explanation blue bibcodes open ADS page with paths to full text
Author name code: uitenbroek
ADS astronomy entries on 2022-09-14
author:"Uitenbroek, Han"
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Title: Evaluating Non-LTE Spectral Inversions with ALMA and IBIS
Authors: Hofmann, Ryan A.; Reardon, Kevin P.; Milic, Ivan; Molnar,
Momchil E.; Chai, Yi; Uitenbroek, Han
2022ApJ...933..244H Altcode: 2022arXiv220508760H
We present observations of a solar magnetic network region in the
millimeter continuum with the Atacama Large Millimeter/submillimeter
Array (ALMA) and in the Ca 8542 and Na 5896 Å spectral lines with
the Interferometric Bidimensional Spectrometer (IBIS). Our goal is
to compare the measurement of local gas temperatures provided by ALMA
with the temperature diagnostics provided by non-LTE inversions using
the STockholm inversion Code (STiC). In performing these inversions,
we find that using column mass as the reference height scale, rather
than optical depth, provides more reliable atmospheric profiles above
the temperature minimum and that the treatment of non-LTE hydrogen
ionization brings the inferred chromospheric temperatures into better
agreement with the ALMA measurements. The Band 3 brightness temperatures
are higher but well correlated spatially with the inversion-derived
temperatures at the height of formation of the Ca 8542 line core. The
Band 6 temperatures instead do not show good correlations with the
temperatures at any specific layer in the inverted atmospheres. We then
performed inversions that included the millimeter-continuum intensities
as an additional constraint. Incorporating Band 3 generally resulted in
atmospheres showing a strong temperature rise in the upper atmosphere,
while including Band 6 led to significant regions of anomalously low
temperatures at chromospheric heights. This is consistent with the
idea that the Band 6 emission can come from a combination of heights
ranging from the temperature minimum to upper chromosphere. The
poor constraints on the chromospheric electron density with existing
inversion codes introduces difficulties in determining the height(s)
of formation of the millimeter continuum as well as uncertainties in
the temperatures derived from the spectral lines.
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Title: The Atmospheric Response to High Nonthermal Electron-beam
Fluxes in Solar Flares. II. Hydrogen-broadening Predictions for
Solar Flare Observations with the Daniel K. Inouye Solar Telescope
Authors: Kowalski, Adam F.; Allred, Joel C.; Carlsson, Mats; Kerr,
Graham S.; Tremblay, Pier-Emmanuel; Namekata, Kosuke; Kuridze, David;
Uitenbroek, Han
2022ApJ...928..190K Altcode: 2022arXiv220113349K
Redshifted components of chromospheric emission lines in the hard X-ray
impulsive phase of solar flares have recently been studied through
their 30 s evolution with the high resolution of the Interface Region
Imaging Spectrograph. Radiative-hydrodynamic flare models show that
these redshifts are generally reproduced by electron-beam-generated
chromospheric condensations. The models produce large ambient electron
densities, and the pressure broadening of the hydrogen Balmer series
should be readily detected in observations. To accurately interpret
the upcoming spectral data of flares with the DKIST, we incorporate
nonideal, nonadiabatic line-broadening profiles of hydrogen into the
RADYN code. These improvements allow time-dependent predictions for
the extreme Balmer line wing enhancements in solar flares. We study two
chromospheric condensation models, which cover a range of electron-beam
fluxes (1 - 5 × 10<SUP>11</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP>) and
ambient electron densities (1 - 60 × 10<SUP>13</SUP> cm<SUP>-3</SUP>)
in the flare chromosphere. Both models produce broadening and
redshift variations within 10 s of the onset of beam heating. In the
chromospheric condensations, there is enhanced spectral broadening due
to large optical depths at Hα, Hβ, and Hγ, while the much lower
optical depth of the Balmer series H12-H16 provides a translucent
window into the smaller electron densities in the beam-heated layers
below the condensation. The wavelength ranges of typical DKIST/ViSP
spectra of solar flares will be sufficient to test the predictions
of extreme hydrogen wing broadening and accurately constrain large
densities in chromospheric condensations.
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Title: DeSIRe: Departure coefficient aided Stokes Inversion based
on Response functions
Authors: Ruiz Cobo, B.; Quintero Noda, C.; Gafeira, R.; Uitenbroek,
H.; Orozco Suárez, D.; Páez Mañá, E.
2022A&A...660A..37R Altcode: 2022arXiv220202226R
Future ground-based telescopes, such as the 4-metre class facilities
DKIST and EST, will dramatically improve on current capabilities for
simultaneous multi-line polarimetric observations in a wide range of
wavelength bands, from the near-ultraviolet to the near-infrared. As a
result, there will be an increasing demand for fast diagnostic tools,
i.e., inversion codes, that can infer the physical properties of the
solar atmosphere from the vast amount of data these observatories
will produce. The advent of substantially larger apertures,
with the concomitant increase in polarimetric sensitivity, will
drive an increased interest in observing chromospheric spectral
lines. Accordingly, pertinent inversion codes will need to take
account of line formation under general non-local thermodynamic
equilibrium (NLTE) conditions. Several currently available codes can
already accomplish this, but they have a common practical limitation
that impairs the speed at which they can invert polarised spectra,
namely that they employ numerical evaluation of the so-called response
functions to changes in the atmospheric parameters, which makes them
less suitable for the analysis of very large data volumes. Here we
present DeSIRe (Departure coefficient aided Stokes Inversion based on
Response functions), an inversion code that integrates the well-known
inversion code SIR with the NLTE radiative transfer solver RH. The
DeSIRe runtime benefits from employing analytical response functions
computed in local thermodynamic equilibrium (through SIR), modified
with fixed departure coefficients to incorporate NLTE effects in
chromospheric spectral lines. This publication describes the operating
fundamentals of DeSIRe and describes its behaviour, robustness,
stability, and speed. The code is ready to be used by the solar
community and is being made publicly available.
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Title: Estimation of projection effects in the solar polar magnetic
flux measurements from an ecliptic view.
Authors: Gosain, Sanjay; Uitenbroek, Han
2021AGUFMSH34D..04G Altcode:
The distribution and evolution of the magnetic field at the solar poles
through a solar cycle is an important parameter in understanding the
solar dynamo. The accurate observations of the polar magnetic flux is
very challenging from the ecliptic view, mainly due to (a) geometric
foreshortening which limits the spatial resolution, and (b) the oblique
view of predominantly vertical magnetic flux elements, which presents
rather small line-of-sight component of the magnetic field towards
the ecliptic. Due to these effects the polar magnetic flux is poorly
measured. Depending upon the measurement technique, longitudinal versus
full vector field measurement, where the latter is extremely sesnitive
to the SNR and azimuth disamiguation problem, the polar magnetic flux
measurements could be underestimated or overestimated. To estimate the
extent of systematic errors in magetic flux measurements at the solar
poles due to aforementioned projection effects we use MHD simulations of
quiet sun network as a reference solar atmosphere. Using the numerical
model of the solar atmosphere we simulate the observations from the
ecliptic as well as from out-of-ecliptic vantage points, such as from
a solar polar orbit at various heliographic latitudes. Using these
simulated observations we make an assessment of the systematic errors
in our measurements of the magnetic flux due to projection effects
and the extent of under- or over estimation.
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Title: Diagnostic capabilities of spectropolarimetric observations for
understanding solar phenomena. I. Zeeman-sensitive photospheric lines
Authors: Quintero Noda, C.; Barklem, P. S.; Gafeira, R.; Ruiz Cobo,
B.; Collados, M.; Carlsson, M.; Martínez Pillet, V.; Orozco Suárez,
D.; Uitenbroek, H.; Katsukawa, Y.
2021A&A...652A.161Q Altcode: 2021arXiv210605084Q
Future ground-based telescopes will expand our capabilities for
simultaneous multi-line polarimetric observations in a wide range of
wavelengths, from the near-ultraviolet to the near-infrared. This
creates a strong demand to compare candidate spectral lines to
establish a guideline of the lines that are most appropriate for each
observation target. We focused in this first work on Zeeman-sensitive
photospheric lines in the visible and infrared. We first examined their
polarisation signals and response functions using a 1D semi-empirical
atmosphere. Then we studied the spatial distribution of the line core
intensity and linear and circular polarisation signals using a realistic
3D numerical simulation. We ran inversions of synthetic profiles, and
we compared the heights at which we obtain a high correlation between
the input and the inferred atmosphere. We also used this opportunity
to revisit the atomic information we have on these lines and computed
the broadening cross-sections due to collisions with neutral hydrogen
atoms for all the studied spectral lines. The results reveal that
four spectral lines stand out from the rest for quiet-Sun and network
conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three
form higher in the atmosphere, and the last line is mainly sensitive to
the atmospheric parameters at the bottom of the photosphere. However,
as they reach different heights, we strongly recommend using at least
one of the first three candidates together with the Fe I 15 648 Å line
to optimise our capabilities for inferring the thermal and magnetic
properties of the lower atmosphere.
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Title: Machine learning initialization to accelerate Stokes profile
inversions
Authors: Gafeira, R.; Orozco Suárez, D.; Milić, I.; Quintero Noda,
C.; Ruiz Cobo, B.; Uitenbroek, H.
2021A&A...651A..31G Altcode: 2021arXiv210309651G
Context. At present, an exponential growth in scientific data
from current and upcoming solar observatories is expected. Most of
the data consist of high spatial and temporal resolution cubes of
Stokes profiles taken in both local thermodynamic equilibrium (LTE)
and non-LTE spectral lines. The analysis of such solar observations
requires complex inversion codes. Hence, it is necessary to develop
new tools to boost the speed and efficiency of inversions and reduce
computation times and costs. <BR /> Aims: In this work we discuss
the application of convolutional neural networks (CNNs) as a tool to
advantageously initialize Stokes profile inversions. <BR /> Methods:
To demonstrate the usefulness of CNNs, we concentrate in this paper on
the inversion of LTE Stokes profiles. We use observations taken with
the spectropolarimeter on board the Hinode spacecraft as a test bench
mark. First, we carefully analyse the data with the SIR inversion code
using a given initial atmospheric model. The code provides a set of
atmospheric models that reproduce the observations well. These models
are then used to train a CNN. Afterwards, the same data are again
inverted with SIR but using the trained CNN to provide the initial
guess atmospheric models for SIR. <BR /> Results: The CNNs allow us
to significantly reduce the number of inversion cycles when used to
compute initial guess model atmospheres (`assisted inversions'),
therefore decreasing the computational time for LTE inversions by
a factor of two to four. CNNs alone are much faster than assisted
inversions, but the latter are more robust and accurate. CNNs also
help to automatically cluster pixels with similar physical properties,
allowing the association with different solar features on the solar
surface, which is useful when inverting huge datasets where completely
different regimes are present. The advantages and limitations of machine
learning techniques for estimating optimum initial atmospheric models
for spectral line inversions are discussed. Finally, we describe a
python wrapper for the SIR and DeSIRe codes that allows for the easy
setup of parallel inversions. The tool implements the assisted inversion
method described in this paper. The parallel wrapper can also be used
to synthesize Stokes profiles with the RH code. <BR /> Conclusions:
The assisted inversions can speed up the inversion process, but the
efficiency and accuracy of the inversion results depend strongly on
the solar scene and the data used for the CNN training. This method
(assisted inversions) will not obviate the need for analysing individual
events with the utmost care but will provide solar scientists with
a much better opportunity to sample large amounts of inverted data,
which will undoubtedly broaden the physical discovery space.
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Title: Infrared diagnostics of the solar magnetic field with Mg I
12 μm lines: forward-model results
Authors: Li, Xin; Song, Yongliang; Uitenbroek, H.; Yang, Xiao; Bai,
Xianyong; Deng, Yuanyong
2021A&A...646A..79L Altcode: 2020arXiv201208912L
Context. The Mg I 12.32 and 12.22 μm lines are a pair of emission
lines that present a great advantage for accurate solar magnetic field
measurement. They potentially contribute to the diagnosis of solar
atmospheric parameters through their high magnetic sensitivity. <BR />
Aims: The goal of this study is to understand the radiation transfer
process of these lines in detail and explore the ability of magnetic
field diagnosis in the infrared. <BR /> Methods: We calculated
the Stokes profiles and response functions of the two Mg I 12 μm
lines based on one-dimensional solar atmospheric models using the
Rybicki-Hummer (RH) radiative transfer code. The integration of
these profiles with respect to the wavelength was used to generate
calibration curves related to the longitudinal and transverse
fields. The traditional single-wavelength calibration curve based on
the weak-field approximation was also tested to determine if it is
suitable for the infrared. <BR /> Results: The 12.32 μm line is more
suitable for a magnetic field diagnosis because its relative emission
intensity and polarization signal are stronger than that of the 12.22
μm line. The result from the response functions illustrates that the
derived magnetic field and velocity with 12.32 μm line mainly originate
from the height of 450 km, while that for the temperature is about
490 km. The calibration curves obtained by the wavelength-integrated
method show a nonlinear distribution. For the Mg I 12.32 μm line,
the longitudinal (transverse) field can be effectively inferred from
Stokes V/I (Q/I and U/I) in the linear range below ∼600 G (∼3000 G)
in quiet regions and below ∼400 G (∼1200 G) in penumbrae. Within
the given linear range, the method is a supplement to the magnetic
field calibration when the Zeeman components are incompletely split.
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Title: Spatially Resolved Ultraviolet Spectroscopy of the Great
Dimming of Betelgeuse
Authors: Dupree, Andrea K.; Strassmeier, Klaus G.; Matthews, Lynn
D.; Uitenbroek, Han; Calderwood, Thomas; Granzer, Thomas; Guinan,
Edward F.; Leike, Reimar; Montargès, Miguel; Richards, Anita M. S.;
Wasatonic, Richard; Weber, Michael
2020ApJ...899...68D Altcode: 2020arXiv200804945D
The bright supergiant Betelgeuse (Alpha Orionis, HD 39801) experienced
a visual dimming during 2019 December and the first quarter of
2020, reaching an historic minimum 2020 February 7-13. During 2019
September-November, prior to the optical dimming event, the photosphere
was expanding. At the same time, spatially resolved ultraviolet spectra
using the Hubble Space Telescope/Space Telescope Imaging Spectrograph
revealed a substantial increase in the ultraviolet spectrum and Mg II
line emission from the chromosphere over the southern hemisphere of the
star. Moreover, the temperature and electron density inferred from the
spectrum and C II diagnostics also increased in this hemisphere. These
changes happened prior to the Great Dimming Event. Variations in the
Mg II k-line profiles suggest material moved outwards in response to
the passage of a pulse or acoustic shock from 2019 September through
November. It appears that this extraordinary outflow of material from
the star, likely initiated by convective photospheric elements, was
enhanced by the coincidence with the outward motions in this phase of
the ∼400 day pulsation cycle. These ultraviolet observations appear
to provide the connecting link between the known large convective cells
in the photosphere and the mass ejection event that cooled to form
the dust cloud in the southern hemisphere imaged in 2019 December, and
led to the exceptional optical dimming of Betelgeuse in 2020 February.
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Title: Focus on Betelgeuse
Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper,
Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka,
Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.;
Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski,
Markus; Matthews, Lynn D.
2020hst..prop16216D Altcode:
Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse,
using STIS will enable spatially resolved measures of chromospheric
structure and mass inflows and outflows. An HST campaign of 3 cycles
will be complemented by multi-frequency photometry, spectroscopy,
interferometry, and polarimetry at radio, infrared, and optical
wavelengths in order to map surface structures and their variability,
and the extended outer atmosphere over both the short (400-day)
and long secondary (2000-day) periods of this supergiant. These
observations, coupled with detailed modeling and simulations, will
probe the structure, the dynamics, and the mass loss from Betelgeuse in
unprecedented detail and provide crucial insights into the atmospheric
physics and wind-driving mechanisms of red supergiants.
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Title: Comparing Radiative Transfer Codes and Opacity Samplings for
Solar Irradiance Reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias; Haberreiter, Margit;
Pereira, Tiago M. D.; Uitenbroek, Han; Fabbian, Damian
2020SoPh..295...50C Altcode:
Some techniques developed to reproduce solar irradiance variations make
use of synthetic radiative fluxes of quiet and magnetic features. The
synthesis of radiative fluxes of astronomical objects is likely
to be affected by uncertainties resulting from approximations and
specific input employed for the synthesis. In this work we compare
spectra obtained with three radiative transfer codes with the
purpose of investigating differences in reproducing solar irradiance
variations. Specifically, we compare spectral synthesis produced in
non-local thermodynamic equilibrium obtained with COSI and RH using
1-D atmosphere models. We also compare local thermodynamic equilibrium
syntheses emerging from 3-D MURaM simulations of the solar atmosphere
obtained with two sets of opacity tables generated with the ATLAS9
package and with the RH code, and test the effects of opacity sampling
on the emergent spectra. We find that, although the different codes
and methodologies employed to synthesize the spectrum reproduce overall
the observed solar spectrum with a similar degree of accuracy, subtle
differences in quiet Sun spectra may translate into larger differences
in the computation of the contrasts of magnetic features, which,
in turn, critically affect the estimates of solar variability.
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Title: Solar Chromospheric Temperature Diagnostics: A Joint ALMA-Hα
Analysis
Authors: Molnar, Momchil E.; Reardon, Kevin P.; Chai, Yi; Gary, Dale;
Uitenbroek, Han; Cauzzi, Gianna; Cranmer, Steven R.
2019ApJ...881...99M Altcode: 2019arXiv190608896M
We present the first high-resolution, simultaneous observations of
the solar chromosphere in the optical and millimeter wavelength
ranges, obtained with the Atacama Large Millimeter Array (ALMA)
and the Interferometric Bidimensional Spectrometer at the Dunn Solar
Telescope. In this paper we concentrate on the comparison between the
brightness temperature observed in ALMA Band 3 (3 mm; 100 GHz) and the
core width of the Hα 6563 Å line, previously identified as a possible
diagnostic of the chromospheric temperature. We find that in the area
of plage, network and fibrils covered by our field of view, the two
diagnostics are well correlated, with similar spatial structures
observed in both. The strength of the correlation is remarkable,
given that the source function of the millimeter radiation obeys local
thermodynamic equilibrium, while the Hα line has a source function that
deviates significantly from the local Planck function. The observed
range of ALMA brightness temperatures is sensibly smaller than the
temperature range that was previously invoked to explain the observed
width variations in Hα. We employ analysis from forward modeling
with the Rybicki-Hummer (RH) code to argue that the strong correlation
between Hα width and ALMA brightness temperature is caused by their
shared dependence on the population number n <SUB>2</SUB> of the first
excited level of hydrogen. This population number drives millimeter
opacity through hydrogen ionization via the Balmer continuum, and
Hα width through a curve-of-growth-like opacity effect. Ultimately,
the n <SUB>2</SUB> population is regulated by the enhancement or lack
of downward Lyα flux, which coherently shifts the formation height
of both diagnostics to regions with different temperature, respectively.
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Title: Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
Authors: Zhu, Yingjie; Kowalski, Adam F.; Tian, Hui; Uitenbroek, Han;
Carlsson, Mats; Allred, Joel C.
2019ApJ...879...19Z Altcode: 2019arXiv190412285Z
Observations from the Interface Region Imaging Spectrograph often
reveal significantly broadened and non-reversed profiles of the Mg II
h, k and triplet lines at flare ribbons. To understand the formation
of these optically thick Mg II lines, we perform plane-parallel
radiative hydrodynamics modeling with the RADYN code, and then
recalculate the Mg II line profiles from RADYN atmosphere snapshots
using the radiative transfer code RH. We find that the current RH
code significantly underestimates the Mg II h and k Stark widths. By
implementing semiclassical perturbation approximation results of
quadratic Stark broadening from the STARK-B database in the RH code,
the Stark broadenings are found to be one order of magnitude larger
than those calculated from the current RH code. However, the improved
Stark widths are still too small, and another factor of 30 has to be
multiplied to reproduce the significantly broadened lines and adjacent
continuum seen in observations. Nonthermal electrons, magnetic fields,
three-dimensional effects, or electron density effects may account
for this factor. Without modifying the RADYN atmosphere, we have also
reproduced non-reversed Mg II h and k profiles, which appear when the
electron beam energy flux is decreasing. These profiles are formed at
an electron density of ∼8 × 10<SUP>14</SUP> cm<SUP>-3</SUP> and a
temperature of ∼1.4 × 10<SUP>4</SUP> K, where the source function
slightly deviates from the Planck function. Our investigation also
demonstrates that at flare ribbons the triplet lines are formed in the
upper chromosphere, close to the formation heights of the h and k lines.
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Title: Focus on Betelgeuse
Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper,
Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka,
Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.;
Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski,
Markus; Matthews, Lynn D.
2019hst..prop15873D Altcode:
Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse,
using STIS will enable spatially resolved measures of chromospheric
structure and mass inflows and outflows. An HST campaign of 3 cycles
will be complemented by multi-frequency photometry, spectroscopy,
interferometry, and polarimetry at radio, infrared, and optical
wavelengths in order to map surface structures and their variability,
and the extended outer atmosphere over both the short (400-day)
and long secondary (2000-day) periods of this supergiant. These
observations, coupled with detailed modeling and simulations, will
probe the structure, the dynamics, and the mass loss from Betelgeuse in
unprecedented detail and provide crucial insights into the atmospheric
physics and wind-driving mechanisms of red supergiants.
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Title: On the Challenges of synthetizing solar and stellar spectra
for Irradiance reconstructions
Authors: Criscuoli, Serena; Rempel, Matthias D.; Haberreiter, Margit;
Pereira, Tiago; Uitenbroek, Han; Fabbian, Damian
2019AAS...23421702C Altcode:
Syntheses of solar and stellar spectra strongly depend on the adopted
approximations and atomic and molecular databases. We compare LTE and
NLTE syntheses of solar spectra obtained with widely used radiative
transfer codes, utilizing both 3D-MHD simulations and 1D-static
atmosphere models. We show that although different codes reproduce
reasonably well the observed spectrum, subtle differences may translate
into discrepancies of several tens of percents in the estimate of
solar and stellar spectral irradiance variability.
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Title: Comparison of Scattering Polarization Signals Observed by
CLASP: Possible Indication of the Hanle Effect
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pomtieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2019ASPC..526..305I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP; Kano et
al. 2012; Kobayashi et al. 2012; Kubo et al. 2014) observed, for the
first time, the linear polarization produced by scattering processes
in the hydrogen Lyman-α (121.57 nm) and Si III (120.56 nm) lines of
the solar disk radiation. The complexity of the observed scattering
polarization (i.e., conspicuous spatial variations in Q/I and U/I
at spatial scales of 10″-20″ and the absence of center-to- limb
variation at the Lyman-α center; see Kano et al. 2017) motivated us
to search for possible hints of the operation of the Hanle effect by
comparing: (a) the Lyman-α line center signal, for which the critical
field strength (B<SUB>H</SUB>) for the onset of the Hanle effect is
53 G, (b) the Lyman-α wing, which is insensitive to the Hanle effect,
and (c) the Si III line, whose B<SUB>H</SUB> = 290 G. We focus on four
regions with different total unsigned photospheric magnetic fluxes
(estimated from SDO/HMI observations), and compare the corresponding
U/I spatial variations in the Lyman-α wing, Lyman-α center, and Si III
line. The U/I signal in the Lyman-α wing shows an antisymmetric spatial
distribution, which is caused by the presence of a bright structure in
all the selected regions, regardless of the total unsigned photospheric
magnetic flux. In an internetwork region, the Lyman-α center shows an
antisymmetric spatial variation across the selected bright structure,
but it does not show it in other more magnetized regions. In the Si III
line, the spatial variation of U/I deviates from the above-mentioned
antisymmetric shape as the total unsigned photospheric magnetic flux
increases. We argue that a plausible explanation of this differential
behavior is the operation of the Hanle effect. <P />This work, presented
in an oral contribution at this Workshop, has been published on The
Astrophysical Journal (Ishikawa et al. 2017).
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Title: Inferring micro-turbulent magnetic fields via the Hanle effect
Authors: Kulkarni, Neeraj; Uitenbroek, Han
2019shin.confE.172K Altcode:
A key research problem in solar physics is to understand the small-scale
magnetic fields that permeate the solar surface. These fields are
too weak to be observed through conventional methods (e.g. Zeeman
splitting in a spectral line). However, the high spatial and spectral
resolution of DKIST will facilitate novel ways of probing the magnetic
field. One such method relies on the Hanle effect. <P />When an
atmosphere is illuminated anisotropically, the emerging radiation
will be polarized. Furthermore, if a magnetic field is present, the
observed polarization is reduced due to the Hanle effect. We aim to
show that the Hanle effect can be used to map out the structure of
microturbulent magnetic fields - potentially providing new insights
into MHD turbulence in the quiet Sun.
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Title: STiC: A multiatom non-LTE PRD inversion code for full-Stokes
solar observations
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2019A&A...623A..74D Altcode: 2018arXiv181008441D
The inference of the underlying state of the plasma in the solar
chromosphere remains extremely challenging because of the nonlocal
character of the observed radiation and plasma conditions in this
layer. Inversion methods allow us to derive a model atmosphere that
can reproduce the observed spectra by undertaking several physical
assumptions. The most advanced approaches involve a depth-stratified
model atmosphere described by temperature, line-of-sight velocity,
turbulent velocity, the three components of the magntic field vector,
and gas and electron pressure. The parameters of the radiative transfer
equation are computed from a solid ground of physical principles. In
order to apply these techniques to spectral lines that sample the
chromosphere, nonlocal thermodynamical equilibrium effects must be
included in the calculations. We developed a new inversion code STiC
(STockholm inversion Code) to study spectral lines that sample the
upper chromosphere. The code is based on the RH forward synthesis code,
which we modified to make the inversions faster and more stable. For
the first time, STiC facilitates the processing of lines from multiple
atoms in non-LTE, also including partial redistribution effects (PRD)
in angle and frequency of scattered photons. Furthermore, we include
a regularization strategy that allows for model atmospheres with a
complex depth stratification, without introducing artifacts in the
reconstructed physical parameters, which are usually manifested in
the form of oscillatory behavior. This approach takes steps toward
a node-less inversion, in which the value of the physical parameters
at each grid point can be considered a free parameter. In this paper
we discuss the implementation of the aforementioned techniques, the
description of the model atmosphere, and the optimizations that we
applied to the code. We carry out some numerical experiments to show
the performance of the code and the regularization techniques that we
implemented. We made STiC publicly available to the community.
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Title: Study of the polarization produced by the Zeeman effect in
the solar Mg I b lines
Authors: Quintero Noda, C.; Uitenbroek, H.; Carlsson, M.; Orozco
Suárez, D.; Katsukawa, Y.; Shimizu, T.; Ruiz Cobo, B.; Kubo, M.; Oba,
T.; Kawabata, Y.; Hasegawa, T.; Ichimoto, K.; Anan, T.; Suematsu, Y.
