lower than those deduced by
Steffens et al. (1996) from a smaller sample, analyzed differently. In
accord with our earlier conclusion from k-omega diagrams (Kneer &
von Uexkuell 1993) and with numerical simulations by Carlsson &
Stein (1997) high-frequency (pseudo-) p-modes can viably explain the
K grains. Yet, the rareness of the K grains may indicate a connection
to magnetic fields. The proper motions of the network bright points
are non-periodic, very impulsive, with velocities of 7-10 km s(-1)
. Estimating the energy flux if these motions are magnetic kink waves
(cf. Choudhuri et al. 1993, Muller et al. 1994), we find it sufficient
to heat the solar corona, but too small to cover the radiative losses
of the chromospheric network.
Title: Spectral line radiation from solar small-scale flux tubes. II
Authors: Hasan, S. S.; Kneer, F.; Kalkofen, W.
Bibcode: 1998A&A...332.1064H
Altcode:
We examine spectral line radiation from small-scale magnetic flux tubes
in the solar atmosphere. This is a continuation of work by Kneer et
al. (1996). The main difference with the previous investigation is in
the choice of the external atmosphere. Earlier we adopted an atmosphere
resembling the empirical quiet Sun model for the ambient medium. In the
present study, we iteratively adjust the temperature structure of the
external atmosphere to fit the Stokes I and V profiles and the average
continuum intensities with those obtained from observations. Our
models are hotter in the uppermost photospheric layers and cooler
in the deeper layers than the quiet Sun model and agree well with
semi-empirical flux tube models.
Title: Ground-based Observation with High Spatial and Spectral
Resolution
Authors: Krieg, J.; Kneer, F.; Koschinsky, M.; Ritter, C.; Starck,
J. -L.
Bibcode: 1998ESASP.417..317K
Altcode: 1998cesh.conf..317K
No abstract at ADS
Title: Dynamics of Solar Fine Structure: Observation with High
Spatial Resolution
Authors: Al, N.; Bendlin, C.; de Boer, C. R.; Denker, C.; Kneer, F.;
Schmitt, D.; Volkmer, R.; Wilken, V.
Bibcode: 1998ASPC..154..553A
Altcode: 1998csss...10..553A
The Sun is an ideal object for studying non-magnetic and magnetic
processes in cool stars. Here, we focus on fine structures of a
few 100 km in the solar atmosphere. Granular overshoot, motions
and waves of magnetic elements in the quiet Sun and in plages,
etc., all affect the atmospheric structure from the bottom of the
photosphere up to the corona and the solar wind. Observations with
high spatial resolution are required to reveal the dynamic behaviour
and to understand the underlying physical processes. During the past
five years, speckle methods have become an excellent tool to obtain
images of solar fine structure with diffraction-limited resolution. We
demonstrate by some examples how one can gain new insights from
speckle interferometry. Likewise, spectroscopy of solar fine structure
is also making rapid progress towards high spatial resolution. Our
two-dimensional, narrow-band spectrometer (Delta\lambda = 20-30 mAA
), working with a scanning Fabry-Perot interferometer, proves very
powerful in several aspects. We present some results obtained in Na D_2
from the quiet solar chromosphere. Using suitable observing techniques
together with image restoration, we aim at achieving diffraction-limited
resolution also for narrow-band spectroscopy.
Title: Universitäts-Sternwarte. Jahresbericht für 1997.
Authors: Kneer, F.
Bibcode: 1998MitAG..81..287K
Altcode:
No abstract at ADS
Title: Spectro-Polarimetry of Polar Faculae
Authors: Homann, T.; Kneer, F.; Makarov, V. I.
Bibcode: 1997SoPh..175...81H
Altcode:
This contribution deals with the properties of small-scale
magnetic elements at the polar caps of the Sun. Spectro-polarimetric
observations, obtained with high spatial resolution with the Gregory
Coudé Telescope at the Observatorio del Teide on Tenerife, were
analysed. We find, though with limited data sets, that polar faculae
differ in two aspects from faculae of the network in non-active
regions near the equator (equatorial faculae): (1) Polar faculae
appear to have the same magnetic polarity as the general polar magnetic
field. Presumably, the latter is rooted in the small-scale faculae. The
equatorial faculae show both magnetic polarities. (2) Polar faculae,
with a size of 3.5'' ± 1.3'', are larger than equatorial faculae with
2.1'' ± 0.4''. Yet as for equatorial faculae, polar faculae possess
kilogauss magnetic fields.
Title: Speckle measurements of the centre-to-limb variation of the
solar granulation.
Authors: Wilken, V.; de Boer, C. R.; Denker, C.; Kneer, F.
Bibcode: 1997A&A...325..819W
Altcode:
The Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife
was used to perform speckle observations of photospheric granulation
from disc centre to limb at λ=550+/-5nm. Images were reconstructed
with the spectral ratio technique (von der Luehe 1984) and the
speckle masking method (Weigelt 1977, Optics Comm. 21, 55, Weigelt
& Wirnitzer 1983, Optics Letters 8, 389, Lohmann et al. 1983,
Applied Optics 22, 4028). The granular rms contrast relative to the
local average intensity decreases monotonically from 13.5+/-1.0%
at disc centre to 8-9% at cosθ=0.1. The granular images as well as
the power spectra of the intensity fluctuations show a fading of the
coarse granular pattern towards the limb while smaller structures
become more pronounced off the disc centre. We identify these as the
bright worm-like structures appearing sometimes at the granular borders
(de Boer et al. 1992A&A...257L...4D). This finding supports the
result from simulations of granular dynamics by Steffen et al. (1994,
Shocks in the solar photosphere and their spectroscopic signature. In:
Schuessler M., Schmidt W. (eds.) Solar Magnetic Fields, Cambridge
Univ. Press, p. 298). There the bright structures are locations of low
pressure between the centres of granules and intergranular spaces with
penetration of hot gas into the lower photosphere.
Title: Flare Multi-Line 2D-SPECTROSCOPY
Authors: Mein, P.; Mein, N.; Malherbe, J. -M.; Heinzel, P.; Kneer,
F.; von Uexkull, M.; Staiger, J.
Bibcode: 1997SoPh..172..161M
Altcode: 1997ESPM....8..161M
A small flare was observed at the Teide Observatory on October 5,
1994. Simultaneous data were obtained at the German Vacuum Tower
Telescope (VTT) with the MSDP spectrograph providing high-resolution
imaging spectroscopy in two chromospheric lines, and the Gregory Coudé
Telescope (GCT) providing information about the magnetic field. Basic
flare characteristics are:
Title: High spatial-resolution spectropolarimetry of small-scale
solar magnetic fields.
Authors: Stolpe, F.; Kneer, F.
Bibcode: 1997A&A...317..942S
Altcode:
We present a study of the structure and the dynamics of small-scale
magnetic elements within solar plage regions. Spectropolarimetric
observations with high spatial and spectral resolutions have been
obtained for three iron lines with different sensitivities to magnetic
field strength and temperature with the Gregory Coud e Telescope at
the Observatorio del Teide/Tenerife. The main results are: (o) With
the present spatial resolution of 0.6-1.0", the continuum intensity
from magnetic areas is close to the average intensity of the quiet
Sun (+/-5%). There is a slight tendency of decreasing intensity with
increasing magnetic flux within the resolution elements. (o) Macroscopic
velocities with rms values of 1.5-2.0km/s as used to explain the widths
of the Stokes V profiles are not seen with the spatial resolution
obtained here. (o) The large observed fluctuations in the wavelength
separation of the red and blue Stokes V maxima suggest that there may
be a considerable variation in the properties of the flux tubes. This
should give constraints for modeling small-scale flux tubes.
Title: Universitäts-Sternwarte. Jahresbericht für 1996.
Authors: Kneer, F.
Bibcode: 1997MitAG..80..257K
Altcode:
No abstract at ADS
Title: 2d-Spectroscopy with the Image Scanner of the
Gregory-Coude-Telescope on Tenerife
Authors: Stolpe, F.; Koschinsky, M.; Kneer, F.
Bibcode: 1997ExA.....7..301S
Altcode:
No abstract at ADS
Title: Methods of Two-Dimensional Spectroscopy
Authors: Kneer, F.
Bibcode: 1997ASPC..118..329K
Altcode: 1997fasp.conf..329K
One of the main fields of solar research is the study of dynamic
processes of small-scale structures. For this purpose, time sequences of
spectroscopic and polarimetric information in two spatial dimensions
with best achievable quality are needed. The present contribution
deals with the ways to obtain images in small wavelength bands. Among
these are image scanners and the MSDP (Multi-Channel Subtractive
Double Pass Spectrograph). Further potential instruments are scanning
Fabry-Perot interferometers (FPI). The principles of such instruments
are discussed. The results obtained hitherto from the FPI in the Vacuum
Tower Telescope at the Observatorio del Teide are promising. Small-band,
two-dimensional spectroscopy with spatial resolution close to the
telescopic diffraction limit seems possible in the near future.
