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Author name code: scharmer
ADS astronomy entries on 2022-09-14
author:"Scharmer, Goran B."
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Title: SSTRED: Data- and metadata-processing pipeline for CHROMIS
and CRISP
Authors: Löfdahl, Mats G.; Hillberg, Tomas; de la Cruz Rodríguez,
Jaime; Vissers, Gregal; Andriienko, Oleksii; Scharmer, Göran B.;
Haugan, Stein V. H.; Fredvik, Terje
2021A&A...653A..68L Altcode: 2018arXiv180403030L
Context. Data from ground-based, high-resolution solar telescopes
can only be used for science with calibrations and processing, which
requires detailed knowledge about the instrumentation. Space-based
solar telescopes provide science-ready data, which are easier to
work with for researchers whose expertise is in the interpretation of
data. Recently, data-processing pipelines for ground-based instruments
have been constructed. <BR /> Aims: We aim to provide observers
with a user-friendly data pipeline for data from the Swedish 1-meter
Solar Telescope (SST) that delivers science-ready data together with
the metadata needed for proper interpretation and archiving. <BR />
Methods: We briefly describe the CHROMospheric Imaging Spectrometer
(CHROMIS) instrument, including its (pre)filters, as well as recent
upgrades to the CRisp Imaging SpectroPolarimeter (CRISP) prefilters and
polarization optics. We summarize the processing steps from raw data
to science-ready data cubes in FITS files. We report calibrations
and compensations for data imperfections in detail. Misalignment
of Ca II data due to wavelength-dependent dispersion is identified,
characterized, and compensated for. We describe intensity calibrations
that remove or reduce the effects of filter transmission profiles
as well as solar elevation changes. We present REDUX, a new version
of the MOMFBD image restoration code, with multiple enhancements and
new features. It uses projective transforms for the registration of
multiple detectors. We describe how image restoration is used with
CRISP and CHROMIS data. The science-ready output is delivered in FITS
files, with metadata compliant with the SOLARNET recommendations. Data
cube coordinates are specified within the World Coordinate System
(WCS). Cavity errors are specified as distortions of the WCS wavelength
coordinate with an extension of existing WCS notation. We establish
notation for specifying the reference system for Stokes vectors with
reference to WCS coordinate directions. The CRIsp SPectral EXplorer
(CRISPEX) data-cube browser has been extended to accept SSTRED output
and to take advantage of the SOLARNET metadata. <BR /> Results: SSTRED
is a mature data-processing pipeline for imaging instruments, developed
and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP
spectropolarimeter. SSTRED delivers well-characterized, science-ready,
archival-quality FITS files with well-defined metadata. The SSTRED
code, as well as REDUX and CRISPEX, is freely available through git
repositories.
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Title: Design and Performance Analysis of a Highly Efficient
Polychromatic Full Stokes Polarization Modulator for the CRISP
Imaging Spectrometer
Authors: de Wijn, A. G.; de la Cruz Rodríguez, J.; Scharmer, G. B.;
Sliepen, G.; Sütterlin, P.
2021AJ....161...89D Altcode: 2021arXiv210201231D
We present the design and performance of a polychromatic polarization
modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot
tunable narrow-band imaging spectropolarimer at the Swedish 1 m Solar
Telescope (SST). We discuss the design process in depth, compare
two possible modulator designs through a tolerance analysis, and
investigate thermal sensitivity of the selected design. The trade-offs
and procedures described in this paper are generally applicable in the
development of broadband polarization modulators. The modulator was
built and has been operational since 2015. Its measured performance
is close to optimal between 500 and 900 nm, and differences between
the design and as-built modulator are largely understood. We show some
example data, and briefly review scientific work that used data from
SST/CRISP and this modulator.
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Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
2020A&A...641A.146R Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.
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Title: Dissecting bombs and bursts: non-LTE inversions of
low-atmosphere reconnection in SST and IRIS observations
Authors: Vissers, G. J. M.; de la Cruz Rodríguez, J.; Libbrecht,
T.; Rouppe van der Voort, L. H. M.; Scharmer, G. B.; Carlsson, M.
2019A&A...627A.101V Altcode: 2019arXiv190502035V
Ellerman bombs and UV bursts are transient brightenings that are
ubiquitously observed in the lower atmospheres of active and emerging
flux regions. As they are believed to pinpoint sites of magnetic
reconnection in reconfiguring fields, understanding their occurrence
and detailed evolution may provide useful insight into the overall
evolution of active regions. Here we present results from inversions
of SST/CRISP and CHROMIS, as well as IRIS data of such transient
events. Combining information from the Mg II h & k, Si IV, and Ca
II 8542 Å and Ca II H & K lines, we aim to characterise their
temperature and velocity stratification, as well as their magnetic
field configuration. We find average temperature enhancements of
a few thousand kelvin, close to the classical temperature minimum
and similar to previous studies, but localised peak temperatures
of up to 10 000-15 000 K from Ca II inversions. Including Mg II
appears to generally dampen these temperature enhancements to below
8000 K, while Si IV requires temperatures in excess of 10 000 K at
low heights, but may also be reproduced with secondary temperature
enhancements of 35 000-60 000 K higher up. However, reproducing Si
IV comes at the expense of overestimating the Mg II emission. The
line-of-sight velocity maps show clear bi-directional jet signatures
for some events and strong correlation with substructure in the
intensity images in general. Absolute line-of-sight velocities range
between 5 and 20 km s<SUP>-1</SUP> on average, with slightly larger
velocities towards, rather than away from, the observer. The inverted
magnetic field parameters show an enhancement of the horizontal
field co-located with the brightenings at heights similar to that of
the temperature increase. We are thus able to largely reproduce the
observational properties of Ellerman bombs with the UV burst signature
(e.g. intensities, profile asymmetries, morphology, and bi-directional
jet signatures), with temperature stratifications peaking close
to the classical temperature minimum. Correctly modelling the Si
IV emission in agreement with all other diagnostics is however an
outstanding issue and remains paramount in explaining its apparent
coincidence with Hα emission. Fine-tuning the approach (accounting for
resolution differences, fitting localised temperature enhancements,
and/or performing spatially coupled inversions) is likely necessary
in order to obtain better agreement between all considered diagnostics.
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Title: Is the sky the limit?. Performance of the revamped Swedish
1-m Solar Telescope and its blue- and red-beam reimaging systems
Authors: Scharmer, G. B.; Löfdahl, M. G.; Sliepen, G.; de la Cruz
Rodríguez, J.
2019A&A...626A..55S Altcode: 2019arXiv190505588S
We discuss the use of measurements of the solar granulation contrast
as a measure of optical quality. We demonstrate that for data recorded
with a telescope that uses adaptive optics and/or post-processing to
compensate for many low- and high-order aberrations, the RMS granulation
contrast is directly proportional to the Strehl ratio calculated
from the residual (small-scale) wavefront error (static and/or from
seeing). We demonstrate that the wings of the high-order compensated
point spread function for the Swedish 1-m Solar Telescope (SST) are
likely to extend to a radius of not more than about 2″, which is
consistent with earlier conclusions drawn from stray-light compensation
of sunspot images. We report on simultaneous measurements of seeing
and solar granulation contrast averaged over 2 s time intervals at
several wavelengths from 525 nm to 853.6 nm on the red-beam (CRISP
beam) and wavelengths from 395 nm to 484 nm on the blue-beam (CHROMIS
beam). These data were recorded with the SST, which has been revamped
with an 85-electrode adaptive mirror and a new tip-tilt mirror, both of
which were polished to exceptionally high optical quality. Compared
to similar data obtained with the previous 37-electrode adaptive
mirror in 2009 and 2011, there is a significant improvement in image
contrast. The highest 2 s average image contrasts measured in April
2015 through 0.3-0.9 nm interference filters at 525 nm, 557 nm, 630
nm, and 853.5 nm with compensation only for the diffraction limited
point spread function of SST are 11.8%, 11.8%, 10.2%, and 7.2%,
respectively. Similarly, the highest 2 s contrasts measured at 395 nm,
400 nm, and 484 nm in May 2016 through 0.37-1.3 nm filters are 16%,
16%, and 12.5%, respectively. The granulation contrast observed with
SST compares favorably to measured values with SOT on Hinode and with
Sunrise as well as major ground-based solar telescopes. Simultaneously
with the above wideband red-beam data, we also recorded narrowband
continuum images with the CRISP imaging spectropolarimeter. We find
that contrasts measured with CRISP are entirely consistent with the
corresponding wideband contrasts, demonstrating that any additional
image degradation by the CRISP etalons and telecentric optical system
is marginal or even insignificant. Finally, we discuss the origin of
the 48 nm RMS wavefront error needed to bring consistency between the
measured granulation contrast and that obtained from 3D simulations
of convection.
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Title: The Dark Side of Penumbral Microjets: Observations in Hα
Authors: Buehler, D.; Esteban Pozuelo, S.; de la Cruz Rodriguez, J.;
Scharmer, G. B.
2019ApJ...876...47B Altcode: 2019arXiv190501245B
We present data of 10 penumbral microjets (PMJs) observed in a Hα,
Ca II 8542 Å, and Fe I 6302 Å line pair with the Swedish 1 m Solar
Telescope (SST) with CRISP and Ca II K with SST/CHROMIS in active region
NOAA 12599 on 2016 October 12 at μ = 0.68. All four Stokes parameters
of the Ca II 8542 Å and Fe I 6302 Å lines were observed and a series
of test pixels were inverted using the Stockholm inversion code. Our
analysis revealed for the first time that PMJs are visible in Hα,
where they appear as dark features with average line-of-sight (LOS)
upflows of 1.1 ± 0.6 km s<SUP>-1</SUP>, matching the LOS velocities
from the inversions. Based on the Hα observations we extend the
previous average length and lifetime of PMJs to 2815 ± 530 km and 163
± 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs
of up to 17 km s<SUP>-1</SUP> tend to give increased velocities with
distance traveled. Furthermore, two of our PMJs with significant Stokes
V signal indicate that the PMJs possess an increased LOS magnetic field
of up to 100 G compared to the local pre-/post- PMJ magnetic field,
which propagates as quickly as the PMJs’ POS velocities. Finally,
we present evidence that PMJs display an on average 1 minute gradual
precursory brightening that only manifests itself in the cores of the
Ca II lines. We conclude that PMJs are not ordinary jets but likely
are manifestations of heat fronts that propagate at the local Alfvén
velocity.
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Title: Observationally Based Models of Penumbral Microjets
Authors: Esteban Pozuelo, S.; de la Cruz Rodríguez, J.; Drews, A.;
Rouppe van der Voort, L.; Scharmer, G. B.; Carlsson, M.
2019ApJ...870...88E Altcode: 2018arXiv181107881E
We study the polarization signals and physical parameters of penumbral
microjets (PMJs) by using high spatial resolution data taken in the
Fe I 630 nm pair, Ca II 854.2 nm, and Ca II K lines with the CRISP
and CHROMIS instruments at the Swedish 1 m Solar Telescope. We
infer their physical parameters, such as physical observables in
the photosphere and chromospheric velocity diagnostics, by different
methods, including inversions of the observed Stokes profiles with
the STiC code. PMJs harbor overall brighter Ca II K line profiles
and conspicuous polarization signals in Ca II 854.2 nm, specifically
in circular polarization that often shows multiple lobes mainly due
to the shape of Stokes I. They usually overlap photospheric regions
with a sheared magnetic field configuration, suggesting that magnetic
reconnections could play an important role in the origin of PMJs. The
discrepancy between their low LOS velocities and the high apparent
speeds reported on earlier, as well as the existence of different
vertical velocity gradients in the chromosphere, indicate that PMJs
might not be entirely related to mass motions. Instead, PMJs could
be due to perturbation fronts induced by magnetic reconnections
occurring in the deep photosphere that propagate through the
chromosphere. This reconnection may be associated with current heating
that produces temperature enhancements from the temperature minimum
region. Furthermore, enhanced collisions with electrons could also
increase the coupling to the local conditions at higher layers during
the PMJ phase, giving a possible explanation for the enhanced emission
in the overall Ca II K profiles emerging from these transients.
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Title: Chromospheric heating during flux emergence in the solar
atmosphere
Authors: Leenaarts, Jorrit; de la Cruz Rodríguez, Jaime; Danilovic,
Sanja; Scharmer, Göran; Carlsson, Mats
2018A&A...612A..28L Altcode: 2017arXiv171200474L
Context. The radiative losses in the solar chromosphere vary from
4 kW m<SUP>-2</SUP> in the quiet Sun, to 20 kW m<SUP>-2</SUP> in
active regions. The mechanisms that transport non-thermal energy to
and deposit it in the chromosphere are still not understood. Aim. We
aim to investigate the atmospheric structure and heating of the solar
chromosphere in an emerging flux region. <BR /> Methods: We have used
observations taken with the CHROMIS and CRISP instruments on the
Swedish 1-m Solar Telescope in the Ca II K , Ca II 854.2 nm, Hα,
and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line
profiles and in addition perform multi-line, multi-species, non-local
thermodynamic equilibrium (non-LTE) inversions to estimate the spatial
and temporal variation of the chromospheric structure. <BR /> Results:
We investigate which spectral features of Ca II K contribute to the
frequency-integrated Ca II K brightness, which we use as a tracer
of chromospheric radiative losses. The majority of the radiative
losses are not associated with localised high-Ca II K-brightness
events, but instead with a more gentle, spatially extended, and
persistent heating. The frequency-integrated Ca II K brightness
correlates strongly with the total linear polarization in the Ca II
854.2 nm, while the Ca II K profile shapes indicate that the bulk
of the radiative losses occur in the lower chromosphere. Non-LTE
inversions indicate a transition from heating concentrated around
photospheric magnetic elements below log τ<SUB>500</SUB> = -3 to a more
space-filling and time-persistent heating above log τ<SUB>500</SUB>
= -4. The inferred gas temperature at log τ<SUB>500</SUB> = -3.8
correlates strongly with the total linear polarization in the Ca
II 854.2 nm line, suggesting that that the heating rate correlates
with the strength of the horizontal magnetic field in the low
chromosphere. <P />Movies attached to Figs. 1 and 4 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201732027/olm">https://www.aanda.org/</A>
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Title: Three-dimensional modeling of the Ca II H and K lines in the
solar atmosphere
Authors: Bjørgen, Johan P.; Sukhorukov, Andrii V.; Leenaarts, Jorrit;
Carlsson, Mats; de la Cruz Rodríguez, Jaime; Scharmer, Göran B.;
Hansteen, Viggo H.
2018A&A...611A..62B Altcode: 2017arXiv171201045B
Context. CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar
Telescope (SST), can observe the chromosphere in the H and K lines of
Ca II at high spatial and spectral resolution. Accurate modeling as
well as an understanding of the formation of these lines are needed to
interpret the SST/CHROMIS observations. Such modeling is computationally
challenging because these lines are influenced by strong departures from
local thermodynamic equilibrium, three-dimensional radiative transfer,
and partially coherent resonance scattering of photons. Aim. We aim to
model the Ca II H and K lines in 3D model atmospheres to understand
their formation and to investigate their diagnostic potential for
probing the chromosphere. <BR /> Methods: We model the synthetic
spectrum of Ca II using the radiative transfer code Multi3D in three
different radiation-magnetohydrodynamic model atmospheres computed with
the Bifrost code. We classify synthetic intensity profiles according
to their shapes and study how their features are related to the
physical properties in the model atmospheres. We investigate whether
the synthetic data reproduce the observed spatially-averaged line
shapes, center-to-limb variation and compare this data with SST/CHROMIS
images. <BR /> Results: The spatially-averaged synthetic line profiles
show too low central emission peaks, and too small separation between
the peaks. The trends of the observed center-to-limb variation of
the profiles properties are reproduced by the models. The Ca II H and
K line profiles provide a temperature diagnostic of the temperature
minimum and the temperature at the formation height of the emission
peaks. The Doppler shift of the central depression is an excellent
probe of the velocity in the upper chromosphere.
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Title: Intermittent Reconnection and Plasmoids in UV Bursts in the
Low Solar Atmosphere
Authors: Rouppe van der Voort, L.; De Pontieu, B.; Scharmer, G. B.;
de la Cruz Rodríguez, J.; Martínez-Sykora, J.; Nóbrega-Siverio,
D.; Guo, L. J.; Jafarzadeh, S.; Pereira, T. M. D.; Hansteen, V. H.;
Carlsson, M.; Vissers, G.