2018MNRAS.481.5675Q Altcode: 2018arXiv181001067Q; 2018MNRAS.tmp.2566Q
The next generation of solar observatories aim to understand the
magnetism of the solar chromosphere. Therefore, it is crucial to
understand the polarimetric signatures of chromospheric spectral
lines. For this purpose, we here examine the suitability of the three
Fraunhofer Mg I b<SUB>1</SUB>, b<SUB>2</SUB>, and b<SUB>4</SUB> lines
at 5183.6, 5172.7, and 5167.3 Å, respectively. We start by describing
a simplified atomic model of only six levels and three line transitions
for computing the atomic populations of the 3p-4s (multiplet number
2) levels involved in the Mg I b line transitions assuming non-local
thermodynamic conditions and considering only the Zeeman effect using
the field-free approximation. We test this simplified atom against
more complex ones finding that, although there are differences in the
computed profiles, they are small compared with the advantages provided
by the simple atom in terms of speed and robustness. After comparing
the three Mg I lines, we conclude that the most capable one is the
b<SUB>2</SUB> line as b<SUB>1</SUB> forms at similar heights and always
shows weaker polarization signals, while b<SUB>4</SUB> is severely
blended with photospheric lines. We also compare Mg I b<SUB>2</SUB>
with the K I D<SUB>1</SUB> and Ca II 8542 Å lines finding that the
former is sensitive to the atmospheric parameters at heights that
are in between those covered by the latter two lines. This makes Mg I
b<SUB>2</SUB> an excellent candidate for future multiline observations
that aim to seamlessly infer the thermal and magnetic properties of
different features in the lower solar atmosphere.
---------------------------------------------------------
Title: STiC: Stockholm inversion code
Authors: de la Cruz Rodríguez, J.; Leenaarts, J.; Danilovic, S.;
Uitenbroek, H.
2018ascl.soft10014D Altcode:
STiC is a MPI-parallel non-LTE inversion code for observed full-Stokes
observations. The code processes lines from multiple atoms in non-LTE,
including partial redistribution effects of scattered photons in
angle and frequency of scattered photons (PRD), and can be used with
model atmospheres that have a complex depth stratification without
introducing artifacts.
---------------------------------------------------------
Title: Influence of the Atmospheric Model on Hanle Diagnostics
Authors: Ishikawa, Ryohko; Uitenbroek, Han; Goto, Motoshi; Iida,
Yusuke; Tsuneta, Saku
2018SoPh..293...74I Altcode:
We clarify the uncertainty in the inferred magnetic field vector via the
Hanle diagnostics of the hydrogen Lyman-α line when the stratification
of the underlying atmosphere is unknown. We calculate the anisotropy of
the radiation field with plane-parallel semi-empirical models under the
nonlocal thermal equilibrium condition and derive linear polarization
signals for all possible parameters of magnetic field vectors based on
an analytical solution of the atomic polarization and Hanle effect. We
find that the semi-empirical models of the inter-network region
(FAL-A) and network region (FAL-F) show similar degrees of anisotropy
in the radiation field, and this similarity results in an acceptable
inversion error (e.g., ∼40 G instead of 50 G in field strength and
∼100<SUP>∘</SUP> instead of 90<SUP>∘</SUP> in inclination) when
FAL-A and FAL-F are swapped. However, the semi-empirical models of FAL-C
(averaged quiet-Sun model including both inter-network and network
regions) and FAL-P (plage regions) yield an atomic polarization that
deviates from all other models, which makes it difficult to precisely
determine the magnetic field vector if the correct atmospheric model
is not known (e.g., the inversion error is much larger than 40% of
the field strength; >70 G instead of 50 G). These results clearly
demonstrate that the choice of model atmosphere is important for
Hanle diagnostics. As is well known, one way to constrain the average
atmospheric stratification is to measure the center-to-limb variation
of the linear polarization signals. The dependence of the center-to-limb
variations on the atmospheric model is also presented in this paper.
---------------------------------------------------------
Title: Harmonizing Mangnetograph Data with end-to-end Instrument
Simulations
Authors: Plowman, J.; Petrie, G. J. D.; Pillet, V. M.; Criscuoli,
S.; Harvey, J. W.; Marble, A.; Uitenbroek, H.
2017AGUFMSH13A2460P Altcode:
There are a number of instruments, such as NSO's GONG and SOLIS/VSM,
which measure the magnetic field of the Sun's photosphere. However,
their measurements are not fully consistent, and the factors responsible
for the differences have yet to be isolated. I report on a new effort
to resolve them - we simulate the observing processes from end to
end, beginning with 3D MHD simulations. This allows us to compare the
synthetic observations produced with the MHD simulations' 'ground truth'
and identify the effects of the observational factors at play. The
result will be the best ever calibration of a magnetograph, giving
magnetic field models based on these data a much firmer foundation.
---------------------------------------------------------
Title: Solar polarimetry through the K I lines at 770 nm
Authors: Quintero Noda, C.; Uitenbroek, H.; Katsukawa, Y.; Shimizu,
T.; Oba, T.; Carlsson, M.; Orozco Suárez, D.; Ruiz Cobo, B.; Kubo,
M.; Anan, T.; Ichimoto, K.; Suematsu, Y.
2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q
We characterize the K I D<SUB>1</SUB> & D<SUB>2</SUB> lines in
order to determine whether they could complement the 850 nm window,
containing the Ca II infrared triplet lines and several Zeeman sensitive
photospheric lines, that was studied previously. We investigate the
effect of partial redistribution on the intensity profiles, their
sensitivity to changes in different atmospheric parameters, and
the spatial distribution of Zeeman polarization signals employing a
realistic magnetohydrodynamic simulation. The results show that these
lines form in the upper photosphere at around 500 km, and that they
are sensitive to the line-of-sight velocity and magnetic field strength
at heights where neither the photospheric lines nor the Ca II infrared
lines are. However, at the same time, we found that their sensitivity
to the temperature essentially comes from the photosphere. Then, we
conclude that the K I lines provide a complement to the lines in the
850 nm window for the determination of atmospheric parameters in the
upper photosphere, especially for the line-of-sight velocity and the
magnetic field.
---------------------------------------------------------
Title: An Inside Look at Sunspot Oscillations with Higher Azimuthal
Wavenumbers
Authors: Jess, David B.; Van Doorsselaere, Tom; Verth, Gary; Fedun,
Viktor; Krishna Prasad, S.; Erdélyi, Robert; Keys, Peter H.; Grant,
Samuel D. T.; Uitenbroek, Han; Christian, Damian J.
2017ApJ...842...59J Altcode: 2017arXiv170506282J
Solar chromospheric observations of sunspot umbrae offer an exceptional
view of magnetohydrodynamic wave phenomena. In recent years, a
wealth of wave signatures related to propagating magneto-acoustic
modes have been presented, which demonstrate complex spatial and
temporal structuring of the wave components. Theoretical modeling has
demonstrated how these ubiquitous waves are consistent with an m = 0
slow magneto-acoustic mode, which is excited by trapped sub-photospheric
acoustic (p-mode) waves. However, the spectrum of umbral waves is broad,
suggesting that the observed signatures represent the superposition
of numerous frequencies and/or modes. We apply Fourier filtering,
in both spatial and temporal domains, to extract chromospheric umbral
wave characteristics consistent with an m = 1 slow magneto-acoustic
mode. This identification has not been described before. Angular
frequencies of 0.037+/- 0.007 {rad} {{{s}}}<SUP>-1</SUP> (2.1+/-
0.4 \deg {{{s}}}<SUP>-1</SUP>, corresponding to a period ≈170 s)
for the m = 1 mode are uncovered for spatial wavenumbers in the range
of 0.45< k< 0.90 arcsec<SUP>-1</SUP> (5000-9000 km). Theoretical
dispersion relations are solved, with corresponding eigenfunctions
computed, which allows the density perturbations to be investigated
and compared with our observations. Such magnetohydrodynamic modeling
confirms our interpretation that the identified wave signatures are
the first direct observations of an m = 1 slow magneto-acoustic mode
in the chromospheric umbra of a sunspot.
---------------------------------------------------------
Title: Indication of the Hanle Effect by Comparing the Scattering
Polarization Observed by CLASP in the Lyα and Si III 120.65 nm Lines
Authors: Ishikawa, R.; Trujillo Bueno, J.; Uitenbroek, H.; Kubo, M.;
Tsuneta, S.; Goto, M.; Kano, R.; Narukage, N.; Bando, T.; Katsukawa,
Y.; Ishikawa, S.; Giono, G.; Suematsu, Y.; Hara, H.; Shimizu, T.;
Sakao, T.; Winebarger, A.; Kobayashi, K.; Cirtain, J.; Champey, P.;
Auchère, F.; Štěpán, J.; Belluzzi, L.; Asensio Ramos, A.; Manso
Sainz, R.; De Pontieu, B.; Ichimoto, K.; Carlsson, M.; Casini, R.
2017ApJ...841...31I Altcode:
The Chromospheric Lyman-Alpha Spectro-Polarimeter is a sounding
rocket experiment that has provided the first successful measurement
of the linear polarization produced by scattering processes in
the hydrogen Lyα line (121.57 nm) radiation of the solar disk. In
this paper, we report that the Si III line at 120.65 nm also shows
scattering polarization and we compare the scattering polarization
signals observed in the Lyα and Si III lines in order to search for
observational signatures of the Hanle effect. We focus on four selected
bright structures and investigate how the U/I spatial variations vary
between the Lyα wing, the Lyα core, and the Si III line as a function
of the total unsigned photospheric magnetic flux estimated from Solar
Dynamics Observatory/Helioseismic and Magnetic Imager observations. In
an internetwork region, the Lyα core shows an antisymmetric spatial
variation across the selected bright structure, but it does not show
it in other more magnetized regions. In the Si III line, the spatial
variation of U/I deviates from the above-mentioned antisymmetric
shape as the total unsigned photospheric magnetic flux increases. A
plausible explanation of this difference is the operation of the Hanle
effect. We argue that diagnostic techniques based on the scattering
polarization observed simultaneously in two spectral lines with very
different sensitivities to the Hanle effect, like Lyα and Si III,
are of great potential interest for exploring the magnetism of the
upper solar chromosphere and transition region.
---------------------------------------------------------
Title: Hydrogen Balmer Line Broadening in Solar and Stellar Flares
Authors: Kowalski, Adam F.; Allred, Joel C.; Uitenbroek, Han; Tremblay,
Pier-Emmanuel; Brown, Stephen; Carlsson, Mats; Osten, Rachel A.;
Wisniewski, John P.; Hawley, Suzanne L.
2017ApJ...837..125K Altcode: 2017arXiv170203321K
The broadening of the hydrogen lines during flares is thought to
result from increased charge (electron, proton) density in the flare
chromosphere. However, disagreements between theory and modeling
prescriptions have precluded an accurate diagnostic of the degree
of ionization and compression resulting from flare heating in the
chromosphere. To resolve this issue, we have incorporated the unified
theory of electric pressure broadening of the hydrogen lines into
the non-LTE radiative-transfer code RH. This broadening prescription
produces a much more realistic spectrum of the quiescent, A0 star Vega
compared to the analytic approximations used as a damping parameter
in the Voigt profiles. We test recent radiative-hydrodynamic (RHD)
simulations of the atmospheric response to high nonthermal electron
beam fluxes with the new broadening prescription and find that
the Balmer lines are overbroadened at the densest times in the
simulations. Adding many simultaneously heated and cooling model
loops as a “multithread” model improves the agreement with the
observations. We revisit the three-component phenomenological flare
model of the YZ CMi Megaflare using recent and new RHD models. The
evolution of the broadening, line flux ratios, and continuum flux
ratios are well-reproduced by a multithread model with high-flux
nonthermal electron beam heating, an extended decay phase model, and a
“hot spot” atmosphere heated by an ultrarelativistic electron beam
with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible
stellar hemisphere, respectively. The new modeling motivates future
work to understand the origin of the extended gradual phase emission.
---------------------------------------------------------
Title: A Chromospheric Flare Model Consisting of Two Dynamical Layers:
Critical Tests from IRIS Data of Solar Flares
Authors: Kowalski, Adam; Allred, Joel C.; Daw, Adrian N.; Cauzzi,
Gianna; Carlsson, Mats; Inglis, Andrew; O'Neill, Aaron; Mathioudakis,
Mihalis; Uitenbroek, Han
2017AAS...22933902K Altcode:
Recent 1D radiative-hydrodynamic simulations of flares have shown that
a heated, chromospheric compression layer and a stationary layer, just
below the compression, are produced in response to high flux electron
beam heating. The hot blackbody-like continuum and redshifted intensity
in singly ionized chromospheric lines in these model predictions are
generally consistent with broad wavelength coverage spectra of M dwarf
flares and with high spectral resolution observations of solar flares,
respectively. We critically test this two-component chromospheric
flare model against the Fe II profiles and NUV continuum brightness
for several X-class solar flares observed with the Interface Region
Imaging Spectrograph (IRIS). We present several new predictions for
the Daniel K. Inoue Solar Telescope (DKIST).
---------------------------------------------------------
Title: Spectropolarimetry of Atomic and Molecular Lines near 4135 nm
Authors: Penn, Matthew James; Uitenbroek, Han; Clark, Alan; Coulter,
Roy; Goode, Phil; Cao, Wenda
2016SoPh..291.2243P Altcode: 2015arXiv151204451P; 2016SoPh..tmp..142P
New spatially scanned spectropolarimetry sunspot observations are made
of photospheric atomic and molecular absorption lines near 4135 nm. The
relative splittings among several atomic lines are measured and shown
to agree with values calculated with configuration interaction and
intermediate coupling. Large splitting is seen in a line identified
with Fe I at 4137 nm, showing multiple Stokes V components and an
unusual linear polarization. This line will be a sensitive probe of
quiet-Sun magnetic fields, with a magnetic sensitivity of 2.5 times
higher than that of the well-known 1565 nm Fe I line.
---------------------------------------------------------
Title: Joint SDO and IRIS Observations of a Novel, Hybrid
Prominence-Coronal Rain Complex
Authors: Liu, Wei; Antolin, Patrick; Sun, Xudong; Gao, Lijia; Vial,
Jean-Claude; Gibson, Sarah; Okamoto, Takenori; Berger, Thomas;
Uitenbroek, Han; De Pontieu, Bart
2016usc..confE..99L Altcode:
Solar prominences and coronal rain are intimately related phenomena,
both involving cool material at chromospheric temperatures within the
hot corona and both playing important roles as part of the return flow
of the chromosphere-corona mass cycle. At the same time, they exhibit
distinct morphologies and dynamics not yet well understood. Quiescent
prominences consist of numerous long-lasting, filamentary downflow
threads, while coronal rain is more transient and falls comparably
faster along well-defined curved paths. We report here a novel, hybrid
prominence-coronal rain complex in an arcade-fan geometry observed
by SDO/AIA and IRIS, which provides new insights to the underlying
physics of such contrasting behaviors. We found that the supra-arcade
fan region hosts a prominence sheet consisting of meandering threads
with broad line widths. As the prominence material descends to the
arcade, it turns into coronal rain sliding down coronal loops with
line widths 2-3 times narrower. This contrast suggests that distinct
local plasma and magnetic conditions determine the fate of the cool
material, a scenario supported by our magnetic field extrapolations
from SDO/HMI. Specifically, the supra-arcade fan (similar to those
in solar flares; e.g., McKenzie 2013) is likely situated in a current
sheet, where the magnetic field is weak and the plasma-beta could be
close to unity, thus favoring turbulent flows like those prominence
threads. In contrast, the underlying arcade has a stronger magnetic
field and most likely a low-beta environment, such that the material
is guided along magnetic field lines to appear as coronal rain. We
will discuss the physical implications of these observations beyond
prominence and coronal rain.
---------------------------------------------------------
Title: Fast inversion of Zeeman line profiles using central
moments. II. Stokes V moments and determination of vector magnetic
fields
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2016A&A...591A..64M Altcode:
Context. In the case of unresolved solar structures or stray light
contamination, inversion techniques using four Stokes parameters
of Zeeman profiles cannot disentangle the combined contributions of
magnetic and nonmagnetic areas to the observed Stokes I. <BR /> Aims:
In the framework of a two-component model atmosphere with filling factor
f, we propose an inversion method restricting input data to Q , U, and
V profiles, thus overcoming ambiguities from stray light and spatial
mixing. <BR /> Methods: The V-moments inversion (VMI) method uses
shifts S<SUB>V</SUB> derived from moments of V-profiles and integrals
of Q<SUP>2</SUP>, U<SUP>2</SUP>, and V<SUP>2</SUP> to determine the
strength B and inclination ψ of a magnetic field vector through
least-squares polynomial fits and with very few iterations. Moment
calculations are optimized to reduce data noise effects. To specify the
model atmosphere of the magnetic component, an additional parameter
δ, deduced from the shape of V-profiles, is used to interpolate
between expansions corresponding to two basic models. <BR /> Results:
We perform inversions of HINODE SOT/SP data for inclination ranges 0
<ψ< 60° and 120 <ψ< 180° for the 630.2 nm Fe I line. A
damping coefficient is fitted to take instrumental line broadening into
account. We estimate errors from data noise. Magnetic field strengths
and inclinations deduced from VMI inversion are compared with results
from the inversion codes UNNOFIT and MERLIN. <BR /> Conclusions:
The VMI inversion method is insensitive to the dependence of Stokes I
profiles on the thermodynamic structure in nonmagnetic areas. In the
range of Bf products larger than 200 G, mean field strengths exceed
1000 G and there is not a very significant departure from the UNNOFIT
results because of differences between magnetic and nonmagnetic model
atmospheres. Further improvements might include additional parameters
deduced from the shape of Stokes V profiles and from large sets of
3D-MHD simulations, especially for unresolved magnetic flux tubes.
---------------------------------------------------------
Title: Interpreting Irradiance Distributions Using High-Resolution
3D MHD Simulations
Authors: Peck, Courtney; Rast, Mark; Criscuoli, Serena; Uitenbroek,
Han; Rempel, Matthias D.
2016SPD....4730302P Altcode:
We present initial results of studies aimed at understanding the
impact of the unresolved magnetic field distribution on solar spectral
irradiance. Using high-resolution 3D MHD simulations (from MURaM code)
and spectral synthesis (with the RH code), we examine the emergent
spectra of two atmospheres with similar mean field strengths but
differing imposed-field conditions at wavelengths spanning from
visible to infrared. Comparing the contrast against the magnetic
field strength for the two magnetic simulations, we find differences
in the distributions of contrasts versus field strength. We repeat
the analysis after convolving the images with the PSF of a typical
solar telescope (1-meter) and discuss the potential implications for
irradiance modeling and future steps.
---------------------------------------------------------
Title: The Effects of Magnetic Field Morphology on the Determination
of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
Criscuoli, Serena; Rast, Mark
2016AAS...22712501M Altcode:
The solar chemical abundance (or a scaled version of it) is
implemented in numerous astrophysical analyses. Thus, an accurate
and precise estimation of the solar elemental abundance is crucial
in astrophysics.We have explored the impact of magnetic fields
on the determination of the solar photospheric oxygen andiron
abundances using 3D radiation-magnetohydrodynamic (MHD) simulations
of convection. Specifically, weexamined differences in abundance
deduced from three classes of atmospheres simulated with the MURaM
code: apure hydrodynamic (HD) simulation, an MHD simulation with
a local dynamo magnetic field that has saturated withan unsigned
vertical field strength of 80 G at the optical depth unity surface,
and an MHD simulation with an initially imposed vertical mean field
of 80 G. We use differential equivalent width analysis for diagnosing
abundances derived from five oxygen and four iron spectral lines of
differing wavelength, oscillator strength, excitation potential, and
Lande g-factor, and find that the morphology of the magnetic field
is important to the outcome of abundance determinations. The largest
deduced abundance differences are found in the vertical mean field
simulations and small scale unresolved field resulting from the local
dynamo has a smaller impact on abundance determinations.
---------------------------------------------------------
Title: New Insights into White-Light Flare Emission from
Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
Authors: Kowalski, Adam F.; Hawley, S. L.; Carlsson, M.; Allred,
J. C.; Uitenbroek, H.; Osten, R. A.; Holman, G.
2015SoPh..290.3487K Altcode: 2015SoPh..tmp...61K; 2015arXiv150307057K
The heating mechanism at high densities during M-dwarf flares is
poorly understood. Spectra of M-dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum
components during the impulsive phase: 1) an energetically dominant
blackbody component with a color temperature of T ≈10<SUP>4</SUP>K
in the blue-optical, 2) a smaller amount of Balmer continuum emission
in the near-ultraviolet at λ ≤3 646 Å, and 3) an apparent
pseudo-continuum of blended high-order Balmer lines between λ =3
646 Å and λ ≈3 900 Å. These properties are not reproduced by
models that employ a typical "solar-type" flare heating level of
≤10<SUP>11</SUP>ergcm−<SUP>2</SUP>s−<SUP>1</SUP> in nonthermal
electrons, and therefore our understanding of these spectra is
limited to a phenomenological three-component interpretation. We
present a new 1D radiative-hydrodynamic model of an M-dwarf flare
from precipitating nonthermal electrons with a high energy flux of
10<SUP>13</SUP>ergcm−<SUP>2</SUP>s−<SUP>1</SUP>. The simulation
produces bright near-ultraviolet and optical continuum emission from a
dense (n >10<SUP>15</SUP>cm−<SUP>3</SUP>), hot (T ≈12 000 -13 500
K) chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T ≈10<SUP>4</SUP>K
blackbody-like continuum component and a low Balmer jump ratio result
from optically thick Balmer (∞ →n =2 ) and Paschen recombination
(∞ →n =3 ) radiation, and thus the properties of the flux spectrum
are caused by blue (λ ≈4 300 Å) light escaping over a larger
physical depth range than by red (λ ≈6 700 Å) and near-ultraviolet
(λ ≈3 500 Å) light. To model the near-ultraviolet pseudo-continuum
previously attributed to overlapping Balmer lines, we include the
extra Balmer continuum opacity from Landau-Zener transitions that
result from merged, high-order energy levels of hydrogen in a dense,
partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated
during dMe and solar flares.
---------------------------------------------------------
Title: Study of resonance scattering polarization in O i 130 nm lines
Authors: Anusha, L. S.; Nagendra, K. N.; Uitenbroek, Han
2015IAUS..305..234A Altcode:
Here we address the importance of frequency cross-redistribution on
the scattering polarization of the O i line at 130.2 nm. We compute
the polarized profiles of this line with ordinary partial frequency
redistribution and cross-redistribution using a two-dimensional
radiative transfer code.
---------------------------------------------------------
Title: Resonant Absorption of Transverse Oscillations and Associated
Heating in a Solar Prominence. II. Numerical Aspects
Authors: Antolin, P.; Okamoto, T. J.; De Pontieu, B.; Uitenbroek,
H.; Van Doorsselaere, T.; Yokoyama, T.
2015ApJ...809...72A Altcode: 2015arXiv150609108A
Transverse magnetohydrodynamic (MHD) waves are ubiquitous in
the solar atmosphere and may be responsible for generating the
Sun’s million-degree outer atmosphere. However, direct evidence
of the dissipation process and heating from these waves remains
elusive. Through advanced numerical simulations combined with
appropriate forward modeling of a prominence flux tube, we provide
the observational signatures of transverse MHD waves in prominence
plasmas. We show that these signatures are characterized by a
thread-like substructure, strong transverse dynamical coherence,
an out-of-phase difference between plane-of-the-sky motions and
line-of-sight velocities, and enhanced line broadening and heating
around most of the flux tube. A complex combination between resonant
absorption and Kelvin-Helmholtz instabilities (KHIs) takes place
in which the KHI extracts the energy from the resonant layer and
dissipates it through vortices and current sheets, which rapidly
degenerate into turbulence. An inward enlargement of the boundary
is produced in which the turbulent flows conserve the characteristic
dynamics from the resonance, therefore guaranteeing detectability of
the resonance imprints. We show that the features described in the
accompanying paper through coordinated Hinode and Interface Region
Imaging Spectrograph observations match the numerical results well.
---------------------------------------------------------
Title: Resonant Absorption of Transverse Oscillations and Associated
Heating in a Solar Prominence. I. Observational Aspects
Authors: Okamoto, Takenori J.; Antolin, Patrick; De Pontieu, Bart;
Uitenbroek, Han; Van Doorsselaere, Tom; Yokoyama, Takaaki
2015ApJ...809...71O Altcode: 2015arXiv150608965O
Transverse magnetohydrodynamic waves have been shown to be ubiquitous
in the solar atmosphere and can, in principle, carry sufficient energy
to generate and maintain the Sun’s million-degree outer atmosphere
or corona. However, direct evidence of the dissipation process of these
waves and subsequent heating has not yet been directly observed. Here we
report on high spatial, temporal, and spectral resolution observations
of a solar prominence that show a compelling signature of so-called
resonant absorption, a long hypothesized mechanism to efficiently
convert and dissipate transverse wave energy into heat. Aside
from coherence in the transverse direction, our observations show
telltale phase differences around 180° between transverse motions
in the plane-of-sky and line-of-sight velocities of the oscillating
fine structures or threads, and also suggest significant heating from
chromospheric to higher temperatures. Comparison with advanced numerical
simulations support a scenario in which transverse oscillations trigger
a Kelvin-Helmholtz instability (KHI) at the boundaries of oscillating
threads via resonant absorption. This instability leads to numerous
thin current sheets in which wave energy is dissipated and plasma is
heated. Our results provide direct evidence for wave-related heating
in action, one of the candidate coronal heating mechanisms.
---------------------------------------------------------
Title: First High-resolution Spectroscopic Observations of an Erupting
Prominence Within a Coronal Mass Ejection by the Interface Region
Imaging Spectrograph (IRIS)
Authors: Liu, Wei; De Pontieu, Bart; Vial, Jean-Claude; Title, Alan
M.; Carlsson, Mats; Uitenbroek, Han; Okamoto, Takenori J.; Berger,
Thomas E.; Antolin, Patrick
2015ApJ...803...85L Altcode: 2015arXiv150204738L
Spectroscopic observations of prominence eruptions associated with
coronal mass ejections (CMEs), although relatively rare, can provide
valuable plasma and three-dimensional geometry diagnostics. We report
the first observations by the Interface Region Imaging Spectrograph
mission of a spectacular fast CME/prominence eruption associated with
an equivalent X1.6 flare on 2014 May 9. The maximum plane-of-sky and
Doppler velocities of the eruption are 1200 and 460 km s<SUP>-1</SUP>,
respectively. There are two eruption components separated by ∼200
km s<SUP>-1</SUP> in Doppler velocity: a primary, bright component
and a secondary, faint component, suggesting a hollow, rather than
solid, cone-shaped distribution of material. The eruption involves
a left-handed helical structure undergoing counterclockwise (viewed
top-down) unwinding motion. There is a temporal evolution from upward
eruption to downward fallback with less-than-free-fall speeds and
decreasing nonthermal line widths. We find a wide range of Mg ii k/h
line intensity ratios (less than ∼2 expected for optically-thin
thermal emission): the lowest ever reported median value of 1.17
found in the fallback material, a comparably high value of 1.63 in
nearby coronal rain, and intermediate values of 1.53 and 1.41 in
the two eruption components. The fallback material exhibits a strong
(\gt 5σ ) linear correlation between the k/h ratio and the Doppler
velocity as well as the line intensity. We demonstrate that Doppler
dimming of scattered chromospheric emission by the erupted material
can potentially explain such characteristics.