Title: Small-scale magnetic fields on the Sun.
Authors: Kneer, F.
Bibcode: 1996NAWG.1996..181K
Altcode:
Spectro-polarimetric observations with high spatial resolution of
small-scale magnetic fields are presented. They were taken from plages
near disc centre of the Sun with the Gregory Coudé Telescope at the
Observatorio del Teide/Tenerife. Stokes I and V profiles calculated from
static flux tubes are compared with the observations. The small-scale
magnetic elements have substructure and are dynamic. They possess
varying gas velocities and temporarily strong magnetic fields.
Title: On the use of Bowen compensators for polarimetry.
Authors: Koschinsky, M.; Kneer, F.
Bibcode: 1996A&AS..119..171K
Altcode:
For high precision polarimetry it is very desirable to observe with a
telescope that is free of instrumental polarization. Yet it is often
inevitable to introduce optical devices which generate crosstalk between
the Stokes Q,U components and the V component due to phase retardations
upon reflection at mirror surfaces. We show that Bowen compensators
are well suited to compensate for these phase changes. They consist
of 2 rotatable λ/8 phase retarder plates, and it can be shown that
they act on polarized light either by a rotation of the axes of the
polarization ellipse followed by a prescribed, desired phase change
or by a phase change followed by rotation of the ellipse. We present
applications of Bowen compensators in the Gregory Coude Telescope at
the Observatorio del Teide/Tenerife.
Title: High Resolution Observations of Small-Scale Magnetic Elements
and Interpretation
Authors: Kneer, F.; Stolpe, F.
Bibcode: 1996SoPh..164..303K
Altcode:
This contribution deals with the properties of small-scale magnetic
elements in plages. Spectro-polarimetric observations, obtained with the
highest possible spatial resolution with the German solar telescopes at
the Observatorio del Teide on Tenerife, were analysed. We conclude from
the spread of line parameters measured in the Stokes I and V profiles
of Fe I and Fe II lines that a wide range of magnetic properties is
realised in the solar atmosphere. The flow velocities in small-scale
magnetic flux tubes, deduced from the zero-crossing of the V profiles
at high spatial resolution, show a fluctuation of vDoppler
= 580 m s-1. This is substantially smaller than the
"turbulent" broadening velocities of vDoppler = 2 - 3 km
s−1 commonly derived by fitting V profiles from flux tube
models to low spatial resolution data, e.g. from a Fourier Transform
Spectrometer. Attempts to explain the high resolution I and V profiles
by models of hydrostatic flux tubes are discussed. It appears impossible
to accomplish agreement between the modeled and observed radiation of
lines with strong and weak magnetic sensitivity at the same time. We
suggest a scenario in which small-scale magnetic elements possess
substructure and are dynamic, with gas flows and magnetic field
strengths varying in space and time.
Title: Spectral line radiation from solar small-scale magnetic
flux tubes.
Authors: Kneer, F.; Hasan, S. S.; Kalkofen, W.
Bibcode: 1996A&A...305..660K
Altcode:
We consider spectral line radiation from small-scale magnetic model flux
tubes in the solar atmosphere. The structure of the tube is determined
from the magnetostatic equations in the thin flux tube approximation. We
assume that the tube is in energy equilibrium and pressure balance
with the ambient medium. For the latter, we construct a quiet sun model
with an artificial heating term in order to reproduce the VAL C model,
treating the medium as a plane-parallel atmosphere. The flux tube models
are parameterized by the plasma β_0_ (the ratio of gas the pressure
to the magnetic pressure), the convective efficiency parameter α,
and the radius R_0_ at height z=0 (τ_5000_=1) in the quiet sun. The
Stokes I and V profiles emerging from the models and averaged over
areas that include the neighbourhood of the flux tube are calculated
for various spectral lines with different sensitivity for magnetic
field strength and temperature. The profiles are compared with high
spatial resolution observations of plages near disc centre that have
been obtained with the Gregory Coude Telescope at the Observatorio del
Teide/Tenerife. The information contained in both I and V profiles is
found to be very useful in constraining the theoretical models. The
best match of models with observations is achieved for values of β_0_
between 0.3 and 0.5. For a sufficiently wide separation of the V extrema
of the strongly split lines, a broadening mechanism is required. Pure
velocity (microturbulent) broadening compatible with observations
of strongly split lines gives too much broadening for weakly split
lines. A broadening that is proportional to the Lande factor, i.e.,
magnetic broadening, appears to be more appropriate. This suggests
dynamic models with temporary enhancement of the magnetic field
strength. The continuum intensity of our models is higher and the
absorption and V amplitude in the FeII 6149A line are stronger than
observed. An improvement in the match between model predictions and
observations is likely to come from models in which the ambient gas
has a lower temperature as well as a lower temperature gradient than
are found in the quiet, field-free sun. Such models are currently
under development for cylindrical flux tubes.
Title: Short-period waves in small-scale magnetic flux tubes on
the Sun.
Authors: Volkmer, R.; Kneer, F.; Bendlin, C.
Bibcode: 1995A&A...304L...1V
Altcode:
We present a time sequence of spectro-polarimetric measurements
of high spatial and spectral resolution. It was obtained in the
Fe I 6302A line from a moderately active region near disc centre
of the Sun. The observation was performed with a two-dimensional
spectrometer which uses a Fabry-Perot interferometer for wavelength
scanning and to which a Stokes-V polarimeter was added. This allows
us to follow horizontal motions of structures, here of small-scale
magnetic elements, to study structural changes, and to measure vertical
velocities of the magnetized plasma. In an isolated magnetic feature
we find quasi-periodic horizontal motions with 2km/s amplitude and,
with high statistical significance, vertical oscillations of 280m/s
rms velocity and with periods near 100s. The energy flux in these
short-period waves is estimated at 1.6-2.3x10^7^ergxcm-2/s, yet only for
this isolated small feature. So the energy supply appears sufficient
for chromospheric heating in bright structures of the Ca network,
but it falls much below the needs to balance the radiative loss in
larger plage areas.
Title: A proposal for a low instrumental polarization coude
telescope. II. The German Gregory-Coude Telescope at the Observatorio
del Teide.
Authors: Sanchez Almeida, J.; Martinez Pillet, V.; Kneer, F.
Bibcode: 1995A&AS..113..359S
Altcode:
We have put into practice the technique to minimize the instrumental
polarization (IP) of coude telescopes proposed by Martinez Pillet &
Sanchez Almeida (1991): a λ/2-plate inserted into the optical path,
with the proper orientation, cancels the IP. The compensation of the
Gregory-Coude Telescope at the Observatorio del Teide turns out to
fulfil theoretical expectations. Empirical tests at 630nm demonstrate
that its IP decreases by a factor ~4. We show that the residual IP is
not intrinsic to the method but it is due to the limited precision of
the retarder presently used. In addition, observations indicate that
the insertion of the λ/2-plate does not noticeably deteriorate the
optical quality of the whole telescope. In short, this work proves
the practical soundness of the λ/2-plate technique to reduce IP.
Title: Speckle masking imaging of the moustache phenomenon.
Authors: Denker, C.; de Boer, C. R.; Volkmer, R.; Kneer, F.
Bibcode: 1995A&A...296..567D
Altcode:
We present high spatial resolution observations of a sunspot near
the solar disc centre obtained with the Vacuum Tower Telescope at the
Observatorio del Teide (Tenerife). Our investigation aims at a better
comprehension of umbral and penumbral fine structures. The speckle
masking image reconstruction technique was applied to narrow-band
filtergrams taken in the red wing of Hα. The reconstructed images
reveal small-scale structures close to the telescopic diffraction
limit of 0.19". Especially, the moustache phenomenon - the intensity
enhancement in the inner wings of strong chromospheric absorbtion lines
- is clearly discernible. The moustaches are concentrated at the outer
border of the penumbra and in its neighbourhood. There, they appear
at locations coinciding with the intergranular lanes. The moustaches
are not circular in shape. Instead they show sharp intensity peaks
surrounded by bright areas with frayed borders. Due to the high spatial
resolution achieved in the restored filtergrams and the correction for
the speckle transfer function, we find much higher intensities than
in previous investigations concerning the moustache phenomenon. The
peak intensities are sometimes larger than 1.7 times the intensity
of the granular background, i.e. the line profiles in the inner wing
of Hα exhibit emission features. Attempts to restore narrow-band
filtergrams with speckle interferometry are relatively new. In this
paper it is shown that the speckle masking technique is capable to
reconstruct images with a low signal-to-noise ratio taken within a
passband of 0.05nm.
Title: Oscillations of the Sun's chromosphere. VII. K grains
revisited.
Authors: von Uexkuell, M.; Kneer, F.