2017ApJ...851L...6R Altcode: 2017arXiv171104581R
Magnetic reconnection is thought to drive a wide variety of dynamic
phenomena in the solar atmosphere. Yet, the detailed physical mechanisms
driving reconnection are difficult to discern in the remote sensing
observations that are used to study the solar atmosphere. In this
Letter, we exploit the high-resolution instruments Interface Region
Imaging Spectrograph and the new CHROMIS Fabry-Pérot instrument at
the Swedish 1-m Solar Telescope (SST) to identify the intermittency
of magnetic reconnection and its association with the formation of
plasmoids in so-called UV bursts in the low solar atmosphere. The Si IV
1403 Å UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular
shapes, in addition to signatures of bidirectional flows. Such profiles
had previously been linked, in idealized numerical simulations, to
magnetic reconnection driven by the plasmoid instability. Simultaneous
CHROMIS images in the chromospheric Ca II K 3934 Å line now provide
compelling evidence for the presence of plasmoids by revealing highly
dynamic and rapidly moving brightenings that are smaller than 0.″2 and
that evolve on timescales of the order of seconds. Our interpretation
of the observations is supported by detailed comparisons with synthetic
observables from advanced numerical simulations of magnetic reconnection
and associated plasmoids in the chromosphere. Our results highlight
how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are
realistic enough to compare with observations can directly reveal the
small-scale physical processes that drive the wide range of phenomena
in the solar atmosphere.
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Title: SST/CHROMIS: a new window to the solar chromosphere
Authors: Scharmer, Goran
2017psio.confE..85S Altcode:
No abstract at ADS
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Title: Recent results and future DMs for astronomy and for space
applications at CILAS
Authors: Sinquin, Jean-Christophe; Bastard, Arnaud; Beaufort, Emmanuel;
Berkefeld, Thomas; Cadiergues, Laurent; Costes, Vincent; Cousty,
Raphaël.; Dekhtiar, Charles; Di Gesu, Frédéric; Gilbert, Xavier;
Grèzes-Besset, Catherine; Groeninck, Denis; Hartung, Markus; Krol,
Hélène; Moreau, Aurélien; Morin, Pierre; Pagès, Hubert; Palomo,
Richard; Scharmer, Göran; Soltau, Dirk; Véran, Jean-Pierre
2014SPIE.9148E..0GS Altcode:
We present recent experimental results obtained with CILAS deformable
mirrors (DMs) or demonstration prototypes in solar and night-time
astronomy (with ground-based telescopes) as well as observation
of the Earth (with space telescopes). These important results have
been reached thanks to CILAS technology range composed of monomorph
and piezostack deformable mirrors, drivers and optical coatings. For
instance, the monomorph technology, due to a simple architecture can
offer a very good reliability for space applications. It can be used
for closed or open loop correction of the primary mirror deformation
(thermal and polishing aberrations, absence of gravity). It can also
allow a real-time correction of wavefront aberrations introduced by
the atmosphere up to relatively high spatial and temporal frequencies
for ground-based telescopes. The piezostack technology is useful for
very high order correction at high frequency and under relatively low
operational temperature (down to -30°C), which is required for future
Extremely Large Telescopes (ELTs). This wide range of applications
is exposed through recent examples of DMs performances in operation
and results obtained with breadboards, allowing promising DMs for
future needs.
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Title: Response to "Stray-light correction in 2D spectroscopy"
by R. Schlichenmaier and M. Franz
Authors: Scharmer, G. B.
2014A&A...561A..31S Altcode: 2013arXiv1310.4630S
We discuss a recent paper by Schlichenmaier & Franz (SF; 2013,
A&A, 555, A84), in which the claim is made that the penumbral
dark downflows detected for the first time with the Swedish 1-m Solar
Telescope (SST) by Scharmer et al. and Joshi et al. could be produced
by overcompensation for straylight. We show that the analysis of SF is
fundamentally flawed, because it ignores the constraints on the strength
of such straylight from 3D convection simulations and on the spatial
extent of the straylight point spread function from the measured minimum
intensity in the sunspot umbra. Furthermore, we show that the claim
made by SF, that the spatial straylight of Hinode is less than 10%,
is false. We conclude that the analysis of SF is of no relevance in
relation to the straylight compensation method applied to the SST data.
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Title: Opposite polarity field with convective downflow and its
relation to magnetic spines in a sunspot penumbra
Authors: Scharmer, G. B.; de la Cruz Rodriguez, J.; Sütterlin, P.;
Henriques, V. M. J.
2013A&A...553A..63S Altcode: 2012arXiv1211.5776S
We discuss NICOLE inversions of Fe i 630.15 nm and 630.25 nm Stokes
spectra from a sunspot penumbra recorded with the CRISP imaging
spectropolarimeter on the Swedish 1-m Solar Telescope at a spatial
resolution close to 0.15 arcsec. We report on narrow, radially extended
lanes of opposite polarity field, located at the boundaries between
areas of relatively horizontal magnetic field (the intra-spines) and
much more vertical field (the spines). These lanes harbor convective
downflows of about 1 km s<SUP>-1</SUP>. The locations of these downflows
close to the spines agree with predictions from the convective gap
model (the "gappy penumbra") proposed six years ago, and more recent
three-dimensional magnetohydrodynamic simulations. We also confirm the
existence of strong convective flows throughout the entire penumbra,
showing the expected correlation between temperature and vertical
velocity, and having vertical root mean square velocities of about
1.2 km s<SUP>-1</SUP>.
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Title: SST/CRISP observations of convective flows in a sunspot
penumbra
Authors: Scharmer, G. B.; Henriques, V. M. J.
2012A&A...540A..19S Altcode: 2011arXiv1109.1301S
Context. Recent discoveries of intensity correlated downflows in the
interior of a sunspot penumbra provide direct evidence for overturning
convection, adding to earlier strong indications of convection from
filament dynamics observed far from solar disk center, and supporting
recent simulations of sunspots. <BR /> Aims: Using spectropolarimetric
observations obtained at a spatial resolution approaching 0.1 arcsec
with the Swedish 1-m Solar Telescope (SST) and its spectropolarimeter
CRISP, we investigate whether the convective downflows recently
discovered in the C i line at 538.03 nm can also be detected in the
wings of the Fe i line at 630.15 nm. <BR /> Methods: We make azimuthal
fits of the measured LOS velocities in the core and wings of the
538 nm and 630 nm lines to disentangle the vertical and horizontal
flows. To investigate how these depend on the continuum intensity,
the azimuthal fits are made separately for each intensity bin. By
using spatially high-pass filtered measurements of the LOS component of
the magnetic field, the flow properties are determined separately for
magnetic spines (relatively strong and vertical field) and inter-spines
(weaker and more horizontal field). <BR /> Results: The dark convective
downflows discovered recently in the 538.03 nm line are evident also
in the 630.15 nm line, and have similar strength. This convective
signature is the same in spines and inter-spines. However, the strong
radial (Evershed) outflows are found only in the inter-spines. <BR />
Conclusions: At the spatial resolution of the present SST/CRISP data,
the small-scale intensity pattern seen in continuum images is strongly
related to a convective up/down flow pattern that exists everywhere
in the penumbra. Earlier failures to detect the dark convective
downflows in the interior penumbra can be explained by inadequate
spatial resolution in the observed data.
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Title: Sources of straylight in the post-focus imaging instrumentation
of the Swedish 1-m Solar Telescope
Authors: Löfdahl, M. G.; Scharmer, G. B.
2012A&A...537A..80L Altcode: 2011arXiv1110.2663L
Context. Recently measured straylight point spread functions (PSFs) in
Hinode/SOT make granulation contrast in observed data and synthetic
magnetohydrodynamic (MHD) data consistent. Data from earthbound
telescopes also need accurate correction for straylight and fixed
optical aberrations. <BR /> Aims: We aim to develop a method for
measuring straylight in the post-focus imaging optics of the Swedish
1-m Solar Telescope (SST). <BR /> Methods: We removed any influence from
atmospheric turbulence and scattering by using an artificial target. We
measured integrated straylight from three different sources in the same
data: ghost images caused by reflections in the near-detector optics,
PSFs corresponding to wavefront aberrations in the optics by using
phase diversity, and extended scattering PSF wings of unknown origin
by fitting to a number of different kernels. We performed the analysis
separately in the red beam and the blue beam. <BR /> Results: Wavefront
aberrations, which possibly originate in the bimorph mirror of the
adaptive optics, are responsible for a wavelength-dependent straylight
of 20-30% of the intensity in the form of PSFs with 90% of the energy
contained within a radius of 0.6 arcsec. There are ghost images that
contribute at the most a few percent of straylight. The fraction of
other sources of scattered light from the post-focus instrumentation
of the SST is only ~10<SUP>-3</SUP> of the recorded intensity. This
contribution has wide wings with a FWHM ~16” in the blue and ~34”
in the red. <BR /> Conclusions: The present method seems to work well
for separately estimating wavefront aberrations and the scattering
kernel shape and fraction. Ghost images can be expected to remain
at the same level for solar observations. The high-order wavefront
aberrations possibly caused by the AO bimorph mirror dominate the
measured straylight but are likely to change when imaging the Sun. We
can therefore make no firm statements about the origin of straylight
in SST data, but strongly suspect wavefront aberrations to be the
dominant source.
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Title: Detection of Convective Downflows in a Sunspot Penumbra
Authors: Scharmer, G. B.; Henriques, V. M. J.; Kiselman, D.; de la
Cruz Rodríguez, J.
2011Sci...333..316S Altcode:
The fine structure and dynamics of sunspots and the strong outflow in
their outer filamentary part—the penumbra—have puzzled astronomers
for more than a century. Recent theoretical models and three-dimensional
numerical simulations explain the penumbral filaments and their
radiative energy output as the result of overturning convection. Here,
we describe the detection of ubiquitous, relatively dark downward
flows of up to 1 kilometer per second (km/s) in the interior penumbra,
using imaging spectropolarimetric data from the Swedish 1-meter Solar
Telescope. The dark downflows are omnipresent in the interior penumbra,
distinguishing them from flows in arched flux tubes, and are associated
with strong (3 to 3.5 km/s) radial outflows. They are thus part of a
penumbral convective flow pattern, with the Evershed flow representing
the horizontal component of that convection.
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Title: Small-scale convection signatures associated with a strong
plage solar magnetic field
Authors: Narayan, G.; Scharmer, G. B.
2010A&A...524A...3N Altcode: 2010arXiv1007.4673N
Context. Solar convection in a strong plage, in which the magnetic
field is vertical and strong over extended regions, but much weaker
than in the umbrae of large sunspots, has so far not been well
studied. This has been mostly because of a lack of spectropolarimetric
data at adequate spatial resolution. The combination of a large solar
telescope, such as the Swedish 1-m Solar Telescope, adaptive optics,
powerful image reconstruction techniques, and a high-fidelity imaging
spectropolarimeter is, however, capable of producing such data. <BR />
Aims: In this work, we study and quantify the properties of strong-field
small-scale convection and compare these observed properties with
those predicted by numerical simulations. <BR /> Methods: We analyze
spectropolarimetric 630.25 nm data from a unipolar ephemeral region
near the Sun center. We use line-of-sight velocities and magnetic field
measurements obtained with Milne-Eddington inversion techniques along
with measured continuum intensities and Stokes V amplitude asymmetry at
a spatial resolution of 0.15 arcsec to establish statistical relations
between the measured quantities. We also study these properties for
different types of distinct magnetic features, such as micropores,
bright points, ribbons, flowers, and strings. <BR /> Results:
We present the first direct observations of a small-scale granular
magneto-convection pattern within extended regions of a strong (more
than 600 G on average) magnetic field. Along the boundaries of the
flux concentrations, we see mostly downflows and asymmetric Stokes
V profiles, consistent with synthetic line profiles calculated from
MHD simulations. We note the frequent occurrence of bright downflows
along these boundaries. In the interior of the flux concentrations,
we observe an up/down flow pattern that we associate with small-scale
magnetoconvection, appearing similar to that of field-free granulation
but with scales 4 times smaller. Measured rms velocities are 70%
of those of nearby field-free granulation, even though the average
radiative flux is not lower than that of the quiet Sun. The interiors
of these flux concentrations are dominated by upflows.
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Title: Striation and convection in penumbral filaments
Authors: Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G.
2010A&A...521A..72S Altcode: 2010arXiv1008.0932S
Observations with the 1-m Swedish Solar Telescope of the flows seen in
penumbral filaments are presented. Time sequences of bright filaments
show overturning motions strikingly similar to those seen along
the walls of small isolated structures in the active regions. The
filaments show outward propagating striations with inclination
angles suggesting that they are aligned with the local magnetic
field. We interpret it as the equivalent of the striations seen in
the walls of small isolated magnetic structures. Their origin is then
a corrugation of the boundary between an overturning convective flow
inside the filament and the magnetic field wrapping around it. The
outward propagation is a combination of a pattern motion due to
the downflow observed along the sides of bright filaments, and the
Evershed flow. The observed short wavelength of the striation argues
against the existence of a dynamically significant horizontal field
inside the bright filaments. Its intensity contrast is explained by
the same physical effect that causes the dark cores of filaments,
light bridges and “canals”. In this way striation represents an
important clue to the physics of penumbral structure and its relation
with other magnetic structures on the solar surface. We put this in
perspective with results from the recent 3-D radiative hydrodynamic
simulations. <P />4 movies are only available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: High-order aberration compensation with multi-frame blind
deconvolution and phase diversity image restoration techniques
Authors: Scharmer, G. B.; Löfdahl, M. G.; van Werkhoven, T. I. M.;
de la Cruz Rodríguez, J.
2010A&A...521A..68S Altcode: 2010arXiv1007.1236S
Context. For accurately measuring intensities and determining magnetic
field strengths of small-scale solar (magnetic) structure, knowledge of
and compensation for the point spread function is crucial. For images
recorded with the Swedish 1-meter Solar Telescope (SST), restoration
with multi-frame blind deconvolution (MFBD) and joint phase diverse
speckle (JPDS) methods lead to remarkable improvements in image quality
but granulation contrasts that are too low, indicating additional stray
light. <BR /> Aims: We propose a method to compensate for stray light
from high-order atmospheric aberrations not included in MFBD and JPDS
processing. <BR /> Methods: To compensate for uncorrected aberrations,
a reformulation of the image restoration process is proposed that allows
the average effect of hundreds of high-order modes to be compensated for
by relying on Kolmogorov statistics for these modes. The applicability
of the method requires simultaneous measurements of Fried's parameter
r<SUB>0</SUB>. The method is tested with simulations as well as real
data and extended to include compensation for conventional stray
light. <BR /> Results: We find that only part of the reduction of
granulation contrast in SST images is due to uncompensated high-order
aberrations. The remainder is still unaccounted for and attributed
to stray light from the atmosphere, the telescope with its re-imaging
system and to various high-altitude seeing effects. <BR /> Conclusions:
We conclude that statistical compensation of high-order modes is a
viable method to reduce the loss of contrast occurring when a limited
number of aberrations is explicitly compensated for with MFBD and JPDS
processing. We show that good such compensation is possible with only
10 recorded frames. The main limitation of the method is that already
MFBD and JPDS processing introduces high-order compensation that,
if not taken into account, can lead to over-compensation.
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Title: Site-seeing measurements for the European Solar Telescope
Authors: Berkefeld, Th.; Bettonvil, F.; Collados, M.; López, R.;
Martín, Y.; Peñate, J.; Pérez, A.; Scharmer, G. B.; Sliepen, G.;
Soltau, D.; Waldmann, T. A.; van Werkhoven, T.
2010SPIE.7733E..4IB Altcode: 2010SPIE.7733E.141B
Seeing measurements are crucial for the optimum design of
(multi-conjugate) adaptive optics systems operating at solar
telescopes. For the design study of the 4-meter European Solar
Telescope, to be located in the Canary Islands, several instruments have
been constructed and operated, at the Observatorio del Roque de los
Muchachos (La Palma) and at the Observatorio del Teide (Tenerife),
to measure the properties of the ground layer and medium-high
altitude turbulence. Several units of short (42.34 cm) and two long
(323.06 cm) scintillometer bars are, or are to be, installed at
both observatories. In addition to them, two wide-field wavefront
sensors will be attached to the optical beams of the Swedish tower,
on La Palma, and of the German VTT, on Tenerife, simultaneously used
with the normal operation of the telescopes. These wavefront sensors
are of Shack-Hartmann type with ~1 arcminute field of view. In this
contribution, the instruments setup and their performance are described.
---------------------------------------------------------
Title: Adaptive optics and MCAO for the 4-m European Solar Telescope
EST
Authors: Soltau, D.; Berkefeld, T.; Sánchez Capuchino, J.; Collados
Vera, M.; Del Moro, D.; Löfdahl, M.; Scharmer, G.