---------------------------------------------------------
Title: Photon Mean Free Paths, Scattering, and Ever-Increasing
Telescope Resolution
Authors: Judge, P. G.; Kleint, L.; Uitenbroek, H.; Rempel, M.;
Suematsu, Y.; Tsuneta, S.
2015SoPh..290..979J Altcode: 2014arXiv1409.7866J; 2015SoPh..tmp....3J
We revisit an old question: what are the effects of observing stratified
atmospheres on scales below a photon mean free path λ? The mean free
path of photons emerging from the solar photosphere and chromosphere
is ≈ 10<SUP>2</SUP> km. Using current 1 m-class telescopes, λ is
on the order of the angular resolution. But the Daniel K. Inoue Solar
Telescope will have a diffraction limit of 0.020″ near the atmospheric
cutoff at 310 nm, corresponding to 14 km at the solar surface. Even
a small amount of scattering in the source function leads to physical
smearing due to this solar "fog", with effects similar to a degradation
of the telescope point spread function. We discuss a unified picture
that depends simply on the nature and amount of scattering in the
source function. Scalings are derived from which the scattering in the
solar atmosphere can be transcribed into an effective Strehl ratio,
a quantity useful to observers. Observations in both permitted (e.g.,
Fe I 630.2 nm) and forbidden (Fe I 525.0 nm) lines will shed light on
both instrumental performance as well as on small-scale structures in
the solar atmosphere.
---------------------------------------------------------
Title: RH 1.5D: a massively parallel code for multi-level radiative
transfer with partial frequency redistribution and Zeeman polarisation
Authors: Pereira, Tiago M. D.; Uitenbroek, Han
2015A&A...574A...3P Altcode: 2014arXiv1411.1079P
The emergence of three-dimensional magneto-hydrodynamic simulations of
stellar atmospheres has sparked a need for efficient radiative transfer
codes to calculate detailed synthetic spectra. We present RH 1.5D, a
massively parallel code based on the RH code and capable of performing
Zeeman polarised multi-level non-local thermodynamical equilibrium
calculations with partial frequency redistribution for an arbitrary
amount of chemical species. The code calculates spectra from 3D, 2D
or 1D atmospheric models on a column-by-column basis (or 1.5D). While
the 1.5D approximation breaks down in the cores of very strong lines
in an inhomogeneous environment, it is nevertheless suitable for
a large range of scenarios and allows for faster convergence with
finer control over the iteration of each simulation column. The code
scales well to at least tens of thousands of CPU cores, and is publicly
available. In the present work we briefly describe its inner workings,
strategies for convergence optimisation, its parallelism, and some
possible applications.
---------------------------------------------------------
Title: The Effects of Magnetic Field Morphology on the Determination
of Oxygen and Iron Abundances in the Solar Photosphere
Authors: Moore, Christopher S.; Uitenbroek, Han; Rempel, Matthias;
Criscuoli, Serena; Rast, Mark P.
2015ApJ...799..150M Altcode:
We have explored the impact of magnetic fields on the determination
of the solar photospheric oxygen and iron abundances using
three-dimensional radiation-magnetohydrodynamic (MHD) simulations
of convection. Specifically, we examined differences in abundance
deduced from three classes of atmospheres simulated with the MURaM
code: a pure hydrodynamic (HD) simulation, an MHD simulation with
a local dynamo magnetic field that has saturated with an unsigned
vertical field strength of 80 G at τ = 1, and an MHD simulation with
an initially imposed vertical mean field of 80 G. We use differential
equivalent width analysis for diagnosing abundances derived from
five oxygen and four iron lines of differing wavelength, oscillator
strength, excitation potential, and Landé g-factor, and find that
the morphology of the magnetic field is important to the outcome of
abundance determinations. The largest deduced abundance differences are
found in the vertical mean field simulations, where the O I and Fe I
abundance corrections compared to the pure HD case are ~+0.011 dex and
+0.065 dex respectively. Small scale unresolved field resulting from
the local dynamo has a smaller impact on abundance determinations,
with corrections of -0.0001 dex and +0.0044 dex in the magnetized
compared to the pure HD simulations. While the overall influence of
magnetic field on abundance estimates is found to be small, we stress
that such estimates are sensitive not only to the magnitude of magnetic
field but also to its morphology.
---------------------------------------------------------
Title: RH 1.5D: Polarized multi-level radiative transfer with partial
frequency distribution
Authors: Pereira, Tiago M. D.; Uitenbroek, Han
2015ascl.soft02001P Altcode:
RH 1.5D performs Zeeman multi-level non-local thermodynamical
equilibrium calculations with partial frequency redistribution for
an arbitrary amount of chemical species. Derived from the RH code
and written in C, it calculates spectra from 3D, 2D or 1D atmospheric
models on a column-by-column basis (or 1.5D). It includes optimization
features to speed up or improve convergence, which are particularly
useful in dynamic models of chromospheres. While one should be aware
of its limitations, the calculation of spectra using the 1.5D or
column-by-column is a good approximation in many cases, and generally
allows for faster convergence and more flexible methods of improving
convergence. RH 1.5D scales well to at least tens of thousands of
CPU cores.
---------------------------------------------------------
Title: Observational Evidence of Resonant Absorption in Oscillating
Prominence
Authors: Okamoto, J.; Antolin, P.; De Pontieu, B.; Uitenbroek, H.;
Van Doorsselaere, T.; Yokoyama, T.
2014AGUFMSH12A..05O Altcode:
Coronal heating and the acceleration of the solar wind are unsolved
problems in solar physics. The propagation of Alfven waves along
magnetic field lines is one of the candidate mechanisms for
carrying energy to large distances from the surface and heat the
coronal plasma. However, the dissipation process is still unclear
in observational aspects.The new NASA's solar physics satellite IRIS
(Interface Region Imaging Spectrograph) provides spectral information of
plasma in the chromosphere and transition region with high-spatial and
high-temporal resolution. Hence, we performed observations of a limb
prominence to find evidence and clues of dissipation in collaboration
with Hinode/SOT and SDO/AIA.In our observations, we found a clear
evidence of resonant absorption that takes place on the surface of
the oscillating prominence flux tubes. This mechanism facilitates
the onset of the Kelvin-Helmholtz instability, which deforms the thin
tube's boundaries and generates thin current sheets and turbulence,
leading to dissipation of the wave energy into heat. In this talk, we
will show the observed phenomena and discuss the dissipation mechanism
compared with numerical simulations of an oscillating prominence.
---------------------------------------------------------
Title: First High-resolution Spectroscopic Observations by IRIS
of a Fast, Helical Prominence Eruption Associated with a Coronal
Mass Ejection
Authors: Liu, W.; De Pontieu, B.; Okamoto, T. J.; Vial, J. C.; Title,
A. M.; Antolin, P.; Berger, T. E.; Uitenbroek, H.
2014AGUFMSH11D..04L Altcode:
High-resolution spectroscopic observations of prominence eruptions and
associated coronal mass ejections (CMEs) are rare but can provide
valuable plasma and energy diagnostics. New opportunities have
recently become available with the advent of the Interface Region
Imaging Spectrograph (IRIS) mission equipped with high resolution of
0.33-0.4 arcsec in space and 1 km/s in velocity, together with the
Hinode Solar Optical Telescope of 0.2 arcsec spatial resolution. We
report the first result of joint IRIS-Hinode observations of a
spectacular prominence eruption occurring on 2014-May-09. IRIS
detected a maximum redshift of 450 km/s, which, combined with the
plane-of-sky speed of 800 km/s, gives a large velocity vector of 920
km/s at 30 degrees from the sky plane. This direction agrees with the
source location at 30 degrees behind the limb observed by STEREO-A
and indicates a nearly vertical ejection. We found two branches of
redshifts separated by 200 km/s appearing in all strong lines at
chromospheric to transition-region temperatures, including Mg II k/h,
C II, and Si IV, suggesting a hollow, rather than solid, cone in the
velocity space of the ejected material. Opposite blue- and redshifts
on the two sides of the prominence exhibit corkscrew variations both
in space and time, suggestive of unwinding rotations of a left-handed
helical flux rope. Some erupted material returns as nearly streamline
flows, exhibiting distinctly narrow line widths (~10 km/s), about
50% of those of the nearby coronal rain at the apexes of coronal
loops, where the rain material is initially formed out of cooling
condensation. We estimate the mass and kinetic energy of the ejected
and returning material and compare them with those of the associated
CME. We will discuss the implications of these observations for CME
initiation mechanisms.
---------------------------------------------------------
Title: Non Coherent Continuum Scattering as a Polarization Mechanism
of the Enigmatic Ba <font size=2>II D<SUB>1</SUB> Line
Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Uitenbroek, H.
2014ASPC..489..107D Altcode:
Line scattering polarization can be strongly affected by Rayleigh
scattering by neutral hydrogen and Thomson scattering by free
electrons. The assumption that continuum polarization can be modeled
as coherent scattering, an excellent approximation far from the
spectral line, yields a continuum depolarization when applied to an
intrinsically unpolarizable spectral line. However, the radiation field
is not always constant over the spectral line and continuum scattering
has to be treated non-coherently. A recent investigation showed that the
redistribution of the spectral line radiation due to the non coherence
of the continuum scattering can significantly modify the shape of
the emergent fractional linear polarization profiles, even yielding
emission Q/I features in intrinsically unpolarizable lines. Here we
show an application to the enigmatic D<SUB>1</SUB> line of Ba <font
size=2>II at 4934 Å, neglecting the hyperfine structure of the 18%
of the barium isotopes whose nuclear spin is non-zero. We show that with
this assumption Q/I signals above the continuum polarization level can
be produced in solar atmospheric models representative of polar faculae.
---------------------------------------------------------
Title: Effect of Cross-redistribution on the Resonance Scattering
Polarization of O I Line at 1302 Å
Authors: Anusha, L. S.; Nagendra, K. N.; Uitenbroek, H.
2014ApJ...794...17A Altcode: 2014arXiv1407.8456A
Oxygen is the most abundant element on the Sun after hydrogen
and helium. The intensity spectrum of resonance lines of neutral
oxygen, namely, O I (1302, 1305, and 1306 Å), has been studied
in the literature for chromospheric diagnostics. In this paper,
we study the resonance scattering polarization in the O I line at
1302 Å using two-dimensional (2D) radiative transfer in a composite
atmosphere constructed using a 2D magneto-hydrodynamical snapshot in
the photosphere and columns of the one-dimensional FALC atmosphere in
the chromosphere. The methods developed by us recently in a series
of papers to solve multi-dimensional polarized radiative transfer
have been incorporated in our new code POLY2D, which we use for our
analysis. We find that multi-dimensional radiative transfer including
XRD effects is important in reproducing the amplitude and shape of
scattering polarization signals of the O I line at 1302 Å.
---------------------------------------------------------
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
2014SoPh..289.2733D Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s<SUP>−1</SUP> velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.
---------------------------------------------------------
Title: A New Solar Fluorine Abundance and a Fluorine Determination
in the Two Open Clusters M67 and NGC 6404
Authors: Maiorca, E.; Uitenbroek, H.; Uttenthaler, S.; Randich, S.;
Busso, M.; Magrini, L.
2014ApJ...788..149M Altcode: 2014arXiv1404.5755M
We present a new determination of the solar fluorine abundance
together with abundance measurements of fluorine in two Galactic open
clusters. We analyzed a sunspot spectrum, observed by L. Wallace
and W. Livingston with the Fourier Transform Spectrometer at the
McMath/Pierce Solar Telescope situated on Kitt Peak, and spectra of four
giants in the old cluster M67 (~4.5 Gyr) and three giants in the young
cluster NGC 6404 (~0.5 Gyr), obtained with the CRIRES spectrograph at
the Very Large Telescope. Fluorine was measured through the synthesis
of the available HF lines. We adopted the recent set of experimental
molecular parameters of HF delivered by the HITRAN database, and found a
new solar fluorine abundance of A(F) = 4.40 ± 0.25, in good agreement
with the M67 average fluorine abundance of A(F) = 4.49 ± 0.20. The
new solar abundance is in a very good agreement with the meteoritic
value. The modern spectrosynthesis tools used and the agreement with
the meteoritic value and with the results in open cluster M67, known
to be a solar analogue, make our solar determination very robust. At
the same time, the fluorine measurement in the above-mentioned open
clusters is the first step toward understanding its evolution during the
last ~10 Gyr in the Galactic disk. In order to develop this project,
a larger sample of open clusters is required, in order to allow us to
trace the evolution of fluorine as a function of time and, in turn,
to better understand its origin.
---------------------------------------------------------
Title: Interpretation of Solar Irradiance Monitor Measurements
through Analysis of 3D MHD Simulations
Authors: Criscuoli, S.; Uitenbroek, H.
2014ApJ...788..151C Altcode: 2014arXiv1404.4651C
Measurements from the Spectral Irradiance Monitor (SIM) on board the
Solar Radiation and Climate Experiment mission indicate that solar
spectral irradiance at visible and IR wavelengths varies in counter
phase with the solar activity cycle. The sign of these variations is not
reproduced by most of the irradiance reconstruction techniques based on
variations of surface magnetism employed so far, and it is not yet clear
whether SIM calibration procedures need to be improved or if instead
new physical mechanisms must be invoked to explain such variations. We
employ three-dimensional magnetohydrodynamic simulations of the solar
photosphere to investigate the dependence of solar radiance in SIM
visible and IR spectral ranges on variations of the filling factor of
surface magnetic fields. We find that the contribution of magnetic
features to solar radiance is strongly dependent on the location on
the disk of the features, which are negative close to disk center and
positive toward the limb. If features are homogeneously distributed over
a region around the equator (activity belt), then their contribution
to irradiance is positive with respect to the contribution of HD
snapshots, but decreases with the increase of their magnetic flux for
average magnetic flux larger than 50 G in at least two of the visible
and IR spectral bands monitored by SIM. Under the assumption that the
50 G snapshots are representative of quiet-Sun regions, we thus find
that the Spectral Irradiance can be in counter-phase with the solar
magnetic activity cycle.
---------------------------------------------------------
Title: The statistical distribution of the magnetic-field strength
in G-band bright points
Authors: Criscuoli, S.; Uitenbroek, H.
2014A&A...562L...1C Altcode: 2013arXiv1312.2611C
Context. G-band bright points are small-sized features characterized
by high photometric contrast. Theoretical investigations indicate
that these features have associated magnetic-field strengths of 1
to 2 kG. Results from observations, however, lead to contradictory
results, indicating magnetic fields of only kG strength in some and
including hG strengths in others. <BR /> Aims: To understand the
differences between measurements reported in the literature, and to
reconcile them with results from theory, we analyzed the distribution
of the magnetic-field strength of G-band bright features identified
in synthetic images of the solar photosphere and its sensitivity
to observational and methodological effects. <BR /> Methods: We
investigated the dependence of magnetic-field strength distributions of
G-band bright points identified in 3D magnetohydrodynamic simulations
on feature selection method, data sampling, alignment, and spatial
resolution. <BR /> Results: The distribution of the magnetic-field
strength of G-band bright features shows two peaks, one at about 1.5
kG and one below 1 hG. The former corresponds to magnetic features,
the second mostly to bright granules. Peaks at several hG are obtained
only on spatially degraded or misaligned data. <BR /> Conclusions:
Simulations show that magnetic G-band bright points have typically
associated field strengths of a few kG. Field strengths in the hG
range can result from observational effects, which explains the
discrepancies presented in the literature. Our results also indicate
that results from spectro-polarimetric inversions with an imposed unit
filling-factor should be employed with great caution.
---------------------------------------------------------
Title: The Atmospheric Response to High Fluxes of Nonthermal Electrons
during M Dwarf Flares
Authors: Kowalski, Adam; Allred, J. C.; Carlsson, M.; Hawley, S. L.;
Holman, G. D.; Mathioudakis, M.; Osten, R. A.; Uitenbroek, H.
2014AAS...22315117K Altcode:
Flares are thought to be the result of magnetic fields in the stellar
corona that undergo reconnection and accelerate charged particles
into the lower atmosphere. Spectra of M dwarf flares in the optical
and near-ultraviolet wavelength regimes can be used to constrain the
heating mechanism of the lower stellar atmosphere. These observations
show several ubiquitous properties of the continuum emission, which
is not reproduced by models that use typical “solar-type” heating
functions. We present results from a grid of new flare models using the
RADYN code, which simultaneously calculates the radiative transfer and
hydrodynamics on short timescales. We explore the atmospheric response
to a short ~2 second burst of a very high heating rate from nonthermal
electrons using a solar-type heating function, and we propose a new
“M dwarf-type” heating variation that explains a range of observed
spectral properties, such as ~10,000 K blackbody emission and a smooth
continuum across the Balmer jump wavelength (3646A).
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. III. Near-ultraviolet
Spectra and Images
Authors: Pereira, T. M. D.; Leenaarts, J.; De Pontieu, B.; Carlsson,
M.; Uitenbroek, H.
2013ApJ...778..143P Altcode: 2013arXiv1310.1926P
The Mg II h&k lines are the prime chromospheric diagnostics
of NASA's Interface Region Imaging Spectrograph (IRIS). In the
previous papers of this series, we used a realistic three-dimensional
radiative magnetohydrodynamics model to calculate the h&k lines
in detail and investigated how their spectral features relate to the
underlying atmosphere. In this work, we employ the same approach to
investigate how the h&k diagnostics fare when taking into account
the finite resolution of IRIS and different noise levels. In addition,
we investigate the diagnostic potential of several other photospheric
lines and near-continuum regions present in the near-ultraviolet
(NUV) window of IRIS and study the formation of the NUV slit-jaw
images. We find that the instrumental resolution of IRIS has a small
effect on the quality of the h&k diagnostics; the relations
between the spectral features and atmospheric properties are mostly
unchanged. The peak separation is the most affected diagnostic, but
mainly due to limitations of the simulation. The effects of noise
start to be noticeable at a signal-to-noise ratio (S/N) of 20, but we
show that with noise filtering one can obtain reliable diagnostics at
least down to a S/N of 5. The many photospheric lines present in the
NUV window provide velocity information for at least eight distinct
photospheric heights. Using line-free regions in the h&k far wings,
we derive good estimates of photospheric temperature for at least
three heights. Both of these diagnostics, in particular the latter,
can be obtained even at S/Ns as low as 5.
---------------------------------------------------------
Title: Ellerman Bombs—Evidence for Magnetic Reconnection in the
Lower Solar Atmosphere
Authors: Nelson, C. J.; Shelyag, S.; Mathioudakis, M.; Doyle, J. G.;
Madjarska, M. S.; Uitenbroek, H.; Erdélyi, R.
2013ApJ...779..125N Altcode: 2013arXiv1310.7756N
The presence of photospheric magnetic reconnection has long been thought
to give rise to short and impulsive events, such as Ellerman bombs (EBs)
and Type II spicules. In this article, we combine high-resolution,
high-cadence observations from the Interferometric BIdimensional
Spectrometer and Rapid Oscillations in the Solar Atmosphere instruments
at the Dunn Solar Telescope, National Solar Observatory, New Mexico,
with co-aligned Solar Dynamics Observatory Atmospheric Imaging Assembly
and Hinode Solar Optical Telescope (SOT) data to observe small-scale
events situated within an active region. These data are then compared
with state-of-the-art numerical simulations of the lower atmosphere
made using the MURaM code. It is found that brightenings, in both the
observations and the simulations, of the wings of the Hα line profile,
interpreted as EBs, are often spatially correlated with increases in
the intensity of the Fe I λ6302.5 line core. Bipolar regions inferred
from Hinode/SOT magnetic field data show evidence of flux cancellation
associated, co-spatially, with these EBs, suggesting that magnetic
reconnection could be a driver of these high-energy events. Through
the analysis of similar events in the simulated lower atmosphere,
we are able to infer that line profiles analogous to the observations
occur co-spatially with regions of strong opposite-polarity magnetic
flux. These observed events and their simulated counterparts are
interpreted as evidence of photospheric magnetic reconnection at scales
observable using current observational instrumentation.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. II. The Formation of the
Mg II h&k Lines in the Solar Atmosphere
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...90L Altcode: 2013arXiv1306.0671L
NASA's Interface Region Imaging Spectrograph (IRIS) small explorer
mission will study how the solar atmosphere is energized. IRIS
contains an imaging spectrograph that covers the Mg II h&k lines
as well as a slit-jaw imager centered at Mg II k. Understanding
the observations requires forward modeling of Mg II h&k line
formation from three-dimensional (3D) radiation-magnetohydrodynamic
(RMHD) models. This paper is the second in a series where we undertake
this modeling. We compute the vertically emergent h&k intensity
from a snapshot of a dynamic 3D RMHD model of the solar atmosphere,
and investigate which diagnostic information about the atmosphere is
contained in the synthetic line profiles. We find that the Doppler
shift of the central line depression correlates strongly with the
vertical velocity at optical depth unity, which is typically located
less than 200 km below the transition region (TR). By combining the
Doppler shifts of the h and k lines we can retrieve the sign of the
velocity gradient just below the TR. The intensity in the central line
depression is anti-correlated with the formation height, especially
in subfields of a few square Mm. This intensity could thus be used to
measure the spatial variation of the height of the TR. The intensity
in the line-core emission peaks correlates with the temperature at
its formation height, especially for strong emission peaks. The peaks
can thus be exploited as a temperature diagnostic. The wavelength
difference between the blue and red peaks provides a diagnostic of the
velocity gradients in the upper chromosphere. The intensity ratio of
the blue and red peaks correlates strongly with the average velocity
in the upper chromosphere. We conclude that the Mg II h&k lines
are excellent probes of the very upper chromosphere just below the
TR, a height regime that is impossible to probe with other spectral
lines. They also provide decent temperature and velocity diagnostics
of the middle chromosphere.
---------------------------------------------------------
Title: The Formation of IRIS Diagnostics. I. A Quintessential
Model Atom of Mg II and General Formation Properties of the Mg II
h&k Lines
Authors: Leenaarts, J.; Pereira, T. M. D.; Carlsson, M.; Uitenbroek,
H.; De Pontieu, B.
2013ApJ...772...89L Altcode: 2013arXiv1306.0668L
NASA's Interface Region Imaging Spectrograph (IRIS) space mission will
study how the solar atmosphere is energized. IRIS contains an imaging
spectrograph that covers the Mg II h&k lines as well as a slit-jaw
imager centered at Mg II k. Understanding the observations will require
forward modeling of Mg II h&k line formation from three-dimensional
(3D) radiation-MHD models. This paper is the first in a series where we
undertake this forward modeling. We discuss the atomic physics pertinent
to h&k line formation, present a quintessential model atom that can
be used in radiative transfer computations, and discuss the effect of
partial redistribution (PRD) and 3D radiative transfer on the emergent
line profiles. We conclude that Mg II h&k can be modeled accurately
with a four-level plus continuum Mg II model atom. Ideally radiative
transfer computations should be done in 3D including PRD effects. In
practice this is currently not possible. A reasonable compromise is to
use one-dimensional PRD computations to model the line profile up to
and including the central emission peaks, and use 3D transfer assuming
complete redistribution to model the central depression.
---------------------------------------------------------
Title: Testing Milne-Eddington Inversion Codes Against One-Dimensional
Model Atmospheres
Authors: Lastufka, Erica; Jaeggli, S. A.; Kankelborg, C.; Uitenbroek,
H.
2013SPD....44..116L Altcode:
Properties of solar vector magnetic fields can be determined by the
inversion of polarization spectra. It is therefore important to have
accurate inversion methods. Milne-Eddington inversions, used almost
exclusively in the photosphere, assume a thin, flat atmosphere and are
one of the most widely used inversion techniques. To investigate the
potential weaknesses of parameterizing a stratified atmosphere using
a single set of properties, we examine the consequences of using a
Milne-Eddington inversion to invert spectra of complex atmospheres. Han
Uitenbroek's Rybicki-Hummer radiative transfer and chemical equilibrium
code was used to generate a series of one-dimensional model atmospheres
with predetermined magnetic field configurations. Atmospheres at the
quiet Sun temperature contained magnetic fields with strengths up
3000 G and inclination and azimuthal angles from 0 to 180 degrees. We
examined the Stokes profiles of the Fe 15648.5 line, which with a
Landé g-factor of 3.0 is very sensitive to the magnetic field. Using a
simple Milne-Eddington inversion code, we examined the ranges in which
the code accurately parameterized the magnetic field. To investigate
the confidence intervals associated with the inverted parameters, we
used the BayesME code developed by Andres Asensio Ramos. We discuss
the key assumptions and limitations of a Milne-Eddington inversion.
---------------------------------------------------------
Title: On Possible Variations of Basal Ca II K Chromospheric Line
Profiles with the Solar Cycle
Authors: Pevtsov, Alexei A.; Bertello, Luca; Uitenbroek, Han
2013ApJ...767...56P Altcode:
We use daily observations of the Ca II K line profiles of the
Sun-as-a-star taken with the Integrated Sunlight Spectrometer from 2006
December through 2011 July to deconvolve the contributions from the
quiet (basal) chromosphere and with magnetic network/plage areas. The
0.5 Å emission index computed from basal profiles shows a significantly
reduced modulation (as compared with one derived from the observed
profiles) corresponding to the Sun's rotation. For basal contribution
of the Ca II K line, the peak in power spectrum corresponding to solar
rotation is broad and not well defined. Power spectra for the plage
contribution show two narrow well-defined peaks corresponding to solar
rotation at two distinct latitudes, in agreement with the latitudinal
distribution of activity on the Sun at the end of Cycle 23 and beginning
of Cycle 24. We use the lack of a signature of solar rotation in the
basal (quiet Sun) component as an indication of a successful removal
of the active Sun (plage) component. Even though the contribution
from solar activity is removed from the basal line profiles, we find
a weak dependency of intensity in the line core (K3) of basal profiles
with the phase of the solar cycle. Such dependency could be the result
of changes in thermal properties of basal chromosphere with the solar
cycle. As an alternative explanation, we also discuss a possibility that
the basal component does not change with the phase of the solar cycle.
---------------------------------------------------------
Title: Effects of Unresolved Magnetic Field on Fe I 617.3 and 630.2
nm Line Shapes
Authors: Criscuoli, S.; Ermolli, I.; Uitenbroek, H.; Giorgi, F.