Bibcode: 1995A&A...294..252V
Altcode:
We analyse time sequences of high spatial resolution filtergrams
obtained simultaneously in Mg b_2_ and Ca K_2v_ from quiet Sun disc
centre with the Vacuum Tower Telescope at Observatorio del Teide,
Tenerife. Special attention is paid to the temporal evolution of
the bright points, or K grains, in the interior of the chromospheric
network. These represent the intensity maxima of brightness oscillations
in K_2v_ with large amplitude. The oscillations last 1 to 5 periods
of 150-240s duration and are strongly non-sinusoidal. They do not
appear to be excited by pulses from below and then to decay. They
rather exhibit the beat phenomenon of a horizontally extended wave
field. The high amplitude oscillations occur only rarely, during 5-10%
of the time, if we pose the limit that high amplitude means that the
maximum intensity is a factor 1.5 larger than the average intensity. The
power spectrum does not show any signature of a chromospheric "3 min"
mode nor of a mode at the acoustic cutoff frequency (period 3.5min),
nor do we find a power ridge at constant frequency. Instead, as in
our earlier finding, the chromospheric oscillations are ordered in
modes in continuation of the 5min modes. Modeling will thus need to
adopt that the waves propagate in a three-dimensional medium with,
at least partially, reflecting boundaries. The bright points of the
chromospheric network behave very differently, more chaotic, like
noise. They show most power at low frequencies which we interpret as
the consequence of the stochastic intensity fluctuations.
Title: Speckle-masking imaging of bright points (moustaches)
Authors: Denker, C.; Deboer, C. R.; Kneer, F.
Bibcode: 1995IAUS..176P..51D
Altcode:
No abstract at ADS
Title: Wellen in der Sonnenchromosphäre.
Authors: Kneer, F.; von Uexküll, M.
Bibcode: 1994S&W....33..871K
Altcode:
No abstract at ADS
Title: On the fluctuation of wing strengths as diagnostics of the
solar atmosphere
Authors: Kneer, F.; Nolte, U.
Bibcode: 1994A&A...286..309K
Altcode:
We investigate the feasibility to obtain information on pressure
fluctuations in the solar atmosphere from the collisionally dominated
damping wings of strong Fraunhofer lines. One expects that the
depression in the wings of lines from neutral atoms of predominantly
ionised species, as Na, Fe, Mg, is proportional to the pressure. Thus
we measured the fluctuations of continuum intensity and wing strength
of Na D_2_ and Mg b_2_ from high spatial resolution observations
obtained with the Gregory Coude Telescope at the Observatorio del
Teide/Tenerife. Strong fluctuations of the wing strengths are indeed
seen on granular scales. They are uncorrelated with the continuum
brightness. Modelling of atmospheric temperature and pressure
fluctuations, including response functions for intensity and wing
strength, shows that temperature fluctuations have larger effects, by
factors 5-6, than pressure perturbations upon both continuum brightness
and wing strength. With precise measurements of temperature fluctuations
and analysing the wing strengths of several lines which differ in
their pressure dependence the detection of pressure fluctuations will
become possible.
Title: Speckle observations of solar granulation
Authors: de Boer, C. R.; Kneer, F.
Bibcode: 1994IAUS..158..398D
Altcode:
No abstract at ADS
Title: Spectro-polarimetry of magnetic elements with high spatial
resolution
Authors: Kneer, F.; Amer, M. A.
Bibcode: 1994smf..conf..319K
Altcode:
No abstract at ADS
Title: Oscillations of the Sun's chromosphere. VI. K grains,
resonances, and gravity waves.
Authors: Kneer, F.; von Uexkuell, M.
Bibcode: 1993A&A...274..584K
Altcode:
We present observations of simultaneous filtergram time sequences in
Mg b2, Ca K and Hα obtained from quiet Sun disc centre with
the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Fourier
analyses are performed to obtain power, coherence and phase spectra in
the k-ω plane. There, the dominant features are the wellknown ridges
of the 5 min resonant modes. Yet in the chromosphere the ridges extend
to high wavenumbers (wavelengths ≍ 1.3 Mm) and to high frequencies
(periods ≍ 105 s). Neither the famous chromospheric "3 min"
oscillations nor an oscillation at the acoustic cutoff frequency
(period 210 s) appear exceptionally pronounced. The signature of
gravity waves is indicated from phase relations. We distinguish
between the behaviour in the interior of the chromospheric network and
on the boundary. The network boundary behaves less oscillatory than
the interior. In snapshots of chromospheric intensities the K grains
(Beckers 1964), or, synonymously the bright cell points, appear in
the cell interior. They represent the phases of high temperature of
a wave field with partly resonant and coherent properties. (We take
intensity fluctuations as proxies for temperature fluctuations.) The
waves are only partly upward propagating p-modes with a multitude of
eigenvalues in frequency and wavenumber, like the subphotospheric
p-modes. We suggest that an excitation mechanism acts within the
chromosphere itself to drive the waves. This could explain the phase
relations between intensity and velocity oscillations.
Title: High spatial resolution spectro-polarimetry of small-scale
magnetic elements on the Sun
Authors: Amer, M. A.; Kneer, F.
Bibcode: 1993A&A...273..304A
Altcode:
Spectro-polarimetric observations from plage regions on the Sun,
obtained with the Gregory Coudé Telescope at Observatorio del
Teide/Tenerife, were analysed. They consist of high spatial resolution
data recorded with a Stokes V polarimeter and a CCD detector from
the magnetically sensitive lines Fe II 6149 (geff =
1.33) and Fe I 6151 (geff = 1.83) simultaneously with the
non-magnetic Fe I 5576 line and from Fe I 6173 (geff = 2.5)
also simultaneously with Fe I 5576. The average asymmetries of
the V profiles are in agreement with previous measurements obtained
with low spatial resolution, and the average wavelength positions
of the zero crossings of the V profiles are consistent with the
picture of zero average velocity within magnetic fluxtubes. Within the
magnetic features and from one feature to the next, there exist strong
fluctuations of the asymmetries of the emergent V profiles and of the
velocities. The latter can be as large as 1 km s-1. Yet
neither a correlation nor an anticorrelation between asymmetry and flow
can be found. Siphon flows may occur across the borders of regions with
opposite polarity. On some locations with high V signal we see very
strong line weakenings and suggest that there the area filling factor of
the magnetic elements amounts to 30 - 50 %. At a height of about 200 km
the tubes merge and loose their identity. The magnetic field strengths
can be derived from the separation of the extrema of the V profiles of
Fe I 6173. They are large, about 2500 gauss (at τ5 = 1 in
the tube). The V profiles observed with high spatial resolution do not
seem to require high non-thermal, micro- or macroturbulent broadening.
Title: On the photospheric temperature in small-scale magnetic flux
concentrations
Authors: Fabiani Bendicho, P.; Kneer, F.; Trujillo Bueno, J.
Bibcode: 1992A&A...264..229F
Altcode:
Results are presented of 2D radiative transfer calculations performed
for geometric configurations that simulate partly evacuated
small-scale magnetic flux sheets embedded in the ambient solar
atmosphere. Temperature distributions in (gray) radiative equilibrium
at low optical depths where radiation transfer dominates the energy
budget are obtained. Two-dimensional radiative equilibrium flux
sheet models are calculated using a novel method which shows that the
temperature enhancement of the upper layers of photospheric magnetic
flux concentrations is due to the radiation channeling effect, i.e.,
that horizontal radiative transfer tends to channel emerging radiation
into the lower opacity regions. The walls of the flux sheets are found
to radiate energy from subphotospheric surrounding layers, giving rise
to a strong heating of the atmosphere of the flux sheets. Radiative
energy migrates horizontally from the heated flux sheets towards the
ambient medium and there it heats the atmosphere at low optical depths.
Title: Speckle observations of abnormal solar granulation
Authors: de Boer, C. R.; Kneer, F.
Bibcode: 1992A&A...264L..24D
Altcode:
We present observations of solar granulation obtained with the Vacuum
Tower Telescope at Observatorio del Teide, Tenerife. Speckle methods
were applied to restore images of abnormal granulation in the vicinity
of a sunspot near disk center. These reconstructions show nests and
chains of bright 'points', whose diameters and spatial distances are
close to the diffraction limit of the telescope (about 0.2 arcsec). The
'points' are not always circular in shape but also elongated. If these
features are identified with the footpoints of small-scale magnetic
fluxtubes, their fast temporal evolution within one or two minutes
and their horizontal motion with velocities up to 2 - 3 km/sec will
influence the understanding of fluxtube dynamics.
Title: Stokes V asymmetry due to stationary siphon flows
Authors: Degenhardt, D.; Kneer, F.
Bibcode: 1992A&A...260..411D
Altcode:
Results are presented of an investigation of theoretically derived
Stokes I and V profiles emerging from an atmosphere which contains
magnetic flux tubes harboring stationary siphon flows. The Stokes I and
V profiles are calculated for three magnetically sensitive lines. The
relative area and amplitude asymmetries of the Stokes V, as well as
the zero-crossing wavelength of the V profile, are calculated for
the upstream part of the flux tube arch as well as for the downstream
part of the arch. It is shown that a wide range of physically realistic
parameters of the simulations yield positive relative area and amplitude
asymmetries with values comparable to the observations.