2010SPIE.7736E..0US Altcode: 2010SPIE.7736E..27S
A consortium of more than 20 European solar physics institution from 15
different countries is conducting a design study for a 4 m class solar
telescope which shall be situated at the Canary Islands. In this paper
we introduce the AO and MCAO design concept for EST. A ground layer
deformable mirror is combined with an arrangement of four deformable
layer mirrors. A combination of Shack-Hartmann wave front sensors with
wide and narrow fields of view is used to control the system and to
achieve a corrected field of view of one arcmin.
---------------------------------------------------------
Title: S-DIMM+ height characterization of day-time seeing using
solar granulation
Authors: Scharmer, G. B.; van Werkhoven, T. I. M.
2010A&A...513A..25S Altcode: 2010arXiv1002.3151S
Context. To evaluate site quality and to develop multi-conjugative
adaptive optics systems for future large solar telescopes,
characterization of contributions to seeing from heights up to at
least 12 km above the telescope is needed. <BR /> Aims: We describe a
method for evaluating contributions to seeing from different layers
along the line-of-sight to the Sun. The method is based on Shack
Hartmann wavefront sensor data recorded over a large field-of-view
with solar granulation and uses only measurements of differential image
displacements from individual exposures, such that the measurements are
not degraded by residual tip-tilt errors. <BR /> Methods: The covariance
of differential image displacements at variable field angles provides
a natural extension of the work of Sarazin and Roddier to include
measurements that are also sensitive to the height distribution of
seeing. By extending the numerical calculations of Fried to include
differential image displacements at distances much smaller and much
larger than the subaperture diameter, the wavefront sensor data can be
fitted to a well-defined model of seeing. The resulting least-squares
fit problem can be solved with conventional methods. The method is
tested with simple simulations and applied to wavefront data from the
Swedish 1-m Solar Telescope on La Palma, Spain. <BR /> Results: We show
that good inversions are possible with 9-10 layers, three of which
are within the first 1.5 km, and a maximum distance of 16-30 km, but
with poor height resolution in the range 10-30 km. <BR /> Conclusions:
We conclude that the proposed method allows good measurements when
Fried's parameter r<SUB>0</SUB> is larger than about 7.5 cm for
the ground layer and that these measurements should provide valuable
information for site selection and multi-conjugate development for the
future European Solar Telescope. A major limitation is the large field
of view presently used for wavefront sensing, leading to uncomfortably
large uncertainties in r<SUB>0</SUB> at 30 km distance.
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Title: Convection and the Origin of Evershed Flows
Authors: Nordlund, Å.; Scharmer, G. B.
2010ASSP...19..243N Altcode: 2009arXiv0905.0918N; 2010mcia.conf..243N
Numerical simulations have by now revealed that the fine scale structure
of the penumbra in general and the Evershed effect in particular is due
to overturning convection, mainly confined to gaps with strongly reduced
magnetic field strength. The Evershed flow is the radial component of
the overturning convective flow visible at the surface. It is directed
outwards - away from the umbra - because of the broken symmetry due
to the inclined magnetic field. The dark penumbral filament cores
visible at high resolution are caused by the "cusps" in the magnetic
field that form above the gaps. Still remaining to be established are
the details of what determines the average luminosity of penumbrae,
the widths, lengths, and filling factors of penumbral filaments, and
the amplitudes and filling factors of the Evershed flow. These are
likely to depend at least partially also on numerical aspects such as
limited resolution and model size, but mainly on physical properties
that have not yet been adequately determined or calibrated, such as
the plasma beta profile inside sunspots at depth and its horizontal
profile, the entropy of ascending flows in the penumbra, etc.
---------------------------------------------------------
Title: Recent Evidence for Convection in Sunspot Penumbrae
Authors: Scharmer, Göran B.
2009SSRv..144..229S Altcode:
Whereas penumbral models during the last 15 years have been successful
in explaining Evershed flows and magnetic field inclination variations
in terms of flux tubes, the lack of contact between these models and a
convective process needed to explain the penumbral radiative heat flux
has been disturbing. We report on recent observational and theoretical
evidence that challenge flux tube interpretations and conclude that
the origin of penumbral filamentary structure is overturning convection.
---------------------------------------------------------
Title: Recent Evidence for Convection in Sunspot Penumbrae
Authors: Scharmer, Göran B.
2009odsm.book..229S Altcode:
Whereas penumbral models during the last 15 years have been successful
in explaining Evershed flows and magnetic field inclination variations
in terms of flux tubes, the lack of contact between these models and a
convective process needed to explain the penumbral radiative heat flux
has been disturbing. We report on recent observational and theoretical
evidence that challenge flux tube interpretations and conclude that
the origin of penumbral filamentary structure is overturning convection.
---------------------------------------------------------
Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz
Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van
Noort, M.; Lagg, A.
2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S
We discuss penumbral fine structure in a small part of a pore,
observed with the CRISP imaging spectropolarimeter at the Swedish
1-m Solar Telescope (SST), close to its diffraction limit of
0.16”. Milne-Eddington inversions applied to these Stokes data
reveal large variations of field strength and inclination angle over
dark-cored penumbral intrusions and a dark-cored light bridge. The
mid-outer part of this penumbra structure shows ~0.3” wide spines,
separated by ~1.6” (1200 km) and associated with 30° inclination
variations. Between these spines, there are no small-scale magnetic
structures that easily can be identified with individual flux tubes. A
structure with nearly 10° more vertical and weaker magnetic field is
seen midway between two spines. This structure is cospatial with the
brightest penumbral filament, possibly indicating the location of a
convective upflow from below.
---------------------------------------------------------
Title: Recent evidence for convection in sunspot penumbrae
Authors: Scharmer, G. B.
2008PhST..133a4015S Altcode: 2008arXiv0812.3971S
Whereas penumbral models during the last 15 years have appeared
successful in explaining Evershed flows and magnetic field inclination
variations in terms of flux tubes, the lack of connection between these
models and a convective process to explain the penumbral radiative
heat flux has been disturbing. We report on recent observational and
theoretical evidence that challenge these flux tube interpretations and
instead suggest overturning convection as the origin of the penumbral
filamentary structure.
---------------------------------------------------------
Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution
Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.;
Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz
Rodríguez, J.
2008ESPM...12..2.5S Altcode:
We present first observations of sunspots with the imaging
spectropolarimeter CRISP, recently installed at the Swedish 1-m
Solar Telescope (SST) on La Palma. This spectropolarimeter is based
on a high-fidelity dual Fabry-Perot filter system. <P />Two liquid
crystals and a polarizing beam splitter are used to reduce seeing
induced I,Q,U,V crosstalk by simultaneously recording images with
two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously
records broadband images through the pre-filter of the FPI filter
system, allowing image reconstruction and co-alignment of images of
different polarization states and at different wavelengths in Zeeman
sensitive spectral lines. <P />The first data, recorded in April 2008,
demonstrate the capability of this system to record high cadence,
high S/N polarimetric data with a spatial resolution at or close to
the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss
the analysis of first spectropolarimetric data for sunspots, based on
Milne-Eddington inversion techniques.
---------------------------------------------------------
Title: SST/CRISP Magnetometry with Fe I 630.2 nm
Authors: Narayan, G.; Scharmer, G. B.; Hillberg, T.; Lofdahl, M.;
van Noort, M.; Sutterlin, P.; Lagg, A.
2008ESPM...122.120N Altcode:
We present recent full Stokes observations in the Fe I 630.2 nm
line with CRISP, an imaging spectropolarimeter at the Swedish 1-m
Solar Telescope (SST). The observations reach a spatial resolution
of 0".16, close to the diffraction limit of the SST, representing
a major improvement over any past ground based or space based
spectropolarimetric data. We describe the data acquisition and reduction
methods and present results of Milne-Eddington(ME) inversions applied
on observations of plage.
---------------------------------------------------------
Title: Convection and the Origin of Evershed Flows in Sunspot
Penumbrae
Authors: Scharmer, G. B.; Nordlund, Å.; Heinemann, T.
2008ApJ...677L.149S Altcode: 2008arXiv0802.1927S
We discuss a numerical 3D radiation-MHD simulation of penumbral fine
structure in a small sunspot. This simulation shows the development of
short filamentary structures with horizontal flows, similar to observed
Evershed flows, and an inward propagation of these structures at a
speed compatible with observations. Although the lengths of these
filaments are much shorter than observed, we conjecture that this
simulation qualitatively reproduces the mechanisms responsible for
filament formation and Evershed flows in penumbrae. We conclude that the
Evershed flow represents the horizontal-flow component of overturning
convection in gaps with strongly reduced field strength. The top of
the flow is always directed outward—away from the umbra—because of
the broken symmetry due to the inclined magnetic field. Upflows occur
in the inner parts of the gaps and most of the gas turns over radially
(outward and sideways), and descends back down again. The ascending,
cooling, and overturning flow tends to bend magnetic field lines down,
forcing a weakening of the field that makes it easier for gas located
in an adjacent layer—farther in—to initiate a similar sequence of
motion, aided by lateral heating, thus causing the inward propagation
of the filament.
---------------------------------------------------------
Title: MHD Simulations of Penumbra Fine Structure
Authors: Heinemann, T.; Nordlund, Å.; Scharmer, G. B.; Spruit, H. C.
2007ApJ...669.1390H Altcode: 2006astro.ph.12648H
We present the results of numerical 3D magnetohydrodynamic (MHD)
simulations with radiative energy transfer of fine structure in a small
sunspot of about 4 Mm width. The simulations show the development
of filamentary structures and flow patterns that are, except for
the lengths of the filaments, very similar to those observed. The
filamentary structures consist of gaps with reduced field strength
relative to their surroundings. Calculated synthetic images show dark
cores like those seen in the observations; the dark cores are the result
of a locally elevated τ=1 surface. The magnetic field in these cores
is weaker and more horizontal than for adjacent brighter structures,
and the cores support a systematic outflow. Accompanying animations
show the migration of the dark-cored structures toward the umbra,
and fragments of magnetic flux that are carried away from the spot by
a large-scale “moat flow.” We conclude that the simulations are in
qualitative agreement with observed penumbra filamentary structures,
Evershed flows, and moving magnetic features.
---------------------------------------------------------
Title: What are 'Faculae'?
Authors: Berger, T. E.; Title, A. M.; Tarbell, T.; Rouppe van der
Voort, L.; Löfdahl, M. G.; Scharmer, G. B.
2007ASPC..369..103B Altcode:
We present very high resolution filtergram and magnetogram observations
of solar faculae taken at the Swedish 1-meter Solar Telescope (SST)
on La Palma. Three datasets with average line-of-sight angles of 16,
34, and 53 degrees are analyzed. The average radial extent of faculae is
at least 400~km. In addition we find that contrast versus magnetic flux
density is nearly constant for faculae at a given disk position. These
facts and the high resolution images and movies reveal that faculae are
not the interiors of small flux tubes - they are granules seen through
the transparency caused by groups of magnetic elements or micropores
“in front of” the granules. Previous results which show a strong
dependency of facular contrast on magnetic flux density were caused
by bin-averaging of lower resolution data leading to a mixture of
the signal from bright facular walls and the associated intergranular
lanes and micropores. The findings are relevant to studies of total
solar irradiance (TSI) that use facular contrast as a function of disk
position and magnetic field in order to model the increase in TSI with
increasing sunspot activity.
---------------------------------------------------------
Title: Recent High Resolution Observations and Interpretations of
Sunspot Fine Structure
Authors: Scharmer, G. B.; Langhans, K.; Kiselman, D.; Löfdahl, M. G.
2007ASPC..369...71S Altcode:
We review analyses made of highly resolved filtergrams, magnetograms
and Dopplergrams of sunspots, recorded with the Swedish 1-meter Solar
Telescope (SSTSST) on La Palma. Dark cores in penumbral filaments are
shown to be directly linked to peripheral umbral dots and to dark
lanes in light bridges, suggesting similar or related underlying
physics. The visibility of dark cores is found to depend strongly
on the azimuth angle already for spots located at small heliocentric
distances. It is shown that dark cores are clearly visible close to the
center of the Ca II H line, formed approximately 150--200~km above the
photosphere. We conclude that the τ = 1 layer of dark-cored filaments
outlines a strongly warped surface, consistent with the finding that
the magnetic field strength is strongly reduced in dark cores. We show
that several properties of dark-cored filaments derived from SSTSST
data are consistent with results of inversions of low-resolution Stokes
spectra, but also find important discrepancies with the interpretation
that penumbral filaments can be identified with flux tubes. Our data
are consistent with the model proposed by Spruit & Scharmer (2006),
explaining dark cores as signatures of field-free convection occurring
just below the visible surface of the penumbra. We discuss recent
simulations of light bridges and umbral dots, providing additional
support to that model.
---------------------------------------------------------
Title: Observations of dark-cored filaments in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.
2007A&A...464..763L Altcode:
Context: The recent discovery of dark-cored penumbral filaments suggests
that we are resolving the building blocks of sunspot penumbrae. Their
properties are largely unknown but provide important clues to
understanding penumbral fine structure. <BR />Aims: Our observations
provide new constraints for the different scenarios put forward to
explain the structure of sunspot penumbrae. <BR />Methods: We present
an analysis of dark-cored penumbral filaments, based on intensity
filtergrams (G-band, continuum and Ca II H line wing), magnetograms
and Dopplergrams, obtained at heliocentric distances between 15° and
55°. <BR />Results: In general, the visibility of dark cores degrades
with increasing heliocentric distance. Based on Ca II H wing images we
conclude that this is due to a geometrical 3D-effect and not due to a
simple formation height effect. Only in the center-side penumbra are
dark-cored filaments visible at all observed heliocentric distances. We
observe that dark-cored filaments frequently split in the umbra, forming
a Y-shape that disappears after a few minutes, leaving a shortened
filamentary structure and a bright dot in the umbra. The dark-cored
filaments have life times ≥ 90 min. The dark cores are related to
a much weaker and a more horizontal magnetic field than their lateral
brightenings. Where the dark-cored filaments appear in the umbra, the
magnetic field is inclined by 40° with respect to the solar surface
normal for both the dark core and the bright edges. With increasing
distance from the umbra, the magnetic field inclination in the dark
cores increases rapidly within a few thousand km. Both the magnetic
field strength and inclination in the lateral brightenings show very
small variations with spot-center radial distance. The velocity field
possesses a strong horizontal component within the dark cores. The
absolute line-of-sight (LOS) velocity is larger within the dark cores
than in their lateral brightenings. The Evershed flow apparently is
present primarily in the dark cores.
---------------------------------------------------------
Title: Ca IIH line wing images of sunspot penumbrae recorded with
the Swedish 1-m Solar Telescope
Authors: Narayan, G.; van Noort, M. J.; Scharmer, G.
2007msfa.conf..213N Altcode:
We present recent Ca IIH images of sunspot penumbrae taken with the
Swedish 1m Solar Telescope (SST) during June-July 2006 and restored to
a resolution close to 0".1. Images were recorded at different Ca IIH
line positions using one tunable filter, one fixed wing filter and
a wide band quasi-continuum filter. Apart from the images recorded
at line center (and formed under non-LTE conditions), this provides
temperature information from approximately the first 200-300 km above
the photosphere.
---------------------------------------------------------
Title: Magnetostatic penumbra models with field-free gaps
Authors: Scharmer, G. B.; Spruit, H. C.
2006A&A...460..605S Altcode: 2006astro.ph..9130S
We present numerical 2D magnetostatic models for sunspot penumbrae
consisting of radially aligned field-free gaps in a potential magnetic
field, as proposed by Spruit & Scharmer (2006, A&A, 447,
343). The shape of the gaps and the field configurations around them
are computed consistently from the condition of magnetostatic pressure
balance between the gap and the magnetic field. The results show that
field-free gaps in the inner penumbra are cusp-shaped and bounded
by a magnetic field inclined by about 70° from the vertical. Here,
the magnetic component has a Wilson depression on the order 200-300
km relative to the top of the field-free gap; the gaps should thus
appear as noticeably elevated features. This structure explains the
large variations in field strength in the inner penumbra inferred from
magnetograms and two-component inversions, and the varying appearance
of the inner penumbra with viewing angle. In the outer penumbra, on the
other hand, the gaps are flat-topped with a horizontal magnetic field
above the middle of the gap. The magnetic field has large inclination
variations horizontally, but only small fluctuations in field strength,
in agreement with observations.
---------------------------------------------------------
Title: Rapid Temporal Variability of Faculae: High-Resolution
Observations and Modeling
Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort,
L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G.