2013ApJ...763..144C Altcode: 2012arXiv1212.2190C
The contribution of the quiet Sun to solar irradiance variability,
either due to changes of the solar effective temperature or
to the presence of unresolved magnetic field, is still poorly
understood. In this study we investigate spectral line diagnostics
that are sensitive to both temperature variations and the presence of
small-scale unresolved magnetic features in these areas of the solar
atmosphere. Specifically, we study the dependence on the magnetic flux
density of three parameters describing the shape of two magnetically
sensitive Fe I lines, at 630.2 nm and 617.3 nm, namely the line core
intensity (IC), the FWHM, and the equivalent width (EQW). To this end
we analyze observations of active region NOAA 11172, acquired with
Interferometric Bidimensional Spectrometer at the Dunn Solar Telescope,
as well as results from numerical synthesis. Our results show that IC
is sensitive to both temperature and magnetic flux density variations,
FWHM is mostly affected by magnetic field changes, and EQW is mostly
sensitive to temperature. Variations of a few percent of the measured
line parameters are found in observational data that were spatially
degraded to represent quiet-Sun, disk-center, medium-resolution
observations. It is therefore possible to disentangle magnetic from
pure thermodynamic effects by the comparison of temporal variations
of the EQW and the FWHM of either of the two Fe I lines.
---------------------------------------------------------
Title: A novel method to estimate temperature gradients in stellar
photospheres.
Authors: Uitenbroek, H.; Criscuoli, S.
2013MmSAI..84..369U Altcode:
Inversions utilizing one-dimensional atmospheric models provide
information about the thermal stratification of stars, but these
models are in general not unique nor sufficiently descriptive of the
physical conditions of a star. Here we propose a novel model-independent
method to better constrain the temperature stratification in a stellar
atmosphere. In our method we employ intensities measured at opacity
conjugate wavelength pairs to improve the estimate of temperature
stratification that is obtained from radiation temperatures in
combination with the Eddington-Barbier relation. This relation can
lead to significant errors because of the non-linear dependence of the
source function on optical depth, even in the case of continua. Such
errors are substantially reduced by combining observations at pairs of
conjugate continua, which have the same H<SUP>-</SUP> opacity between
them, and therefore pairwise form at the same height.
---------------------------------------------------------
Title: On the sensitivity of FeI 617.3 and 630.2 nm line shapes to
unresolved magnetic fields
Authors: Criscuoli, S.; Ermolli, I.; Uitenbroek, H.; Giorgi, F.
2013MmSAI..84..335C Altcode:
Our study was aimed at obtaining line diagnostics sensitive to
effects of small scale magnetic features that are unresolved in
observations. We studied the dependence on the magnetic flux of
parameters describing the two Fe I lines at 630.2 and 617.3 nm. In
particular, we analyzed the line core intensity (IC), full width
half maximum (FWHM), and equivalent width (EQW) of Stokes I in NOAA
11172 observed with IBIS at the Dunn Solar Telescope on March 17th,
2011. Our results show that IC is sensitive to both temperature and
magnetic flux variations, while FWHM is sensitive mostly to magnetic
flux variations. The EQW is almost insensitive to magnetic flux and
mostly sensitive to temperature. Variations of a few percents of line
parameters are found in data spatially degraded to represent quiet
Sun, disk-centre conditions in medium resolution observations. Such
variations can be observed with instruments as SOLIS/VSM, SDO/HMI,
HINODE/SOT. Shapes of investigated lines can therefore be employed to
investigate physical properties of quiet Sun regions, and in particular
to disentangle magnetic and thermodynamic effects an d their variations
over the magnetic cycle.
---------------------------------------------------------
Title: Eyes on the Sun: Solar Instrumentation
Authors: Uitenbroek, H.
2013ASPC..470...83U Altcode:
Solar Physics is at the threshold of a new era of high spatial
resolution observations with a number of large aperture facilities
coming on-line. These facilities will allow us to come closer to
resolving phenomena that result from the interaction of plasma with
magnetic fields at their natural spatial and temporal scales, an
opportunity that is unique in astrophysics. In this paper I review
what makes solar telescopes, special, what a typical solar telescope
looks like, and how future facilities will be constructed. The design
and development of the Utrecht built Dutch Open Telescope (DOT) has
contributed significantly to the new directions that the designs of
these new facilities have taken.
---------------------------------------------------------
Title: The Sunspot Penumbra in the Photosphere: Results from Forward
Synthesized Spectroscopy
Authors: Tritschler, A.; Uitenbroek, H.; Rempel, M.
2012ASPC..463...89T Altcode:
We present first results from a spectral synthesis of the
Zeeman-insensitive Fe 1 557.6 nm line for two different viewing angles
(0° and 30°) using numerical simulations of a sunspot as an input
model. We performed a bisector analysis to calculate two-dimensional
maps of line-of-sight Doppler velocities and the line width. We analyze
azimuthal cuts of the LOS velocity at different penumbral radii and
calculate the radial behavior of azimuthal averages of line width and
intensity. Both are compared with observational results. The properties
of dark cores in penumbral filaments are discussed briefly. Within the
limitations of this study, we find that the results from the forward
synthesized spectroscopy are in good agreement with the observations,
corroborating that the photospheric structure and dynamics of the
penumbra is a signature of overturning anisotropic magneto-convection.
---------------------------------------------------------
Title: The Influence of Molecular Lines on the Measurement of
Photospheric Velocities
Authors: Uitenbroek, H.; Dumont, N.; Tritschler, A.
2012ASPC..463...99U Altcode:
We use different solar atmospheric models to investigate the influence
of molecular lines on the accuracy with which line-of-sight velocities
of mass flows can be measured from Doppler shifts. Particularly in
relatively cool atmospheres molecules become more abundant and give
rise to a thick forest of perturbing lines. Using bisectors we estimate
the apparent shift introduced in the positions of the C I 538.0 nm
and Fe I 557.6 nm lines by molecular lines in one-dimensional models
with different effective temperatures, ranging from 3750 K to 6250 K,
and in a three-dimensional section from a solar magneto-convection
simulation. We find that the core of the iron line is mostly unaffected,
and that by contrast the carbon line is severely compromised by
molecular lines, even in environments with effective temperatures
similar to those in the quiet Sun.
---------------------------------------------------------
Title: The Visible Broadband Imager: The Sun at High Spatial and
Temporal Resolution
Authors: Wöger, F.; McBride, W.; Ferayorni, A.; Gregory, S.; Hegwer,
S.; Tritschler, A.; Uitenbroek, H.
2012ASPC..463..431W Altcode:
The Visible Broadband Imager (VBI) will be the primary first-light
instrument for the Advanced Technology Solar Telescope (ATST). It is
designed to observe the solar atmosphere at heights ranging from the
photosphere to chromosphere. High frame-rate detectors that sample
the FOV of up to 2.8 arcmin in diameter critically at the diffraction
limit of ATST's 4 meter aperture will provide near real-time speckle
reconstruction imaging. With its focus on high-spatial resolution, the
VBI will be addressing scientific questions related to the smallest
structures visible in the solar atmosphere with high photometric
precision. The capability to observe the solar atmosphere with a
cadence of about 3 seconds per reconstructed image will enable the VBI
to temporally resolve fast evolving structures. In this contribution we
present the major aspects of the current design of the VBI and highlight
some scientific questions related to fast evolving, small-scale features
within the solar atmosphere that the VBI will address.
---------------------------------------------------------
Title: Observing strategies for future solar facilities: the ATST
test case
Authors: Uitenbroek, H.; Tritschler, A.
2012IAUSS...6E.401U Altcode:
Traditionally solar observations have been scheduled and performed
very differently from night time efforts, in particular because we have
been observing the Sun for a long time, requiring new combinations of
observables to make progress, and because solar physics observations
are often event driven on time scales of hours to days. With the
proposal pressure that is expected for new large-aperture facilities,
we can no longer afford the time spent on custom setups, and will have
to rethink our scheduling and operations. We will discuss our efforts
at Sac Peak in preparing for this new era, and outline the planned
scheduling and operations planning for the ATST in particular.
---------------------------------------------------------
Title: Fast approximation of angle-dependent partial redistribution
in moving atmospheres
Authors: Leenaarts, J.; Pereira, T.; Uitenbroek, H.
2012A&A...543A.109L Altcode: 2012arXiv1205.5110L
<BR /> Aims: Radiative transfer modeling of spectral lines including
partial redistribution (PRD) effects requires the evaluation of
the ratio of the emission to the absorption profile. This quantity
requires a large amount of computational work if one employs the
angle-dependent redistribution function, which prohibits its use in
3D radiative transfer computations with model atmospheres containing
velocity fields. We aim to provide a method to compute the emission
to absorption profile ratio that requires less computational work but
retains the effect of angle-dependent scattering in the resulting line
profiles. <BR /> Methods: We present a method to compute the profile
ratio that employs the angle-averaged redistribution function and
wavelength transforms to and from the rest frame of the scattering
particles. We compare the emergent line profiles of the Mg II k and
Lyα lines computed with angle-dependent PRD, angle-averaged PRD and
our new method in two representative test atmospheres. <BR /> Results:
The new method yields a good approximation of true angle-dependent
profile ratio and the resulting emergent line profiles while keeping
the computational speed and simplicity of angle-averaged PRD theory.
---------------------------------------------------------
Title: The Imaging Vector Magnetograph at Haleakalā IV: Stokes
Polarization Spectra in the Sodium D<SUB>1</SUB> 589.6 nm Spectral
Line
Authors: Leka, K. D.; Mickey, Donald L.; Uitenbroek, Han; Wagner,
Eric L.; Metcalf, Thomas R.
2012SoPh..278..471L Altcode:
The Imaging Vector Magnetograph (IVM) at the Mees Solar Observatory,
Haleakalā, Maui, Hawai'i, obtained many years of vector magnetic-field
data in the photospheric Fe I 630.25 nm line. In the latter period
of its operation, the IVM was modified to allow routine observations
in the chromospheric Na I D<SUB>1</SUB> line, as well as the Fe I
line. We describe the sodium observational data in detail, including the
data-reduction steps that differ from those employed for the Fe I 630.25
nm line, to obtain calibrated Stokes polarization spectra. We have
performed a systematic comparison between the observational data and
synthetic NLTE Na I D<SUB>1</SUB> Stokes spectra derived for a variety
of solar-appropriate atmospheric and magnetic configurations. While
the Na I D<SUB>1</SUB> Stokes polarization signals from the solar
atmosphere are expected to be weak, they should generally be within the
IVM capability. A comparison between synthetic spectra and observational
data indicates that this is indeed the case.
---------------------------------------------------------
Title: Inversion of Zeeman Line Profiles Using Central Moments
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2012EAS....55...83M Altcode:
A new inversion method derived from central moments of Zeeman line
profiles (ICM), is used to determine magnetic field vectors (Mein et
al. 2011). Two quantities A<SUB>1</SUB> and A<SUB>2</SUB> combining
moments of profiles I ± S (S = Q,U,V) are nearly linear functions
of the longitudinal and transverse components and lead to the field
components through very fast iterations. Optimized exponents reduce
noise effects. The ICM inversion does not require Milne Eddington
approximation and can be used in a wide range of solar models.
---------------------------------------------------------
Title: Three-dimensional Radiative Transfer applied to the Diagnostics
of Magnetic Fields
Authors: Uitenbroek, H.
2012EAS....55...35U Altcode:
With three examples of forward modeling of spectral intensity formation
we illustrate some of the difficulties encountered in the use of spectra
for the determination of physical properties of highly structured
magnetized plasmas in general, and the Sun in particular. We demonstrate
that the average spectrum of a structured atmosphere cannot be use
for an accurate determination of the atmosphere's average properties,
show that why it is difficult to measure the chromospheric magnetic
vector field from Zeeman polarization, and make clear from forward
modeling of spectral lines that it will be hard to fully recover the
physical structure of a sunspot.
---------------------------------------------------------
Title: Comparison of Multi-Height Observations with a 3D MHD
Sunspot Model
Authors: Jaeggli, S. A.; Lin, H.; Uitenbroek, H.; Rempel, M.
2012ASPC..456...67J Altcode:
In sunspots the contribution to the horizontal pressure support from the
curvature force and the geometrical height of formation which magnetic
field measurements sample are poorly constrained observationally due to
the effect of radiative transfer. In cool atmospheres, observations of
the sunspot photosphere probe geometrically deeper layers, information
on the magnetic field gradients cannot be easily derived even using
multi-wavelength, multi-height observations. Recent MHD atmosphere
models of sunspots analyzed with the Rybiki-Hummer radiative transfer
code allow for direct comparison with simultaneous multi-height
observations of the Fe I magnetic field diagnostics at 1565 and 630.2
nm in sunspots observed using the Facility Infrared Spectropolarimeter
at the Dunn Solar Telescope.
---------------------------------------------------------
Title: Chromospheric backradiation in ultraviolet continua and Hα
Authors: Rutten, R. J.; Uitenbroek, H.
2012A&A...540A..86R Altcode: 2012arXiv1203.0396R
A recent paper states that ultraviolet backradiation from the solar
transition region and upper chromosphere strongly affects the degree of
ionization of minority stages at the top of the photosphere, i.e., in
the temperature minimum of the one-dimensional static model atmospheres
presented in that paper. We show that this claim is incompatible with
observations and we demonstrate that the pertinent ionization balances
are instead dominated by outward photospheric radiation, as in older
static models. We then analyze the formation of Hα in the above model
and show that it has significant backradiation across the opacity gap
by which Hα differs from other strong scatttering lines.
---------------------------------------------------------
Title: Spectropolarimetry in the Sodium 589.6nm D1 line: Evaluating
the Resulting Chromospheric (?) Vector Field Maps.
Authors: Leka, K. D.; Barnes, G.; Stockwell, R. G.; Wagner, E. L.;
Uitenbroek, H.; Derouich, M.
2012decs.confE..79L Altcode:
Pioneering work by T. R. Metcalf almost two decades ago pointed to
the Na 589.6nm D1 line as a contender for providing chromospheric
vector magnetic field measurements (using the Zeeman effect). We
report here on a systematic examination of what can be expected from
Sodium 589.6nm spectropolarimetry, with respects to polarization-signal
amplitudes and retrieval, and the implementation of the inversion for
this line based on the Jeffries, Lites & Skumanich Weak-Field
Approximation algorithm. The analysis is performed using both
synthetic data and observations from the Imaging Vector Magnetograph,
for which a large dataset of Sodium 589.6nm vector spectropolarimetry
exists. The synthetic data are based on a 3-D field extrapolated from
photospheric vector magnetograms of two active regions, four distinct
model atmospheres coupled with NLTE synthesis of the emergent NaI
D1 Stokes polarization spectra, computed for a variety of viewing
angles. In this manner, a broad representation of active-region
features, field strengths and observing angles are tested using “hare
& hound” approaches, including evaluating algorithm performance in
the presence of noise and instrumental effects. We compare retrieval
algorithms for the very weak (as expected) polarization signals, and
evaluate the retrieved vector magnetic field at a range of inferred
heights. Finally, we provide an example from the IVM and discuss the
prospects for obtaining and interpreting chromospheric vector magnetic
field maps. Support for this work comes from NASA NAG5-12466, NASA
NNH09CE60C, AFOSR F49620-03-C-0019, NSF/NSWP ATM-0519107, NSF/SHINE
ATM-0454610, and NSF CRG ATM-0551055.
---------------------------------------------------------
Title: The RH suite of radiative transfer programs: a tutorial
Authors: Uitenbroek, Han
2012decs.confE..31U Altcode:
The RH suite of radiative transfer programs derives its name from
the Rybicky-Hummer multi-level accelerated lambda iteration (MALI)
formalism it employs for the general solution on Non-LTE problems
in a given atmospheric model. The suite provides separate programs
for Non-LTE solutions in 1-D, 2-D, and 3-D Cartesian, and spherical
geometry, including the effects of partial frequency redistribution
(PRD) and Zeeman-induced polarization. The code is flexible through
the use of structured input files, and allows for the calculation of
both atomic and molecular diagnostics. I will give a short tutorial
on the structure of the code, the principles on which it is build,
how to set up simple problems, and how to use the IDL-based graphical
user interface to look at output results. The code is available for
download for those interested in using it.
---------------------------------------------------------
Title: Potential for diagnostics with IRIS and Mg II lines
Authors: Pereira, Tiago M. D.; Carlsson, Mats; Leenaarts, Jorrit;
Uitenbroek, Han; De Pontieu, Bart; Martinez-Sykora, Juan
2012decs.confE..13P Altcode:
The IRIS mission will open up a new window into the solar chromosphere
and transition region. An important diagnostic that IRIS will bring
is the Mg II H and K lines. Radiation from these lines is believed
to be come from a wide range of formation depths, from the higher
photosphere to the onset of the transition region. With a complex
formation mechanism, Mg II H and K suffer from departures from LTE
and partial redistribution (PRD). In this preliminary analysis we will
look into the potential for diagnostics of Mg II H and K. Using a new
parallel version of the RH code we synthesised Mg II H and K spectra
from 3D rMHD simulations of the solar atmosphere. We will discuss
the relevance of several approximations on the final observables,
and will compare the Mg II H and K filtergrams with those of Ca II H,
a robust chromospheric diagnostic line widely used with Hinode/SOT/BFI.
---------------------------------------------------------
Title: On Molecular Hydrogen Formation and the Magnetohydrostatic
Equilibrium of Sunspots
Authors: Jaeggli, S. A.; Lin, H.; Uitenbroek, H.
2012ApJ...745..133J Altcode: 2011arXiv1110.0575J
We have investigated the problem of sunspot magnetohydrostatic
equilibrium with comprehensive IR sunspot magnetic field survey
observations of the highly sensitive Fe I lines at 15650 Å and nearby
OH lines. We have found that some sunspots show isothermal increases
in umbral magnetic field strength which cannot be explained by the
simplified sunspot model with a single-component ideal gas atmosphere
assumed in previous investigations. Large sunspots universally
display nonlinear increases in magnetic pressure over temperature,
while small sunspots and pores display linear behavior. The formation
of molecules provides a mechanism for isothermal concentration of
the umbral magnetic field, and we propose that this may explain the
observed rapid increase in umbral magnetic field strength relative to
temperature. Existing multi-component sunspot atmospheric models predict
that a significant amount of molecular hydrogen (H<SUB>2</SUB>) exists
in the sunspot umbra. The formation of H<SUB>2</SUB> can significantly
alter the thermodynamic properties of the sunspot atmosphere and
may play a significant role in sunspot evolution. In addition to the
survey observations, we have performed detailed chemical equilibrium
calculations with full consideration of radiative transfer effects
to establish OH as a proxy for H<SUB>2</SUB>, and demonstrate that a
significant population of H<SUB>2</SUB> exists in the coolest regions
of large sunspots.
---------------------------------------------------------
Title: Fast inversion of Zeeman line profiles using central moments
Authors: Mein, P.; Uitenbroek, H.; Mein, N.; Bommier, V.; Faurobert, M.
2011A&A...535A..45M Altcode:
Context. Many inversion techniques derive vector magnetic fields and
other parameters of the solar atmosphere from Stokes profiles with an
iterative process. <BR /> Aims: We propose a new inversion method, using
functions derived from central moments (ICM), to determine magnetic
field vectors with very few iterations. <BR /> Methods: Two quantities
A<SUB>1</SUB> and A<SUB>2</SUB> that combine moments of profiles I ±
S (S = Q,U,V) are proposed. They are nearly linear functions of the
longitudinal and transverse components of the magnetic field, and lead
to estimates of the field components through a least-squares polynomial
fit. A third quantity A<SUB>D</SUB> can be used to interpolate between
expansions that correspond to two basic models. Exponents β<SUB>1</SUB>
and β<SUB>2</SUB> in the moment expressions are adjusted to minimize
the sensitivity to data noise. <BR /> Results: Inversion coefficients
are computed for magnetic fields up to 3000 G in the case of the 630.2
Fe i line by forward modeling in two selected 1D model atmospheres
(FALC and MALTM). After inversion of synthetic profiles computed with
four models at disk center (FALA, FALC, FALF, MALTM), the mean standard
deviations with respect to the input fields do not exceed 5 G for both
components over the full range 0-3000 G. A comparison of ICM results
with inversion by the UNNOFIT code of profiles observed with THEMIS/MTR
shows good agreement. The typical computing time for a solar map of 100
000 points is less than 30 s. <BR /> Conclusions: The ICM inversions
are almost insensitive to thermodynamic properties and solve for vector
magnetic fields in a wide range of solar conditions, ranging from plage
to spot, with very little computational effort. They are, therefore,
extremely suitable for large data sets. Further improvements should
take into account instrumental profiles and effects of limited spatial
resolution by using filling factors. Extensions using more parameters
and models with large departures from the Milne Eddington approximation
could also be considered.
---------------------------------------------------------
Title: Why One-dimensional Models Fail in the Diagnosis of Average
Spectra from Inhomogeneous Stellar Atmospheres
Authors: Uitenbroek, Han; Criscuoli, Serena
2011ApJ...736...69U Altcode: 2011arXiv1101.2643U
We investigate the feasibility of representing a structured
multi-dimensional stellar atmosphere with a single one-dimensional
average stratification for the purpose of spectral diagnosis of the
atmosphere's average spectrum. In particular, we construct four
different one-dimensional stratifications from a single snapshot
of a magnetohydrodynamic simulation of solar convection: one by
averaging its properties over surfaces of constant height and three
by averaging over surfaces of constant optical depth at 500 nm. Using
these models, we calculate continuum and atomic and molecular line
intensities and their center-to-limb variations. From an analysis of
the emerging spectra, we identify three main reasons why these average
representations are inadequate for accurate determination of stellar
atmospheric properties through spectroscopic analysis. These reasons are
nonlinearity in the Planck function with temperature, which raises the
average emergent intensity of an inhomogeneous atmosphere above that of
an average-property atmosphere, even if their temperature-optical depth
stratification is identical; nonlinearities in molecular formation with
temperature and density, which raise the abundance of molecules of an
inhomogeneous atmosphere over that in a one-dimensional model with the
same average properties; and the anisotropy of convective motions,
which strongly affects the center-to-limb variation of line-core
intensities. We argue therefore that a one-dimensional atmospheric
model that reproduces the mean spectrum of an inhomogeneous atmosphere
necessarily does not reflect the average physical properties of that
atmosphere and is therefore inherently unreliable.
---------------------------------------------------------
Title: An Observational Study of the Formation and Evolution of
Sunspots
Authors: Jaeggli, Sarah A.; Lin, H.; Uitenbroek, H.
2011SPD....42.0302J Altcode: 2011BAAS..43S.0302J
It is well known that the thermal-magnetic relation in sunspots can
be non-linear. Previous investigations ascribe the non-linearity
of the relation to changing geometrical height of the measurement
due to radiative transfer effects (Wilson Depression) and the poorly
determined magnetic field curvature force. However, the very coolest
regions of some sunspots show a rapid increase in umbral magnetic
field strength relative to temperature which cannot be explained
by the simplified sunspot model with single-component ideal gas
atmosphere which has been previously assumed. This represents a
fundamental flaw in our understanding of the sunspot equilibrium
problem. Existing multiple-component sunspot atmospheric models
predict that a large amount of molecular hydrogen (H2) exists in
the sunspot umbra. The formation of molecules provides a mechanism
for isothermal concentration of the umbral magnetic field which may
explain the observed rapid increase in umbral magnetic field strength
relative to temperature. We have characterized the equilibrium forces
in sunspots using simultaneous visible and IR sunspot magnetic field
survey observations of the highly sensitive Fe I lines at 6302 and
15650 Angstroms and nearby OH lines which have been conducted with
the new Facility Infrared Spectropolarimeter (FIRS) at the Dunn Solar
Telescope. We have performed detailed chemical equilibrium calculations
with full consideration of radiative transfer effects to establish OH
as a proxy for H2, and demonstrate that a significant population of H2
exists in the coolest regions of large and more mature sunspots. We
further point out that the formation of H2 can significantly alter
the thermodynamic properties of the sunspot atmosphere and may play
a significant role in sunspot evolution.
---------------------------------------------------------
Title: The Observed Red Asymmetry in the Bisectors of the
Chromospheric CaII 854.2 nm Line
Authors: Burleigh, Kaylan; Tritschler, A.; Uitenbroek, H.
2011SPD....42.0304B Altcode: 2011BAAS..43S.0304B
The bisector analysis of chromospheric spatially and temporally
unresolved Ca II atlas profiles reveals a red asymmetry of the
Doppler core in form of an "inverse C” (Uitenbroek, 2005). The
origin of this red asymmetry is yet unknown. We use spatially and
temporally resolved 2D spectroscopic chromospheric (CaII 854.2 nm)
observations of the quiet and more active sun obtained with the Dunn
Solar Telescope's Interferometric BIdimensional Spectrometer (IBIS) to
determine where the inverse C-shape appears with respect to granules,
inter-granular lanes, and magnetic features. To this end we generate
masks of the spatial location of the red asymmetry. We also examine
the temporal behavior of profiles showing this red asymmetry. In the
chromosphere, we find the red asymmetry most concentrated in dark region
outside of magnetic networks; it avoids nearly all bright regions. It
disappears almost entirely within magnetic networks which suggests
magnetic activity "damps out” the red asymmetry. Relative to the
underlying photosphere, the red asymmetry preferentially occurs over
or just slightly offset from inter-granular lanes; very rarely does
it occur over granules. The temporal behavior of at least one red
asymmetry profile shows a periodicity near 3 min. We speculate that
the red asymmetry forms from upward traveling acoustic shock waves. <P
/>This work was supported by the National Solar Observatory's Research
Experiences for Undergraduate (REU) program which is co-funded by
the Department of Defense in partnership with the National Science
foundation REU Program.
---------------------------------------------------------
Title: The Visible Broadband Imager: The Sun at High Spatial and
Temporal Resolution
Authors: Friedrich, Woeger; Tritschler, A.; Uitenbroek, H.; Rimmele, T.
2011SPD....42.2001F Altcode: 2011BAAS..43S.2001F
The Visible Broadband Imager (VBI) will be the first of the five
first-light instruments for the Advanced Technology Solar Telescope
(ATST). It is designed to observe the solar atmosphere at heights
ranging from photosphere to chromosphere. High frame-rate detectors
that sample the FOV of up to 2.8 arcmin in diameter critically near or
at the diffraction limit of ATST's 4 meter aperture will facilitate near
real-time speckle reconstruction imaging. With its focus on high-spatial
resolution, the VBI will be addressing scientific questions related to
the smallest structures visible in the solar atmosphere today with high
photometric precision. The capability to observe the solar atmosphere
with a cadence of about 3 seconds per reconstructed image will enable
the VBI to temporally resolve fast evolving structures. <P />In this
contribution we present the current design of the VBI and highlight
some scientific questions related to fast evolving, small-scale features
within the solar atmosphere that the VBI will address.
---------------------------------------------------------
Title: Measurement of Line Formation Depths from a Super Resolving
Analysis of Photospheric Layers
Authors: Faurobert, M.; Aime, C.; Ricort, G.; Uitenbroek, H.; Grec, C.