Title: High resolution observations of the Evershed flow
Authors: Boerner, P.; Kneer, F.
Bibcode: 1992A&A...259..307B
Altcode:
Observations with high spatial resolution of the Evershed effect in
a sunspot near the limb are presented. They were obtained with the
Gregory Coude Telescope at Observatorio del Teide, Tenerife, and consist
of photographic spectrograms in the wavelength region 5160-5180A and
corresponding slit-jaw images in white light at various positions of the
sunspot image on the slit of the spectrograph. The measurement of the
velocity field covers a height range of approximately 100 km to 700 km
in the Mg b2 line. The Evershed flow is inhomogeneous at all heights,
though decreasing in amplitude with increasing height. We find maximum
velocities of 4 km/s at the lowest layer. The flow goes clearly beyond
the outer penumbra border. In Mg b2 the Evershed effect is inverse,
on average, while at the 500 km level it is not. We suggest that in
one fortunate case, with slit orientation parallel to a flow tube, we
see siphon flows outward, reaching to the height of the Mg b2 layer
and ending in photospheric faculae. The stationarity of the flow on
small scales is questioned.
Title: A new instrument for high resolution, two-dimensional solar
spectroscopy.
Authors: Bendlin, C.; Volkmer, R.; Kneer, F.
Bibcode: 1992A&A...257..817B
Altcode:
A two-dimensional spectrometer suitable for solar observations of high
spatial, spectral, and temporal resolution was built and successfully
tested with the German Vacuum Tower Telescope at the Observatorio
del Teide/Tenerife. Using a universal birefringent filter and a
Fabry-Perot interferometer (FPI), narrow-band (⪉30 mÅ) filtergrams
can be obtained. With this instrument, it is possible to scan through
a Fraunhofer line at an appropriate number of wavelength settings
within a few seconds. Here, fast scanning is accomplished by tuning
only the FPI. The images are taken by two CCDs. One of them is coupled
with an image intensifier to achieve short integration times. Due to
the instrument's design, the adequate field of view has a diameter
of about 20-40 arsec. The spectrometer can be tuned to virtually any
wavelength in the visible spectrum.
Title: Speckle observations of solar granulation.
Authors: de Boer, C. R.; Kneer, F.; Nesis, A.
Bibcode: 1992A&A...257L...4D
Altcode:
We present observations of solar granulation in a plage region near
disc center obtained with the Vacuum Tower Telescope at Observatorio
del Teide, Tenerife. Speckle methods were employed for data acquisition
and data reduction. The images show small-scale structures of the size
near the telescopic diffraction limit of 0.2 arcsec. We call attention
to bright lanes at the borders between granules and intergranular
areas. Conceivably, they are the intensity signature of strong upflows
at the border of granules or of shocks in supersonic convection which
are predicted by computer simulations of the granular phenomenon.
Title: Small-scale waves and motions in photosphere and chromosphere
of the Sun.
Authors: Kneer, F.
Bibcode: 1992AnGeo..10...47K
Altcode:
This is a review on motions and waves in the solar atmosphere. The
discussion is limited to non-active phenomena in the solar
atmosphere. Firstly, the gross structure - the average temperature
profile in the photosphere, chromosphere, transition region and corona
- is outlined and the energetic needs to maintain this structure are
addressed. Then the various small-scale phenomena and processes in the
photosphere and chromosphere are described. Secondary motions such
as gravity waves and acoustic waves as well as waves involving the
magnetic field, are excited by the granulation and may carry energy
upward. The chromosphere shows several distinct features and dynamical
processes. Within the cell interior of the chromospheric network,
grains with quasi-periodic brightening are observed. At the cell
boundaries, where magnetic fields are concentrated, less oscillatory
motions but more stochastic variations than those seen in the cell
interior are observed. For the boundaries as well as for the spicules,
the magnetic field dominates the dynamics and may provide the means
of chromospheric heating.
Title: Universitäts-Sternwarte. Jahresbericht für 1991.
Authors: Kneer, F.
Bibcode: 1992MitAG..75..165K
Altcode:
No abstract at ADS
Title: Sonnenbeobachtung bei hoher Auflösung.
Authors: Kneer, F.
Bibcode: 1992GGMit..29....5K
Altcode:
No abstract at ADS
Title: Fluctuation of wing strengths as diagnostic tool for the
structure of granulation.
Authors: Nolte, U.; Kneer, F.; Wiehr, E.
Bibcode: 1992AGAb....7..150N
Altcode:
No abstract at ADS
Title: On the structure of spectral line gap regions
Authors: Kneer, F.; von Uexkuell, M.
Bibcode: 1991A&A...247..556K
Altcode:
The analysis of a photographic spectrogram of the plage region near
the disk center of the sun is undertaken to study small-scale magnetic
features with special attention given to the structure of fluxtubes,
and a related model is developed. Line-gap regions are identified in
the Fe-I line which are characterized by redshifts and fluctuation
velocity as well as an asymmetric profile indicative of a direct
relationship between increasing downflow and increasing depth. The
observations are compared to calculated line profiles in which a
magnetic tube under certain conditions is present. The pressure and
density are calculated for the tube under the assumption of hydrostatic
equilibrium, and the observed intensity profiles can be reproduced
when the correct parameters are used. The calculations demonstrate
that a strong kilogauss magnetic field can be introduced to explain
the line gap in the Fe-II 6149-A line.
Title: Universitäts-Sternwarte. Jahresbericht für 1990.
Authors: Kneer, F.
Bibcode: 1991MitAG..74..163K
Altcode:
No abstract at ADS
Title: MgH as a diagnostic tool for fluctuations in the solar
photosphere
Authors: Kulaczewski, J.; Degenhardt, D.; Kneer, F.
Bibcode: 1990A&A...234..530K
Altcode:
A linear perturbation method is used to calculate the behavior of an
MgH spectral line at 5175 A as a function of temperature and pressure
fluctuations. Particular attention is given to the MgH spectral lines of
the A2Pi-X2Sigma(+) (0,0)-system in solar active and quiet regions. It
is found that Mgh 5175 A is sensitive to temperature perturbations in
the height range of 0 to 300 km with a peak in the response function
near 100 km.
Title: Stability of cool flux tubes in the solar chromosphere. II -
Non-linear dynamical behaviour
Authors: Hassan, S. S.; Kneer, F.
Bibcode: 1990A&A...232..536H
Altcode:
A single vertical cool flux tube in the solar chromosphere is focused
upon for stability studies. The analysis of a previous study by Hasan
and Kneer (1986) is extended to the nonlinear regime with a view
to examining the consequences of having self-exciting mechanisms of
oscillations above the photosphere. In particular, the possibility
of whether the motions driven by the convective instability caused by
the presence of CO could extract sufficient energy from the radiation
field near the Tmin region of empirical models and deposit it in
higher layers to produce chromospheric heating is investigated. The
time evolution of this instability is followed by solving the MHD
equations in the thin flux tube approximation. The analysis includes
energy exchange with the radiation field. The simulations of a flux
tube with a transmitting upper boundary show that the average energy
flux in the oscillations is inadequate for chromospheric heating.
Title: Multidimensional radiative transfer in stratified
atmospheres. VI - Radiative instabilities
Authors: Trujillo Bueno, J.; Kneer, F.
Bibcode: 1990A&A...232..135T
Altcode:
The possibility that radiative instabilities in stellar atmospheres
are driven by transfer of energy processes between stellar gas and the
radiation field is considered. Harmonic temperature fluctuations in
gray radiative equilibrium atmospheres are introduced, and the linear
response of the radiation field to the ensuing Planck-function and
opacity fluctuations is investigated. Analytical and numerical
calculations are performed, emphasizing the influence of the
multidimensional radiative transfer (MRT) effects of opacity
fluctuations on the radiative relaxation time as a function of the
wavenumber of the perturbations. Quantitative examples are given
for stellar atmospheres with solarlike T(eff) and gravitational
stratification. It is concluded that, while the MRT effects of
B fluctuations generally tend to be stabilizing, the MRT effects
of chi fluctuations are optically important for driving radiative
instabilities.
Title: Universitäts-Sternwarte. Jahresbericht für 1989.
Authors: Kneer, F.
Bibcode: 1990MitAG..73...97K
Altcode:
No abstract at ADS
Title: Waves in active regions: observations.
Authors: Kneer, F.
Bibcode: 1990PDHO....7..277K
Altcode:
This review deals with oscillations in prominences, with the 5 min p
modes in active regions, and with waves in sunspots.
Title: Observations of Magnetic Features with the German Solar
Telescopes at the Observatorio-Del / Tenerife
Authors: Kneer, F.; Soltau, D.; Wiehr, E.