2006ApJ...646.1405D Altcode:
We present high-resolution G-band observations (obtained with the
Swedish 1 m Solar Telescope) of the rapid temporal variability of
faculae, which occurs on granular timescales. By combining these
observations with magnetoconvection simulations of a plage region, we
show that much of this variability is not intrinsic to the magnetic
field concentrations that are associated with faculae, but rather
a phenomenon associated with the normal evolution and splitting of
granules. We also show examples of facular variability caused by
changes in the magnetic field, with movies of dynamic behavior of
the striations that dominate much of the facular appearance at 0.1"
resolution. Examples of these dynamics include merging, splitting,
rapid motion, apparent fluting, and possibly swaying.
---------------------------------------------------------
Title: Large bearings with incorporated gears, high stiffness,
and precision for the Swedish Solar Telescope (SST) on La Palma
Authors: Hammerschlag, Robert H.; Bettonvil, Felix C. M.; Jägers,
Aswin P. L.; Scharmer, Göran B.
2006SPIE.6273E..15H Altcode: 2006SPIE.6273E..34H
The 1-meter Swedish Solar Telescope (SST) obtains images of the solar
surface with an unprecedented resolution of 0.1 arcsec. It consists
of a relatively slender tower with on top only the vacuum turret for
reflecting downward the solar beam and no protective dome. This is a
favourable situation to get good local seeing. Just in the case of some
wind, seeing is best for daytime observations, therefore the precision
bearings and drives of the elevation- and azimuth axis of the turret
have to be stiff against wind. This requires line contact between the
meshing teeth of the large gear wheel and the pinion. High preload
forces to achieve line contact are not allowed because of appearing
stick-slip effects. To reduce the risk on stick-slip a special design
of the teeth for high stiffness combined with low friction and smooth
transition from one tooth to the next was made. Furthermore, extreme
precision in the fabrication was pursued such that relatively small
contact forces give already line contact. This required a special order
of the successive fabrication steps of the combination of bearing and
gear teeth. An additional problem was the relatively thin section
of the bearings required for a compact turret construction, needed
for best local seeing and minimum wind load. Solutions for all these
problems will be discussed. For the large gears the exceptional good
DIN quality class 4 for the pitch precision and straightness plus
direction of the teeth faces was achieved.
---------------------------------------------------------
Title: Comments on the optimization of high resolution Fabry-Pérot
filtergraphs
Authors: Scharmer, G. B.
2006A&A...447.1111S Altcode:
In this paper we present results of simulations of Fabry-Pérot (FPI)
system performance that address two entirely separate aspects. The
first concerns the image quality of FPIs used in telecentric optical
setups. We show that the image degradation from phase errors found by
von der Lühe & Kentischer (2000, A&AS, 146, 499) can largely be
compensated by refocusing and that the major cause of image degradation
is the pupil apodization discussed by Beckers (1998, A&AS, 120,
191). We also discuss the optimization of FPI systems, considering the
effects of cavity errors on the homogeneity of the instrument function
(transmission profile) across the FOV when two FPIs are combined in a
telecentric reimaging system. Our conclusions from several numerical
experiments are that telecentric FPI systems that use two or more FPIs
with high (94-95%) reflectivities are likely to show large variations
in the instrument profile across the FOV unless the cavity errors are
significantly smaller than 2.0 nm. We show, that such homogeneity can
be obtained at relatively low "cost" in terms of increased parasitic
light levels by lowering the reflectivity of the FPI with the smallest
cavity separation rather then lowering the reflectivity of both FPIs. We
also demonstrate that the choice of cavity ratios has a strong impact
on the homogeneity of the spectral transmission profile across the FOV
and that double FPI systems with cavity ratios around 0.3 perform much
better than at around 0.6.
---------------------------------------------------------
Title: Fine structure, magnetic field and heating of sunspot penumbrae
Authors: Spruit, H. C.; Scharmer, G. B.
2006A&A...447..343S Altcode: 2005astro.ph..8504S
We interpret penumbral filaments as due to convection in field-free,
radially aligned gaps just below the visible surface of the penumbra,
intruding into a nearly potential field above. This solves the
classical discrepancy between the large heat flux and the low vertical
velocities observed in the penumbra. The presence of the gaps causes
strong small-scale fluctuations in inclination, azimuth angle and
field strength. The field is nearly horizontal in a region around
the cusp-shaped top of the gap, thereby providing an environment for
Evershed flows. We identify this region with the recently discovered
dark penumbral cores. Its darkness has the same cause as the dark lanes
in umbral light-bridges, reproduced in numerical simulations by Nordlund
& Stein (2005, in preparation). We predict that the large vertical
and horizontal gradients of the magnetic field inclination and azimuth
in the potential field model will produce the net circular polarization
seen in observations. The model also explains the significant elevation
of bright filaments above their surroundings. It predicts that dark
areas in the penumbra are of two different kinds: dark filament cores
containing the most inclined (horizontal) fields, and regions between
bright filaments, containing the least inclined field lines.
---------------------------------------------------------
Title: Simulated Solar Plages
Authors: Stein, R. F.; Carlsson, M.; de Pontieu, B.; Scharmer, G.;
Nordlund, Å.; Benson, D.
2006apri.meet...30S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Inclination of magnetic fields and flows in sunspot penumbrae
Authors: Langhans, K.; Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.;
Berger, T. E.
2005A&A...436.1087L Altcode:
An observational study of the inclination of magnetic fields and
flows in sunspot penumbrae at a spatial resolution of 0.2 arcsec is
presented. The analysis is based on longitudinal magnetograms and
Dopplergrams obtained with the Swedish 1-m Solar Telescope on La Palma
using the Lockheed Solar Optical Universal Polarimeter birefringent
filter. Data from two sunspots observed at several heliocentric
angles between 12 ° and 39 ° were analyzed. We find that the
magnetic field at the level of the formation of the Fe i-line wing
(630.25 nm) is in the form of coherent structures that extend radially
over nearly the entire penumbra giving the impression of vertical
sheet-like structures. The inclination of the field varies up to
45 ° over azimuthal distances close to the resolution limit of the
magnetograms. Dark penumbral cores, and their extensions into the outer
penumbra, are prominent features associated with the more horizontal
component of the magnetic field. The inclination of this dark penumbral
component - designated B - increases outwards from approximately 40 °
in the inner penumbra such that the field lines are nearly horizontal
or even return to the solar surface already in the middle penumbra. The
bright component of filaments - designated A - is associated with the
more vertical component of the magnetic field and has an inclination
with respect to the normal of about 35 ° in the inner penumbra,
increasing to about 60 ° towards the outer boundary. The magnetogram
signal is lower in the dark component B regions than in the bright
component A regions of the penumbral filaments. The measured rapid
azimuthal variation of the magnetogram signal is interpreted as being
caused by combined fluctuations of inclination and magnetic field
strength. The Dopplergrams show that the velocity field associated with
penumbral component B is roughly aligned with the magnetic field while
component A flows are more horizontal than the magnetic field. The
observations give general support to fluted and uncombed models of
the penumbra. The long-lived nature of the dark-cored filaments makes
it difficult to interpret these as evidence for convective exchange
of flux tubes. Our observations are in broad agreement with the two
component model of Bellot Rubi et al. (2003), but do not rule out the
embedded flux tube model of Solanki & Montavon (1993).
---------------------------------------------------------
Title: Solar magnetic elements at 0.1 arcsec resolution. General
appearance and magnetic structure
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Löfdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A.; Scharmer, G.
2004A&A...428..613B Altcode:
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were obtained at
the recently commissioned Swedish 1-m Solar Telescope on La Palma. We
examine a single 430.5 nm G-band filtergram that resolves ∼70 km
(0.1 arcsec) structures and find new forms of magnetic structures
in this particular region. A cotemporal Ca II H-line image is used
to examine the low-chromosphere of network elements. A cotemporal Fe
I 630.25 nm magnetogram that resolves structures as small as 120 km
(0.18 arcsec) FWHM with a flux sensitivity of approximately 130 Mx
cm<SUP>-2</SUP> quantifies the magnetic structure of the region. A
Ni I 676.8 nm Dopplergram establishes relative velocity patterns
associated with the network features with an accuracy of about 300 m
s<SUP>-1</SUP>. We find that magnetic flux in this region as seen in
both the magnetogram and the G-band image is typically structured into
larger, amorphous, “ribbons” which are not resolved into individual
flux tubes. The measured magnetic flux density in the ribbon structures
ranges from 300 to 1500 Mx cm<SUP>-2</SUP>, the higher values occurring
at localized concentrations embedded within the ribbons. The Dopplergram
indicates relative downflows associated with all magnetic elements
with some indication that higher downflows occur adjacent to the peak
magnetic flux location. The mean absolute magnetic flux density of the
remnant plage network is about 130 Mx cm<SUP>-2</SUP>; in the lowest
flux regions of the field-of-view, the mean absolute flux density is
approximately 60 Mx cm<SUP>-2</SUP>. Within these quiet regions we do
not find evidence of pervasive kilo-gauss strength magnetic elements
as seen in recent high resolution internetwork studies. In general,
the observations confirm recent 3-dimensional numerical simulations
which show that the magnetic field in high-density regions such as
plage is concentrated in complex structures that are not generally
composed of discrete magnetic flux tubes. <P />Appendices are only
available in electronic form at http://www.edpsciences.org
---------------------------------------------------------
Title: Observational Manifestations of Solar Magnetoconvection:
Center-to-Limb Variation
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
2004ApJ...610L.137C Altcode: 2004astro.ph..6160C
We present the first center-to-limb G-band images synthesized from
high-resolution simulations of solar magnetoconvection. Toward the
limb the simulations show “hilly” granulation with dark bands on
the far side, bright granulation walls, and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G band, the contrast is further increased
by the destruction of CH in the low-density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements' optical surface is hidden by
the granules and the bright points are no longer visible, except where
the “magnetic valleys” are aligned with the line of sight. Toward
the limb, the low density in the strong magnetic elements causes unit
line-of-sight optical depth to occur deeper in the granule walls behind
than for rays not going through magnetic elements, and variations
in the field strength produce a striated appearance in the bright
granule walls.
---------------------------------------------------------
Title: Three-Dimensional Structure of the Active Region Photosphere
as Revealed by High Angular Resolution
Authors: Lites, B. W.; Scharmer, G. B.; Berger, T. E.; Title, A. M.
2004SoPh..221...65L Altcode:
Blue continuum images of active regions at ∼ 60° from the center
of the solar disk obtained with the new Swedish 1-m Solar Telescope
reveal heretofore unreported structure of the magnetized solar
atmosphere. Perhaps the most striking aspect of these images is
that, at an angular resolution of 0.12″, they show clearly the
three-dimensional structure of the photosphere. In particular,
the Wilson depression of the dark floors of pores is readily
apparent. Conversely, the segmented structure of light bridges running
through sunspots and pores reveal that light bridges are raised
above the dark surroundings. The geometry of light bridges permits
estimates of the height of their central (slightly darker) ridge:
typically in the range 200-450 km. These images also clearly show
that facular brightenings outside of sunspots and pores occur on the
disk-center side of those granules just limbward of intergranular lanes
that presumably harbor the associated plage magnetic flux. In many cases
the brightening extends 0.5″ or more over those granules. Furthermore,
a very thin, darker lane is often found just centerward of the facular
brightening. We speculate that this feature is the signature of cool
down flows that surround flux tubes in dynamical models. These newly
recognized observational aspects of photospheric magnetic fields
should provide valuable constraints for MHD models of the magnetized
photosphere, and examination of those models as viewed from oblique
angles is encouraged.
---------------------------------------------------------
Title: G-band Images from MHD Convection Simulations
Authors: Stein, R. F.; Carlsson, M.; Nordlund, A.; Scharmer, G.
2004AAS...204.8804S Altcode: 2004BAAS...36..820S
High resolution magneto-convection simulations are used to calculate
G-band and G-continuum images at various angles. Towards the limb
the simulations show "hilly" granulation, bright granulation walls,
intergranular striations and "sticking out" G-band bright features
similar to observations. The increased brightness in magnetic
elements is due to their lower density compared with the surrounding
intergranular medium, so that one sees deeper layers where the
temperature is higher. At a given geometric height, the magnetic
elements are not hotter than the surrounding medium. In the G-band,
the contrast is further increased by the destruction of CH in the
low density magnetic elements. The optical depth unity surface is
very corrugated. Bright granules have their continuum optical depth
unity 80 km above the mean surface, the magnetic elements 200-300 km
below. At large angles, the deep lying magnetic elements are hidden
by the granules and the bright points are no longer visible. Where
the "magnetic valleys" are aligned with the line of sight, they are
visible as elongated structures seemingly "sticking out". Even when
the deep hot surface is hidden, the low density in the strong magnetic
elements causes unit line-of-sight optical depth to occur deeper in
the granule walls behind then for rays not going through magnetic
elements. Flux concentrations in intergranular lanes therefore cause
a striped intensity pattern. This work is funded by NSF grants AST
0205500 and ATM 99881112 and NASA grants NAG 5 12450 and NNGO4GB92G.
---------------------------------------------------------
Title: Observations of solar magnetic elements with 0.1" resolution
Authors: Berger, T. E.; Rouppe van der Voort, L. H. M.; Lofdahl,
M. G.; Carlsson, M.; Fossum, A.; Hansteen, V. H.; Marthinussen, E.;
Title, A. M.; Scharmer, G.
2004AAS...204.2005B Altcode: 2004BAAS...36..686B
New observations of solar magnetic elements in a remnant active region
plage near disk center are presented. The observations were taken with
the Swedish 1-meter Solar Telescope on La Palma. We examine a single
430.5 nm G-band filtergram that resolves ∼70 km (0.”1) structures
and find new forms of magnetic structures in this particular region. A
simultaneous Ca II H-line image is used to examine the low-chromosphere
of network elements. A simultaneous Fe I 630.25 nm magnetogram
that resolves structures as small as 120 km (0.”18) FWHM with a
flux sensitivity of approximately 130 Mx cm<SUP>-2</SUP> quantifies
the magnetic structure of the region. A Ni I 676.8 nm Dopplergram
establishes relative velocity patterns associated with the network
features with an accuracy of about 300 m s<SUP>-1</SUP>. Magnetic flux
in this region as seen in both the magnetogram and the G-band image
is typically structured into larger, amorphous, “ribbons” with a
wide range of flux density values, rather than isolated kilogauss
flux tubes. We also present filtergrams and magnetograms of magnetic
elements at the solar limb showing that solar faculae are resolved
into bright granular walls that appear to project 350 to 500 km above
the photosphere.
---------------------------------------------------------
Title: Penumbral structure at 0.1 arcsec resolution. I. General
appearance and power spectra
Authors: Rouppe van der Voort, L. H. M.; Löfdahl, M. G.; Kiselman,
D.; Scharmer, G. B.
2004A&A...414..717R Altcode:
We analyse sunspot filtergrams of unprecedented quality obtained by
\citet{scharmer02dark} with the Swedish 1-m Solar Telescope on La
Palma. The observations comprise images in three different wavelength
bands: 488, 436, and 430 nm (G-band). We find that there are still
unresolved penumbral filaments which must have widths smaller than
80 km. The fine structuring along the filaments is limited. Penumbral
grains have internal structure and look like they are split or crossed
by narrow dark structures. We calculate intensity power spectra of the
penumbra from images that are corrected for seeing using the Phase
Diversity technique. The effects of high order aberrations that are
not corrected for are estimated to be too low to be consistent with
a flat power spectrum. The penumbral power spectra do not show any
signs of bumps or peaks that could correspond to a preferred scale
at 0\farcs35 for the width of penumbral filaments. We argue that the
power spectrum is not a very reliable source of information concerning
preferred scales.
---------------------------------------------------------
Title: High resolution limb images synthesized from 3D MHD simulations
Authors: Carlsson, Mats; Stein, Robert F.; Nordlund, Åke; Scharmer,
Göran B.
2004IAUS..223..233C Altcode: 2005IAUS..223..233C
We present the first center-to-limb G-band images synthesized from
high resolution simulations of solar magneto-convection. Towards the
limb the simulations show "hilly" granulation with dark bands on the
far side, bright granulation walls and striated faculae, similar
to observations. At disk center G-band bright points are flanked
by dark lanes. The increased brightness in magnetic elements is due
to their lower density compared with the surrounding intergranular
medium. One thus sees deeper layers where the temperature is higher. At
a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G-band, the contrast is further increased
by the destruction of CH in the low density magnetic elements. The
optical depth unity surface is very corrugated. Bright granules have
their continuum optical depth unity 80 km above the mean surface,
the magnetic elements 200-300 km below. The horizontal temperature
gradient is especially large next to flux concentrations. When viewed
at an angle, the deep magnetic elements optical surface is hidden by
the granules and the bright points are no longer visible, except where
the "magnetic valleys" are aligned with the line of sight. Towards
the limb, the low density in the strong magnetic elements causes
unit line-of-sight optical depth to occur deeper in the granule
walls behind than for rays not going through magnetic elements and
variations in the field strength produce a striated appearance in the
bright granule walls.