2011ASPC..437...51F Altcode:
We present the results of an experiment aimed at measuring the formation
depths of the Fe I line pair at 630 nm in the solar photosphere. We use
images of the granulation obtained at different levels in the lines,
from line center up to the continuum level. When we observe out of
disc center their difference in formation depths is projected into a
radial shift of the images by the perspective effect. We measure this
shift by implementing a cross-spectral method similar to a technique
previously developed for stellar applications (Aime et al. 1984). The
signal-to-noise ratio is increased by averaging the cross spectra
over a large number of images. This technique allows us to measure
very small displacements, below the telescope resolution. <P />We
show results we obtained on HINODE/SP observations and compare them to
numerical simulations. The difference of formation depths between the
two line cores is determined with a very high accuracy and compares
quite well to LTE model calculation using either 1D solar model
or full 3D calculations in snapshots of the granulation. However
it shows significant variations in magnetic regions. The difference
between line core and continuum formation depths is more difficult to
measure precisely because line core and continuum images are not well
correlated, due to contrast inversion and depth-dependence of granular
structures. We solve this problem by measuring the perspective shifts
between similar enough images taken at successive steps along the
line profile, and by integrating the shifts from the continuum level
to the line center forming layer.
---------------------------------------------------------
Title: Detection of Chromospheric Magnetic Fields: A Forward Modeling
Approach
Authors: Uitenbroek, H.
2011ASPC..437..439U Altcode:
We show that circular and linear polarization resulting from the
Zeeman effect in chromospheric spectral lines is generally expected
to be small because these lines form at high temperatures and arise
from light elements. To illustrate this point we solve two-dimensional
non-LTE radiative transfer in the Ca II 854.21 nm and Na I D<SUB>2</SUB>
lines through a magnetostatic flux concentration model and calculate
the expected polarization. Finally, we show that the vertical magnetic
field on the axis of the concentration can be recovered by measuring
the bisector separation of the left- and right-hand circularly polarized
emergent profiles.
---------------------------------------------------------
Title: Molecule Formation and Magnetic Field Evolution in Sunspots
Authors: Jaeggli, S. A.; Lin, H.; Uitenbroek, H.
2011ASPC..437..473J Altcode:
In a sunspot the magnetic field provides horizontal support to keep
the warm, high pressure photosphere from intruding into the cool,
low pressure sunspot atmosphere. In the very coolest regions of
the umbra a rapid increase in magnetic field strength relative to
temperature has been observed by many authors although its origins
have remained unknown. In these regions the magnetic and gas pressure
forces have found a different state which the current simplified theory
of magnetohydrostatic equilibrium cannot quantitatively describe. It
is well known that molecules form in sunspots. The formation of a
large molecular fraction would alter the physical characteristic of
the gas, providing a mechanism for concentrating the umbral magnetic
field. The formation of molecules may be responsible for the observed
rapid increase of the magnetic field strength observed in the coolest
regions of the sunspots and may play a significant role in sunspot
evolution. We investigate this problem with atmospheric models and
present preliminary results from observations taken with the new
Facility Infrared Spectropolarimeter on the Dunn Solar Telescope.
---------------------------------------------------------
Title: Line Shape Effects on Intensity Measurements of Solar Features:
Brightness Correction to SOHO MDI Continuum Images
Authors: Criscuoli, S.; Ermolli, I.; Del Moro, D.; Giorgi, F.;
Tritschler, A.; Uitenbroek, H.; Vitas, N.
2011ApJ...728...92C Altcode: 2010arXiv1012.2561C
Continuum intensity observations obtained with the Michelson
Doppler Imager (MDI) on board the SOHO mission provide long time
series of filtergrams that are ideal for studying the evolution of
large-scale phenomena in the solar atmosphere and their dependence on
solar activity. These filtergrams, however, are not taken in a pure
continuum spectral band, but are constructed from a proxy, namely a
combination of filtergrams sampling the Ni I 6768 Å line. We studied
the sensitivity of this continuum proxy to the shape of the nickel line
and to the degradation in the instrumental transmission profiles. We
compared continuum intensity measurements near the nickel line with
MDI proxy values in three sets of high-resolution spectro-polarimetric
data obtained with the Interferometric Bidimensional Spectrometer,
and in synthetic data, obtained from multi-dimensional simulations of
magneto-convection and one-dimensional atmosphere models. We found that
MDI continuum measurements require brightness corrections which depend
on magnetic field strength, temperature and, to a smaller extent, plasma
velocity. The correction ranges from 2% to 25% in sunspots, and is,
on average, less than 2% for other features. The brightness correction
also varies with position on the disk, with larger variations obtained
for sunspots, and smaller variations obtained for quiet Sun, faculae,
and micropores. Correction factors derived from observations agree
with those deduced from the numerical simulations when observational
effects are taken into account. Finally, we found that the investigated
potential uncertainties in the transmission characteristics of MDI
filters only slightly affect the brightness correction to proxy
measurements.
---------------------------------------------------------
Title: Magnetic Field Measurements at the Photosphere and Coronal Base
Authors: Judge, P. G.; Centeno, R.; Tritschler, A.; Uitenbroek, H.;
Jaeggli, S.; Lin, H.
2010AGUFMSH31A1783J Altcode:
We have obtained vector polarimetric measurements in lines of Fe I
(630nm), Ca II (854nm) and He I (1083nm) of several active regions
during 3-14 June 2010. The measurements were made at the Dunn Solar
Telescope at Sacramento Peak Observatory, using the FIRS and IBIS
instruments simultaneously. We discuss these and SDO data for NOAA
11076. The seeing was very good or excellent and the adaptive
optics system functioned well. In this preliminary analysis we
compare extrapolations of photospheric fields with the constraints
available from Stokes polarimetry, including the morphology and
kinematic properties of fibrils. Connections to the corona will also be
discussed. The implications for field extrapolations from photospheric
measurements will be discussed. We will make the reduced data freely
available on the web for interested researchers.
---------------------------------------------------------
Title: Radiative emission of solar features in the Ca II K line:
comparison of measurements and models
Authors: Ermolli, I.; Criscuoli, S.; Uitenbroek, H.; Giorgi, F.;
Rast, M. P.; Solanki, S. K.
2010A&A...523A..55E Altcode: 2010arXiv1009.0227E
Context. The intensity of the Ca II K resonance line observed with
spectrographs and Lyot-type filters has long served as a diagnostic of
the solar chromosphere. However, the literature contains a relative
lack of photometric measurements of solar features observed at this
spectral range. <BR /> Aims: We study the radiative emission of various
types of solar features, such as quiet Sun, enhanced network, plage,
and bright plage regions, identified on filtergrams taken in the Ca II
K line. <BR /> Methods: We analysed full-disk images obtained with the
PSPT, by using three interference filters that sample the Ca II K line
with different bandpasses. We studied the dependence of the radiative
emission of disk features on the filter bandpass. We also performed a
non-local thermal equilibrium (NLTE) spectral synthesis of the Ca II
K line integrated over the bandpass of PSPT filters. The synthesis
was carried out by utilizing the partial frequency redistribution
(PRD) with the most recent set of semi-empirical atmosphere models
in the literature and some earlier atmosphere models. As the studied
models were computed by assuming the complete redistribution formalism
(CRD), we also performed simulations with this approximation for
comparison. <BR /> Results: We measured the center-to-limb variation
of intensity values for various solar features identified on PSPT
images and compared the results obtained with those derived from the
synthesis. We find that CRD calculations derived using the most recent
quiet Sun model, on average, reproduce the measured values of the
quiet Sun regions slightly more accurately than PRD computations with
the same model. This may reflect that the utilized atmospheric model
was computed assuming CRD. Calculations with PRD on earlier quiet Sun
model atmospheres reproduce measured quantities with a similar accuracy
as to that achieved here by applying CRD to the recent model. We
also find that the median contrast values measured for most of the
identified bright features, disk positions, and filter bandpasses
are, on average, a factor ≈1.9 lower than those derived from PRD
simulations performed using the recent bright feature models. The
discrepancy between measured and modeled values decreases by ≈12%
after taking into account straylight effects on PSPT images. When
moving towards the limb, PRD computations display closer agreement with
the data than performed in CRD. Moreover, PRD computations on either
the most recent or the earlier atmosphere models of bright features
reproduce measurements from plage and bright plage regions with a
similar accuracy. <P />Appendix A is only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Magnetic field measurements at the photosphere and coronal base
Authors: Judge, Philip; Centeno, R.; Tritschler, A.; Uitenbroek, H.;
Jaeggli, S.; Lin, H.
2010shin.confE..56J Altcode:
We have obtained vector polarimetric measurements in lines of Fe I
(630nm), Ca II (854nm) and He I (1083) of several active regions during
3-14 June 2010. The measurements were made at the Dunn Solar Telescope
at Sacramento Peak Observatory, using the FIRS and IBIS instruments
simultaneously. We discuss data for NOAA 11076 observed on 4 June
2010. The seeing was very good or excellent and the adaptive optics
system functioned well. In this preliminary analysis we compare linear
extrapolations of photospheric fields with the constraints available
from Stokes polarimetry, including the morphology and kinematic
properties of fibrils. The implications for field extrapolations from
photospheric measurements will be discussed. We will make the reduced
data freely available on the web for interested researchers.
---------------------------------------------------------
Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with
the Imaging Vector Magnetograph II: The Magnetic Maps
Authors: Derouich, M.; Leka, K. D.; Mickey, D. L.; Uitenbroek, H.;
Metcalf, T. R.
2010shin.confE...5D Altcode:
Following Poster I (Leka et al), we focus here on recent progress
regarding the inversion algorithms and interpretation of Zeeman
Na D1 589.6nm Stokes Spectra observed using the Imaging Vector
Magnetograph. We present systematic tests of the inversion procedures
and our interpretation of the results, relying on synthetic Na D1
589.6nm Stokes Spectra generated using known magnetic and atmospheric
models described in Poster I. In this second poster, we present the
results of "hare and hound" exercises which focus on (1) the optimal
fitting and inversion algorithm for the Na D1 589.6nm Stokes spectra,
and (2), the interpretation of the resulting active magnetic field
"maps", especially as a function of inferred height of the solar
atmosphere sampled by these Zeeman-polarization spectra. <P />This
work has been supported by AFOSR contract F49620-03-C-0019, NSF space
weather program grant ATM-0519107 and NSF SHINE grant ATM-0454610.
---------------------------------------------------------
Title: The ATST visible broadband imager: a case study for real-time
image reconstruction and optimal data handling
Authors: Wöger, Friedrich; Uitenbroek, Han; Tritschler, Alexandra;
McBride, William; Elmore, David; Rimmele, Thomas; Cowan, Bruce;
Wampler, Steve; Goodrich, Bret
2010SPIE.7735E..21W Altcode: 2010SPIE.7735E..69W
At future telescopes, adaptive optics systems will play a role beyond
the correction of Earth's atmosphere. These systems are capable of
delivering information that is useful for instrumentation, e.g. if
reconstruction algorithms are employed to increase the spatial
resolution of the scientific data. For the 4m aperture Advanced
Technology Solar Telescope (ATST), a new generation of state-of-the-art
instrumentation is developed that will deliver observations of the solar
surface at unsurpassed high spatial resolution. The planned Visual
Broadband Imager (VBI) is one of those instruments. It will be able
to record images at an extremely high rate and compute reconstructed
images close to the telescope's theoretical diffraction limit using
a speckle interferometry algorithm in near real-time. This algorithm
has been refined to take data delivered by the adaptive optics system
into account during reconstruction. The acquisition and reconstruction
process requires the use of a high-speed data handling infrastructure
to retrieve the necessary data from both adaptive optics system and
instrument cameras. We present the current design of this infrastructure
for the ATST together with a feasibility analysis of the underlying
algorithms.
---------------------------------------------------------
Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with
the Imaging Vector Magnetograph I: Polarization Spectra
Authors: Leka, K. D.; Mickey, D. L.; Uitenbroek, H.; Derouich, M.;
Metcalf, T. R.
2010shin.confE...4L Altcode:
We report on progress made recently on validating and interpreting
Stokes spectra from the Na D1 589.6nm line observed using the Imaging
Vector Magnetograph at Mees Solar Observatory, Haleakala, Maui. While
preliminary results from the dataset (which comprises daily observations
of active regions spanning 2000 - 2005, plus a few additional special
observation campaigns) have been reported previously (e.g., Leka
& Metcalf 2003; Metcalf Leka & Mickey 2005), we focus here
on systematic tests of the observed data and our interpretation of
them. In this first poster, we present Non-LTE synthetic Na D1 589.6nm
spectra, computed using known underlying magnetic and atmospheric
models, which form the basis for various "hare and hound" exercises
to test (1) the observed degree of polarization in (2) the context
of the instrument response and photon noise. While we find generally
excellent agreement, there are some systematic differences between
the synthetic and observational data. We summarize our understandings
of these differences and attempts to mitigate their effects in the
context of the inversion to a magnetic field map (see Poster II,
Derouich et al). <P />This work has been supported by AFOSR contract
F49620-03-C-0019, NSF space weather program grant ATM-0519107 and NSF
SHINE grant ATM-0454610.
---------------------------------------------------------
Title: Measuring line formation depths by cross-spectral
analysis. Numerical simulations for the 630 nm Fe I line pair
Authors: Grec, C.; Uitenbroek, H.; Faurobert, M.; Aime, C.
2010A&A...514A..91G Altcode:
Context. Numerical three-dimensional simulations of the solar
photosphere have progressed greatly in the last 15 years. Observational
tests are needed to independently verify the realism of these
simulations. <BR /> Aims: We aim to measure the perspective shift
between monochromatic images at different wavelengths taken away from
disk center. We investigate the feasibility of our method by applying
it to simulated spectra of the Fe i line pair at 630.15 and 630.25 nm
calculated from several snapshots of a three-dimensional simulation
of solar magneto-convection. <BR /> Methods: We present a method
to determine line formation depths from spectroscopic observations
without relying on assumptions about an atmospheric model. Our method is
based on the measurement of a perspective shift, which is detected as a
linear phase term in the cross-spectrum of the images. In principle this
detection is independent of the spatial resolution of the observations,
and provides a valuable test for numerical simulations of the solar
photosphere. <BR /> Results: To obtain accurate formation heights we
need to correct spectra for convective Doppler shifts, and we need to
accumulate successive phase shifts between images in nearby wavelengths,
rather than compare images from the continuum and core directly. The
comparison of images provides large dissimilarities, which result from
the temperature contrast inversion in the granulation with height. We
verify that the cross-spectrum phase of the simulated images shows
the expected linear behavior with spatial frequency when considering
two close enough wavelengths in a spectral line profile. This linear
behavior is however only obtained at small spatial frequencies,
i.e. for large granular structures. Derived line formation heights of
the two lines range from 239 and 287 km above the continuum formation
height for the 630.15 nm line, and from 138 to 201 km for the 630.25
nm line, with significant variation between snapshots. Formation
height estimates from optical depth unity give on average 319 km and
244 km respectively. <BR /> Conclusions: Our numerical tests validate
measurements of line formation depths from cross-spectra between images
at different wavelengths and stress the value of measuring the phase
of the cross-spectra as an important test for numerical simulations.
---------------------------------------------------------
Title: Delving into the Chromosphere: New Observational Tools
Authors: Reardon, Kevin P.; Cauzzi, G.; Tritschler, A.; Uitenbroek, H.
2010AAS...21630503R Altcode:
The chromosphere lies at the boundary between the near-equilibrium
photosphere and the hot, expanding corona. This region combines both
large interconnecting magnetic structures, and fine-scaled dynamics into
an intriguingly complex whole. Studying this behavior is a significant
observational challenge, requiring sizable fields of view (60-90")
sampled at high spatial (< 0.3") and temporal resolution (< 30
seconds), with full spectral information in multiple lines. We will
describe how instruments based on Fabry-Perot interferometers have
recently begun to routinely deliver such observations. We will review
some of the most exciting results obtained and the deeper insights
they have provided into the characteristics of the solar chromosphere.
---------------------------------------------------------
Title: Fabry-Pérot Versus Slit Spectropolarimetry of Pores and
Active Network: Analysis of IBIS and Hinode Data
Authors: Judge, Philip G.; Tritschler, Alexandra; Uitenbroek, Han;
Reardon, Kevin; Cauzzi, Gianna; de Wijn, Alfred
2010ApJ...710.1486J Altcode: 2010arXiv1001.0561J
We discuss spectropolarimetric measurements of photospheric (Fe I
630.25 nm) and chromospheric (Ca II 854.21 nm) spectral lines in and
around small magnetic flux concentrations, including a pore. Our
long-term goal is to diagnose properties of the magnetic field
near the base of the corona. We compare ground-based two-dimensional
spectropolarimetric measurements with (almost) simultaneous space-based
slit spectropolarimetry. We address the question of noise and crosstalk
in the measurements and attempt to determine the suitability of Ca II
measurements with imaging spectropolarimeters for the determination
of chromospheric magnetic fields. The ground-based observations
were obtained 2008 May 20, with the Interferometric Bidimensional
Spectrometer (IBIS) in spectropolarimetric mode operated at the Dunn
Solar Telescope at Sunspot, NM. The space observations were obtained
with the Spectro-Polarimeter of the Solar Optical Telescope aboard the
Japanese Hinode satellite. The agreement between the near-simultaneous
co-spatial IBIS and Hinode Stokes-V profiles at 630.25 nm is
excellent, with V/I amplitudes compatible to within 1%. The IBIS QU
measurements are affected by residual crosstalk from V, arising from
calibration inaccuracies, not from any inherent limitation of imaging
spectroscopy. We use a Principal Component Analysis to quantify the
detected crosstalk. QU profiles with V crosstalk subtracted are in
good agreement with the Hinode measurements, but are noisier owing to
fewer collected photons. Chromospheric magnetic fields are notoriously
difficult to constrain by polarization of Ca II lines alone. However,
we demonstrate that high cadence, high angular resolution monochromatic
images of fibrils in Ca II and Hα, seen clearly in IBIS observations,
can be used to improve the magnetic field constraints, under conditions
of high electrical conductivity. Such work is possible only with time
series data sets from two-dimensional spectroscopic instruments such
as IBIS, under conditions of good seeing.
---------------------------------------------------------
Title: Chromospheric Structure and Dynamics. From Old Wisdom to
New Insights
Authors: Tritschler, A.; Reardon, K.; Uitenbroek, H.
2010MmSAI..81..533T Altcode: 2010MmSAI..81..533R
No abstract at ADS
---------------------------------------------------------
Title: Detection of chromospheric magnetic fields: a forward modeling
approach .
Authors: Uitenbroek, H.
2010MmSAI..81..701U Altcode:
We show that circular and linear polarization in chromospheric
spectral lines, in particular that resulting from the Zeeman effect,
is generally expected to be small because these lines form at high
temperatures and arise from light elements. To illustrate these points
we solve two-dimensional non-LTE radiative transfer in the Ca II 854.21
nm line through a magnetostatic flux concentration model and calculate
the expected polarization. Finally, we show that the vertical magnetic
field on the axis of the concentration can be recovered by measuring
the bisector separation of the left- and right-hand circularly polarized
emergent profiles.
---------------------------------------------------------
Title: Radiative emission of solar features in Ca II K
Authors: Criscuoli, S.; Ermolli, I.; Fontenla, J.; Giorgi, F.; Rast,
M.; Solanki, S. K.; Uitenbroek, H.
2010MmSAI..81..773C Altcode: 2010arXiv1002.0244C
We investigated the radiative emission of different types of solar
features in the spectral range of the Ca II K line. We analyzed
full-disk 2k × 2k observations from the Precision Solar Photometric
Telescope (PSPT). The data were obtained by using three narrow-band
interference filters that sample the Ca II K line with different pass
bands. Two filters are centered in the line core, the other in the red
wing of the line. We measured the intensity and contrast of various
solar features, specifically quiet Sun (inter-network), network,
enhanced network, plage, and bright plage (facula) regions. Moreover,
we compared the results obtained with those derived from the numerical
synthesis performed for the three PSPT filters with a widely used
radiative code on a set of reference semi-empirical atmosphere models.
---------------------------------------------------------
Title: Dual-Line Spectral Imaging of the Chromosphere
Authors: Cauzzi, G.; Reardon, K.; Rutten, R. J.; Tritschler, A.;
Uitenbroek, H.
2010ASSP...19..513C Altcode: 2010mcia.conf..513C
Hα filtergrams are notoriously difficult to interpret, "beautiful
to view but not fit for analysis." We try to remedy this by using
the IBIS bi-dimensional spectrometer at the Dunn Solar Telescope at
NSO/Sacramento Peak to compare the quiet-sun chromosphere observed in
Hα to what is observed simultaneously in Ca II 854.2 nm, sampling both
lines with high angular and spectral resolution and extended coverage
of space, time, and wavelength. Per (x, y, t) pixel we measured the
intensity and Dopplershift of the minimum of each line's profile at
that pixel, as well as the width of their inner chromospheric cores. A
paper submitted to A&A (December 2008) compares these measurements
in detail.
---------------------------------------------------------
Title: Recovering the line-of-sight magnetic field in the chromosphere
from Ca II IR spectra
Authors: Wöger, F.; Wedemeyer-Böhm, S.; Uitenbroek, H.; Rimmele, T.
2010MmSAI..81..598W Altcode: 2009arXiv0912.3467W
We propose a method to derive the line-of-sight magnetic flux density
from measurements in the chromospheric Ca II IR line at 854.2 nm. The
method combines two well-understood techniques, the center-of-gravity
and bisector method, in a single hybrid technique. The technique
is tested with magneto-static simulations of a flux tube. We apply
the method to observations with the Interferometric Bidimensional
Spectrometer (IBIS) installed at the Dunn Solar Telescope of the NSO/SP
to investigate the morphology of the lower chromosphere, with focus on
the chromospheric counterparts to the underlying photospheric magnetic
flux elements.
---------------------------------------------------------
Title: Service-Mode Observations for Ground-Based Solar Physics
Authors: Reardon, K. P.; Rimmele, T.; Tritschler, A.; Cauzzi, G.;
Wöger, F.; Uitenbroek, H.; Tsuneta, S.; Berger, T.
2009ASPC..415..332R Altcode: 2009arXiv0909.1522R
There are significant advantages in combining Hinode observations
with ground-based instruments that can observe additional spectral
diagnostics at higher data rates and with greater flexibility. However,
ground-based observations, because of the random effects of weather
and seeing as well as the complexities data analysis due to changing
instrumental configurations, have traditionally been less efficient
than satellite observations in producing useful datasets. Future large
ground-based telescopes will need to find new ways to optimize both
their operational efficiency and scientific output. <P />We have begun
experimenting with service-mode or queue-mode observations at the Dunn
Solar Telescope using the Interferometric Bidimensional Spectrometer
(IBIS) as part of joint Hinode campaigns. We describe our experiences
and the advantag es of such an observing mode for solar physics.
---------------------------------------------------------
Title: Morphology and Dynamics of the Low Solar Chromosphere
Authors: Wöger, F.; Wedemeyer-Böhm, S.; Uitenbroek, H.; Rimmele,
T. R.
2009ApJ...706..148W Altcode: 2009arXiv0910.1381W
The Interferometric Bidimensional Spectrometer (IBIS) installed at
the Dunn Solar Telescope of the NSO/SP is used to investigate the
morphology and dynamics of the lower chromosphere and the virtually
non-magnetic fluctosphere below. The study addresses in particular the
structure of magnetic elements that extend into these layers. We choose
different quiet-Sun regions inside and outside the coronal holes. In
inter-network regions with no significant magnetic flux contributions
above the detection limit of IBIS, we find intensity structures with the
characteristics of a shock wave pattern. The magnetic flux elements in
the network are long lived and seem to resemble the spatially extended
counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight
magnetic field strength and explain some of the difficulties in deriving
the magnetic field vector from observations of the fluctosphere.
---------------------------------------------------------
Title: Direct measurement of the formation height difference of the
630 nm Fe I solar lines
Authors: Faurobert, M.; Aime, C.; Périni, C.; Uitenbroek, H.; Grec,
C.; Arnaud, J.; Ricort, G.
2009A&A...507L..29F Altcode:
Context: Spectral lines formed over a limited height range in either
a stellar or planetary atmosphere provide us with information about
the physical conditions within this height range. In this context,
an important quantity is the so-called line formation depth. It is
usually determined from numerical calculation of the atmospheric
opacity in the line of interest and then converted into geometrical
depth by using atmospheric models. <BR />Aims: We develop a radically
different approach, which allows us to measure directly line formation
depths from spectroscopic observations without relying on assumptions
about an atmospheric model. This method requires spatially resolved
observations, which up to now have been available only for solar
or planetary studies. We apply this method to images of the solar
granulation. <BR />Methods: The method was presented and tested
numerically in previous papers. It is based on the measurement
of the perspective shift between images at different wavelengths,
formed at different heights, when they are observed away from disk
center. Because of the Fourier transform properties, this shift gives
rise to a deterministic linear phase term in the cross spectrum of the
images. <BR />Results: The method is applied to observations of solar
quiet regions performed with the SOT spectropolarimeter on HINODE in
the Fe i line pair at 630.15 and 630.25 nm. We derive the difference
in formation heights between the two lines and its center-to-limb
variations. We show that the high sensitivity of the measurements allows
us to detect variations in the line formation heights between magnetized
and non-magnetized regions of the solar atmosphere. <BR />Conclusions:
Our results are the first direct measurements of line formation depths
in the solar photosphere. Cross spectral analysis provides us with a
new observable quantity, which may be measured with an accuracy well
bellow the spatial resolution of the observations. We recall that
the Fe i line pair at 630.15 and 630.25 nm is often used to determine
solar magnetic fields by spectropolarimetric observations and inversion
methods. The difference in the line formation heights that we measure
should be taken into account in the inversion procedures.
---------------------------------------------------------
Title: The solar chromosphere at high resolution with
IBIS. IV. Dual-line evidence of heating in chromospheric network
Authors: Cauzzi, G.; Reardon, K.; Rutten, R. J.; Tritschler, A.;
Uitenbroek, H.
2009A&A...503..577C Altcode: 2009arXiv0906.2083C
The structure and energy balance of the solar chromosphere remain
poorly known. We used the imaging spectrometer IBIS at the Dunn Solar
Telescope to obtain fast-cadence, multi-wavelength profile sampling
of Hα and Ca ii 854.2 nm over a sizable two-dimensional field of view
encompassing quiet-Sun network. We provide a first inventory of how the
quiet chromosphere appears in these two lines by comparing basic profile
measurements in the form of image displays, temporal-average displays,
time slices, and pixel-by-pixel correlations. We find that the two lines
can be markedly dissimilar in their rendering of the chromosphere,
but that, nevertheless, both show evidence of chromospheric heating,
particularly in and around network: Hα in its core width and Ca ii
854.2 nm in its brightness. We discuss venues for improved modeling.