Bibcode: 1990IAUS..142..113K
Altcode:
No abstract at ADS
Title: Oscillations of the sun's chromosphere. V - Importance of
network dynamics for chromospheric heating
Authors: von Uexkuell, M.; Kneer, F.; Malherbe, J. M.; Mein, P.
Bibcode: 1989A&A...208..290V
Altcode:
A 64-min time sequence of disk center H-alpha spectrograms taken
with the MSDP spectrograph at the Observatoire du Pic du Midi
is analyzed. This type of spectrograph allows spectroscopy of a
two-dimensional field of view. From the H-alpha line profiles at each
pixel, line-shift and minimum intensity fluctuation are determined as
functions of spatial and temporal coordinates. A frequency analysis
with standard Fourier techniques is performed. It is confirmed that
in the interior of the chromospheric network cells the oscillatory
behavior dominates, whereas at the boundaries one generally finds random
motions on scales of 2-10 arcsec. The random behavior of the H-alpha
structures outlines the permanent rearrangement of the magnetic field
lines pushed around by the subphotospheric granular flow.
Title: The Gregory-Coudé- Telescope at the Observatorio Del Teide
Tenerife
Authors: Kneer, F.; Wiehr, E.
Bibcode: 1989ASIC..263...13K
Altcode: 1989ssg..conf...13K
No abstract at ADS
Title: Universitäts-Sternwarte. Jahresbericht für 1988.
Authors: Kneer, F.
Bibcode: 1989MitAG..72..107K
Altcode:
No abstract at ADS
Title: Spatially Resolved Spectra of Solar Granules
Authors: Holweger, H.; Kneer, F.
Bibcode: 1989ASIC..263..173H
Altcode: 1989ssg..conf..173H
No abstract at ADS
Title: On the Influence of Opacity Fluctuations on the Energy Transfer
by Radiation
Authors: Trujillo-Bueno, J.; Kneer, F.
Bibcode: 1989ASIC..263..441T
Altcode: 1989ssg..conf..441T
No abstract at ADS
Title: Spectroscopy of the solar photosphere with high spatial
resolution
Authors: Wiehr, E.; Kneer, F.
Bibcode: 1988A&A...195..310W
Altcode:
A high-resolution spectrogram from the solar photosphere obtained with
the recently installed Gregory-Coude telescope at the Canary Islands
is analyzed. Continuum intensity fluctuations are seen at scales down
to 0.53 arcsec, which demonstrates the high spatial resolution. Peak
velocities reach 1.5 km/s, while rms velocities range from 350 to
490 m/s. It is found that, for deeply-formed spectral lines, shifts
are well correlated with continuum intensity fluctuations at scales
between 1.5 and 6 arcsec.
Title: Two-dimensional spectroscopy of the Sun in Hα.
Authors: von Uexküll, M.; Kneer, F.; Mein, P.
Bibcode: 1988AGAb....1...11V
Altcode:
No abstract at ADS
Title: The Gregory-Coudé-Telescope in the Observatorio del Teide,
Izaña.
Authors: Kneer, F.; Wiehr, E.; Wittmann, A. D.
Bibcode: 1988AGAb....1...46K
Altcode:
No abstract at ADS
Title: Multidimensional radiative transfer in stratified atmospheres
Authors: Kneer, F.; Trujillo-Bueno, J.
Bibcode: 1987A&A...183...91K
Altcode:
Energy transport by radiation is an important contribution to the energy
budget in stellar atmospheric structures. In this paper, radiative
relaxation of small-scale structures is investigated. The authors
show in a linear analysis: (1) Already at structural lengths of 10
opacity scale heights, horizontal photon exchange is important for the
energy budget. (2) In atmospheric layers near continuum optical depth
τc = 1 and below, the continuum absorption and emission
processes dominate the radiative relaxation. (3) Weak spectral lines or
lines with σlɛ ≤ 1 have little influence on the energy
exchange. (4) At large heights, transport in few spectral lines with
σlɛ very large 1 can compete with continuum processes.
Title: Multidimensional radiative transfer in stratified
atmospheres. IV - Radiative cooling by LTE and non-LTE spectral lines
Authors: Trujillo-Bueno, J.; Kneer, F.
Bibcode: 1987A&A...174..183T
Altcode:
The question of efficiency of radiative energy losses in spectral
lines is addressed. In a semi-infinite atmosphere with constant
temperature, the total radiative energy loss (integrated over all
depths) in a spectral line without continuum is infinite, in both
LTE and non-LTE. Thus, only local energy balances may be considered
with such models. The authors give radiative cooling functions for
various non-LTE parameters and structural lengths of a two-dimensional
stratified atmosphere. At the surface, cooling is less efficient in
non-LTE than in LTE. At large optical depths, both become equal and
are non-negligible. In these layers horizontal transfer effects become
important for the energy balance.
Title: Einige Aspekte der Erfordernisse und Möglichkeiten zeitlicher
Auflösung in der optischen Sonnenphysik
Authors: Kneer, F.; Knölker, M.
Bibcode: 1987MitAG..68..167K
Altcode:
No abstract at ADS
Title: Observations from Space vs. Ground Based Observations:
Advantages and Disadvantages
Authors: Kneer, F.
Bibcode: 1987LNP...292..159K
Altcode: 1987ssp..conf..159K
A comparison of solar observations from space and from the ground
is made.
Title: Radiative Relaxation in Small Scale Structures
Authors: Trujillo-Bueno, J.; Kneer, F.
Bibcode: 1987rfsm.conf..281T
Altcode:
The authors discuss the effects of multidimensional radiative transfer
on the energy exchange by radiation in small-scale structures.
Title: Construction of the German Solar Telescopes at Observatorio
del Teide/Tenerife: The Gregory-Coude Telescope
Authors: Kneer, F.; Schmidt, W.; Wiehr, E.; Wittmann, A. D.
Bibcode: 1987MitAG..68..181K
Altcode:
No abstract at ADS
Title: Stability of cool flux tubes in the solar chromosphere. Linear
analysis.
Authors: Hasan, S. S.; Kneer, F.
Bibcode: 1986A&A...158..288H
Altcode:
The linear stability of cool flux tubes in the solar chromosphere
which are initially in radiative equilibrium is examined. Owing to
the presence of carbon monoxide, there exists a narrow region near the
temperature minimum where the temperature gradient becomes steep enough
to drive a convective instability. The thin flux tube equations are
used and in a simple manner radiative heat exchange with the ambient
medium are included. Initial states of constant beta (where beta is
the ratio of gas to magnetic pressure) are considered. Results for
various values of beta are presented. It is found that for beta less
than 5.7 the tube is overstable with periods in the range 300-600 s. At
beta = 5.7 a bifurcation occurs into two purely growing modes. Growth
rates, eigenvectors of the fundamental modes are calculated and phase
relationships are examined. It is suggested that overstable oscillations
should invariably be associated with cool flux tubes. These oscillations
transport energy and can thus change the thermodynamic structure of flux
tubes. It is conjectured that the CO overstability may be responsible
for spicules.
Title: Energy Transport by Radiation
Authors: Kneer, F.
Bibcode: 1986ssmf.conf..147K
Altcode:
The exchange of energy by radiation is an important contribution
to the energy budget of solar fine structures. The short horizontal
distances necessitate a multidimensional treatment of the radiation
transfer. Several aspects are discussed. (1) The equations and their
properties are described in various geometries. (2) A review on
earlier work is given. (3) Calculations of first order perturbations
in a gravitationally stratified atmosphere are presented and compared
with Spiegel's (1957) analytic result. (4) The radiative stability of
a stratified atmosphere is discussed.
Title: Oscillations of the sun's chromosphere. IV - Temporal evolution
of H-alpha profile
Authors: Kneer, F.; von Uexkuell, M.
Bibcode: 1986A&A...155..178K
Altcode:
H-alpha observations of the quiet solar chromosphere at disk center are
presented, including a 9 min time sequence of photographic filtergrams
in line center and + or - 0.5 A and a 54 min sequence of photographic
spectrograms. The very different dynamical behavior of the interior and
boundary of the network cells is emphasized. The dominant processes
in the interior are waves in the 5 min period range and with shorter
periods. The energy flux in these waves is estimated to be too small to
account for the radiative losses from standard chromospheric models. In
the cell boundaries, the material flows down, on time average, near the
centers of rosettes. The short time fluctuations are mainly stochastic
while the oscillations are reduced. This suggests that the energy
balance in the cell boundaries is determined by MHD turbulence.
Title: Multidimensional Radiative Transfer in Stratified Atmospheres:
Radiative Cooling by LTE and non-LTE Spectral Lines
Authors: Trujillo-Bueno, J.; Kneer, F.
Bibcode: 1986MitAG..67..304T
Altcode:
A detailed paper has been submitted to Astron. Astrophys.