---------------------------------------------------------
Title: Observations of magnetoconvection in Sunspots with 100 km
resolution
Authors: Berger, T. E.; Löfdahl, M. G.; Scharmer, G.; Title, A. M.
2003SPD....34.1108B Altcode: 2003BAAS...35..828B
We present new observations from the Swedish 1-meter Solar Telescope
(SST) on La Palma with ∼0.1 arcsecond ( ∼100 km) resolution:
the highest resolution yet achieved in solar observations. We focus
on sunspot and active region magnetoconvective phenomena using G-band
4305 Å, 4877 Å continuum, 7507 Å TiO bandhead, and Ca II 3968 Å
H-line filtergram movies. The G-band data are post-processed using
Joint Phase Diverse Speckle wavefront restoration to create a full
diffraction limited time series. Sunspot light-bridges are shown to
have dark lanes less than 300 km in width that are coherent along
the entire length of the bridge. Similarly, we find elongated dark
“canals” in plage regions, particularly near pores, that appear to be
highly modified intergranular downflow lanes. The canals are less than
200 km in width and are much more coherent than intergranular lanes
in non-magnetic regions, often retaining their basic structure for
more than one granular turn-over time. Both the light-bridge central
lane and the canals appear to be the result of highly constrained
flow structure in strong magnetic field regions -- an aspect of solar
magnetoconvection that has not previously been observed. This reseach
was supported by funding from the Royal Swedish Academy of Sciences,
a SOHO Guest Investigator subcontract to California State University
Northridge, and the NASA TRACE contract NAS5-38099 at Lockheed Martin.
---------------------------------------------------------
Title: The 1-meter Swedish solar telescope
Authors: Scharmer, Goran B.; Bjelksjo, Klas; Korhonen, Tapio K.;
Lindberg, Bo; Petterson, Bertil
2003SPIE.4853..341S Altcode:
We describe the 1-meter Swedish solar telescope which replaces the
former 50-cm solar telescope (SVST) in La Palma. The un-obscured
optics consists of a singlet lens used as vacuum window and two
secondary optical systems. The first of these enables narrow-band
imaging and polarimetry with a minimum of optical surfaces. The second
optical system uses a field mirror to re-image the pupil on a 25 cm
corrector which provides a perfectly achromatic image, corrected also
for atmospheric dispersion. The adaptive optics system is integrated
with the design of the telescope but is sufficiently flexible to allow
future upgrades. It consists of a low-order bimorph modal mirror with
37 electrodes, allowing near-diffraction-limited imaging a reasonable
fraction of the observing time on La Palma. The new telescope became
operational at the end of May 2002 and has already proven to be the
most highly resolving solar telescope ever built. In this paper, we
describe its mechanical and optical design, the polishing and testing of
the optics and the instrumentation in use or planned for this telescope.
---------------------------------------------------------
Title: Adaptive optics system for the new Swedish solar telescope
Authors: Scharmer, Goran B.; Dettori, Peter M.; Lofdahl, Mats G.;
Shand, Mark
2003SPIE.4853..370S Altcode:
The 1-meter Swedish solar telescope is a new solar telescope that was
put in operation on the island of La Palma in the Canary Islands at the
end of May 2002. The goal of this telescope is to reach its diffraction
limited resolution of 0.1 arcsec in blue light. This has already
been achieved by use of a low-order adaptive optics (AO)system. This
paper describes the AO system initially developed for the former
50-cm Swedish Vacuum Solar Telescope (SVST) and further improved for
the new telescope. Both systems use a combination of bimorph modal
mirrors and Shack-Hartmann wavefront sensors. Unique to these systems
are that they rely on a single workstation or a PC to do all the
computations required to extract and pre-process the images, measure
their positions using cross correlation techniques and for controlling
the deformable mirror. This is in the present system possible by using
the PERR instruction available on Compaq's Alpha architecture and in
the new system using the PSADDBW instruction, available on Pentium 4
and Athlon processors. We describe both these systems with an emphasis
on the performance, the ease of support and upgrades of performance. We
also describe the optimization of the electrode geometry for the new
37-electrode bimorph mirror, supplied by AOPTIX Technologies, Inc.,
for controlling Karhunen--Loeve modes. Expected performance, based on
closed-loop simulations, is discussed.
---------------------------------------------------------
Title: Phase diverse speckle inversion applied to data from the
Swedish 1-meter solar telescope
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
2003SPIE.4853..567L Altcode:
We report on the use of a new joint phase diverse speckle code,
an implementation of a method where a single object and individual
phases are estimated from several pairs of phase diverse data. The
code was used on 430.5 nm G-band data collected with the newly
installed Swedish 1-meter solar telescope in La Palma, equipped with
a low-order adaptive optics system. We describe the algorithm briefly,
show wavefront statistics and object estimates from the processing and
discuss the results. We demonstrate a resolution of 0.12 arc seconds
for a time sequence and a large field of view, which is a break-through
for ground based solar telescopes.
---------------------------------------------------------
Title: First Results from the Swedish 1-m Solar Telescope
Authors: Scharmer, G. B.; Kiselman, D.; Löfdahl, M. G.; Rouppe van
der Voort, L. H. M.
2003ASPC..307....3S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dark cores in sunspot penumbral filaments
Authors: Scharmer, Göran B.; Gudiksen, Boris V.; Kiselman, Dan;
Löfdahl, Mats G.; Rouppe van der Voort, Luc H. M.
2002Natur.420..151S Altcode:
Sunspot umbrae-the dark central regions of the spots-are surrounded
by brighter filamentary penumbrae, the existence of which remains
largely inexplicable. The penumbral filaments contain magnetic fields
with varying inclinations and are associated with flowing gas, but
discriminating between theoretical models has been difficult because
the structure of the filaments has not hitherto been resolved. Here
we report observations of penumbral filaments that reveal dark cores
inside them. We cannot determine the nature of these dark cores,
but their very existence provides a crucial test for any model of
penumbrae. Our images also reveal other very small structures, in line
with the view that many of the fundamental physical processes in the
solar photosphere occur on scales smaller than 100km.
---------------------------------------------------------
Title: Workstation-based solar/stellar adaptive optics system
Authors: Scharmer, Goran B.; Shand, Mark; Lofdahl, Mats G.; Dettori,
Peter M.; Wei, Wang
2000SPIE.4007..239S Altcode:
The microprocessors used in off-the-shelf workstations double in
performance every eighteen months. The Swedish Vacuum Solar Tower (SVST)
uses off-the-shelf workstations for all aspects of its on-line telescope
control and data acquisition. Since 1995 workstation performance
has been adequate for a correlation tracker of solar granulation
controlling a tip- tilt corrector. In 2000 workstation performance
permits the construction of a 20 - 50 subimage Shack-Hartmann based low-
latency adaptive optics system. It is argued that workstations provide a
cost-effective, upgradable, low-risk and flexible means of construction
of stellar and solar adaptive optics systems. We give an overview
of the adaptive optics system installed at the SVST in May 1999. The
system uses a bimorph modal mirror with 19 electrodes from Laplacian
Optics. For use with extended targets, such as solar fine structure,
cross- correlations with 16 X 16-pixel sub-images are used. For use with
point sources, a centroiding algorithm is implemented. The work station
used is capable of completing all processing required by the adaptive
optics system in 0.5 ms (cross-correlations) or 0.3 ms (centroiding),
with potential for significant performance improvements.
---------------------------------------------------------
Title: Predictor approach to closed-loop phase-diversity wavefront
sensing
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
2000SPIE.4013..737L Altcode:
We present a novel and fast method for utilizing wavefront information
in closed-loop phase-diverse image data. We form a 2D object-independent
error function using the images at different focus positions together
with OTFs of the diffraction limited system. Each coefficient in an
expansion of the wavefront is estimated quickly and independently by
calculating the inner produce of a corresponding predictor function and
the error function. This operation is easy to parallelize. The main
computational burden is in pre- processing, when the predictors are
formed. This makes this method fast and therefore attractive for closed
loop operation. Calculating the predictors involves error function
derivatives with respect to the wavefront parameters, statistics of
the parameters, noise levels and other known characteristics of the
optical system. The predictors are optimized so that the RMS error in
the wavefront parameters is minimized rather than consistency between
estimated quantities with image data. We present simulation results that
are relevant to the phasing of segmented mirrors in a space telescope,
such as the NGST.
---------------------------------------------------------
Title: Coordinated MDI/TRACE/SVST Observations of Sunspots
Authors: Bush, R. I.; Shine, R. A.; Brandt, P.; Sobotka, M.;
Scharmer, G.
2000SPD....31.0122B Altcode: 2000BAAS...32R.804B
During the first two weeks of June 1999, coordinated observations
of sunspots were made by the MDI instrument on the SOHO spacecraft,
the TRACE satellite and the Swedish Vacuum Solar Tower (SVST) at La
Palma. The MDI instrument was operating in a "high resolution" mode
and was obtaining 1.2 arc-second resolution magnetograms and 2.4
arc-second resolution dopplergrams at a one minute cadence. TRACE
observations were made in Fe IX/X 171 Angstroms and/or Fe XII 195
Angstroms together with the 1600 Angstroms and continuum bands at a
1.0 arc-second resolution. The SVST data consist of filtergrams taken
with three 2Kx2K CCD cameras with about 0.4 arc-second resolution
operating in frame selection mode. The first camera used a fixed G
band (4305 Angstroms) filter, the second a fixed blue continuum filter
(4507 Angstroms), and the third a narrow band tunable filter which was
cycled through several positions in the Hα line and the 6302 Angstroms
Fe ~I line. The goal of this investigation is to detail the evolution
of stable sunspots in order to understand the detailed correlation of
photospheric flows and magnetic features. The high time and spatial
resolution of these measurements provides a unique opportunity to
explore the interactions of plasma and magnetic field at the solar
surface. This work was supported by NASA grant NAG5-3077 at Stanford
and NASA contract NAS5-38099 at Lockheed Martin.
---------------------------------------------------------
Title: Calibration of a Deformable Mirror and Strehl Ratio
Measurements by Use of Phase Diversity
Authors: Löfdahl, Mats G.; Scharmer, Göran B.; Wei, Wang
2000ApOpt..39...94L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Observations of Transition Region Dynamics using
TRACE and the SVST
Authors: Berger, T.; de Pontieu, B.; Schrijver, C.; Title, A.;
Scharmer, G.
1999ASPC..183..365B Altcode: 1999hrsp.conf..365B
No abstract at ADS
---------------------------------------------------------
Title: The New Swedish Solar Telescope
Authors: Scharmer, G.; Owner-Petersen, M.; Korhonen, T.; Title, A.
1999ASPC..183..157S Altcode: 1999hrsp.conf..157S
No abstract at ADS
---------------------------------------------------------
Title: Optimized Shack-Hartmann Wavefront Sensing for Adaptive Optics
and Post Processing
Authors: Scharmer, G.; Blomberg, H.
1999ASPC..183..239S Altcode: 1999hrsp.conf..239S
No abstract at ADS
---------------------------------------------------------
Title: Object-Independent Fast Phase-Diversity
Authors: Scharmer, G.
1999ASPC..183..330S Altcode: 1999hrsp.conf..330S
No abstract at ADS
---------------------------------------------------------
Title: Correlation Tracking and Adaptive Optics Control Using
Off-The-Shelf Workstation Technology
Authors: Shand, M.; Scharmer, G.; Wei, W.
1999ASPC..183..231S Altcode: 1999hrsp.conf..231S
No abstract at ADS
---------------------------------------------------------
Title: LPSP & TIP: Full Stokes Polarimeters for the Canary
Islands Observatories
Authors: Mártinez Pillet, V.; Collados, M.; Sánchez Almeida, J.;
González, V.; Cruz-Lopez, A.; Manescau, A.; Joven, E.; Paez, E.;
Diaz, J.; Feeney, O.; Sánchez, V.; Scharmer, G.; Soltau, D.
1999ASPC..183..264M Altcode: 1999hrsp.conf..264M
No abstract at ADS
---------------------------------------------------------
Title: Fine Structures of Magnetic Field in Solar Quiet Region
Authors: Zhang, H.; Scharmer, G.; Lofdahl, M.; Yi, Z.
1998SoPh..183..283Z Altcode:
In this paper, we present a time series of Fe i λ5250.2 Å photospheric
filtergrams and corresponding magnetograms in a quiet region. The
relationship between fine structures of granulation and magnetic fields
is analyzed. It is found that although most bright filigree features in
photospheric filtergrams are related to corresponding magnetic features,
they are generally not cospatial. It is also found that some bright
features and their corresponding photospheric magnetic fields show
fast changes within several minutes.
---------------------------------------------------------
Title: The Swedish Vacuum Solar Telescope data-acquisition and
control systems
Authors: Shand, Mark; Scharmer, Göran B.
1998NewAR..42..481S Altcode:
The performance of commodity computer systems doubles approximately
every 18 months. Traditionally, the design of scientific
data-acquisition and control systems has tended to ignore this fact,
relying instead on custom hardware developments using the technology
available at the time of instrument specification. Moreover,
development manpower is usually limited, causing relatively long
development cycles. Often the the result is that an instrument is
technologically obsolete quite early in its projected lifetime. In
contrast, all the digital processing for data acquisition and control at
the Swedish Vacuum Solar Telescope (SVST) on La Palma (Canary Islands)
is performed with commodity workstations. The result is a flexible
system with low development costs that can easily take advantage
of the latest microprocessor advances. The SVST's use of commodity
workstations in on-line real-time tasks is in large part made possible
by its use of reconfigurable interface technology. Indeed the SVST
has been a valuable proving ground for this technology. This article
summarizes the instrumentation of the SVST and illustrates examples
of data recorded with this instrumentation.
---------------------------------------------------------
Title: Fast phase diversity wavefront sensing for mirror control
Authors: Lofdahl, Mats G.; Duncan, Alan L.; Scharmer, Goran B.
1998SPIE.3353..952L Altcode:
We show with simulation experiments that closed-loop phase- diversity
can be used without numerical guard-bands for wavefront sensing
of low-order wavefronts from extended objects using broad-band
filters. This may allow real-time correction at high bandwidth for
certain applications. We also present a proper maximum likelihood
treatment of Shack- Hartmann data, which includes an imaging model to
extract curvature information from the lenslet images. We demonstrate
by simple simulations that this approach should allow higher-order
wavefront information to be extracted than with traditional
Shack-Hartmann wavefront sensing for a given number of lenslets.
---------------------------------------------------------
Title: Comparison of Granulation Correlation Tracking (CT) and
Feature Tracking (FT) Results from SOHO/MDI and the Swedish Vacuum
Solar Telescope on La Palma
Authors: Shine, R.; Strous, L.; Simon, G.; Berger, T.; Hurlburt, N.;
Tarbell, T.; Title, A.; Scharmer, G.
1997SPD....28.0262S Altcode: 1997BAAS...29Q.904S
We have computed photospheric velocity flow maps from simultaneous
observations taken with MDI and at the Swedish Vacuum Solar Tower
(SVST) on La Palma on August 15, 1996. Both sets consist of a series
of photospheric images, and flow maps are computed by following the
local motions of granules. The MDI data have the important advantages
of very stable images and longer continuous coverage of the same area
of the solar surface. This longer coverage is necessary to study the
evolution of mesogranules, supergranules, and to detect possible low
amplitude motions on scales larger than supergranules. However, the
high resolution mode of MDI is limited by the small telescope size to
about 1.2 arc seconds angular resolution and uses a 0.6 arc second pixel
size. This is adequate to show granulation but has the rms constrast
significantly reduced to about 2%. Early efforts adapting techniques
that were successful with higher resolution ground based images gave
poor results and although new methods have now been developed, there
are still some problems with accuracy. On the other hand, the SVST
images have much higher angular resolution (as good as 0.2 arc second)
but suffer from variable atmospheric distortion. They also have a much
smaller field of view. By detailed comparison of the two data sets
and by using CT and FT techniques to track the motions, we hope to
understand the sources of any differences between them and to develop
credible correction parameters to the MDI data sets if necessary. This
work was supported by NASA Grant NAG5-3077 at Stanford and Lockheed
Martin, by AFOSR and the Fellows Program of AF Phillips Lab at NSO/SP,
and by the Swedish Royal Academy of Sciences.
---------------------------------------------------------
Title: The IAC Solar Polarimeters: Goals and Review of Two Ongoing
Projects
Authors: Sanchez Almeida, J.; Collados, M.; Martinez Pillet, V.;
Gonzalez Escalera, V.; Scharmer, G. B.; Shand, M.; Moll, L.; Joven,
E.; Cruz, A.; Diaz, J. J.; Rodriguez, L. F.; Fuentes, J.; Jochum,
L.; Paez, E.; Ronquillo, B.; Carranza, J. M.; Escudero-Sanz, I.