---------------------------------------------------------
Title: Spectropolarimetry of Ca II 8542: Probing the Chromospheric
Magnetic Field
Authors: Kleint, L.; Reardon, K.; Stenflo, J. O.; Uitenbroek, H.;
Tritschler, A.
2009ASPC..405..247K Altcode:
We present spectropolarimetric observations of the chromospheric Ca II
8542 and photospheric Fe I 6302 lines obtained with the Interferometric
Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope. The
high spatial resolution over a large field of view (FOV) allows us to
connect the observed profiles to the overall topology of the target
region. After suitable calibrations we can extract Stokes profiles
for each point in the FOV. The Stokes V profiles observed in the Ca II
line show a large variety of shapes, indicating widely varying vertical
behavior of the field strength, velocity, and temperature. We examine
the center-of-gravity method for determining a representative field
strength from the observed profiles and use it to directly compare
photospheric and chromospheric magnetic fields.
---------------------------------------------------------
Title: Influence of Coronal EUV Irradiance on the Stokes Profiles
of the He I 10830 Å Multiplet
Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M.
2009ASPC..405..297C Altcode:
One of the most useful spectral windows for spectropolarimetric
investigations of the solar chromosphere is the one provided by the
spectral lines of the He I 10830 Å multiplet, whose polarization
signals are sensitive to the Hanle and Zeeman effects. However, in
order to be able to carry out reliable diagnostics of the dynamic
and magnetic properties of the solar outer atmosphere it is crucial
to have a good physical understanding of the sensitivity of the
observed spectral line radiation to the various competing triggering
mechanisms. Here we report a series of on-disk and off-the-limb
non-LTE calculations of the 10830 Å absorption and emission profiles,
focusing our investigation on their sensitivity to the EUV coronal
irradiation and the model atmosphere used in the calculations. We show
in what respects the on-disk case sensitivity of the polarization
signals induced by the Zeeman effect to the EUV coronal irradiance,
and investigate whether or not inversions based on the Milne-Eddington
model are reliable. Concerning the off-the-limb case we demonstrate
that the intensity ratio of the blue to the red components of the
He I 10830 Å multiplet is a sensitive function of the amount of EUV
coronal illumination. Therefore, measurements of this observable as
a function of the distance to the limb and its confrontation with
radiative transfer modeling might give us valuable information on the
physical properties of the solar atmosphere and on the amount of EUV
radiation penetrating the chromosphere from above.
---------------------------------------------------------
Title: The solar chromosphere at high resolution with
IBIS. III. Comparison of Ca II K and Ca II 854.2 nm imaging
Authors: Reardon, K. P.; Uitenbroek, H.; Cauzzi, G.
2009A&A...500.1239R Altcode: 2008arXiv0810.5260R
Aims: Filtergrams obtained in Ca II H, Ca II K, and Hα are often
employed as diagnostics of the solar chromosphere. The vastly disparate
appearance between the typical filtergrams in these different lines
calls into question the nature of what is actually being observed. We
investigate the lack of obvious structures of magnetic origin such as
fibrils and mottles in on-disk Ca II H and K images. <BR />Methods: We
directly compare a temporal sequence of classical Ca II K filtergrams
with a co-spatial and co-temporal sequence of spectrally resolved
Ca II 854.2 images obtained with the Interferometric Bidimensional
Spectrometer (IBIS), considering the effect of both the spectral and
spatial smearing. We analyze the temporal behavior of the two series
by means of Fourier analysis. <BR />Results: The lack of fine magnetic
structuring in Ca II K filtergrams, even with the narrowest available
filters, is due to observational effects, primarily contributions from
the bright, photospheric wings of the line that swamp the small and
dark chromospheric structures. Signatures of fibrils remain, however,
in the temporal evolution of the filtergrams, in particular with
the evidence of magnetic shadows around the network elements. The
Ca II K filtergrams do not appear, however, to properly reflect the
high-frequency behavior of the chromosphere. Using the same analysis,
we find no significant chromospheric signature in the Hinode/SOT Ca II
H quiet-Sun filtergrams. <BR />Conclusions: The picture provided by Hα
and Ca II 854.2, which show significant portions of the chromosphere
dominated by magnetic structuring, appears to reflect the true and
essential nature of the solar chromosphere. Data that do not resolve
this aspect, whether spatially or spectrally, may misrepresent the
behavior the chromosphere.
---------------------------------------------------------
Title: The Solar Chromosphere: Old Challenges, New Frontiers
Authors: Ayres, T.; Uitenbroek, H.; Cauzzi, G.; Reardon, K.; Berger,
T.; Schrijver, C.; de Pontieu, B.; Judge, P.; McIntosh, S.; White,
S.; Solanki, S.
2009astro2010S...9A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evidence for a Current Sheet above a Sunspot Umbra
Authors: Tritschler, A.; Uitenbroek, H.; Reardon, K.
2008ApJ...686L..45T Altcode:
We present observational evidence for the existence of a current
sheet in the chromosphere above a sunspot umbra based on high angular
resolution two-dimensional spectroscopic observations in the Ca II
854.21 nm line. In the core of this line we observe a very stable bright
ribbon-like structure separating magnetic field configurations that
connect to different parts of the active region. We make plausible
that the structure is a string of sheets carrying vertical currents
that result from dissipation when the different parts of the active
region are moved around in the photosphere. To our knowledge this is
the first direct observation of the heating caused by the dissipation
in such a current sheet in the chromosphere.
---------------------------------------------------------
Title: Imaging Spectropolarimetry of the Photosphere and Chromosphere
with IBIS
Authors: Reardon, K.; Tritschler, A.; Uitenbroek, H.; et al.
2008ESPM...12.2.31R Altcode:
We present recent results based on high-resolution spectropolarimetry
using IBIS, a dual Fabry-Perot imaging spectrometer. We describe
the characteristics of the instrument and its capability to measure
the full Stokes vector in a range of photospheric and chromospheric
spectral lines. <P />Since late 2006 IBIS has been regularly used in
spectropolarimetric mode and observations have included solar pores,
quiet sun network and internetwork areas, and the disk passage of active
regions NOAA 10941 and 10940. Measurements are primarily performed in
the Fe I 630.2 and the Ca II 854.2 nm lines to provide information on
both photospheric to chromospheric heights. We present results on the
highly dynamical nature of both the vertical and horizontal components
of quiet Sun magnetic fields. We also show the fine structure and
height variation of the magnetic field in a large sunspot.
---------------------------------------------------------
Title: Solar Chromospheric Dynamics: Onwards and Upwards
Authors: Cauzzi, G.; Reardon, K.; Rimmele, T.; Rutten, R.; Tritschler,
A.; Uitenbroek, H.; Woeger, F.
2008AGUSMSP41B..03C Altcode:
We present a study of chromospheric dynamics and its relation with the
driving photospheric magneto-convection in a variety of solar targets,
from quiet Sun to more active regions. To this end high resolution
observations were obtained in CaII 854.2 nm, Hα, and photospheric
FeI lines with the Interferometric BIdimensional Spectrometer (IBIS)
installed at the Dunn Solar Telescope of the NSO. The availability of
full spectroscopic information on extended fields of view allows us
to derive a comprehensive view of the intrinsically 3-D chromospheric
scene. A coherent picture is emerging that involves the propagation
and dissipation of photospheric acoustic waves into the chromospheric
layers, but selected and guided by the local and highly variable
magnetic topology. In particular, ubiquitous fibrilar structures,
apparently originating from even the smallest magnetic elements,
appear an integral part of the dynamic chromosphere.
---------------------------------------------------------
Title: The Influence of Coronal EUV Irradiance on the Emission in
the He I 10830 Å and D<SUB>3</SUB> Multiplets
Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M.
2008ApJ...677..742C Altcode: 2007arXiv0712.2203C
Two of the most attractive spectral windows for spectropolarimetric
investigations of the physical properties of the plasma structures
in the solar chromosphere and corona are the ones provided by the
spectral lines of the He I 10830 and 5876 Å (or D<SUB>3</SUB>)
multiplets, whose polarization signals are sensitive to the Hanle and
Zeeman effects. However, in order to be able to carry out reliable
diagnostics, it is crucial to have a good physical understanding
of the sensitivity of the observed spectral line radiation to the
various competing driving mechanisms. Here we report a series of
off-the-limb non-LTE calculations of the He I D<SUB>3</SUB> and 10830
Å emission profiles, focusing our investigation on their sensitivity
to the EUV coronal irradiation and the model atmosphere used in the
calculations. We show in particular that the intensity ratio of the
blue to the red components in the emission profiles of the He I 10830
Å multiplet turns out to be a good candidate as a diagnostic tool for
the coronal irradiance. Measurements of this observable as a function of
the distance to the limb and its confrontation with radiative transfer
modeling might give us valuable information on the physical properties
of the solar atmosphere and on the amount of EUV radiation at relevant
wavelengths penetrating the chromosphere from above.
---------------------------------------------------------
Title: The solar chromosphere at high resolution with IBIS. I. New
insights from the Ca II 854.2 nm line
Authors: Cauzzi, G.; Reardon, K. P.; Uitenbroek, H.; Cavallini, F.;
Falchi, A.; Falciani, R.; Janssen, K.; Rimmele, T.; Vecchio, A.;
Wöger, F.
2008A&A...480..515C Altcode: 2007arXiv0709.2417C
Context: The chromosphere remains a poorly understood part of the solar
atmosphere, as current modeling and observing capabilities are still
ill-suited to investigating its fully 3-dimensional nature in depth. In
particular, chromospheric observations that can preserve high spatial
and temporal resolution while providing spectral information over
extended fields of view are still very scarce. <BR />Aims: In this
paper, we seek to establish the suitability of imaging spectroscopy
performed in the Ca II 854.2 nm line as a means of investigating
the solar chromosphere at high resolution. <BR />Methods: We utilize
monochromatic images obtained with the Interferometric BIdimensional
Spectrometer (IBIS) at multiple wavelengths within the Ca II 854.2 nm
line and over several quiet areas. We analyze both the morphological
properties derived from narrow-band monochromatic images and the
average spectral properties of distinct solar features such as network
points, internetwork areas, and fibrils. <BR />Results: The spectral
properties derived over quiet-Sun targets are in full agreement with
earlier results obtained with fixed-slit spectrographic observations,
highlighting the reliability of the spectral information obtained
with IBIS. Furthermore, the very narrowband IBIS imaging reveals very
clearly the dual nature of the Ca II 854.2 nm line. Its outer wings
gradually sample the solar photosphere, while the core is a purely
chromospheric indicator. The latter displays a wealth of fine structures
including bright points akin to the Ca II H{2V} and K{2V} grains, and
as fibrils originating from even the smallest magnetic elements. The
fibrils occupy a large fraction of the observed field of view, even
in the quiet regions, and clearly outline atmospheric volumes with
different dynamical properties, strongly dependent on the local magnetic
topology. This highlights how 1D models stratified along the vertical
direction can provide only a very limited representation of the actual
chromospheric physics. <BR />Conclusions: Imaging spectroscopy in the
Ca II 854.2 nm line currently represents one of the best observational
tools for investigating the highly structured and highly dynamical
chromospheric environment. A high-performance instrument such as IBIS
is crucial in achieving the necessary spectral purity and stability,
spatial resolution, and temporal cadence. <P />Two movies are only
available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: The Discrepancy in G-Band Contrast: Where is the Quiet Sun?
Authors: Uitenbroek, H.; Tritschler, A.; Rimmele, T.
2007ApJ...668..586U Altcode: 2007arXiv0704.3637U
We compare the rms contrast in observed speckle reconstructed G-band
images with synthetic filtergrams computed from two magnetohydrodynamic
simulation snapshots. The observations consist of 103 bursts of 80
frames each, taken at the Dunn Solar Telescope (DST), sampled at twice
the diffraction limit of the telescope. The speckle reconstructions
account for the actions of the adaptive optics (AO) system at the DST in
order to supply reliable photometry. We find a considerable discrepancy
between the observed rms contrast of 14.1% for the best reconstructed
images and the synthetic rms contrast of 21.5% in a simulation snapshot
thought to be representative of the quiet Sun. The areas of features
in the synthetic filtergrams that have positive or negative contrast
beyond the minimum and maximum values in the reconstructed images have
spatial scales that should be resolved. This leads us to conclude that
there are fundamental differences in the rms G-band contrast between
observed and computed filtergrams. On the basis of the substantially
reduced granular contrast of 16.3% in the synthetic plage filtergram,
we speculate that the quiet Sun may contain more weak magnetic field
than previously thought.
---------------------------------------------------------
Title: Two-dimensional Spectropolarimetry At The Dunn Solar Tower
Authors: Uitenbroek, Han; Tritshler, A.; Reardon, K.; Kleint, L.
2007AAS...210.2605U Altcode: 2007BAAS...39..324U
Measurement of the solar magnetic field within individual atmospheric
structures is a desirable, but persistently challenging goal, in
particular in chromospheric layers. Successful measurements over
different heights would provide an important contribution to our
understanding of the solar atmosphere and would provide valuable
input for theoretical modeling. We provide a short description of the
capabilities of the Interferometric BIdimensional Spectrometer (IBIS),
which has recently been upgraded to full Stokes capabilities. IBIS
is installed at the Dunn Solar Tower (DST) at the Sacramento Peak
observatory operated by NSO. Using IBIS we achieve high spatial
resolution over a large field of view in both the photosphere and
the chromosphere, which allows us to connect the observed profiles
to the overall topology of the target region. After performing
suitable calibrations for the telescope and instrument polarization
properties, we can extract Stokes profiles for each point in the
field of view. Stokes V profiles observed in the Ca II 854.2 nm line
show a large variety of forms, indicating widely varying vertical
behavior of the field strength, velocity, and temperature. We examine
the center-of-gravity method for determining a representative field
strength from the observed profiles looking at observations and
comparing with simulated profiles.
---------------------------------------------------------
Title: Narrow-band imaging in the CN band at 388.33 nm
Authors: Uitenbroek, H.; Tritschler, A.
2007A&A...462.1157U Altcode: 2006astro.ph.11407U
Aims:We promote the use of narrow-band (0.05-0.20 nm FWHM) imaging in
the molecular CN band head at 388.33 nm as an effective method for
monitoring small-scale magnetic field elements because it renders
them with exceptionally high contrast. <BR />Methods: We create
synthetic narrow-band CN filtergrams from spectra computed from a
three-dimensional snapshot of a magnetohydrodynamic simulation of the
solar convection to illustrate the expected high contrast and explain
its nature. In addition, we performed observations with the horizontal
slit spectrograph at the Dunn Solar Tower at 388.3 nm to experimentally
confirm the high bright-point contrast, and to characterize and optimize
the transmission profile of a narrow-band (0.04 FWHM) Lyot filter, which
was built by Lyot and tailored to the CN band at Sacramento Peak in
the early 70's. <BR />Results: The presented theoretical computations
predict that bright-point contrast in narrow-band (0.04 FWHM) CN
filtergrams is more than 3 times higher than in CN filtergrams taken
with 1 nm FWHM wide filters, and in typical G-band filtergrams. Images
taken through the Lyot filter after optimizing its passband confirm that
the filter is capable of rendering small-scale magnetic elements with
contrasts that are much higher than in traditional G-band imaging. The
filter will be available as an user instrument at the Dunn Solar Tower.
---------------------------------------------------------
Title: On the fine structure of the quiet solar Ca II K atmosphere
Authors: Tritschler, A.; Schmidt, W.; Uitenbroek, H.; Wedemeyer-Böhm,
S.
2007A&A...462..303T Altcode: 2006astro.ph.11402T
Aims:We investigate the morphological, dynamical, and evolutionary
properties of the internetwork and network fine structure of the quiet
sun at disk centre. <BR />Methods: The analysis is based on a ~6 h
time sequence of narrow-band filtergrams centred on the inner-wing
Ca II K2v reversal at 393.3 nm. To examine the temporal evolution of
network and internetwork areas separately we employ a double-Gaussian
decomposition of the mean intensity distribution. An autocorrelation
analysis is performed to determine the respective characteristic
time scales. In order to analyse statistical properties of the fine
structure we apply image segmentation techniques. <BR />Results: The
results for the internetwork are related to predictions derived from
numerical simulations of the quiet sun. The average evolutionary time
scale of the internetwork in our observations is 52 s. Internetwork
grains show a tendency to appear on a mesh-like pattern with a
mean cell size of ~4-5 arcsec. Based on this size and the spatial
organisation of the mesh we speculate that this pattern is related to
the existence of photospheric downdrafts as predicted by convection
simulations. The image segmentation shows that typical sizes of both
network and internetwork grains are in the order of 1.6 arcsec.
---------------------------------------------------------
Title: Solar MHD Theory and Observations: A High Spatial Resolution
Perspective
Authors: Leibacher, John; Stein, Robert F.; Uitenbroek, Han
2006ASPC..354.....L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Diagnostics
Authors: Uitenbroek, H.
2006ASPC..354..313U Altcode:
Several methods for estimating the formation heights of spectral
features are explored. For the general non-LTE conditions that prevail
in the solar chromosphere the method of response functions seems to be
the most appropriate. Examples of the response function for different
chromospheric lines in one- and two-dimensional models of the solar
atmosphere are shown to illustrate the usefulness of this method. From
the shape of the response function for H α we conclude that current
dynamical models are still lacking relevant physics, in particular
the influence of magnetic forces, in their upper layers above 500 km.
---------------------------------------------------------
Title: The Contrast of Magnetic Flux Concentrations at Near-Infrared
and Visible Wavelengths
Authors: Tritschler, A.; Uitenbroek, H.
2006ApJ...648..741T Altcode:
To reconcile discrepancies in the observed contrast of magnetic
flux concentrations, we compute synthetic filtergrams from solar
magnetoconvection models in four different passbands: two continuum
bands, at 1626 and 575 nm, the G band, and the line wing of the Ca
II K line. Magnetic flux concentrations are selected by their G-band
brightness. In the near-infrared the selected flux concentrations
appear inconspicuous with intensities that are on average below that of
the synthetic average quiet Sun, while their contrast in the visible
passband is mostly positive. When we account for limited telescope
resolution and imperfect seeing, the contrasts of bright points in the
visible become increasingly negative as a result of mixing with the
dark intergranular lanes in which they are embedded. The correlation in
bright-point intensity between different passbands becomes increasingly
poor from the G band to the visible and the near-infrared. This is
caused in part by the varying amount of spectral lines in each of the
bands. Further, we find that the largest magnetic field concentrations,
representative of micropores or magnetic knots, are dark in all
four passbands. The brightenings in the Ca II K line wing do not
coincide with magnetic flux concentrations on the spatial scale of
the simulation. Finally, we find that the rms intensity contrast in
all four passbands depends on the amount of magnetic field present
in the simulation, with higher contrast for lower average magnetic
field strength.
---------------------------------------------------------
Title: Evidence for a Siphon Flow Ending near the Edge of a Pore
Authors: Uitenbroek, H.; Balasubramaniam, K. S.; Tritschler, A.
2006ApJ...645..776U Altcode:
Observations of NOAA AR 9431, taken with the Vacuum Tower at Kitt
Peak on 2001 April 18 in the Ca II 854.21 nm line in both circular
polarizations, show evidence for a strong supersonic downflow ending
near the edge of a magnetic pore. The observed supersonic motion is
interpreted as a siphon flow along a magnetic loop connecting a patch
of weaker field to the pore of opposite polarity in the same active
region. The 854.21 nm line data reveal the upflow at one footpoint of
the loop, as well as the acceleration of the flow toward the footpoint
at the pore, where the flow reaches line-of-sight velocities of well
over 20 km s<SUP>-1</SUP>, substantially larger than the critical
speed. Numerical radiative transfer modeling of the 854.21 nm line
indicates the presence of a strong discontinuity in the flow velocity,
which we interpret as evidence for a tube shock in the downwind leg
of the siphon.
---------------------------------------------------------
Title: A comparison of solar proxy-magnetometry diagnostics
Authors: Leenaarts, J.; Rutten, R. J.; Carlsson, M.; Uitenbroek, H.
2006A&A...452L..15L Altcode:
Aims.We test various proxy-magnetometry diagnostics, i.e., brightness
signatures of small-scale magnetic elements, for studying magnetic
field structures in the solar photosphere.<BR /> Methods: .Images are
numerically synthesized from a 3D solar magneto-convection simulation
for, respectively, the G band at 430.5 nm, the CN band at 388.3 nm,
and the blue wings of the H α, H β, Ca ii H, and Ca ii 854.2 nm
lines.<BR /> Results: .Both visual comparison and scatter diagrams of
the computed intensity versus the magnetic field strength show that,
in particular for somewhat spatially extended magnetic elements, the
blue H α wing presents the best proxy-magnetometry diagnostic, followed
by the blue wing of H β. The latter yields higher diffraction-limit
resolution.<BR /> Conclusions: .We recommend using the blue H α wing
to locate and track small-scale photospheric magnetic elements through
their brightness appearance.
---------------------------------------------------------
Title: The visible-light broad-band imager for ATST: preliminary
design
Authors: Uitenbroek, Han; Tritschler, Alexandra; An, Hyun Kyoung;
Berger, Thomas
2006SPIE.6269E..61U Altcode: 2006SPIE.6269E.193U
This poster outlines the conceptual design of the Visible-light
Broad-band Imager (VBI) instrument for the Advanced Technology Solar
Telescope (ATST) as it follows from scientific requirements. The VBI
is scheduled to be the first-light instrument of the ATST, highlighting
the telescope's high spatial resolution capabilities.
---------------------------------------------------------
Title: Narrow-band Imaging In Ihe Cn Band Head
Authors: Uitenbroek, Han; Tritschler, A.
2006SPD....37.0717U Altcode: 2006BAAS...38..662U
We report on results of an observing campaign intended to revive an old
CN Lyot filter originally built by Bernhard Lyot himself, but modified
at Sacramento Peak. The filter has two band-width settings of 0.025
nm and 0.05 nm which can be fine tuned thermally. We characterise
the passband of the Lyot filter and the employed prefilter based
on osbervations performed with a spectrograph. We also performed an
imaging experiment in an attempt to obtain data visualizing the imaging
capability of the filter. Our results show that the CN filter is in
a surprisingly good condition and is most suited for observations
to verify theoretical predictions about the brightness of magnetic
elements in the CN bandhead at 388.3\,nm.
---------------------------------------------------------
Title: DOT tomography of the solar atmosphere. VI. Magnetic elements
as bright points in the blue wing of Hα
Authors: Leenaarts, J.; Rutten, R. J.; Sütterlin, P.; Carlsson, M.;
Uitenbroek, H.
2006A&A...449.1209L Altcode:
High-resolution solar images taken in the blue wing of the Balmer H
α line with the Dutch Open Telescope show intergranular magnetic
elements as strikingly bright features, similar to, but with
appreciably larger contrast over the surrounding granulation than
their more familiar manifestation as G-band bright points. Part of
this prominent appearance is due to low granular contrast, without
granule/lane brightness reversal as, e.g., in the wings of Ca II H
& K. We use 1D and 2D radiative transfer modeling and 3D solar
convection and magnetoconvection simulations to reproduce and explain
the H α wing images. We find that the blue H α wing obeys near-LTE
line formation. It appears particularly bright in magnetic elements
through low temperature gradients. The granulation observed in the blue
wing of H α has low contrast because of the lack of H α opacity in the
upper photosphere, Doppler cancellation, and large opacity sensitivity
to temperature working against source function sensitivity. We conclude
that the blue H α wing represents a promising proxy magnetometer to
locate and track isolated intermittent magnetic elements, a better one
than the G band and the wings of Ca II H & K although less sharp
at given aperture.
---------------------------------------------------------
Title: The Inverse-C Shape of Solar Chromospheric Line Bisectors
Authors: Uitenbroek, H.
2006ApJ...639..516U Altcode:
Spatially averaged intensity profiles of the chromospheric Na I D and
Ca II infrared lines exhibit a pronounced red asymmetry in their cores
with bisectors in the shape of an inverse C. This shape stands in stark
contrast to the regular C shape of photospheric spectral line bisectors,
which on average exhibit a blue shift as a result of the asymmetry in
surface area subtended by convective upflows over downflows. The nature
of the inverse chromospheric C shape is investigated by comparing
spatially averaged profiles of the Na I D and Ca II infrared lines
with mean profiles computed through three-dimensional snapshots of a
hydrodynamic convection simulation and a one-dimensional simulation
of chromospheric radiation hydrodynamics. In part the red asymmetry is
the result of the asymmetry in time the atmosphere spends in downward
motion compared to upward motion when it is traversed by acoustic
shocks. Profiles from convection simulations without shocks suggest that
convective motions play a limited role in shaping the chromospheric line
asymmetry. Further simulations that include effects of both convection
and shock wave formation are needed to reach a definitive conclusion
on the origin of the inverse-C shaped bisectors.
---------------------------------------------------------
Title: The Contrast of Magnetic Elements in Synthetic CH- and CN-Band
Images of Solar Magnetoconvection
Authors: Uitenbroek, H.; Tritschler, A.
2006ApJ...639..525U Altcode: 2005astro.ph.10333U
We present a comparative study of the intensity contrast in synthetic
CH-band and violet CN-band filtergrams computed from a high-resolution
simulation of solar magnetoconvection. The underlying simulation
has an average vertical magnetic field of 250 G with kilogauss fields
concentrated in its intergranular lanes and is representative of a plage
region. To simulate filtergrams typically obtained in CH- and CN-band
observations we computed spatially resolved spectra in both bands and
integrated these spectra over 1 nm FWHM filter functions centered at
430.5 and 388.3 nm, respectively. We find that the average contrast
of magnetic bright points in the simulated filtergrams is lower in
the CN band by a factor of 0.96. In comparison, earlier semiempirical
modeling and recent observations both estimated that the bright point
contrast in the CN band is higher by a factor of 1.4. We argue that
the near equality of the bright point contrast in the two bands in
the present simulation is a natural consequence of the mechanism that
causes magnetic flux elements to be particularly bright in the CN and
CH filtergrams, namely, the partial evacuation of these elements and
the concomitant weakening of molecular spectral lines in the filter
passbands. We find that the rms intensity contrast in the whole field
of view of the filtergrams is 20.5% in the G band and 22.0% in the CN
band and conclude that this slight difference in contrast is caused
by the shorter wavelength of the latter. Both the bright point and rms
intensity contrast in the CN band are sensitive to the precise choice
of the central wavelength of the filter.
---------------------------------------------------------
Title: Evaluation and Selection of Solar Observing Programs
Authors: Uitenbroek, Han
2006ASSL..335..117U Altcode: 2006osa6.book..117U
Solar observing programs are different from their night-time
counterparts. The need to obtain a unique dataset in a long-established
field drives a very flexible setup of instrumention at solar
telescopes. This in turn requires heavy involvement of the user in
customized instrument definition and layout. The instrument setup,
selection procedures, and user statistics at the Dunn Solar Tower
(DST) of the National Solar Observatory (NSO) at Sacramento Peak are
discussed as a typical example of a solar observing program.