Title: Interpretation of High Resolution Observations
Authors: Kneer, F.
Bibcode: 1985tphr.conf...63K
Altcode:
No abstract at ADS
Title: Oscillations of the sun's chromosphere. III - Simultaneous
H-alpha observations from two sites
Authors: von Uexkuell, M.; Kneer, F.; Mattig, W.; Nesis, A.;
Schmidt, W.
Bibcode: 1985A&A...146..192V
Altcode:
The authors analyze time sequences of Hα filtergrams taken
simultaneously from two distant observatories, Capri and Izaña. By
means of a coherence analysis the authors discriminate between
instrumental effects including seeing and truly solar intensity
fluctuations. Waves with periods as short as 60 s are present in the
solar chromosphere; the lower limit is set by the time resolution of
the observations.
Title: Oscillations of the sun's chromosphere. II - H-alpha line
centre and wing filtergram time sequences
Authors: Kneer, F.; von Uexkuell, M.
Bibcode: 1985A&A...144..443K
Altcode:
In order to investigate the dynamics of the solar chromosphere we
perform a Fourier analysis of time sequences (total duration 128 min)
of Hα photographic filtergrams taken simultaneously at disc centre
in line centre and ±0.5 Å from the line centre. The results are: (i)
At low frequencies (periods >450 s) the brightness fluctuations are
caused by the temporal evolution of Hα structures at the boundaries
of the chromospheric network. We observe that much of the coarse
chromospheric structure survives the 128 min time span. We derive
lifetimes of 2-8 min for the small-scale structure and 5-20 min for the
larger structures. (ii) The modal structure of the 5 min oscillation
is clearly visible in the power spectra of the three filtergram. The
fundamental (f) mode can be followed to high horizontal wavenumbers
kh ≍3.7 Mm-1 and follows the expected
relationship ω2f = gkh, where g is
the surface gravity. (iii) A chromospheric resonant mode cannot be
found in the k - ω plane. (iv) The higher resonant p modes reach into
the acoustic wave domain (periods T ≍ 150 s) and thus require the
transition zone as the upper reflecting layer. (v) We find no evidence
for internal gravity waves. (vi) From a coherence and phase analysis we
conclude that the brightness fluctuations of the chromospheric structure
seen in -0.5 Å lead those in +0.5 Å by 2 min at kh =2 Mm
-1 and by 4 min at kh =0.5 Mm-1. (vii)
From the same coherence and phase analysis we can identify acoustic
waves in the solar atmosphere with periods as short as 80 s. They
possess as much power as the 5 min oscillations and are seen better
outside the chromospheric network than within the network. (viii) The
phase difference between intensity in Hα line centre and velocity,
constructed from the two Hα wing filtergrams, decreases from about 90°
at low frequencies and high wavenumbers to 0° at high frequencies and
low wavenumbers. Tentatively we interpret this as a change from mainly
standing waves for the low order p modes to mainly upward propagating
waves in the acoustic wave domain. Acoustic waves are thus candidates
for the heating process of the inter-network regions.
Title: Report of the working group on future observations of the sun.
Authors: Dere, K.; Kneer, F.; Landman, D.
Bibcode: 1985cdm..proc...14D
Altcode:
No abstract at ADS
Title: Radiative equilibrium models and heating requirements.
Authors: Kneer, F.
Bibcode: 1985cdm..proc..252K
Altcode:
Strong cooling in the IR rotation-vibration bands of carbon monoxide
makes the solar atmosphere convectively unstable near the temperature
minimum of the empirical models. Some of the calculated radiative
equilibrium and heated models are bistable. The author speculates that
the instability due to molecule formation contributes to the production
of chromospheres.
Title: Waves in the chromosphere observed in Hα.
Authors: Kneer, F.; von Uexküll, M.
Bibcode: 1985cdm..proc..282K
Altcode:
The authors analyze Hα filtergram time sequences and emphasize the
further development and use of filter spectroscopy for studies on the
dynamics of the solar atmosphere.
Title: Interpretation of high-resolution observations.
Authors: Kneer, F.
Bibcode: 1985MPARp.212...63K
Altcode:
The path on which one arrives from observation to knowledge is highly
recursive. This contribution is intended to draw attention to the
ways of specification of the assumptions necessarily made to solve
the diagnostic problem. Both observational conditions and earlier
experience enter the assumptions about the complexity, initial and
boundary conditions of the process under consideration. High-resolution
observations reveal a wealth of small-scale, dynamic phenomena whose
properties must be taken into account when diagnosing by way of
simulation and calculation of emergent intensities.
Title: Chromospheric umbral oscillations.
Authors: Mattig, W.; von Uexküll, M.; Kneer, F.
Bibcode: 1984ESASP.220...59M
Altcode: 1984ESPM....4...59M
No abstract at ADS
Title: Interpretation of High-Resolution Measurements, Selected
Problems (Keynote)
Authors: Kneer, F.
Bibcode: 1984ssdp.conf..110K
Altcode:
This review deals with four problems bearing on small-scale dynamical
processes: (1) small-scale intensity fluctuations, (2) small-scale
velocities, (3) average models of the solar atmosphere, and (4)
small-scale magnetic fields.
Title: A possible explanation of the Wilson-Bappu relation and the
chromospheric temperature rise in late-type stars.
Authors: Kneer, F.
Bibcode: 1983A&A...128..311K
Altcode:
IR vibration-rotation bands of CO are noted to cause a radiative
instability due to the extreme temperature sensitivity of molecule
formation, in the course of an attempt to construct stellar atmospheres
which are in radiative equilibrium for the case of a T(eff) of 5800 K,
which approximates the T(eff) of the sun. It is accordingly suggested
that such stellar atmospheres, in principle, may not exist. CO
molecule-based arguments can be extended to other molecular species
which should destabilize the atmospheres of cooler stars. This implies
that, for the sun, the instability occurs at the mass column density
of the temperature minimum deduced from observations. For sunspot
chromospheres, the predicted critical mass column density agrees with
that of the onset of the chromospheric temperature rise in current
sunspot models. It is further suggested that the chromospheres of
late-type stars are produced inevitably, and that their seed is located
at their base.
Title: The chromosphere above sunspot umbrae. IV. Frequency analysis
of umbral oscillations.
Authors: von Uexküll, M.; Kneer, F.; Mattig, W.
Bibcode: 1983A&A...123..263V
Altcode:
The authors analyse time sequences of photographic spectrograms in Ca II
H, K, 8498 Å, and 8542 Å, in Sodium D1 and D2,
in Ni I 5893 Å and Hα, obtained from two sunspots. The frequency
analysis of line intensities and shifts for the umbral chromospheric
oscillations in the period range 110 s-200 s is discussed.
Title: The chromosphere above sunspot umbrae. IV - Frequency analysis
of umbral oscillations
Authors: Uexkuell, M. V.; Kneer, F.; Mattig, W.
Bibcode: 1983A&A...123..263U
Altcode:
An analysis is made of time sequences of photographic spectrograms in Ca
II H, K, 8498 A, and 8542 A, in sodium D1 and D2, and in Ni I 5893 A and
H-alpha obtained from two sunspots with the Vacuum Tower Telescope at
Sacramento Peak Observatory. The two sunspot chromospheres are found to
behave similarly with regard to their phase relationships but somewhat
differently with regard to the oscillation resonant frequencies. In one
and the same umbra, oscillation modes having different frequencies may
be excited. The vertical velocity of propagation of the phase in umbral
chromospheres, which is 10-25 km/s, decreases with increasing frequency
and is much lower than the Alfven speed. The umbral chromospheric
oscillations are thus slow mode waves. The downward motion enhances
the intensity by some 65 deg (H-alpha) to 85 deg (D1), indicating
dissipation of mechanical energy. It is also found that the mechanical
energy flux of the umbral chromospheric oscillations is not sufficient
to account for the chromospheric radiative losses.
Title: Brightness oscillations of the sun's chromosphere in K
and H-alpha
Authors: Kneer, F.; von Uexkuell, M.
Bibcode: 1983A&A...119..124K
Altcode:
The authors have performed a power analysis of time sequences of
photographic K and Hα filtergrams from disc center of the sun. The
results are: In the k-ω-diagram from the Ca II K filtergrams, the modal
structure of the 5-min-oscillation is clearly seen. The 5-min modes
are difficult to detect in the k-ω-diagrams from the Hα sequences. In
both lines, K and Hα, the authors observe ridges in the k-ω-diagrams
running parallel to the kh-axis. Their periods correspond to
the chromospheric 3-min-oscillation. But power appears also, in discrete
ridges, at shorter periods P = 60 s...150 s, where the lower limit is
given by the Nyquist frequency of the observations. After subtraction
of the time averaged intensity at each position in the filtergrams,
the rms fluctuations are 3.5% in K and 4.0% in Hα.
Title: Zur Aufheizung der Chromosphäre später Sterne
Authors: Kneer, F.
