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Author name code: deboer
ADS astronomy entries on 2022-09-14
=author:"De Boer, C.R."
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Title: The hot prominence periphery in EUV lines
Authors: de Boer, C. R.; Stellmacher, G.; Wiehr, E.
1998A&A...334..280D Altcode:
Two sets of He I and metallic lines were observed with the EUV
spectrograph SUMER in a quiescent prominence. H, He, and Ca II
lines were observed simultaneously with both German telescopes on
Tenerife. The visible lines from elements with different atomic
weights yield thermal and non-thermal broadening parameters of 7500
<= T<SUB>kin</SUB> <= 8000 K and 2.5 <xi <5.0 km/s for the
cool prominence body. The EUV lines, however, show line widths which
correspond to much higher temperatures and non-thermal velocities. If
the calculated formation temperature for every individual ion is
assumed, the observed line widths require non-thermal velocities of 14 -
25 km/s. The narrowest reduced widths of the EUV He I 584 and He I 537
lines are 3.1 and 2.9 times broader than those of the visible He D_3
and He 3888 lines. If this is due to optical depth effects in the EUV
lines, one obtains tau_0 (584) ~ 8*10(3) and tau_0 (537) ~ 2*10(3)
, respectively. The emission ratios of the Ca II-to-Balmer lines
vary little inside the prominence, indicating a largely constant gas
pressure. The ratios of the visual He-to-Balmer lines as well as those
of the EUV He-to-metallic lines show a significant branching between
peripheral and central prominence regions. The total emissions in the
main prominence body amount to 13, 0.3, and 4 [Watt/ (m(2*) ster)]
for the 584, 537, and D_3 lines, respectively. The observed emission
ratio E(He 584)/E(He 537)= 45 agrees with model calculations whereas
their total emissions are about 37 times higher than calculated. The
observed ratio E(He D_3)/ E(He 584)~0.3 is about 15 times smaller
than model predictions. The observations indicate that the emissions
of different ions originate from individual (isothermal) threads with
different temperatures between 10(4) and 10(5) K.
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Title: Prominence Emissions with SUMER and Optical Telescopes
Authors: Wiehr, E.; Stellmacher, G.; de Boer, C. R.; Sütterlin, P.
1998ASPC..155..331W Altcode: 1998sasp.conf..331W
No abstract at ADS
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Title: Prominence Emission Lines Observed with SUMER and Two
Ground-Based Telescopes
Authors: de Boer, C. R.; Stellmacher, G.; Wiehr, E.
1998ASPC..150..196D Altcode: 1998npsp.conf..196D; 1998IAUCo.167..196D
No abstract at ADS
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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.
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.
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Title: Speckle measurements of the centre-to-limb variation of the
solar granulation.
Authors: Wilken, V.; de Boer, C. R.; Denker, C.; Kneer, F.
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.
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Title: Three colour photometry of solar limb faculae.
Authors: de Boer, C. R.; Stellmacher, G.; Wiehr, E.
1997A&A...324.1179D Altcode:
The contrasts of limb faculae at various heliocentric angles are
observed in three continuum windows largely free of absorption
lines. Observations with interference filters show a contrast decrease
with wavelength which is not found when using an `UBF' to select the
shortest continuum window. Limb faculae in speckle-reconstructed images
yield contrasts being typically 1.2 times higher than those deduced from
the `best images' of the corresponding bursts. The fit of the wavelength
dependence (`colour index') to a black-body law is significantly better
for the contrasts deduced from the reconstructed images than for the
contrasts from the best images. The uncorrected contrasts yield a mean
facular temperature excess of 200-300K; those from the reconstructed
images yield about 470 K. The center-to-limb variation is found to be
much smaller than the fluctuations between individual faculae.
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Title: Emission Lines in a Quiescent Prominence Observed with the
SUMER, VTT, a nd Gregory Telescopes
Authors: Wiehr, E.; de Boer, C. R.; Stellmacher, G.; Sutterlin, P.
1997ASPC..118..294W Altcode: 1997fasp.conf..294W
A number of emission lines have been observed in a quiescent prominence
with the two German telescopes on Tenerife and with SUMER. The Gregory
telescope monitored simultaneously the lines H_8 3889, He 3888, Ca(+\:)
8498, the VTT simultaneously the lines H_β, He-D_3, Ca(+\:) 8542, and
H_α slit jaw pictures on 1024 x 1024 CCDs. At the same time, SUMER
monitored successively three combinations of EUV lines containing He
584, He 537 in the second, and Ar, N(+) , C, C(++) , O, O(+) , O(++)
, S(++) , S(+++) lines in the first order. Two dimensional images in
the various EUV lines establish the anti-relation between temperature
and Balmer brightness.
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Title: Noise filtering in solar speckle masking reconstructions.
Authors: de Boer, C. R.
1996A&AS..120..195D Altcode:
A new method of obtaining a sensitive noise filter for solar speckle
masking reconstructions is presented below. This filter separates the
true image information from noise most reliably. Its efficiency is
demonstrated by some representative examples considering observed and
artificial image data which were generated in a computer. The latter
set of data also suffered realistic degradations by the influence of
seeing and noise taken from suitable observations.
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Title: Untersuchung von Feinstrukturen der Sonne.
Authors: Denker, C.; de Boer, C. R.; Volkmer, R.
1996S&W....35..184D Altcode:
Fine structures are elements in the solar photosphere with sizes smaller
than 1 arcsec. The observation of such structures requires exquisite
instrumentation and reduction methods. In this article two methods are
represented: image reconstruction by means of speckle-interferometry
and two-dimensional spectro-polarimetry.
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Title: Empirical speckle transfer function measurements from partial
eclipse observations of the Sun.
Authors: de Boer, C. R.
1995A&AS..114..387D Altcode:
Empirical speckle transfer functions are derived from time series
obtained during the partial eclipse on May 10, 1994, using the lunar
limb as a reference structure. These functions are compared with
theoretical transfer functions usually applied to solar data. Moreover,
the observations are suitable for testing the spectral ratio method
which is often used to estimate the seeing-dependent Fried parameter
r_0_. The results indicate that the spectral ratio method overestimates
the Fried parameter. The empirical speckle transfer functions exhibit
a steeper decrease at spatial frequencies above the seeing-cutoff where
speckle methods restore the usually lost image structures. This steeper
decrease was taken into account for correcting the theoretical transfer
functions. Applications of these new speckle transfer functions to solar
data are presented. In comparison with previous reconstructions based
on theoretical transfer functions, the sharpness of the reconstructed
pictures is significantly increased. Besides, the contrast of solar
structures can be estimated more reliably.
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Title: Speckle masking imaging of the moustache phenomenon.
Authors: Denker, C.; de Boer, C. R.; Volkmer, R.; Kneer, F.
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.
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Title: Speckle observations of solar granulation
Authors: de Boer, C. R.; Kneer, F.
1994IAUS..158..398D Altcode:
No abstract at ADS
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Title: Speckle-Interferometrie und ihre Anwendungen auf die
Sonnenbeobachtung
Authors: de Boer, C. R.
1993PhDT.......155D Altcode:
No abstract at ADS
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Title: Speckle observations of abnormal solar granulation
Authors: de Boer, C. R.; Kneer, F.
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.
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Title: Speckle observations of solar granulation.
Authors: de Boer, C. R.; Kneer, F.; Nesis, A.
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.