---------------------------------------------------------
Title: Small Scale Magnetic Elements as Bright Points in the Blue
Hα Wing
Authors: Leenaarts, J.; Sütterlin, P.; Rutten, R. J.; Carlsson, M.;
Uitenbroek, H.
2005ESASP.596E..15L Altcode: 2005ccmf.confE..15L
No abstract at ADS
---------------------------------------------------------
Title: Response functions of chromospheric lines to changes in
temperature and magnetic field
Authors: Uitenbroek, H.
2005AGUSMSH12A..02U Altcode:
Typical densities in the chromosphere are four orders of magnitude
smaller than in the underlying photosphere. For this reason the
chromosphere is mostly transparent in the optical, aside from
wavelengths in the centres of a handful of strong lines. Few viable
diagnostics are, therefore, available for routine chromospheric
temperature, velocity and magnetic field measurements. Interpretation
of these diagnostics is problematic because the low density environment
favours radiative line excitation over collisional excitation. As a
result, chromospheric lines generally require non-LTE radiative transfer
solutions to determine the population of their upper and lower levels,
and their formation heights. I will present Non-LTE response functions
of several chromospheric lines to investigate the sensitivity of the
intensity profile of these lines to changes in temperature and magnetic
field. Although these response functions are model dependent they give
a first indication of the heights at which we can expect to measure
magnetic field strength and geometry in the chromosphere. We will
also discuss the sensitivity of the response functions to different
solar models
---------------------------------------------------------
Title: Spectro-polarimetry of the G band
Authors: Uitenbroek, H.; Balasubramaniam, K.; Tritschler, A.
2005AGUSMSP41B..03U Altcode:
Narrow-band filter imaging in the G band at 430 nm has been used to
track the evolution of small-scale magnetic field elements for more
than two decades. Because of the presence of many lines of the CH
molecule, and the relatively high contrast at this short wavelength
the G-band region is exceptionally suitable for this task. However,
despite the frequent use of G-band brightness as magnetic field proxy
it has not yet been well established what the precise mechanism is that
makes the small scale magnetic elements appear bright. In particular,
it is unclear why there is no one-to-one correlation between G-band
brightness and magnetic field, as established from co-spatial
magnetograms in atomic lines. To obtain a better understanding of
the elusive G-band brightening mechanism we obtained high spatial-
and spectral resolution spectra of the G-band region in Stokes I and V
at the Dunn Solar Telescope on Sacramento Peak. We use the molecular
Zeeman effect to determine line-of-sight magnetic field strength
directly in the CH lines that provide most of the opacity in the G
band, avoiding difficulties with co-aligning images and magnetograms
taken seperately. We compare our observations with radiative transfer
modeling of the Stokes profiles in snapshots of a magneto-hydrodynamic
simulation of solar convection.
---------------------------------------------------------
Title: How reliable is the inversion of Stokes profiles?
Authors: McMillan, M. T.; Sankarasubramaniam, K.; Uitenbroek, H.
2004AAS...205.1003M Altcode: 2004BAAS...36.1349M
Numerical methods inversion methods are routinely used to obtain the
structure of the magnetic field on the solar surface from observed
spectra. These codes necessarily include appoximations of the
magnetic field structure and the spectral line formation process,
and make a trade-off between complexity and numerical expediency. To
evaluate the accuracy with which magnetic field structure can be
recovered under various assumptions we performed an accurate forward
calculation of Stokes profiles from a two-dimensional cross section of
a magneto-hydrodynamic simulation of solar convection. The calculated
profiles were then analyzed with several different inversion codes and
the recovered field strengths and orientations were then compared with
the original values. We conclude that the results of Stokes profile
inversions have to be treated the necessary skepticism, especially
when complex field geometries are present.
---------------------------------------------------------
Title: Spectro-polarimetry of the G band
Authors: Uitenbroek, H.; Balasubramaniam, K. S.
2004AAS...205.4302U Altcode: 2004BAAS...36.1411U
Narrow-band filter imaging in the G band at 430 nm has been used to
track the evolution of small-scale magnetic field elements for more
than two decades. Because of the presence of many lines of the CH
molecule, and the relatively high contrast at this short wavelength
the G-band region is exceptionally suitable for this task. Howeve,
despite the frequent use of G-band brightness as magnetic field proxy
it has not yet been well established what the precise mechanism is that
makes the small scale magnetic elements appear bright. In particular,
it is unclear why there is no one-to-one correlation between magnetic
field and G-band brightness, as has been established from co-spatial
magnetograms in atomic lines. <P />To obtain a better understanding
of the elusive G-band brightening mechanism we obtained high spatial-
and spectral resolution spectra of the G-band region in Stokes I and V
at the Dunn Solar Telescope on Sacramento Peak. We use the molecular
Zeeman effect to determine line-of-sight magnetic field strength
directly in the CH lines that provide most of the opacity in the G
band, avoiding difficulties with co-aligning images and magnetograms
taken seperately. We compare our observations with radiative transfer
modeling of the Stokes profiles in snapshots of a magneto-hydrodynamic
simulation of solar convection.
---------------------------------------------------------
Title: Observation of the Molecular Zeeman Effect in the G Band
Authors: Asensio Ramos, A.; Trujillo Bueno, J.; Bianda, M.; Manso
Sainz, R.; Uitenbroek, H.
2004ApJ...611L..61A Altcode: 2004astro.ph..7332A; 2004astro.ph..7332R
Here we report on the first observational investigation of the
Zeeman effect in the G band around 4305 Å. Our spectropolarimetric
observations of sunspots with the Zürich Imaging Polarimeter at the
Istituto Ricerche Solari Locarno confirm our previous theoretical
prediction that the molecular Zeeman effect produces measurable
circular polarization signatures in several CH lines that are not
overlapped with atomic transitions. We also find both circular and
linear polarization signals produced by atomic lines whose wavelengths
lie in the G-band spectral region. Together, such molecular and atomic
lines are potentially important for empirical investigations of solar
and stellar magnetism. For instance, a comparison between observed and
calculated Stokes profiles suggests that the thermodynamical and/or
magnetic properties of the photospheric regions of sunspot umbrae are
horizontally structured with a component that might be associated with
umbral dots.
---------------------------------------------------------
Title: Promises and Pitfalls of Solar Hα Zeeman Spectropolarimetry
Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H.
2004ApJ...606.1233B Altcode:
Zeeman spectropolarimetry of the solar disk in the Hα line is a
daunting, yet promising diagnostic for measuring solar chromospheric
magnetic fields. As a demonstration of its potential we used the
National Solar Observatory (NSO)/High Altitude Observatory (HAO)
Advanced Stokes Polarimeter to obtain simultaneous measurements
of polarimetric signals from the photosphere in the Fe I 6301.5,
6302.5 Å lines and from the chromosphere in Hα. With these
measurements, we explore the promises and pitfalls of Hα Zeeman
spectropolarimetry. Remarkable features observed in Hα include
a reversal of Zeeman polarity in the emission core across flaring
active regions and highly redshifted and smeared Stokes V profiles in
prominences. We reproduce the Hα Zeeman reversals using radiative
transfer diagnostics, and we compare photospheric and chromospheric
line-of-sight magnetic fields.
---------------------------------------------------------
Title: The Zeeman Effect in the G Band
Authors: Uitenbroek, H.; Miller-Ricci, E.; Asensio Ramos, A.; Trujillo
Bueno, J.
2004ApJ...604..960U Altcode: 2004astro.ph..1467U
We investigate the possibility of measuring magnetic field strength in
G-band bright points through the analysis of Zeeman polarization in
molecular CH lines. To this end we solve the equations of polarized
radiative transfer in the G band through a standard plane-parallel
model of the solar atmosphere with an imposed magnetic field
and through a more realistic snapshot from a simulation of solar
magnetoconvection. This region of the spectrum is crowded with many
atomic and molecular lines. Nevertheless, we find several instances of
isolated groups of CH lines that are predicted to produce a measurable
Stokes V signal in the presence of magnetic fields. In part this is
possible because the effective Landé factors of lines in the stronger
main branch of the CH A<SUP>2</SUP>Δ-X<SUP>2</SUP>Π transition
tend to zero rather quickly for increasing total angular momentum J,
resulting in a Stokes V spectrum of the G band that is less crowded than
the corresponding Stokes I spectrum. We indicate that, by contrast,
the effective Landé factors of the R and P satellite subbranches of
this transition tend to +/-1 for increasing J. However, these lines
are in general considerably weaker and do not contribute significantly
to the polarization signal. In one wavelength location near 430.4 nm,
the overlap of several magnetically sensitive and nonsensitive CH lines
is predicted to result in a single-lobed Stokes V profile, raising
the possibility of high spatial resolution narrowband polarimetric
imaging. In the magnetoconvection snapshot we find circular polarization
signals of the order of 1%, prompting us to conclude that measuring
magnetic field strength in small-scale elements through the Zeeman
effect in CH lines is a realistic prospect.
---------------------------------------------------------
Title: On the Diagnostic Potential of Hα for Chromospheric Magnetism
Authors: Socas-Navarro, H.; Uitenbroek, H.
2004ApJ...603L.129S Altcode:
We investigate the feasibility of measuring line-of-sight magnetic
field strength in the solar chromosphere through Stokes polarimetry in
the hydrogen Hα line. Because of the large intrinsic Doppler width
of the hydrogen line, the weak-field approximation applies even for
the strongest magnetic fields in the solar atmosphere. We calculate
the Hα response functions of Stokes I and V to perturbations in the
magnetic field and temperature in two different one-dimensional solar
models representing the average quiet Sun and a sunspot umbra. These
response functions show that the Hα line exhibits large photospheric
sensitivities in these solar models in addition to its chromospheric
sensitivity. One particularly striking example is the Stokes V response
in the quiet Sun, which is mostly photospheric when the field decreases
with height.
---------------------------------------------------------
Title: Chromospheric Heating and CO Simulations
Authors: Uitenbroek, H.
2004IAUS..219..103U Altcode: 2003IAUS..219E.112U
No abstract at ADS
---------------------------------------------------------
Title: Observational Aspects of Waves in the Chromosphere
Authors: Uitenbroek, H.
2004ESASP.547..107U Altcode: 2004soho...13..107U
It is far from straightforward to infer the values of physical
quantities as a function of height and/or horizontal position in
the solar atmosphere from observed data. This is especially true in
the chromosphere, where low density conditions prevail and lead to a
decoupling of the radiation field from local conditions. The formation
height of a given spectral feature in the atmosphere is not a priori
known, but can only be calculated via a physical model. In general,
formation heights will fluctuate in time and vary with position in the
atmosphere, making inversions difficult. The nature of such problems
in relation to the detection of waves in the chromosphere is discussed
in this paper. Some specific examples of chromospheric diagnostics
are discussed.
---------------------------------------------------------
Title: The Accuracy of the Center-of-Gravity Method for Measuring
Velocity and Magnetic Field Strength in the Solar Photosphere
Authors: Uitenbroek, H.
2003ApJ...592.1225U Altcode:
I investigate the accuracy with which the line-of-sight velocity and
magnetic field strength in the solar photosphere can be recovered from
spatially resolved spectral line profiles with the center-of-gravity
(COG) method. For this purpose, theoretical non-LTE polarized
line profiles of a series of Fe I lines were calculated through a
two-dimensional slice from a snapshot of a three-dimensional solar
magnetoconvection simulation. The calculated profiles were analyzed
with the COG method for all positions along the slice, and retrieved
values of velocity and field strength were compared with actual values
at the heights of formation of the lines. The average formation
heights of the employed lines range from 60 to almost 400 km above
the average photospheric level. The COG method appears reliable for
measuring velocities in the lower half of these formation heights and
for measuring field strength over the whole range of heights, for
fields up to intermediate strength. Moreover, it is shown that the
COG determination is independent of spectral resolution, making it
particularly suitable for applications that require high throughput
and a correspondingly large spectral bandpass, such as high spatial
resolution observations with a large-diameter telescope. Finally,
the effect of broad-angle scattering, which includes a schematic
representation of image deterioration through seeing, on the retrieved
velocity and field strength was investigated.
---------------------------------------------------------
Title: SUMI - The Solar Ultraviolet Magnetograph Investigation
Authors: Porter, J. G.; West, E. A.; Davis, J. M.; Gary, G. A.; Noble,
M. W.; Thomas, R. J.; Rabin, D. M.; Uitenbroek, H.
2003SPD....34.2015P Altcode: 2003BAAS...35..847P
Solar physics has been successful in characterizing the full vector
magnetic field in the photosphere, where the ratio of gas pressure to
magnetic pressure (β ) is greater than 1. However, at higher levels
in the atmosphere, where β is much less than 1 and flares and CMEs
are believed to be triggered, observations are difficult, severely
limiting the understanding of these processes. In response to this
situation, we are developing SUMI (the Solar Ultraviolet Magnetograph
Investigation) a unique instrument designed to measure the circular
and linear polarization of upper chromospheric Mg II lines (280 nm) and
circular polarization of transition region C IV lines (155 nm). To date
the telescope mirrors have been built, tested and coated with dielectric
stacks designed to reflect only the wavelengths of interest. We have
also developed a unique UV polarimeter and completed the design of a
high-resolution spectrograph that uses dual toroidal varied-line-space
(TVLS) gratings. Incorporating measurements of those components
developed so far, the revised estimate of the system throughput exceeds
our original estimate by more than an order of magnitude. A sounding
rocket flight is anticipated in 2006. Our objectives and progress
are detailed in this presentation. <P />This work is supported by
NASA SR&T.
---------------------------------------------------------
Title: Observing MHD Oscillations in Sunspot
Authors: Norton, A. A.; Uitenbroek, H.
2003PADEU..13..109N Altcode:
Attempts to detect magnetohydrodynamic waves in the solar photosphere by
identifying oscillations in the magnetic field have proved problematic
due to suspected contributions from systematic temperature and density
fluctuations causing the spectral line formation height to vary, which
in turn samples a vertical gradient in the magnetic field strength. We
investigate this effect in sunspot umbrae and penumbrae through the
analysis of data obtained with the Advanced Stokes Polarimeter in
spectral lines with notoriously different temperature sensitivities. The
temporal behavior of the magnetic field strength in sunspot is presented
with special consideration to line formation physics occurring in
the dynamic solar atmosphere. These results are compared to forward
modeling of Stokes profiles with a radiative transfer code given a
sunspot atmosphere perturbed by an MHD oscillation.
---------------------------------------------------------
Title: Simultaneous Chromospheric and Photospheric Spectropolarimetry
of a Sunspot
Authors: Balasubramaniam, K. S.; Christopoulou, E. B.; Uitenbroek, H.
2003ASPC..286..227B Altcode: 2003ctmf.conf..227B
No abstract at ADS
---------------------------------------------------------
Title: Multi-level Accelerated Lambda Iteration with Partial
Redistribution
Authors: Uitenbroek, H.
2003ASPC..288..597U Altcode: 2003sam..conf..597U
When parts of a spectral line form (i.e., have optical depth near unity)
in a region of a stellar atmosphere where radiative excitation in
the line dominates over collisional excitation, effects of coherent
scattering have to be taken into account. In this paper we will
discuss the problem coherent scattering poses for multi-level
radiative transfer solutions and how this problem can be solved
efficiently. <P />Several examples will be discussed. Among them are
a comparison of radiative cooling rates due to the calcium H and K
lines computed with angle-dependent and angle-averaged redistribution,
and complete redistribution, and a demonstration of the importance of
cross-redistribution for the formation of the oxygen resonance triplet
at 130 nm.
---------------------------------------------------------
Title: Current Theoretical Models and Future High Resolution Solar
Observations: Preparing for ATST
Authors: Pevtsov, Alexei A.; Uitenbroek, Han
2003ASPC..286.....P Altcode: 2003ctmf.conf.....P
No abstract at ADS
---------------------------------------------------------
Title: Radiative Transfer Modeling of Magnetic Fluxtubes (Invited
review)
Authors: Uitenbroek, H.
2003ASPC..286..403U Altcode: 2003ctmf.conf..403U
No abstract at ADS
---------------------------------------------------------
Title: Observing MHD oscillations: the effects of vertical magnetic
gradients and thermodynamic fluctuations
Authors: Norton, A. A.; Uitenbroek, H.
2002ESASP.505..281N Altcode: 2002solm.conf..281N; 2002IAUCo.188..281N
Attempts to detect magnetohydrodynamic waves in the solar photosphere by
identifying oscillations in the magnetic field have proved problematic
due to suspected contributions from systematic temperature and density
fluctuations causing the spectral line formation height to vary, which
in turn samples a vertical gradient in the magnetic field strength. We
investigate this effect through the analysis of data obtained with the
Advanced Stokes Polarimeter in spectral lines with notoriously different
temperature sensitivities. The temporal behavior of the magnetic field
strength in sunspot and plage is presented with special consideration to
line formation physics occurring in the dynamic solar atmosphere. These
results are compared to forward modeling of Stokes profiles with a
radiative transfer code given a sunspot atmosphere perturbed by an MHD
oscillation. The possibility of a thermodynamic diagnostic is discussed.
---------------------------------------------------------
Title: Modeling the O I resonance triplet with partial redistribution
Authors: Miller-Ricci, E.; Uitenbroek, H.
2002AAS...200.3901M Altcode: 2002BAAS...34..701M
We present a slight modification to an existing multi-level radiative
transfer code that allows us to include the effects of frequency
cross-redistribution (XRD) between lines sharing the same upper
level. With this improved code we calculate theoretical profiles of
the O I resonance triplet lines at 130 nm through a hydrostatic model
of the average quiet Sun. The width of the calculated XRD profiles
show good agreement with an observed spatially averaged disk-center
spectrum obtained with the HRTS spectrograph. This is in stark contrast
to profiles calculated with complete frequency redistribution (CRD) and
ordinary partial frequency redistribution (PRD), which have Lorentzian
wings that are too broad. We find deep central reversals, contrary to
most observed profiles, but note that this discrepancy is to a large
degree the result of limited instrumental spectral resolution.
---------------------------------------------------------
Title: The Diagnostic Potential of the CaII 8542 spectral line for
Stokes I, V Spectropolarimetry
Authors: Balasubramaniam, K. S.; Uitenbroek, H.; Havey, J. W.;
Jones, H.
2002AAS...200.3807B Altcode: 2002BAAS...34R.699B
The number of available spectral lines to probe magnetic fields in the
solar chromosphere from the ground is practically limited to resonance
lines of hydrogen, calcium, sodium and magnesium and the multiplets of
helium. The range of heights over which each these spectral lines form
extends over several hundred kilometers and temperatures over several
hundreds of degrees, in a significantly non-local thermodynamic
equilibrium atmosphere. In this paper we explore the diagnostic
potential of the CaII 8542 A spectral line and the inference of active
region magnetic and velocity fields. We will provide a NLTE analysis of
the radiative transfer of this spectral line in the presence of magnetic
fields and compare theoretical and measured spectral line profiles
(observed using the NSO/KP Vacuum Telescope), to infer magnetic field
strengths using the center-of-gravity separation of the Stokes (I+V),
(I-V) components, and using the weak field approximation.
---------------------------------------------------------
Title: Chromospheric constraints from observations and modeling of
CO lines
Authors: Uitenbroek, H.
2002AAS...200.5306U Altcode: 2002BAAS...34..730U
The dark cores of infrared rotation-vibration lines of the CO molecule
that are observed close to the solar limb pose a major difficulty
for for efforts to model the solar chromosphere as one-dimensional
hydrostatic layer. If temperatures are allowed to go well below 4000K
as seems to be required by the CO lines, the reulting intensities in
the UV continua and in the Ca II resonance lines turn out to be much
lower than observed. I will discuss the severe constraints that the CO
lines pose on the one-dimensional semi-empirical modeling, and discuss
possible solutions in terms of multi-dimensional structure and/or time
dependent models. In both types of models the spectrum is averaged in
a non-linear way because of the way the formation height of radiation
is affected by the conditions in the plasma. This provides ways for
the plasma to exhibit both the qualities of hot and cold temperatures
at the same time in the case of spatial inhomogeneities, or on average
in case of time-dependent conditions.
---------------------------------------------------------
Title: The Effect of Coherent Scattering on Radiative Losses in the
Solar Ca II K Line
Authors: Uitenbroek, H.
2002ApJ...565.1312U Altcode:
We investigate the influence of partial frequency redistribution (PRD)
on radiative cooling due to the Ca II K line in a hydrostatic model of
the quiet Sun, and in a series of 20 snapshots from a chromospheric
radiation-hydrodynamics simulation. The net radiative rates in the
K line were calculated through these models with three different
frequency-redistribution formalisms: complete redistribution (CRD),
angle-averaged PRD, and angle-dependent PRD. It is found that the
approximation of CRD generally leads to an overestimate of the net
radiative rates because of its neglect of coherent scattering. Compared
to the more realistic full angle-dependent solution, angle-averaged
PRD provides accurate emergent profiles and cooling rates in the
hydrostatic model and also provides accurate instantaneous profiles in
the dynamical snapshots, even when considerable macroscopic velocities
are present. Instantaneous angle-averaged cooling rates in the dynamical
snapshots may be different at times, but on average, cooling is very
similar to the angle-dependent case. In the temperature minimum,
the Ca II K line provides, on average, more cooling in the dynamic
chromosphere at the same column mass than in the hydrostatic model. It
provides less cooling in the chromosphere. Finally, we formulate an
approximation for angle-averaged redistribution in a moving atmosphere,
employing the unshifted redistribution function. This approximation
is easy to implement in existing numerical codes and should provide an
efficient method for PRD transfer solutions in dynamic models without
loss of accuracy in emergent profiles and radiative cooling rates.
---------------------------------------------------------
Title: Improving the Numerical Modeling of the O I Resonance Triplet
in the Solar Spectrum
Authors: Miller-Ricci, Eliza; Uitenbroek, H.
2002ApJ...566..500M Altcode:
We present a slight modification to an existing multilevel radiative
transfer code that allows us to include the effects of frequency
cross redistribution (XRD) between lines sharing the same upper
level. With this improved code, we calculate theoretical profiles of
the O I resonance triplet lines at 130 nm through a hydrostatic model
of the average quiet Sun. The width of the calculated XRD profiles
show good agreement with an observed, spatially averaged disk-center
spectrum obtained with the high-resolution telescope spectrometer
(HRTS) spectrograph. This is in stark contrast to profiles calculated
with complete frequency redistribution (CRD) and ordinary partial
frequency redistribution (PRD), which have Lorentzian wings that are
too broad. We find deep central reversals, contrary to most observed
profiles, but we note that this discrepancy is to a large degree the
result of limited instrumental spectral resolution.
---------------------------------------------------------
Title: Multilevel Radiative Transfer with Partial Frequency
Redistribution
Authors: Uitenbroek, H.
2001ApJ...557..389U Altcode:
A multilevel accelerated lambda iteration (MALI) method for radiative
transfer calculations with partial frequency redistribution (PRD) is
presented. The method, which is based on Rybicki & Hummer's complete
frequency redistribution (CRD) formalism with full preconditioning,
consistently accounts for overlapping radiative transitions. Its
extension to PRD is implemented in a very natural way through the use
of the Ψ operator operating on the emissivity rather than the commonly
used Λ operator, which operates on the source function. Apart from
requiring an additional inner computational loop to evaluate the PRD
emission-line profiles with fixed population numbers, implementation
of the presented method requires only a trivial addition of computer
code. Since the presented method employs a diagonal operator, it
is easily extended to different geometries. Currently, it has been
implemented for one-, two-, and three-dimensional Cartesian grids and
spherical symmetry. In all cases, the speed of convergence with PRD is
very similar to that in CRD, with the former sometimes even surpassing
the latter. Sample calculations exhibiting the favorable convergence
behavior of the PRD code are presented in the case of the Ca II H and
K lines, the Mg II h and k lines, and the hydrogen Lyα and Lyβ lines
in a one-dimensional solar model and the Ca II resonance lines in a
two-dimensional flux-sheet model.
---------------------------------------------------------
Title: Two K Giants with Supermeteoritic Lithium Abundances: HDE
233517 and HD 9746
Authors: Balachandran, Suchitra C.; Fekel, Francis C.; Henry, Gregory
W.; Uitenbroek, Han
2000ApJ...542..978B Altcode:
Two unusual Li-rich K giants, HDE 233517 and HD 9746, have been
studied. Optical spectroscopy and photometry have been obtained
to determine the fundamental parameters of HDE 233517, a single
K2 III with an extremely large infrared excess. The spectra yield
T<SUB>eff</SUB>=4475 K, logg=2.25, [Fe/H]=-0.37, vsini=17.6 km
s<SUP>-1</SUP>, and a non-LTE logɛ(<SUP>7</SUP>Li)=4.22. Photometric
observations reveal low-amplitude light variability with a period
of 47.9 days. Combined with other parameters, this results in a
minimum radius of 16.7 R<SUB>solar</SUB> and minimum distance of 617
pc. Comparison of spectra obtained in 1994 and 1996 show profile
variations in Hα and the Na D lines indicative of changing mass
loss. Optical spectra of HD 9746, a chromospherically active giant,
were analyzed. The T<SUB>eff</SUB>=4400 K and revised Hipparcos-based
gravity of logg=2.30 lead to a non-LTE logɛ(<SUP>7</SUP>Li)=3.75. The
Li abundances in both stars are supermeteoritic. By the inclusion and
exclusion of <SUP>6</SUP>Li in the syntheses, we show that consistent
<SUP>7</SUP>Li abundances are obtained only when <SUP>6</SUP>Li
is absent in the synthetic fit. This provides evidence for fresh
<SUP>7</SUP>Li production and excludes both preservation of primordial
Li and planetary accretion as viable scenarios for the formation
of Li-rich giants. Both stars lie in close proximity to the red
giant luminosity bump supporting the hypothesis that <SUP>7</SUP>Li
production is caused by the same mixing mechanism that later results
in CN processing and lowers the <SUP>12</SUP>C/<SUP>13</SUP>C ratio
to nonstandard values.
---------------------------------------------------------
Title: The CO Fundamental Vibration-Rotation Lines in the Solar
Spectrum. II. Non-LTE Transfer Modeling in Static and Dynamic
Atmospheres
Authors: Uitenbroek, H.