1997ASPC..118..366S Altcode: 1997fasp.conf..366S
The IAC is currently developing two similar polarimeters, one for
optical wavelengths and one for near infra-red wavelengths (1.5
mu m). Both instruments will provide spectra of the four Stokes
parameters over 2D solar regions. The visible spectro-polarimeter will
be operated at the Swedish Tower (La Palma), and it is being developed
in collaboration with the Royal Swedish Academy of Sciences. We intend
to take advantage of the excellent seeing conditions at this telescope,
while maintaining high polarimetric precision. The IR polarimeter
is being designed for the German VTT (Tenerife) which has better
angular resolution in the near infra-red. This report describes the
goals and technical solutions. It also briefs on the current status
of the projects.
---------------------------------------------------------
Title: Evaluation of Phase-Diversity Techniques for Solar-Image
Restoration
Authors: Paxman, Richard G.; Seldin, John H.; Loefdahl, Mats G.;
Scharmer, Goeran B.; Keller, Christoph U.
1996ApJ...466.1087P Altcode:
Phase-diversity techniques provide a novel observational method for
overcoming the effects of turbulence and instrument-induced aberrations
in ground-based astronomy. Two implementations of phase-diversity
techniques that differ with regard to noise model, estimator,
optimization algorithm, method of regularization, and treatment of edge
effects are described. Reconstructions of solar granulation derived
by applying these two implementations to common data sets are shown to
yield nearly identical images. For both implementations, reconstructions
from phase-diverse speckle data (involving multiple realizations of
turbulence) are shown to be superior to those derived from conventional
phase-diversity data (involving a single realization). Phase-diverse
speckle reconstructions are shown to achieve near diffraction-limited
resolution and are validated by internal and external consistency tests,
including a comparison with a reconstruction using a well-accepted
speckle-imaging method.
---------------------------------------------------------
Title: TRACE: the Transition Region and Coronal Explorer
Authors: Schrijver, C.; Title, A.; Acton, L.; Bruner, M.; Fischer,
R.; Golub, L.; Harrison, R.; Lemen, J.; Rosner, R.; Scharmer, G.;
Scherrer, P.; Strong, K.; Tarbell, T.; Wolfson, J.
1996AAS...188.6704S Altcode: 1996BAAS...28..934S
The TRACE mission is designed to obtain images of the solar
transition region and corona of unprecedented quality. With these
images we will be able to explore quantitatively the connections
between the photospheric magnetic field and the associated hot and
tenuous structures in the outer atmosphere. The TRACE telescope has
an aperture of 30 cm, and will observe an 8.5 x 8.5 arcminute field of
view with a resolution of one arcsecond. Finely tuned coatings on four
quadrants on the primary and secondary normal--incidence mirrors will
allow observations in narrow EUV and UV spectral bands. The passbands
are set to Fe IX, XII, and XV lines in the EUV band, while filters
allow observations in C IV, Ly alpha , and the UV continuum using
the UV mirror quadrant. The data thus cover temperatures from 10(4)
K up to 10(7) K. The Sun--synchronous orbit allows long intervals of
uninterrupted viewing. Observations at different wavelengths can be
made in rapid succession with an alignment of 0.1 arcsec. Coordinated
observing with TRACE, SoHO and YOHKOH will give us the first opportunity
to observe all temperature regimes in the solar atmosphere, including
magnetograms, simultaneously from space. TRACE is currently scheduled
to be launched in October 1997. More information can be found on the
web at “http://pore1.space.lockheed.com/TRACE/welcome.html”.
---------------------------------------------------------
Title: Photospheric Surface Flows and Small Magnetic Structures in
Sunspot Moats
Authors: Shine, R. A.; Title, A.; Frank, Zoe; Scharmer, G.
1996AAS...188.3501S Altcode: 1996BAAS...28Q.871S
We have computed horizontal flow maps of the photosphere around
and within three different sunspots using high spatial resolution
continuum movies obtained at the Swedish Vacuum Solar Telescope on
La Palma on June 5, 1993, August 29, 1993, and July 14, 1994. Two
of the data sets also included magnetograms and dopplergrams. A new
feature found in the flow maps is azimuthal structure in the moat
flows surrounding sunspots. Instead of a simple radial flow, there
are zones of azimuthal divergence and convergence resulting in radial
“spokes” of convergence. These are not uniformly distinct around the
entire circumference but are seen in part of the surrounding area for
all 3 sunspots. The angular spacing is about 10 to 20 degrees and the
pattern persists for several hours. For one of the sunspots, we have
concurrent movies made with a 3 Angstrom wide K line filter. A time
average of these images shows bright spokes in the K line congruent with
the convergence spokes. Much of the magnetic flux that is continually
emerging and moving outward in the moat is in the vicinity of the
“spokes.” In addition, these data show several good examples of
so called “streakers.” These are small bright structures seen in
continuum movies that appear to be emitted from the outer edge of the
penumbral and travel a few thousand km at about 2 to 3 km/s before
fading. We find that these are associated with a magnetic field of
opposite polarity to the sunspot and that they travel toward another
larger and slower moving magnetic feature with the same polarity as
the sunspot. When the streaker catches up, it disappears, sometimes
coincident with a brightening of the merged feature which continues
outward at the previous velocity, about 0.5 km/s. This work was
supported by NSF grant ATM-9213879, NASA contracts NAS8-39746 and
NAS8-39747, Lockheed Independent Research Funds, and the Swedish Royal
Academy of Sciences.
---------------------------------------------------------
Title: Phenomena in an emerging active region. I. Horizontal dynamics.
Authors: Strous, L. H.; Scharmer, G.; Tarbell, T. D.; Title, A. M.;
Zwaan, C.
1996A&A...306..947S Altcode:
Horizontal dynamics in observations of NOAA AR 5617 are studied by
tracking individual elements through the field of view. Small magnetic
elements of both magnetic polarities occur everywhere in the active
region, and define unipolar thread-like concentrations of magnetic
field of up to 15Mm length. The horizontal granular flow field in
the active region is divergent (e-time scale 2.1hours) and clockwise
(time scale 32hours). Facular elements are tracers of (clumps of)
fluxtubes. A hierarchy of movement of magnetic elements appears:
Facular elements everywhere in the active region move obliquely toward
the edges of the active region of the same polarity as their own, faster
than those edges (as defined by strings of pores) move apart. The pores
move along the edges toward the major sunspots of their own polarity,
and the major sunspots of either polarity move apart. The separation
velocity of both polarities of facular elements is about 0.84km/s, of
pores about 0.73km/s, and that of the major sunspots is about 0.50km/s.
---------------------------------------------------------
Title: Motion and Evolution of Solar Magnetic Elements
Authors: Berger, T. E.; Schrijver, C. J.; Shine, R. S.; Tarbell,
T. D.; Title, A. M.; Scharmer, G.
1995AAS...18710104B Altcode: 1995BAAS...27.1426B
The dynamics of sub-arcsecond solar magnetic flux tubes are analyzed
based on very-high resolution movies of photospheric bright points
obtained in 1994 at the 50-cm Swedish Solar Vacuum Telescope (SVST)
on the island of La Palma, Spain. The bright points are imaged using
a 12 Angstroms bandpass interference filter centered at 4305 Angstroms
in the “G Band” molecular bandhead of the CH molecule. The image sets
typically consist of up to 4 hours of consecutive images taken at a 10
to 20 second cadence. Spatial resolution throughout the movies averages
less than 0\arcsec.5 and many frames in the sets exhibit resolution
down to 0\arcsec.25. Magnetic flux elements in the photosphere are
shown to move continually along the intergranular lanes at speeds
of up to 5 km/sec and ranges up to several thousand km. Evolution of
individual magnetic elements is dominated by the local evolution of
surrounding granules. Fragmentation and merging is the fundamental
mode of evolution of the majority of magnetic elements seen in our
data. Rotation and folding of chains or groups of elements is also
frequently observed. The time scale for the fragmentation/merging
evolution of the elements is on the order of the lifetime of granulation
(6--8 minutes), but significant morphological changes are seen to
occur on time scales as short as 100 seconds. The concept of a stable,
isolated, sub-arcsecond magnetic flux element in the solar photosphere
is inconsistent with the observations presented here.
---------------------------------------------------------
Title: New Observations of Subarcsecond Photospheric Bright Points
Authors: Berger, T. E.; Schrijver, C. J.; Shine, R. A.; Tarbell,
T. D.; Title, A. M.; Scharmer, G.
1995ApJ...454..531B Altcode:
We have used an interference filter centered at 4305 Å within the
bandhead of the CH radical (the "G band") and real-time image selection
at the Swedish Vacuum Solar Telescope on La Palma to produce very
high contrast images of subarcsecond photospheric bright points at all
locations on the solar disk. During the 6 day period of 1993 September
15-20 we observed active region NOAA 7581 from its appearance on the
East limb to a near disk-center position on September 20. A total of
1804 bright points were selected for analysis from the disk center image
using feature extraction image processing techniques. The measured FWHM
distribution of the bright points in the image is subnormal with a modal
value of 220 km (0".30) and an average value of 250 km (0".35). The
smallest measured bright point diameter is 120 km (0".17) and the
largest is 600 km (0".69). Approximately 60% of the measured bright
points are circular (eccentricity ∼1.0), the average eccentricity
is 1.5, and the maximum eccentricity corresponding to filigree in
the image is 6.5. The peak contrast of the measured bright points is
normally distributed. The contrast distribution variance is much greater
than the measurement accuracy, indicating a large spread in intrinsic
bright-point contrast. When referenced to an averaged "quiet-Sun area 1n
the image, the modal contrast is 29% and the maximum value is 75%; when
referenced to an average intergranular lane brightness in the image,
the distribution has a modal value of 61 % and a maximum of 119%. The
bin-averaged contrast of G-band bright points is constant across the
entire measured size range. The measured area of the bright points,
corrected for population and selection effects, covers about 1.8% of
the total image area. Large pores and micropores occupy an additional
2% of the image area, implying a total area fraction of magnetic
proxy features in the image of 3.8%. We discuss the implications of
this area fraction measurement in the context of previously published
measurements which show that typical active region plage has a magnetic
filling factor on the order of 10% or greater. The results suggest that
in the active region analyzed here, less than 50% of the small-scale
magnetic flux tubes are demarcated by visible proxies such as bright
points or pores.
---------------------------------------------------------
Title: 3.8-ms latency correlation tracker for active mirror control
based on a reconfigurable interface to a standard workstation
Authors: Shand, Mark; Wei, Wang; Scharmer, Goran B.
1995SPIE.2607..145S Altcode:
We describe the use of a reconfigurable interface board based on FPGAs
and a UNIX workstation to implement a correlation tracker with 3.8ms
latency. The correlation tracker is part of an active mirror system in
use at the Swedish Vacuum Solar Telescope, La Palma, Canary Islands. The
reconfigurable interface is used to leverage the workstation CPU,
relieving it of tasks that it performs poorly such as rapid context
switching and low-level bit manipulation. The reconfigurable interface
handles control of external devices, high- performance input (16 MB/s)
and data preformatting. The workstation CPU, a 64-bit microprocessor,
performs the bulk of the computation. For the key computations of the
correlation tracker we are able to treat 8 pixels in parallel in the
CPU's 64-bit integer datapath. We present the structure of the CCD
interface configuration and the implementations of the key algorithms
on the workstation CPU. We describe the design trade-offs that arose
during the development of the system, and demonstrate the symbiosis
between components implemented in software and configurable hardware.
---------------------------------------------------------
Title: Frame Selection Techniques for Solar Movies
Authors: Shine, R. A.; Tarbell, T.; Title, A.; Scharmer, G.; Simon,
G.; Brandt, P.; Berger, T.
1995SPD....26..506S Altcode: 1995BAAS...27..957S
No abstract at ADS
---------------------------------------------------------
Title: Properties of Sub-Arcsecond Facular Bright Points
Authors: Berger, T.; Schrijver, C.; Shine, R.; Tarbell, T.; Title,
A.; Scharmer, G.
1995SPD....26..505B Altcode: 1995BAAS...27..957B
No abstract at ADS
---------------------------------------------------------
Title: On the Relation Between Facular Bright Points and the
Magnetic Field
Authors: Berger, Thomas; Shine, Richard; Tarbell, Theodore; Title,
Alan; Scharmer, Goran
1994AAS...185.8607B Altcode: 1994BAAS...26.1465B
Multi-spectral images of magnetic structures in the solar photosphere
are presented. The images were obtained in the summers of 1993 and
1994 at the Swedish Solar Telescope on La Palma using the tunable
birefringent Solar Optical Universal Polarimeter (SOUP filter), a 10
Angstroms wide interference filter tuned to 4304 Angstroms in the band
head of the CH radical (the Fraunhofer G-band), and a 3 Angstroms wide
interference filter centered on the Ca II--K absorption line. Three
large format CCD cameras with shuttered exposures on the order of
10 msec and frame rates of up to 7 frames per second were used to
create time series of both quiet and active region evolution. The
full field--of--view is 60times 80 arcseconds (44times 58 Mm). With
the best seeing, structures as small as 0.22 arcseconds (160 km) in
diameter are clearly resolved. Post--processing of the images results
in rigid coalignment of the image sets to an accuracy comparable to the
spatial resolution. Facular bright points with mean diameters of 0.35
arcseconds (250 km) and elongated filaments with lengths on the order
of arcseconds (10(3) km) are imaged with contrast values of up to 60
% by the G--band filter. Overlay of these images on contemporal Fe I
6302 Angstroms magnetograms and Ca II K images reveals that the bright
points occur, without exception, on sites of magnetic flux through the
photosphere. However, instances of concentrated and diffuse magnetic
flux and Ca II K emission without associated bright points are common,
leading to the conclusion that the presence of magnetic flux is a
necessary but not sufficient condition for the occurence of resolvable
facular bright points. Comparison of the G--band and continuum images
shows a complex relation between structures in the two bandwidths:
bright points exceeding 350 km in extent correspond to distinct
bright structures in the continuum; smaller bright points show no
clear relation to continuum structures. Size and contrast statistical
cross--comparisons compiled from measurements of over two-thousand
bright point structures are presented. Preliminary analysis of the time
evolution of bright points in the G--band reveals that the dominant mode
of bright point evolution is fission of larger structures into smaller
ones and fusion of small structures into conglomerate structures. The
characteristic time scale for the fission/fusion process is on the
order of minutes.
---------------------------------------------------------
Title: Wavefront sensing and image restoration from focused and
defocused solar images.
Authors: Löfdahl, M. G.; Scharmer, G. B.
1994A&AS..107..243L Altcode:
We have implemented a least-squares technique for recovering phase
information from simultaneously recorded focused and defocused
images. The inversions are made from small subfields in order to
deal with anisoplanatic image formation, such as occurring through
the Earth's atmosphere. Parameters corresponding to the alignment
of the focused-defocused images are determined simultaneously with
aberration parameters. Simulations show that the method can recover
wavefronts of up to 1/2 wave rms and that 15-21 Zernike coefficients
can be obtained from 3"x3"-5"x5" solar granulation images using a 50 cm
telescope and with noise levels of 0.4% of the average intensity. In
general, the accuracy of the restored images is better than expected
from the number of Zernike polynomials used to represent the wave
front. We have applied the method to sequences of 100 8-bit solar
granulation images obtained with the Swedish Vacuum Solar Telescope
(SVST) in La Palma using subfields of 3"x3"-5"x5". These data enable
a number of consistency tests, all of which demonstrate that the
technique works on real data. Using averaged images obtained from each
sequence we find that derived alignment parameters are consistent to
within 0.02" and that wavefronts derived from different subfields and
different sequences recorded close in time are virtually identical. The
wavefronts derived from averaged images are also virtually identical
to the average of wavefronts derived from individual images. These
measurements of telescope aberrations suggest that astigmatism and
coma are the major aberrations. These aberrations vary with time
in a way which is consistent with a major contribution from the
telescope objective and/or the first folding mirror of the alt-az
tower telescope. Wavefronts derived from individual images show 50-90%
correlation between Zernike coefficients 4-14 derived from nearby
but independent subfields. Mosaics of 13 x 11 independently derived
wavefronts from single images show smooth variations across a 12"x10"
field-of-view. These results are consistent with the impression
that degradation of image quality is more or less uniform across the
image. Restored images in a sequence show a high degree of consistency
and much more fine structure than the corresponding observed images, but
occasional fringe-like artifacts can be seen. Using the results of two
inversions to restore the scene removes such artifacts. We conclude that
this technique provides adequate wavefront information on telescopic
and atmospheric wavefront aberrations and substantial improvements
in image quality. The ease of implementation as well as its tolerance
to experimental errors and low cost makes it an excellent complement
to or even substitute for adaptive optics for many applications. The
technique is particularly well-suited for solar telescopes, where
wavefront sensing over a large field-of-view is important.