Bibcode: 1983MitAG..60..301K
Altcode:
No abstract at ADS
Title: New features of the oscillation spectrum of the sun
Authors: Kneer, F.; von Uexkuell, M.; Newkirk, G., Jr.
Bibcode: 1982A&A...113..129K
Altcode:
The results of a Fourier analysis of observations of brightness
fluctuations in a sequence of CN 3883 Angstrom filtergrams are
presented. From the 75 min time interval and field of view of 170 x
220 sq arcsec, a 40 sec sampling interval was examined, maintaining
a spatial resolution as high as 1600 km. The data is filtered for a
high wavenumber power spectrum, and an average power coefficient is
calculated by examining power spectra for each of the sub-arrays. It
is concluded that the 5 min velocity oscillation modal structure is
easily detected in brightness fluctuations in the CN band, the surface
gravity mode is the strongest single feature aside from the zero
frequency ridge caused by convection, and the trapped chromospheric
mode may be represented by a constant frequency ridge. Improved broad
band filtergrams may be used to analyze little explored regions of
the oscillation spectrum and to interpret intensity fluctuations as
temperature fluctuations.
Title: Zur Struktur der Chromosphäre über Sonnenflecken
Authors: Kneer, F.; Mattig, W.; von Uexküll, M.
Bibcode: 1982MitAG..55...65K
Altcode:
No abstract at ADS
Title: Chromosphärische Helligkeits-oszillationen
Authors: Kneer, F.; Newkirk, G.; von Uexküll, M.
Bibcode: 1982MitAG..55...70K
Altcode:
No abstract at ADS
Title: The analysis of solar limb observations. II - Geometrical
smearing
Authors: Durrant, C. J.; Kneer, F.; Maluck, G.
Bibcode: 1981A&A...104..211D
Altcode:
Center-of-disk and limb observations of photospheric brightness
fluctuations reveal a systematic suppression of the values at the
limb. This is a geometrical effect arising from the line of sight at the
limb passing through various structures across the surface. This effect
is examined here in the light of recent granular temperature models
using a simple model incorporating the dominant granular scale. The
geometry is found to have little influence on the deep-seated granular
brightness field but to strongly suppress the brightness fluctuations
caused by the upper photospheric temperature field. The significance
of these results for the restoration of limb observations is discussed.
Title: The chromosphere above sunspot umbrae. III - Spatial and
temporal variations of chromospheric lines
Authors: Kneer, F.; Mattig, W.; v. Uexkuell, M.
Bibcode: 1981A&A...102..147K
Altcode:
The lines Ca II H, K, 8542 A, Na I D(1) and D(2), Ni I 5893 A, and
H-alpha, were analyzed in photographic spectrograms obtained from a
sunspot. The following results are found for the umbra: (1) the H and K
lines are highly correlated, with an emission cores maximum-intensities
ratio of 1.2; (2) the K intensities are poorly correlated with those
of 8542, D(2), and H-alpha; this behavior is attributed to atmospheric
structure horizontal fluctuations having little correlations between
those in high and low layers, (3) the periodic temporal fluctuations
of the K emission core with umbral flashes are identified, and found to
occur throughout the umbral area covered by the spectrograph slit; and
(4) fluctuations in the photospheric Ni I 5893 A line cannot be matched
to the chromospheric fluctuations, in accord with Giovanelli (1978).
Title: OSO-8 Observations of CAII H and K MGII H and K Lyman-Alpha
and Lyman-Beta above a Sunspot
Authors: Kneer, F.; Scharmer, G.; Mattig, W.; Wyller, A.; Artzner,
G.; Lemaire, P.; Vial, J. C.
Bibcode: 1981SoPh...69..289K
Altcode:
Observations with the French (L.P.S.P.) experiment on board OSO-8 of
a sunspot and nearby plage region are described. The behaviour of the
emission cores of the Ca II H and K and Mg II h and k resonance lines is
very similar and the correspondence in intensity between the four lines
persists in all observed features. In contrast, the Lyman lines show
little correlation with the other lines. Their emission regions appear
broader in the spectroheliograms than the underlying sunspot structure
and must not necessarily possess a counterpart in lower layers. From
the central intensity of Lα above the umbra an electron density of
4.3 × 1010 cm-3 ≲ne*
≲2.3 × 1011 cm-3 at 20 000 K is estimated.
Title: The Chromosphere above Sunspot Umbrae - Part Three - Spatial
and Temporal Variations of Chromospheric Lines
Authors: Kneer, F.; Mattig, W.; Uexkull, M. V.
Bibcode: 1981phss.conf..318K
Altcode:
No abstract at ADS
Title: Multidimensional radiative transfer in stratified
atmospheres. III - Non-LTE line formation
Authors: Kneer, F.
Bibcode: 1981A&A....93..387K
Altcode:
Multidimensional radiative transfer under non-LTE conditions is
investigated. Line formation strictly by two-level atoms is treated
with both coherent scattering and complete redistribution and with
non-LTE parameters ranging from epsilon 0.1 to 0.000001. Lateral
transfer effects increase with increasing non-LTE conditions and may,
in the solar case, compete with observational limitation of angular
resolution. Calculations of line transfer in simple model structures
in the solar chromosphere show that horizontal migration of photons
may diminish the contrast by an order of magnitude compared to that
expected in a one-dimensional treatment.
Title: Sunspot chromospheres and their relation to the chromospheres
of laten main sequence stars.
Authors: Mattig, W.; Kneer, F.
Bibcode: 1981A&A....93...20M
Altcode:
The sunspot umbra parameters such as spectral type, effective
temperature, and chromospheric Ca II H and K line emission are related
to the same parameters of luminosity class V stars of spectral type
G 8 to M 2. The sunspots follow the Wilson-Bappu relation. It is
concluded that the strong magnetic fields in umbrae may control the
nonradiative energy supply of the sunspot chromosphere; however, the
heating mechanism is not reflected in the average appearance of the
H and K line cores.
Title: Coherence Analysis of Granular Intensity
Authors: Kneer, F. J.; Mattig, W.; Nesis, A.; Werner, W.
Bibcode: 1980SoPh...68...31K
Altcode:
A high resolution spectrogram of the Mg b2 line from the
quiet Sun disc centre is subjected to a coherence analysis. We find
that the coherence between intensity fluctuations in the continuum
and the wings of the line breaks down at a distance Δλ = 0.35
Å from line centre. From this and the r.m.s. intensity contrast
as a function of Δλ we are led to the following simple model of
temperature fluctuation δT in the solar photosphere: A lower part
(below 50 km, or τ5000 > 0.25) with strongly inward
increasing δT and an upper part (above 50 km) with constant δT =
75 K. The two parts are supposed to fluctuate incoherently.
Title: On time-dependent ionization in stellar chromospheres
Authors: Kneer, F.
Bibcode: 1980A&A....87..229K
Altcode:
This investigation deals with the question of whether ionization
equilibrium in stellar chromospheres is adjusted quickly or slowly
compared with dynamical time scales. It is shown that the adjustment
time for ionization of hydrogen may be of the order of 10 to the
2nd s (middle solar chromosphere) to 10 to the 3rd s (upper solar
chromosphere). This has consequences on the structure of chromospheres;
for instance, stationary flows - which are limited to subsonic speed
in plane-parallel geometry - drastically change the electron density
in comparison to the static case. Unless confirmed by consistent
dynamical calculations, chromospheric models based on the assumption
of statistical steady state should be taken as rough estimates of
chromospheric structure.
Title: Observations of Chromospheric Lines from OSO-8
Authors: Grossmann-Doerth, U.; Kneer, F.; Uexkuell, M.; Artzner,
G. E.; Vial, J. C.
Bibcode: 1980SoPh...66....3G
Altcode:
The line profiles of Lα, Ca II K and Mg IIk were measured with the
spectrometer of the `Laboratoire de Physique Stellaire et Planétaire'
on board of OSO-8. The results of these measurements are presented.
Title: Report from the discussion group on speckle interferometry
and adaptive optics
Authors: Kneer, F.
Bibcode: 1980fsoo.conf..298K
Altcode:
No abstract at ADS
Title: What are the resolution limits given by the solar atmosphere
itself?
Authors: Kneer, F.
Bibcode: 1980fsoo.conf..204K
Altcode:
No abstract at ADS
Title: Multidimensional radiative transfer in stratified atmospheres:
gray radiative equilibrium.
Authors: Kneer, F.; Heasley, J. N.
Bibcode: 1979A&A....79...14K
Altcode:
This paper tests the validity of the multidimensional Eddington,
or diffusion, approximation in radiative transfer in a gray
radiative-equilibrium atmosphere with opacity increasing exponentially
toward the stellar interior. The diffusion approximation is unacceptable
at small optical depths. The height dependence of intensity fluctuations
is studied systematically by adopting the above simplified model
atmosphere for the solar photosphere. Lateral radiative exchange is
efficient and drastically damps the fluctuations in the uppermost
layers.
Title: Mehrdimensionaler Strahlungstransport im
Strahlungsgleichgewicht
Authors: Kneer, F.
Bibcode: 1979MitAG..45...95K
Altcode:
No abstract at ADS
Title: Report from discussion group on speckle interferometry and
adaptive optics.
Authors: Kneer, F.
Bibcode: 1979MmArc.106..298K
Altcode:
No abstract at ADS
Title: What are the resolution limits given by the solar atmosphere
itself?
Authors: Kneer, F.
Bibcode: 1979MmArc.106..204K
Altcode:
No abstract at ADS
Title: The Chromosphere above Sunspot Umbrae II. The Interpretation
of the H, K and IR Lines of CA II
Authors: Kneer, F.; Mattig, W.
Bibcode: 1978A&A....65...17K
Altcode:
Summary. The chromospheres above sunspot umbrae are investigated
by comparing calculated and observed profiles of the Ca ii H, K,
and infra-red lines. Statistical steady state is assumed for the
level populations. We test several model chromospheres in hydrostatic
equilibrium. We distinguish between chromospheres which are optically
thick and optically thin at the centre of the K line. In view of the
observed intensity ratio 1K3/1H3 1.15 we are forced to adopt a thick
model as reference chromosphere. Key words: chromosphere - sunspot
umbra - Ca ii lines
Title: The chromosphere above sunspot umbrae. I. Observations of
the emission cores in the Ca II H- and K-lines.
Authors: Mattig, W.; Kneer, F.
Bibcode: 1978A&A....65...11M
Altcode:
Summary. Photographic spectra of the Ca ii H- and K- lines in sunspot
umbrae are analysed. The emission features in the line profiles may be
classified into minimum emission profiles that also lack self-reversal,
and profiles which change rapidly in time and exhibit self-reversals
(umbral flashes) that are most often strongly asymmetric. Average
intensity profiles from the minimum emission parts of the umbra and
characteristic intensities of the more active parts of the umbra are
given. Key words: chromosphere - sunspot umbra - Ca ii lines
Title: Chromosphärenmodell von Sonnenflecken
Authors: Kneer, F.; Mattig, W.
Bibcode: 1978MitAG..43..141K
Altcode:
No abstract at ADS
Title: Report from the discussion group on speckle interferometry
and adaptive optics
Authors: Kneer, F.
Bibcode: 1978fsoo.conf..298K
Altcode:
No abstract at ADS
Title: What are the resolution limits given by the solar atmosphere
itself?
Authors: Kneer, F.
Bibcode: 1978fsoo.conf..204K
Altcode:
No abstract at ADS
Title: The effects of partial redistribution on facular K line
profiles.
Authors: Heasley, J. N.; Kneer, F.; Chapman, G. A.
Bibcode: 1977SoPh...52..309H
Altcode:
We present theoretical Ca II K-line profiles and filtergram contrasts
for several recent models of solar faculae. The line profiles vary
greatly between models and between complete and partial frequency
redistribution non-LTE calculations for any given model. The filtergram
contrasts are relatively insensitive to the line formation theory which
greatly simplifies the calculation for comparison with observations. All
of the models considered exhibit K-line contrasts smaller than the
mean value observed by Mehltretter.
Title: The sun's chromospheric velocity field as inferred from the
Ca II K line.
Authors: Durrant, C. J.; Grossmann-Doerth, U.; Kneer, F. J.
Bibcode: 1976A&A....51...95D
Altcode:
Theory and observation of the solar Ca II K line under high spatial
resolution are briefly reviewed. It is shown that contrary to popular
belief, present theory is not capable of explaining the observed
features of the K line. An attempt is made to develop a schematic model
of the temporal behavior of the spatially resolved K line, based on
Leibacher's (1971) investigation of waves in the solar atmosphere
Title: Formation of spectral lines with partial frequency
redistribution.
Authors: Heasley, J. N.; Kneer, F.
Bibcode: 1976ApJ...203..660H
Altcode:
A method for treating the effects of partial frequency redistribution
(PRD) in non-LTE radiative-transfer problems is presented in which the
rate equations may be kept in their usual form and the PRD effects may
be included by altering the form of the radiative-transfer equation. The
required modifications of the transfer and statistical-equilibrium
equations are outlined along with the changes these necessitate in
the complete linearization method. The formulation is compared with
that of Milkey et al. (1973), and sample PRD calculations are given
for the solar chromospheric Ly-alpha and Ca II K line profiles. The
results are found to be in excellent quantitative agreement with
previous computations.
Title: Radiative hydrodynamics of chromospheric transients.
Authors: Kneer, F.; Nakagawa, Y.
Bibcode: 1976A&A....47...65K
Altcode:
A self-consistent method for the numerical modeling of transient
phenomena in stellar atmospheres is developed. The nonlinear equations
of hydrodynamics are solved together with the equations of radiative
transfer and non-LTE ionization equilibrium, and consideration is given
to one-dimensional (vertical) motions in a plane-parallel atmosphere
which is initially in hydrostatic and statistical equilibrium. The
radiative-transfer problem is treated for a two-level-plus-continuum
hydrogen atom. The coupled partial differential equations are solved
through complete linearization of the corresponding difference
equations. The effects of a thermal pulse introduced at the bottom of
a model solar chromosphere are calculated. The temperature perturbation
produces a pressure pulse which runs upwards through the atmosphere and
generates complex variations of temperature, velocity, and radiation.
Title: Wechselwirkung zwischen Strahlungstransport und Gasdynamik
in der Sonnenchromosphäre.
Authors: Kneer, F.; Nakagawa, Y.
Bibcode: 1976MitAG..38..203K
Altcode:
No abstract at ADS
Title: Comments on the redistribution function of Jefferies and White.
Authors: Kneer, F.
Bibcode: 1975ApJ...200..367K
Altcode:
The simplified form of the angle-averaged redistribution function R"(v',
v) for coherent scattering in the atom's frame, suggested by Jefferies
and White (1960), is neither normalized nor symmetric. It is shown
how these requirements can be met by a simple change, retaining the
idea that photons are completely redistributed in the Doppler core and
coherently scattered in the line wings. Subject headings: functions -
line formation - radiative transfer
Title: Properties of the solar Ca ii K-Line at high spatial
resolution
Authors: Grossmann-Doerth, U.; Kneer, F.; Uexküll, M. v.
Bibcode: 1974SoPh...37...85G
Altcode:
The analysis of three Ca II K-spectra with spatial resolution of
∼1″ is described and its results presented. The comparison of
the observed single peak line profiles with model computation leads
to some conclusions regarding the non-uniform large-scale velocity
fields in the chromosphere.
Title: On Some Characteristics of Umbral Fine Structure
Authors: Kneer, F.
Bibcode: 1973SoPh...28..361K
Altcode:
Photographic spectra of the umbra of a sunspot (1971, August 24,
Rome No. 6205) around 6150 Å show fine bright threads which were
identified as the spectra of a lightbridge, of the bright end of a
penumbral filament and of umbral dots, respectively. It was found,
in agreement with the results of other authors, that the magnetic
field in bright structures is considerably weaker than in dark umbral
material. Analysis of line profiles of Fe II 6149.2 Å in umbral dots
indicates (a) a fieldstrength reduced by a factor ≈2 compared to
the surroundings, (b) an outflow with v≈3.0 km s−1
relative to the penumbra and (c) possibly photospheric temperatures
in umbral dots.
Title: Profiles of Magnetically Split Lines in Sunspots
Authors: Kneer, F.
Bibcode: 1972A&A....18...47K
Altcode:
No abstract at ADS
Title: Line Profiles in Sunspots
Authors: Kneer, F.
Bibcode: 1972A&A....18...39K
Altcode:
No abstract at ADS
Title: Linienprofile in Sonnenflecken
Authors: Kneer, F.
Bibcode: 1972MitAG..31..149K
Altcode:
No abstract at ADS
Title: Linienprofile in Sonnenflecken : ein Beitrag zur Aufstellung
von Umbramodellen Title: Linienprofile in Sonnenflecken : ein Beitrag
zur Aufstellung von Umbramodellen Title: Line profiles in sunspots :
a contribution to the establishment of umbra models;
Authors: Kneer, Franz
Bibcode: 1970PhDT........99K
Altcode:
No abstract at ADS
Title: Sunspot Intensities and their Correction for Scattered Light
Authors: Kneer, F.; Mattig, W.
Bibcode: 1968SoPh....5...42K
Altcode:
Photoelectric measurements of monochromatic spot intensities obtained
with the domeless Coudé refractor in Anacapri are given. The scattered
light superimposed on the spot, as deduced from measurements outside
the sun's limb, amounts on the average to about 4% of the photospheric
intensity. The accuracy of the derived spot intensities is better than
10%. Two of four investigated spots yielded temperatures below 3900K
(ΔΘ > 0.5).