---------------------------------------------------------
Title: Application of phase-diversity to solar images
Authors: Lofdahl, Mats G.; Scharmer, Goran B.
1994SPIE.2302..254L Altcode:
We have implemented a least-squares technique for recovering phase
information and alignment parameters from simultaneously obtained
focused and defocused solar images. Small subfields are used,
in order to deal with anisoplanatism. The method is applied to
sequences of 100 8-bit solar granulation images. These data enable
a number of consistency tests, all of which demonstrate that the
technique works. Alignment parameters derived from averaged images
in a sequence are highly consistent and wavefronts derived from
different subfields and different sequences recorded close in time are
virtually identical. The wavefronts derived from averaged images are
also virtually identical to the average of wavefronts derived from
individual images. These aberrations vary with time in a way which
is consistent with a major contribution from the moving elements of
the alt-az tower telescope. Independently derived wavefronts from
single images show high correlation between neighboring subfields and
smooth variations across large fields-of-view, consistent with the
impression that the image quality is more or less uniform across the
image. Restored images in a sequence show a high degree of consistency
and much more fine structure than the corresponding observed images.
---------------------------------------------------------
Title: High-Resolution Observations of the Evershed Effect in Sunspots
Authors: Shine, R. A.; Title, A. M.; Tarbell, T. D.; Smith, K.; Frank,
Z. A.; Scharmer, Goran
1994ApJ...430..413S Altcode:
High spatial resolution movies of sunspots taken at the Swedish
Solar Observatory on La Palma reveal that the Evershed effect is time
dependent. Outward proper motions are visible in both the continuum
and Dopplergrams. These can be tracked over most of the width of
the penumbra and overlap regions that show inward moving penubral
grains. The radial spacing between the moving structures is about 2000
km, and they exhibit irregular repetitive behavior with a typical
interval of 10 minutes. These are probably the cause of 10 minutes
oscillations sometimes seen in a penumbral power spectra. Higher
velocities are spatially correlated with the relatively darker regions
between bright filaments. Regions with a strong variation in the Doppler
signal show peak-to-peak modulations of 1 km/s on an average velocity
of about 3-4 km/s. The proper motion velocity is approximately constant
from the iner penumbra and generally larger than the Doppler velocity
when both are interpreted as projections of horizontal motion. Regions
where thay are consistent suggest a typical horizontal velocity of
3.5 km/s. Some proper motion velocites as high as 7 km/s are seen,
but these are less certain. The temporal behavoir shows a correlation
between increased Doppler signal and increased continuum intensity,
the opposite of the spatial correlation. When spatially averaged
across filaments and over time, the averaged Evershed effect has a peak
horizontal component near the outer edge of the penumbra of 2.0 km/s
with evidence for a 200-400 m/s upward component. The latter depends
on an uncertain absolute velocity calibration. If real it could be
an actual upward component or a penumbral analogue of the convective
blueshift seen in the quiet Sun.
---------------------------------------------------------
Title: Dynamics of the Evershed effect
Authors: Shine, R. A.; Title, A. M.; Tarbell, T. D.; Smith, K.; Frank,
Z. A.; Scharmer, G.
1994ASIC..433..197S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Large-scale photospheric motions: first results from an
extraordinary eleven-hour granulation observation
Authors: Simon, G. W.; Brandt, P. N.; November, L. J.; Scharmer,
G. B.; Shine, R. A.
1994ASIC..433..261S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Phase-Diversity Restoration of Solar Images
Authors: Löfdahl, Mats G.; Scharmer, Göran B.
1993rtpf.conf...89L Altcode:
We have implemented a least-squares linearization technique for
recovering phase information from simultaneously recorded focused
and defocused solar images. The inversion is made from small
subfields, in order to deal with anisoplanatic image formation,
such as occuring through the earth's atmosphere. Simulations show
that the method can recover wavefronts of approximately 1/4 wave
rms and that noise levels corresponding to 8-bit images allow better
reconstruction of the images than of the derived wavefronts. We find
that cross-correlation techniques cannot be used to align the focused
and defocused images because of cross-talk with coma and similar
asymmetric aberrations. Therefore we simultaneously determine alignment
and aberration parameters. We have applied the method to 8-bit solar
granulation images obtained with the Swedish Vacuum Solar Telescope
in La Palma using subfields of 3".1 x 3".1. The least-squares fits
indicate wavefronts of approximately 0.12-0.16 waves rms. Derived
alignment parameters from different subfields and different images
are consistent, indicating that the wavefront is well determined by
the data. Wavefronts derived from different sub fields of a single
image in general show similar aberrations whereas derived wavefronts
from different images only show occasional similarities, indicating
a significant wavefront aberration from the lower atmosphere. The
reconstructed granulation images show enhanced fine structure and
an increase of the rms contrast by ~2% as compared to the observed
focused image, but the rms contrasts of consequtive images indicate
small errors in the derived rms wavefronts.
---------------------------------------------------------
Title: High Resolution Observations of the Evershed Flow in Sunspots
Authors: Shine, R.; Title, A.; Smith, K.; Scharmer, G.
1993BAAS...25.1183S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: La Palma Observations During the CoMStOC'92 Campaign
Authors: Shine, R.; Tarbell, T.; Topka, K.; Frank, Z.; Title, A.;
Scharmer, G.
1993BAAS...25S1223S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Magnetic and Velocity Field Geometry of Simple Sunspots
Authors: Title, Alan M.; Frank, Zoe A.; Shine, Richard A.; Tarbell,
Theodore D.; Topka, Kenneth P.; Scharmer, Goran; Schmidt, Wolfgang
1993ApJ...403..780T Altcode:
It is presently shown that a simple sunspot model with azimuthal
variations in inclination, but lacking azimuthal field-strength
variations, is free from azimuthal Lorentz forces. The meridional
currents arising from the inclination variations are parallel to the
field lines, suggesting that a cylindrically symmetric magnetostatic
sunspot model can be perturbed into one with azimuthal variations in
inclination with adjustment of the meridional force balance.
---------------------------------------------------------
Title: On the Differences between Plage and Quiet Sun in the Solar
Photosphere
Authors: Title, Alan M.; Topka, Kenneth P.; Tarbell, Theodore D.;
Schmidt, Wolfgang; Balke, Christiaan; Scharmer, Goran
1992ApJ...393..782T Altcode:
Time sequences of interleaved observations of the continuum intensity,
longitudinal magnetic field, vertical velocity in the midphotosphere,
and the line-center intensity in Ni I 6768 A were obtained in an
active-region plage and the surrounding relatively field-free area near
disk center. Spacetime Fourier filtering techniques are used to separate
the convective and oscillatory components of the solar atmosphere. The
properties of the photosphere are found to differ qualitatively and
quantitatively between the plage, where the field is 150 G or more,
and its quiet surroundings. The scale of granulation is smaller, the
contrast lower, and the temporal evolution slower in the plage than the
quiet sun. In the plage, the vertical velocity is reduced in amplitude
compared to the quiet sun, and there is little evidence of a granulation
pattern, while in the quiet sun the vertical flow pattern is similar in
size and shape to the underlying granulation pattern in the continuum.
---------------------------------------------------------
Title: Very high spatial resolution two-dimensional solar spectroscopy
with video CCDs
Authors: Johanneson, A.; Bida, T.; Lites, B.; Scharmer, G. B.
1992A&A...258..572J Altcode:
We have developed techniques for recording and reducing spectra of
solar fine structure with complete coverage of two-dimensional areas
at very high spatial resolution and with a minimum of seeing-induced
distortions. These new techniques permit one, for the first time,
to place the quantitative measures of atmospheric structure that are
afforded only by detailed spectral measurements into their proper
context. The techniques comprise the simultaneous acquisition of
digital spectra and slit-jaw images at video rates as the solar scene
sweeps rapidly by the spectrograph slit. During data processing the
slit-jaw images are used to monitor rigid and differential image motion
during the scan, allowing measured spectrum properties to be remapped
spatially. The resulting quality of maps of measured properties from
the spectra is close to that of the best filtergrams. We present the
techniques and show maps from scans over pores and small sunspots
obtained at a resolution approaching 1/3 arcsec in the spectral
region of the magnetically sensitive Fe I lines at 630.15 and 630.25
nm. The maps shown are of continuum intensity and calibrated Doppler
velocity. More extensive spectral inversion of these spectra to yield
the strength of the magnetic field and other parameters is now underway,
and the results of that analysis will be presented in a following paper.
---------------------------------------------------------
Title: High Resolution Observations of the Magnetic and Velocity
Field of Simple Sunspots
Authors: Title, Alan M.; Frank, Zoe A.; Shine, Richard A.; Tarbell,
T. D.; Topka, K. P.; Scharmer, Goran; Schmidt, Wolfgang
1992ASIC..375..195T Altcode: 1992sto..work..195T
We have observed the disk passage of relatively simple round sunspots
using a narrowband filter and a large format CCD detector and have
created magnetograms, Dopplergrams, and continuum images nearly
simultaneously. In addition the spectral resolution of the filter
allows the construction of 'spectra' for all points in the field of
view. The mean inclination of the magnetic field increases from 45-50
deg to 70-75 deg across the penumbra and there is a fluctuation of the
inclination angle about the mean of about 4 +/- 18 deg. The variation in
inclination is large enough that substantial amounts of magnetic field
are parallel to the solar surface from the mid to outer penumbra. The
Evershed flow tends to occur in the regions where the magnetic field
is horizontal. This suggests that the Evershed flow is confined to the
regions of horizontal fields. We show that a simple sunspot model with
azimuthal variations in inclination but no azimuthal variations of
field strength is free from azimuthal Lorentz forces. The meridional
component of the currents which arise from the azimuthal variation
in inclination are parallel to the field lines. This suggests that a
cylindrically symmetric magnetostatic sunspot model can be perturbed
into one with azimuthal variations in inclination with some adjustment
in the meridional force balance.
---------------------------------------------------------
Title: Properties of the Smallest Magnetic Elements on the Sun
Authors: Topka, K. P.; Smith, K. L.; Tarbell, T. D.; Title, A. M.;
Scharmer, G.
1991BAAS...23.1388T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High-Resolution Spectra of Solar Magnetic
Features. II. Magnetic Fields of Umbral Brightenings
Authors: Lites, Bruce W.; Bida, Thomas A.; Johannesson, A.; Scharmer,
G. B.
1991ApJ...373..683L Altcode:
The spectra of Fe I and Fe II Zeeman-sensitive lines enhanced by video
processing of CCD images are considered. The magnetic-field variation
within umbras is obtained from the nearly complete Zeeman splitting
of the Stokes I profile. It is shown that small brightenings within
umbrae have magnetic fields nearly equal to or slightly smaller than
that of the darker surroundings; these features are also nearly at rest
with respect to their surroundings. It is noted that the absence of
significant motions in umbral dots implies that radiation transports
most of the energy at and immediately below the surface. The small
size of the dots implies that, if convective plumes transport energy
below the surface of sunspot umbrae, they should have both a lateral
extent and a depth of their upper boundary comparable to or smaller
than the size of the dot.
---------------------------------------------------------
Title: Magnetograph Observations with the Swedish Solar Telescope
on La-Palma
Authors: Lundstedt, Henrik; Johannesson, Anders; Scharmer, Göran;
Stenflo, Jan Olof; Kusoffsky, Ulf; Larsson, Birgitta
1991SoPh..132..233L Altcode:
A high-resolution videomagnetograph that records the images of
opposite circular polarization simultaneously has been constructed
for the Swedish vacuum solar telescope at La Palma. Magnetograms are
obtained by off-line integration of bursts consisting of typically
50 frames of 20 ms exposures, with bad frames rejected, and the
frame-to-frame image motion of the remaining frames compensated for by
cross-correlation techniques. The short exposures combined with frame
selection and elimination of image motion optimizes the resolution
and thereby also the S/N, allowing good magnetograms to be obtained
with an effective exposure time of less than 1 s at an image scale of
0.1″ pixel<SUP>−1</SUP>. The advantages and limitations of the
system are discussed and compared with other techniques of making
filter magnetograms are discussed.
---------------------------------------------------------
Title: Fine-Scale Magnetic Field in a Sunspot Penumbra and Adjacent
Photosphere
Authors: Frank, Z. A.; Scharmer, G. B.; Keller, C.; Lundstedt, H.
1991BAAS...23.1052F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Power Spectra of Flows and Magnetic Fields in the Solar
Photosphere
Authors: Tarbell, T. D.; Slater, G. L.; Frank, Z. A.; Topka, K. P.;
Scharmer, G.; Schmidt, W.
1991BAAS...23.1048T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stokes Polarimetry of a Sunspot from the Swedish Solar
Observatory at La Palma
Authors: Topka, K. P.; Frank, Z. A.; Tarbell, T. D.; Title, A. M.;
Scharmer, G.
1991BAAS...23Q1052T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Variation of granulation properties on a mesogranular scale
Authors: Brandt, P. N.; Ferguson, S.; Shine, R. A.; Tarbell, T. D.;
Scharmer, G. B.
1991A&A...241..219B Altcode:
A 79 min series of CCD images of the solar granulation with subarcsec
spatial resolution is analyzed. Local correlation techniques are applied
to determine horizontal displacements of the granular intensity pattern
in a 14.2 by 12.2 sq arcsec field of view. The divergence of the average
horizontal flow field shows mean cell sizes of 5 to 7 arcsec. Granules
are selected with respect to area, brightness, lifetime, and expansion
rate. It is shown that small, faint, short-lived, and fast collapsing
granules are located preferentially in the negative divergence
(down-draft) regions of the mesogranular flow field, while bright,
long-lived, and rapidly expanding granules populate preferentially
the positive divergence (up-draft) regions. The differences in area
coverage range between + or - 6 percent and + or - 9 percent.
---------------------------------------------------------
Title: Swedish solar telescope: Short summary of instrumentation
and observation techniques
Authors: Scharmer, Goran; Lofdahl, Mats
1991AdSpR..11e.129S Altcode: 1991AdSpR..11..129S
A short summary of the design concepts of the Swedish Solar Telescope
at La Palma, is given along with the most important parts of the
instrumentation and observing techniques. Our experiences from using
high-speed read-out CCDs for solar observations are also discussed. The
advantages of this data aquisition system are that it allows real-time
frame selection for achieving high spatial resolution, that several
cameras can be slaved by one seeing monitor, and that bursts of digital
images can be recorded for full spatial coverage of small parts of
active regions.
---------------------------------------------------------
Title: SOUP Observations of Solar Activity
Authors: Shine, R. A.; Scharmer, G.; Tarbell, T. D.; Title, A. M.;
Topka, K. P.
1991max..conf..295S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High-Resolution Spectra of Solar Magnetic Features. I. Analysis
of Penumbral Fine Structure
Authors: Lites, B. W.; Scharmer, G. B.; Skumanich, A.
1990ApJ...355..329L Altcode:
The Swedish Vacuum Telescope on La Palma was used to obtain spectra
of the magnetic-sensitive Fe I 630.25 nm line under conditions
of exceptional angular resolution (0.32 arcsec) and high spectral
resolution (FWHM 2.5 pm). Simultaneous 0.02 s CCD exposures of both the
spectrum and the slit-jaw image effectively 'freeze' the atmospheric
seeing motions and permit unambiguous identification of the spectra
of the various penumbral structures. These spectra reveal the magnetic
field strength in penumbral filaments through an intensity fit of the
Zeeman splitting of this line. The observations show that: (1) the field
strength varies from about 2100 G near the umbra-penumbra boundary
to about 900 G at the outer edge of the penumbra, (2) the observed
fluctuation of penumbral magnetic field is much less dramatic than the
fluctuation in intensity, (3) there is a suggestion of a rapid change
in field inclination between some light and dark filaments near the
edge of the penumbra, and (4) there is no obvious correlation between
Doppler shift (in part due to the Evershed flow) and filament intensity.
---------------------------------------------------------
Title: Detailed Comparison of Quiet and Magnetic Sun
Authors: Topka, K.; Ferguson, S.; Shine, R.; Tarbell, T.; Title, A.;
Balke, C.; Scharmer, G.; Schmidt, W.
1990BAAS...22R.879T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Penumbral Flows and Magnetic Fields
Authors: Shine, R.; Smith, K.; Tarbell, T.; Title, A.; Scharmer, G.
1990BAAS...22..878S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Spectroscopy of Penumbral Fine Structure
Authors: Bida, T.; Lites, B. W.; Johannesson, A.; Scharmer, G.
1990BAAS...22..880B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Intermittency of Fine Scale Solar Magnetic Fields in the
Photosphere
Authors: Tarbell, T.; Acton, S.; Topka, K.; Title, A.; Schmidt, W.;
Scharmer, G.
1990BAAS...22..878T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic flux tubes and their relation to continuum and
photospheric features
Authors: Title, A.; Tarbell, T.; Topka, K.; Cauffman, D.; Balke, C.;
Scharmer, G.
1990GMS....58..171T Altcode:
An investigation is made of the relationship between photospheric
'filigree' light points, line-center brightness, and magnetic field,
as inferred from sets of individual images and films showing a distinct
difference between two classes of magnetic regions. While in the first
such region the vertical velocity field is average and the magnetic
field is mostly confined in narrow lanes, the granulation pattern
of the second scale is much smaller, the vertical velocity is lower,
and the magnetic field is less compact. Where granulation is normal,
excellent correlation is obtained between bright continuum, line-center,
and magnetic field line structure.
---------------------------------------------------------
Title: High-Resolution Observations of Emerging Magnetic Fields and
Flux Tubes in Active Region Photosphere
Authors: Tarbell, T.; Ferguson, S.; Frank, Z.; Shine, R.; Title, A.;
Topka, K.; Scharmer, G.
1990IAUS..138..147T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Observations of the Photosphere
Authors: Title, A. M.; Shine, R. A.; Tarbell, T. D.; Topka, K. P.;
Scharmer, G. B.
1990IAUS..138...49T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Intial Results of the Lockheed 1989 La Palma Observing Campaign
Authors: Topka, K.; Frank, Z.; Shine, R.; Smith, K.; Tarbell, T.;
Title, A.; Scharmer, G.
1989BAAS...21.1111T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High resolution spectra of umbral fine structure from the
Swedish solar observatory at La Palma
Authors: Lites, B. W.; Scharmer, G.
1989hsrs.conf..286L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Variation of granulation properties on a meso-granular scale
Authors: Brandt, P. N.; Ferguson, S.; Scharmer, G. B.; Shine, R. A.;
Tarbell, T. D.; Title, A. M.; Topka, K.
1989hsrs.conf..473B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Short Term Evolution of Fine Scale Magnetic Structures
Authors: Topka, K.; Frank, Z.; Shine, R.; Tarbell, T.; Title, A.;
Scharmer, G.; Balke, A.
1989BAAS...21..842T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Field Inclination in Penumbra of a Round Sunspot
Observed at Very High Spatial Resolution
Authors: Title, A. M.; Frank, Z. A.; Shine, R. A.; Tarbell, T. D.;
Scharmer, G.
1989BAAS...21Q.837T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Modeling the Flow in Solar Vortices
Authors: Simon, G. W.; Weiss, N. O.; Scharmer, G. B.
1989BAAS...21Q.829S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Running Penumbral Waves
Authors: Shine, R.; Tarbell, T.; Title, A.; Topka, K.; Frank, Z.;
Scharmer, G.
1989BAAS...21..837S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Magnetic Field Strength of Umbral Dots
Authors: Lites, B. W.; Bida, T. A.; Scharmer, G. B.
1989BAAS...21..854L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Granulation Observations from La Palma:
Techniques and First Results
Authors: Scharmer, G. B.
1989ASIC..263..161S Altcode: 1989ssg..conf..161S
No abstract at ADS
---------------------------------------------------------
Title: Examples of high resolution observations of solar fine
structures made at La Palma.
Authors: Jensen, E.; Engvold, O.; Scharmer, G. B.
1989ftsa.conf...67J Altcode:
Two video films from the Swedish Vacuum Solar Telescope at La Palma
were shown. Some basic parameters of the telescope are given.
---------------------------------------------------------
Title: Vortex Motion of the Solar Granulation
Authors: Brandt, P. N.; Scharmer, G. B.; Ferguson, S. H.; Shine,
R. A.; Tarbell, T. D.; Title, A. M.
1989ASIC..263..305B Altcode: 1989ssg..conf..305B
No abstract at ADS
---------------------------------------------------------
Title: Constraints Imposed by Very High Resolution Spectra and Images
on Theoretical Simulations of Granular Convection
Authors: Lites, B. W.; Nordlund, Å.; Scharmer, G. B.
1989ASIC..263..349L Altcode: 1989ssg..conf..349L
No abstract at ADS
---------------------------------------------------------
Title: Fractal Geometry of Convective Flows and Magnetic Fields in
the Solar Atmosphere
Authors: Tarbell, T.; Ferguson, S.; Title, A.; Scharmer, G.; Brandt, P.
1988BAAS...20Q1010T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Granulation in Quiet and Magnetic Sun from
the Swedish Solar Observatory on LaPalma
Authors: Topka, K.; Ferguson, S.; Frank, Z.; Shine, R.; Tarbell, T.;
Title, A.; Wolfson, J.; Scharmer, G.; Brandt, P.
1988BAAS...20S1010T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Activity and Flare Observations from the Swedish Solar
Observatory on La Palma
Authors: Wolfson, J.; Ferguson, S.; Frank, Z.; Shine, R.; Tarbell,
T.; Title, A.; Topka, K.; Scharmer, G.; Brandt, P.; Gurman, J.
1988BAAS...20..978W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Measurements of Turbulent Diffusion by Solar Granulation in
Quiet and Magnetic Areas
Authors: Title, A.; Ferguson, S.; Tarbell, T.; Scharmer, G.; Brandt, P.
1988BAAS...20R1010T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Vortex flow in the solar photosphere
Authors: Brandt, P. N.; Scharmer, G. B.; Ferguson, S.; Shine, R. A.;
Tarbell, T. D.; Title, A. M.
1988Natur.335..238B Altcode:
Convective flow fields in the solar atmosphere play a key role in the
concentration and dispersal of magnetic flux<SUP>1</SUP>, but because
the individual flow elements-the solar granules-are a few arcsec or less
in size, studies of their motions have been limited by the distortion
and blurring of the Earth's atmosphere ('seeing'). We report here a
very high-quality series of granulation images taken at the new Swedish
Solar Observatory on La Palma (Canary Islands) which have permitted
flow measurements at the sub-arcsec level. These movies show a vortex
structure which visibly dominates the motion of the granules in its
neighbourhood and persists for the 1.5 h duration of the movie. If such
vortices are a common feature of the solar convective zone, they may
provide an important mechanism for the heating of stellar chromospheres
and coronae by twisting the footprints of magnetic flux tubes.
---------------------------------------------------------
Title: High Resolution Observations of Penubral Magnetic Fields
Authors: Lites, B. W.; Scharmer, G. B.
1988BAAS...20..681L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Granulation Movies of Exceptional Spatial Resolution:
Observations and Simulations of Horizontal Convective Flows
Authors: Title, A.; Shine, R.; Ferguson, S.; Tarbell, T.; Brandt,
P.; Scharmer, G.
1988BAAS...20R.679T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Vortex Flow in Granulation
Authors: Scharmer, G.; Brandt, P.; Title, A.; Shine, R.; Ferguson, S.
1987BAAS...19Q1118S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Power Spectra of Solar Granulation
Authors: Acton, D. S.; Brandt, P.; Scharmer, G.; Dunn, D.; Tarbell,
T. D.; Title, A. M.; Smithson, R. C.
1987BAAS...19.1118A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Swedish 50 cm Vacuum Solar Telescope: Concepts and
Auxiliary Instrumentation
Authors: Scharmer, G. B.
1987rfsm.conf..349S Altcode:
The paper reviews the concepts of the Swedish 50 cm vacuum solar
telescope. The auxiliary instrumentation, including the Littrow
spectrograph, the Image Sharpness Selector and the image acquisition
system, are also discussed briefly. The strengths and weaknesses of the
entire system are pointed out in order to guide future users towards
optimized observing programs and procedures.
---------------------------------------------------------
Title: Concepts for the Swedish 50-cm vacuum solar telescope.
Authors: Scharmer, G. B.; Brown, D. S.; Pettersson, L.; Rehn, J.
1985ApOpt..24.2558S Altcode:
The concepts of a new major but medium-sized solar telescope are
described. Located at one of the astronomical sites, La Palma,
the telescope uses a doublet lens as the image forming element to
minimize the number of reflecting surfaces, antireflection coatings
to minimize stray light, and a separate guiding telescope outside
the aperture of the telescope to avoid image degradation due to
diffraction effects. Mechanically, the telescope uses the Sacramento
Peak turret design which produces a minimum of local seeing effects. A
short spectrograph, designed for minimum scattered light along and
perpendicular to the slit, is also described. In several respects
the telescope is different from other recently constructed or planned
solar telescopes.
---------------------------------------------------------
Title: Steady flows in active regions observed with the HeI 10830
Å line
Authors: Lites, B. W.; Keil, S. L.; Scharmer, G. B.; Wyller, A. A.
1985SoPh...97...35L Altcode:
We show that the He I 10830 A line gives reliable Doppler shift
measurements in the upper chromosphere above active regions. Persistent
flow patterns in active regions observed near the solar limb show
features previously noted in Dopplergrams using the CIV transition
region ultraviolet emission line. Unlike the CIV measurements, however,
the He I absorption shows a strong correlation with the line-of-sight
velocity images in certain regions of some active regions.
---------------------------------------------------------
Title: A new approach to multi-level non-LTE radiative transfer
problems.
Authors: Scharmer, G. B.; Carlsson, M.
1985JCoPh..59...56S Altcode:
A new approach to the numerical solution of multilevel, non-LTE
problems is described. The standard non-LTE multilevel problem is
formulated, and the statistical equilibrium equations and the radiative
transfer equation are linearized. It is shown how to precondition the
statistical equilibrium equations and the radiative transfer equation
in a way which enables the solution of problems with strong numerical
cancellation which arise from 'passive' scatterings at large optical
depth. Simplifications in the numerical representation of radiative
transfer processes are introduced which lead to rapid methods for
setting up and solving the statistical equilibrium equations. Some
calculations which have been made to test the convergence properties
of the present scheme are described, and some generalizations of the
present methods are briefly outlined.
---------------------------------------------------------
Title: Steady flows in active regions observed with the He I 10830
Å line.
Authors: Lites, B. W.; Keil, S. L.; Scharmer, G. B.; Wyller, A. A.
1985cdm..proc..287L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sweden's solar and stellar telescopes on La Palma
Authors: Wyller, A. A.; Scharmer, G. B.
1985VA.....28..467W Altcode:
The astrophysical research station on La Palma of the Swedish Royal
Academy of Sciences contains an 0.6m stellar telescope, used for
photometry, and a solar telescope. The latter is being upgraded from
a 44cm Cassegrain telescope via a 25cm lens test telescope to a 50cm
lens telescope. An account is given of the new design, with attention
to its principal aims of minimum reduction in practice of the MTF of
the theoretical optical design and control of seeing degradation. The
instruments of both the stellar telescope and the solar telescope,
including the URSIES spectrograph, are described, and an account is
given of some results from the first years of operation.
---------------------------------------------------------
Title: Effects of meso-scale velocity fields on the solar Ca II
spectral lines.
Authors: Carlsson, M.; Scharmer, G. B.
1985cdm..proc..137C Altcode:
The effects of random mesoscale velocity fields on the solar Ca II
K-line and the infrared 8542 A line are discussed, and average profiles
and radiative cooling functions are compared with those obtained from
a combination of microturbulence and macroturbulence. Profiles in
the mesoscale regime could not be reproduced with any combination of
micro- and macroturbulence owing to the fact that mesoscale velocity
fields give core intensities that are much higher than those of static
atmospheres. Calculations of radiative cooling in the two lines were
different when mesoscale velocity fields were taken into account;
this difference was due mainly to a redistribution of the radiative
cooling between the lines.
---------------------------------------------------------
Title: A new method for solving multi-level non-LTE problems.
Authors: Scharmer, G. B.; Carlsson, M.
1985ASIC..152..189S Altcode: 1985pssl.proc..189S
A new scheme for solving multi-level non-LTE problems is described. This
method uses an approximate operator for the relation between the
intensity and the source function. This operator results in a
matrix equation for the population numbers which has a simple and
characteristic structure. Solutions are obtained such that the results
are "exact", irrespective of the choice of the approximate operator.
---------------------------------------------------------
Title: Pannel discussion on radiative transfer methods.
Authors: Kalkofen, W.; Linsky, J.; Rybicki, G.; Scharmer, G.;
Weherse, R.
1985ASIC..152..233K Altcode: 1985pssl.proc..233K
No abstract at ADS
---------------------------------------------------------
Title: Accurate solutions to non-LTE problems using approximate
Lambda operators.
Authors: Scharmer, G. B.
1984mrt..book..173S Altcode: 1984mrt..conf..173S
A numerical method for the approximate solution of non-LTE problems
is described. The essence of this method is the evaluation of an
approximate operator, relating the intensity I<SUB>ν</SUB> to the
source function S<SUB>ν</SUB> along a ray. Using this operator, an
approximate matrix equation for the line-source function is derived. The
solution of this equation generally gives less than 10 - 30% errors
in the line-source function even when there are large gradients
in the Doppler width or in the macroscopic velocity. Furthermore,
a linearization method for the exact solution on non-LTE problems
is presented.
---------------------------------------------------------
Title: A Linearization Method for Solving Partial Redistribution
Problems
Authors: Scharmer, G. B.
1983A&A...117...83S Altcode:
A linearization method for solving partial redistribution (PRD)
problems is presented. The basic idea of this method, due to Cannon
et al. (1975), is the evaluation of an approximate operator which
corresponds to assuming that radiation is completely redistributed
over the line profile. Using this approximate operator, corrections
to the line-source function are obtained iteratively with very small
amounts of computing time. The present method uses a Rybicki-type
of elimination scheme which requires small core storage even when the
number of frequency-angle points is large. <P />The linearization method
for solving PRD problems is combined with the linearization method
of Scharmer (1981) and Scharmer and Nordlund (1982), used to solve
complete redistribution problems. This decreases the computing time
required to solve a given problem. <P />These methods for solving PRD
problems are particularly efficient when the number of frequency-angle
points is large and can be used even on very small computers. Existing
CRD programs for solving complete redistribution problems can easily
be modified to incorporate PRD.
---------------------------------------------------------
Title: DQPT: a computer program for solving non-LTE problems for
two-level atoms in one-dimensional semi-infinite media with velocity
fields.
Authors: Scharmer, G. B.; Nordlund, Å.
1982StoOR..19.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solutions to radiative transfer problems using approximate
lambda operators
Authors: Scharmer, G. B.
1981ApJ...249..720S Altcode:
An approximate analytical method and an exact numerical method for
solving non-LTE problems are presented. The analytical method relies
on concepts of radiative transfer theory such as the core-saturation
approximation, the Eddington-Barbier relation, and the upper boundary
condition, to derive a simple first-order differential equation for the
line source function. The more efficient technique is developed using
a linear test function and a one-point quadrature relation between
the specific intensity and the source function. A 1% accuracy in the
line source function can be obtained after four iterations, with a
very rapid convergence and no evidence of numerical instabilities.
---------------------------------------------------------
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.
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 × 10<SUP>10</SUP> cm<SUP>-3</SUP> ≲n<SUB>e</SUB><SUP>*</SUP>
≲2.3 × 10<SUP>11</SUP> cm<SUP>-3</SUP> at 20 000 K is estimated.
---------------------------------------------------------
Title: The Wilson-Bappu effect as a result of supersonic turbulence.
Authors: Scharmer, G. B.
1976A&A....53..341S Altcode:
An analysis is performed to investigate qualitatively the possibility
that a stellar chromosphere is dominated by the presence of supersonic
turbulence. It is assumed that a chromosphere transforms only mechanical
energy into radiation and is optically thin in the main energy-loss
spectral region, that turbulence energy is distributed throughout the
chromosphere in the form of turbulent eddies, and that the turbulence
is supersonic. Momentum and energy conservation equations are combined
with a boundary condition for the mechanical-energy input to obtain
an expression for the turbulent velocity, the mechanical-energy
flux is normalized relative to the photospheric radiative flux,
and the dependence of chromospheric properties on surface gravity and
effective temperature is examined. Assuming that the turbulent velocity
is proportional to the line width of chromospheric Ca II K emission,
it is concluded that the Wilson-Bappu effect implies temporally and
spatially inhomogeneous chromospheres. The results are applied to the T
Tauri star RU Lup, and observed widths of chromospheric emission lines
are interpreted as being due to a supersonic turbulent velocity field.