2000ApJ...536..481U Altcode:
We present a numerical method for solving radiative transfer in
molecular vibration-rotation bands that allows for departures
from local thermodynamic equilibrium (LTE) while accurately
including a large number of lines. The method is applied to
the formation of the CO fundamental vibration-rotation bands in
several plane-parallel hydrostatic models and in a sequence of 20
snapshots from a radiation-hydrodynamics simulation of chromospheric
dynamics. Calculations for the hydrostatic models performed with
different values of the collisional coupling between different
vibrational states confirm earlier results in the literature showing
that the CO lines have LTE source functions in the solar atmosphere,
so emergent CO intensities reflect actual temperatures therein. Only
if the canonical collisional strengths are too large by more than 2
orders of magnitude would it be possible to explain the low temperatures
derived from CO line core intensities at the solar limb by scattering
in an atmosphere with much higher temperatures, consistent with the
values derived from UV line and continuum and Ca II resonance line
diagnostics. An interesting feature in the wavelength structure of the
CO vibration-rotation bands is pointed out, in which pairs of lines can
be found in different bands but of similar strength and wavelength. In
principle such pairs provide a diagnostic for departures from LTE
in the CO lines. CO line core intensity variations computed from the
sequence of dynamical snapshots, which represent a typical episode in
the chromospheric dynamics simulation, have an amplitude that is 2.5
times higher than observed. It is shown that this large amplitude
is due in part to the up and down shift of the CO line formation
region during the evolution of the atmosphere and is related to the
assumption of instantaneous chemical equilibrium that was assumed to
calculate CO concentrations. This suggests that the CO concentration is
not in equilibrium, may be lower than would be expected on the basis
of chemical equilibrium at the time-averaged mean temperature of the
atmosphere, and may have reduced variations compared to instantaneous
chemical equilibrium values at the local temperatures.
---------------------------------------------------------
Title: The CO Fundamental Vibration-Rotation Lines in the Solar
Spectrum. I. Imaging Spectroscopy and Multidimensional LTE Modeling
Authors: Uitenbroek, H.
2000ApJ...531..571U Altcode:
Spectroscopic imaging observations of the CO fundamental
vibration-rotation transitions at 4.6 μm, obtained at the Kitt Peak
McMath-Pierce facility, show that the dynamics of both the solar
granulation and, to a lesser extent, the 5 minute oscillations play
an important role in CO line formation. Spectroheliograms made in
the cores of strong CO lines display an inverted granular contrast
with dark areas corresponding to granule centers and a bright network
corresponding to the intergranular lanes. This observation is confirmed
by multidimensional radiative transfer modeling of CO line formation
in a solar convection-simulation snapshot. Unfortunately, current
granulation simulations do not extend to high enough layers in the
atmosphere to model formation of CO lines into the chromosphere
and close to the solar limb where they exhibit their anomalous
temperature behavior. The presented transfer calculations facilitate
the interpretation of the observed pattern, predicting that the darkest
CO line cores at disk center are associated with the strong adiabatic
expansion and cooling that occurs over granule centers when warm
upflowing material runs into the steep density gradient of the stable
layer above the photosphere. The calculated granulation intensity
contrast in the CO line cores is considerably higher than observed,
and the calculated spatially averaged line profiles at disk center are
deeper than the observed ones. It is speculated that both discrepancies
result from the assumption of instantaneous chemical equilibrium which
may not be valid in the convective flows. If the CO concentration in the
hot convective upflow cannot increase fast enough to adjust to the lower
temperatures in the radiatively cooled layer above the photosphere, CO
lines would form deeper in the atmosphere, have higher core intensities,
and show less contrast, more in agreement with observations.
---------------------------------------------------------
Title: Imaging Spectroscopy of CO Lines Compared with
Three-Dimensional Radiative Transfer
Authors: Uitenbroek, H.
1999ASPC..183..486U Altcode: 1999hrsp.conf..486U
No abstract at ADS
---------------------------------------------------------
Title: Spatially Resolved Hubble Space Telescope Spectra of the
Chromosphere of alpha Orionis
Authors: Uitenbroek, H.; Dupree, A. K.; Gilliland, R. L.
1998AJ....116.2501U Altcode:
Spatially resolved UV spectra of the supergiant alpha Orionis
(Betelgeuse) obtained in 1995 March with the Goddard High Resolution
Spectrograph (GHRS) on board the Hubble Space Telescope (HST) show
that the chromospheric emission in the Mg II h and k (around 280 nm)
lines reaches a diameter of at least ~270 milliarcseconds, about twice
the size of ultraviolet continuum images obtained at the same time with
the HST Faint Object Camera (FOC), and about 5 times the star's optical
diameter. The signature of the unresolved bright spot observed in the
1995 March FOC images occurs in the spectrum as an asymmetry in the
intensity measured across the disk at constant wavelength. Because the
spectra were obtained scanning across the stellar image, it is possible
to measure a differential line shift across the disk of the star that
can be interpreted as being due to rotation. If this interpretation
is correct, we can determine the axis of rotation of Betelgeuse and
estimate its rotational speed to be 1.2 x 10^-8 rad s^-1, corresponding
to a rotation period of 17 yr. In addition, it is plausible that
the 1995 March bright spot is congruent with the pole of the star,
suggesting that star's angle of inclination is ~20 deg to the line of
sight. Considering the small number of bright spots that are present
at any one time on the surface of Betelgeuse, and the signature of
the 1995 March spot in the Mg II resonance lines as observed with the
GHRS, it appears that such spots are not the consequence of convective
flows. Differences in the spatial distribution of the flux emerging
from different layers of the atmosphere and the line asymmetries of
the Mg II h and k lines suggest that the spot is the signature of an
outwardly propagating shock wave in an atmosphere modified by rotation.
---------------------------------------------------------
Title: The Effect of Photospheric Granulation on the Determination
of the Lithium Abundance in Solar-Type Stars
Authors: Uitenbroek, H.
1998ApJ...498..427U Altcode:
I investigate non-local thermodynamic equilibrium (non-LTE) formation
of the 670.6 nm Li I resonance doublet in the presence of convective
surface inhomogeneities in solar-type stars. This doublet is widely
used for lithium abundance determination in stars. It has been suggested
that the presence of hot and cool elements in a stellar atmosphere due
to convective heat transport might lead to an underestimate of lithium
abundance by as much as a factor of 10 when the equivalent width of
the doublet is analyzed in terms of a one-dimensional plane-parallel
model atmosphere. <P />To explore this possibility, I solved the
two-dimensional non-LTE radiative transfer equations for a sufficiently
large lithium model atom in a hydrodynamic simulation snapshot of the
solar granulation. This was done for different values of the lithium
abundance ranging from A<SUB>Li</SUB> = 0.0 to 3.3. <P />In all cases
the effects of the inhomogeneities in the atmosphere on lithium line
strength are small, never amounting to more than 0.1 dex in the derived
abundance. This occurs mainly for three reasons. First, because of the
exponential decrease of density with height in the gravitationally
stratified stellar atmosphere, radiation escapes mostly vertically
with little horizontal exchange. Some lateral transfer does occur at
the boundaries between hot and cold elements, but the effect of this
exchange on the spatially averaged line strength cancels out. It leads
to a smoothing over the surface rather than to a diminishing overall
strength of the doublet. Second, the sharp drop in temperature over
hot upwelling material, in contrast to the much shallower gradient over
the dark intergranular lanes, causes the 670.6 nm doublet to be deeper
and narrower in the former and broader in the latter. Consequently,
the contrast of equivalent line width between profiles emerging over
hot upflows and cold downflows is small. Finally, because of its small
abundance the opacity scale in lithium ionizing continua is mostly
set by H<SUP>-</SUP> bound-free processes. Optical depth unity at the
photoionization edges, therefore, follows the contours of electron
temperature, moderating contrast in the ionizing radiation field.
---------------------------------------------------------
Title: Study of Magnetic Structure in the Solar Photosphere and
Chromosphere
Authors: Noyes, Robert W.; Avrett, Eugene; Nisenson, Peter; Uitenbroek,
Han; van Ballegooijen, Adriaan
1998nasa.reptV....N Altcode:
This grant funded an observational and theoretical program to study the
structure and dynamics of the solar photosphere and low chromosphere,
and the spectral signatures that result. The overall goal is to learn
about mechanisms that cause heating of the overlying atmosphere, and
produce variability of solar emission in spectral regions important
for astrophysics and space physics. The program exploited two new
ground-based observational capabilities: one using the Swedish Solar
Telescope on La Palma for very high angular resolution observations
of the photospheric intensity field (granulation) and proxies of the
magnetic field (G-band images); and the other using the Near Infrared
Magnetograph at the McMath-Pierce Solar Facility to map the spatial
variation and dynamic behavior of the solar temperature minimum
region using infrared CO lines. We have interpreted these data using
a variety of theoretical and modelling approaches, some developed
especially for this project. Previous annual reports cover the work
done up to 31 May 1997. This final report summarizes our work for the
entire period, including the period of no-cost extension from 1 June
1997 through September 30 1997. In Section 2 we discuss observations
and modelling of the photospheric flowfields and their consequences
for heating of the overlying atmosphere, and in Section 3 we discuss
imaging spectroscopy of the CO lines at 4.67 mu.
---------------------------------------------------------
Title: Are the cool CO clouds produced by the solar granulation?
Authors: Uitenbroek, H.
1998AAS...192.1505U Altcode: 1998BAAS...30..840U
The low temperatures measured in the cores of strong vibration--rotation
lines of molecular CO near the solar limb do not agree with temperatures
measured in most other spectral features like the UV continuum,
the mm wavelength continuum and strong ionic lines like the Ca II
H & K lines, which all form at similar altitudes in the solar
atmosphere. However, the interpretation of observed intensities in
terms of atmospheric temperatures is strongly model dependent, and
with more detailed observations the one-dimensional plane-parallel
hydrostatic models used to interpret solar spectra have been shown to
be more and more at odds with the inherent dynamic and inhomogeneous
nature of the solar atmosphere. Time resolved long slit observations
of the solar CO lines near 4.6 mu m with the Near Infrared Magnetograph
(NIM) at the National Solar observatory at Kitt Peak have revealed that
the dynamics of both the five-minute oscillations and the granulation
play an important role in CO line formation. In these observations
the darkest (coolest) elements seem to be associated with strong
overshooting granules. Hydrodynamic granulation simulations have shown
that over such granules the atmosphere is cooled by the rapid horizontal
expansion of the upflowing material which is forced by the steep drop
in density with height. I have performed three-dimensional, Non-LTE
CO line formation calculations in a snapshot taken from a granulation
simulation, and will compare the calculated profiles with observed
spectra to see whether the adiabatic cooling over upwelling granules
can explain the dark CO line cores, or whether further dynamic effects
like a disparity in the chemical formation and destruction time scales
play a more decisive role.
---------------------------------------------------------
Title: Hubble Space Telescope Observations of Betelgeuse
Authors: Dupree, A. K.; Uitenbroek, H.; Gilliland, R. L.
1998psrd.conf...51D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effect of Surface Inhomogeneities on the Determination
of the Lithium Abundance in Cool Stellar Atmospheres
Authors: Uitenbroek, H.
1998ASPC..154..979U Altcode: 1998csss...10..979U
I investigate the formation of the 670.6 nm LiI resonance doublet
in the presence of stellar surface inhomogeneities. This doublet is
widely used for lithium abundance determination in stars. To explore
the possibility that the presence of hot and cool elements in a stellar
atmosphere due to convective heat transport might lead to an under-
or overestimate of lithium abundance if the equivalent width of the
doublet is analyzed in terms of a one-dimensional plane-parallel model
atmosphere, I solved the two-dimensional non-LTE radiative transfer
equations for lithium in a hydrodynamic simulation snapshot of the
solar granulation. For different lithium abundances the effects of the
inhomogeneities in the atmosphere on lithium line-strength is small,
never amounting to more than 0.1 dex in the derived abundance. This is
mainly for three reasons: - Due to the exponential decrease of density
with height in the gravitationally stratified stellar atmosphere,
radiation escapes mostly vertically with little horizontal exchange. -
The sharp drop in temperature over hot upwelling material, in contrast
to the much shallower gradient over the dark intergranular lanes,
causes the 670.6 nm doublet to be deeper and narrower in the first and
broader in the latter. Consequently, the contrast of equivalent line
width between profiles emerging over hot upflows and cold downflows is
small. - Because of its small abundance the opacity scale in lithium
ionizing continua is mostly set by H^- bound-free processes. Optical
depth unity at the photoionization edges, therefore, follows the
contours of electron temperature, moderating contrast in the ionizing
radiation field.
---------------------------------------------------------
Title: Imaging Spectroscopy of Betelgeuse in the Ultraviolet
Authors: Uitenbroek, H.; Dupree, A. K.; Gilliland, R. L.
1998ASPC..154..393U Altcode: 1998csss...10..393U
The bright supergiant Betelgeuse has been imaged in the ultraviolet
continuum and with spectroscopic resolution using the Faint Object
Camera (FOC) and the Goddard High Resolution Spectrograph (GHRS)
on the Hubble Space Telescope (HST). FOC images were obtained on two
separate occasions, in March 1995 and October 1996. A single bright
unresolved area is found in both sets of observations, although with
different position and contrast. Spatially resolved spectroscopy
obtained with the GHRS in March 1995 shows the chromospheric emission
in the Mg 2 h and k lines reaches a diameter of ~300 mas, about
twice the size of the ultraviolet continuum images. The signature
of the bright spot observed in the March 1995 FOC images occurs in
the spectrum as an asymmetry in the intensity measured across the
disk at constant wavelength. On the basis of the small number of such
hotspots that are present at any one time, and their signature in the
Mg 2 resonance lines, as observed with the GHRS, we argue that these
spots are not the consequence of convective flows as hypothesized by
\activecite{Schwarzschild1975}. Differences in the spatial distribution
of the flux between the h and k lines, rather suggest that we are
observing a non-spherically symmetric shock wave that propagates
radially outward. Because the spectra were obtained scanning across
the stellar image, it is possible to determine the axis of rotation of
Betelgeuse and estimate its rotational speed. The bright spot in March
1995 appears congruent with the pole of the star suggesting that its
angle of inclination is ~20^\circ to the line of sight.
---------------------------------------------------------
Title: Electron Temperature Distribution in Coronal Holes
Authors: Halas, C. D.; Habbal, S. R.; Penn, M.; Uitenbroek, H.; Esser,
R.; Altrock, R.; Guhathakurta, M.
1997AAS...191.7413H Altcode: 1997BAAS...29Q1326H
Knowledge of the electron temperature within coronal holes is extremely
important for constructing solar wind models and for understanding
the coronal heating process. We report on the two-dimensional CCD
observations of the Fe IX 4585, Fe X 6374, Fe XI 7892 and Fe XIV 5303
{ Angstroms} emission lines made using a coronagraph at the National
Solar Observatory at Sacramento Peak. These iron lines, which have
a peak formation temperature of 5 10(5) , 10(6) , 1.2 10(6) , and 2
10(6) K, respectively, allow the examination of different temperature
plasmas within the same large scale magnetic structure. To account for
possible line of sight ambiguities from hot material in the foreground
or background of the coronal hole, Yohkoh data were used to determine
the extent of the coronal hole along the line of sight. Intensities and
widths of these spectral lines as a function of heliocentric distance
out to 1.15 R_s will be presented. A comparison will be made between
coronal hole and streamer observations.
---------------------------------------------------------
Title: THE SOLAR Mg II h AND k LINES - Observations and Radiative
Transfer Modeling
Authors: Uitenbroek, H.
1997SoPh..172..109U Altcode: 1997ESPM....8..109U
This paper presents observations of the MgII h and k lines obtained with
the UVSP instrument that flew aboard the SMM satellite. Both spatially
averaged and resolved observations are compared with calculated profiles
of the lines from standard, plane-parallel solar models. The radiative
transfer calculations presented take full account of partial frequency
redistribution and wavelength overlap of the h and k lines. A comparison
between theoretical and observed wing profiles indicates that current
one-dimensional models underestimate the temperature in the middle
photosphere. The cores of spatially resolved dark intra-network
profiles are well reproduced by the model calculations, while the
spatially averaged profiles have in general broader emission peaks,
indicating that the additional broadening is due to a contribution of
magnetic network profiles.
---------------------------------------------------------
Title: Spatially Resolved HST Spectra of alpha Orionis' Chromosphere
Authors: Uitenbroek, H.; Dupree, A. K.; Gilliland, R. L.
1996AAS...188.7106U Altcode: 1996BAAS...28T.942U
The Hubble Space Telescope, for the first time, provides us with the
opportunity to obtain direct images of the surface of a star. Such
images in two UV wavelength bands around 255 and 280 nm respectively
have been obtained of the red supergiant alpha Ori (Betelgeuse;
HD 39801). They show a chromosphere that extends over a diameter
of at least 125 milliarcsec (mas), far beyond the 55 mas optical
disk, and they also show the presence of an unresolved bright spot
in the South-West quadrant of the disk (Gilliland &\ Dupree,
ApJ Letters, in press). In addition, spatially resolved spectra of
the Mg II resonance doublet were obtained with the GHRS by scanning
the small science aperture across the stellar disk in perpendicular
directions with 27.5 mas steps. We have analyzed these spectra and
present exciting new results here. The chromosphere as observed in the
Mg II h&k emission stretches over a diameter of at least 270 mas,
even further than the filter band images suggest. The h&k lines
(at 280.2 and 279.5 nm respectively) show blue/red asymmetries in the
height of their emission; the h line has more emission in the blue
peak, while the opposite is true for the k line. The signature of the
bright spot is revealed as an asymmetry in the flux measured across
the disk at constant wavelength. Such asymmetry is present both at
continuum wavelengths and in the h-line emission, but not in the k-line
emission. This together with the observed red/blue asymmetries prompts
us to suggest that we are observing a non-spherically symmetric shock
wave that propagates radially outward. The break in spherical symmetry
may be induced by rotation in which case we expect the bright spot
to coincide with one of the poles of the supergiant where the shock
breaks out first. Shifts we measure in the position of photospheric
lines indicate a rotation axis that is consistent with this proposition.
---------------------------------------------------------
Title: Spatially resolved HST spectra of α Orionis' chromosphere.
Authors: Uitenbroek, H.; Dupree, A. K.; Gilliland, R. L.
1996BAAS...28..942U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: PRD vs. CRD CaII K Stokes profiles from solar plage
Authors: Bruls, J. H. M. J.; Solanki, S. K.; Uitenbroek, H.
1996ASPC..109..113B Altcode: 1996csss....9..113B
No abstract at ADS
---------------------------------------------------------
Title: Temporal Variations in Solar Chromospheric Modeling.
Authors: Avrett, E.; Hoeflich, P.; Uitenbroek, H.; Ulmschneider, P.
1996ASPC..109..105A Altcode: 1996csss....9..105A
No abstract at ADS
---------------------------------------------------------
Title: Infrared MG I lines in cool giant and supergiant stars
Authors: Uitenbroek, H.; Noyes, R. W.
1996ASPC..109..723U Altcode: 1996csss....9..723U
No abstract at ADS
---------------------------------------------------------
Title: The MG i lambda 285.21 Nanometer Line: an Example of Non-LTE
Line Formation
Authors: Uitenbroek, Han; Briand, Carine
1995ApJ...447..453U Altcode:
We discuss how the Mg I λ285.21 nm line is formed in the context of
standard plane-parallel modeling. The line appears to be very sensitive
to the nonlocal radiation field determining the balance between neutral
and singly ionized magnesium. We resolve between conflicting results
in earlier λ285.21 nm line modeling by showing that, in the quite
Sun, the line forms at sufficiently low density for partial frequency
redistribution to take effect and give rise to small emission reversals
in the core. We find this to be true only if we take proper account
of UV line blanketing at the relevant Mg I ionization edges. In
this case there is good agreement between theoretical line profiles
and spatially averaged spectra from the Solar Maximum Mission (SMM)
Ultraviolet Spectrometer and Polarimeter (UVSP) and from the French
RASOLBA balloon experiment. Spatially resolved spectra obtained with
the latter instrument show considerable variation in the line core,
with emission present only in some locations and absent in others.
---------------------------------------------------------
Title: Is the Solar Chromospheric Magnetic Field Force-free?
Authors: Metcalf, Thomas R.; Jiao, Litao; McClymont, Alexander N.;
Canfield, Richard C.; Uitenbroek, Han
1995ApJ...439..474M Altcode:
We use observations of the Na I lambda 5896 spectral line, made with
the Stokes Polarimeter at Mees Solar Observatory, to measure the
chromospheric vector magnetic field in NOAA active region 7216. We
compute the magnetic field from observations of the Stokes parameters
at six wavelengths within this spectral line using a derivative method
and calculate the height dependence of the net Lorentz force in the
photosphere and low chromosphere. We conclude that the magnetic field
is not force-free in the photosphere, but becomes force-free roughly
400 km above the photosphere.
---------------------------------------------------------
Title: Imaging spectroscopy of the solar CO lines at 4.67 microns
Authors: Uitenbroek, Han; Noyes, R. W.; Rabin, Douglas
1994ApJ...432L..67U Altcode:
We analyze spatially and temporally resolved spectra of the fundamental
vibration-rotation transitions of carbon monoxide (CO) in the solar
spectrum at 4.67 micrometers. Our observations imply that, in the quiet
Sun, spatial variations in CO intensity are largely dynamical in nature,
reinforcing the suggestion that dynamical effects play a key role in
the formation of the dark CO cores. Time sequences of resolved spectra
exhibit mainly 3 minute power in line-core intensity but mainly a 5
minute period in Doppler shift. The weak 7-6 R68 line shows normal
Evershed flow in the penumbra of a sunspot; we find evidence for the
onset of inverse Evershed flow in the strong 3-2 R14 line. Spectra at
the limb indicate that 3-2 R14 emission extends approximately 360 km
beyond the continuum limb.
---------------------------------------------------------
Title: The upper photosphere and lower chromosphere of small-scale
magnetic features
Authors: Solanki, S. K.; Bruls, J. H. M. J.; Steiner, O.; Ayres, T.;
Livingston, W.; Uitenbroek, H.
1994ASIC..433...91S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New insight in the solar Tmin region from the CO lines at
4.67 micron
Authors: Uitenbroek, H.; Noyes, R. W.
1994chdy.conf..129U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Recent array-detector Observations of the solar CO Fundamental
vibration--rotation Transitions at 4.67 microns
Authors: Uitenbroek, H.; Noyes, R. W.; Rabin, D.
1993AAS...183.5902U Altcode: 1993BAAS...25.1386U
We present recent observations of lines of the fundamental
vibration--rotation transitions of carbon monoxide (CO) in the solar
atmosphere obtained with the 256(2) infrared array detector at the
McMath telescope on Kitt Peak. Standard, plane parallel, solar models
have these lines form in LTE around the temperature minimum region;
they should be indicative of electron temperatures there. However,
matching observed line profiles in a standard solar model requires
temperatures as low as 3700 K which are not confirmed by any other
spectral diagnostic. We investigate whether this discrepancy can be
solved by invoking spatial inhomogeneities or temporal variations
or a combination of both. To this end we obtained series of
spectra-spectroheliograms at different positions on the disk as well
as time series of slit-spectra at a single position. The former type
of observations allow us to study spatial inhomogeneities in stronger
and weaker lines and the IR continuum at 4.6 microns and to distinguish
between variations due to the 5-minute oscillations and the more steady
patterns due to magnetic fields by comparing heliograms taken several
minutes apart. We also obtained spectra with the slit crossing the limb
giving us a more rigid registration of the intensity variations above
the limb as compared to previous single-detector measurements. Early
analysis shows that high and low excitation lines behave differently
at the limb which may bear information on the temperature structure
of the atmosphere just above the minimum.
---------------------------------------------------------
Title: On the Measurement of the Chromospheric Magnetic Field Using
the Na I γ5896Å Spectral Line
Authors: Jiao, L.; Metcalf, T. R.; Uitenbroek, H.
1993BAAS...25.1206J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The formation of helioseismology lines. III. Partial
redistribution effects in weak solar resonance lines.
Authors: Uitenbroek, H.; Bruls, J. H. M. J.
1992A&A...265..268U Altcode:
This paper investigates the formation of the Na I D<SUB>2</SUB>,
K I λ769.9 nm and Ba II λ455.4 nm resonance lines in the solar
atmosphere allowing for partial frequency redistribution (PRD). The
authors show that the influence of PRD is negligible on the solar
disk in all three lines for a model of the average quiet-Sun, and
even for the more extreme case of a model with a steeper photospheric
temperature decline. At the solar limb PRD does affect the intensity
profile of the Na I and Ba II resonance lines, but not that of the K
I line. Finally, it is found that effects of PRD are more pronounced
at the limb in the Na I D<SUB>2</SUB> and Ba II λ455.4 nm lines when
the photospheric temperature gradient is steep.
---------------------------------------------------------
Title: CA II K Line Diagnostics of Two Dimensional Models of the
Solar Chromosphere
Authors: Solanki, S. K.; Buente, M.; Steiner, O.; Uitenbroek, H.
1992ASPC...26..294S Altcode: 1992csss....7..294S
No abstract at ADS
---------------------------------------------------------
Title: The MG II H & K Lines as Diagnostics of the Solar
Chromosphere
Authors: Uitenbroek, H.
1992ASPC...26..564U Altcode: 1992csss....7..564U
No abstract at ADS
---------------------------------------------------------
Title: Partial redistribution radiative transfer with MULTI: Method
and application to solar Mg I and II resonance lines
Authors: Uitenbroek, H.
1992sccw.conf...69U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Ca uc(ii) H<SUB>2v</SUB> and K<SUB>2v</SUB> cell grains
Authors: Rutten, Robert J.; Uitenbroek, Han
1991SoPh..134...15R Altcode:
The bright Ca II H<SUB>2v</SUB> and K<SUB>2v</SUB> grains, which
are intermittently present in the interiors of network cells in
quiet-Sun areas, should provide important diagnostics of the dynamical
interaction between the quiet photosphere and the chromosphere
above it, but their nature has so far eluded identification. We
review the extensive observational literature on these grains and on
related phenomena. We resolve various contradictions, connect hitherto
unconnected observations, distill new constraints and relate signatures
in the measurement domain to signatures in the Fourier domain. We then
review interpretations and simulation efforts, adding computations of
our own to illustrate modeling options. We conclude that the grains are
a hydrodynamical phenomenon in which magnetic fields do not play a major
role. The grains are due to interference between a pervasive standing
oscillation with about a 180 s periodicity and an 8 Mm horizontal
wavelength in the chromosphere and the wave trains of the evanescent
p-mode interference pattern in the upper photosphere. The roles of
short-period waves, shock formation and granular piston excitation
and the issue of long-lived patterning remain open; we suggest avenues
for further research.
---------------------------------------------------------
Title: Partial Redistribution Modeling of the Ca II K Line Numerical
Method and Solar Applications
Authors: Uitenbroek, H.
1991BAAS...23.1047U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The K Line of Ca II in Chromospheric Bright Points
Authors: Rossi, P.; Kalkofen, W.; Uitenbroek, H.; Bodo, G.;
Massaglia, S.
1991BAAS...23Q1050R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: K<SUB>2V</SUB> Cell Grains and Chromospheric Heating (With
1 Figure)
Authors: Rutten, R. J.; Uitenbroek, H.
1991mcch.conf...48R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Partial redistribution modeling of the Ca II K line: Numerical
method and solar applications
Authors: Uitenbroek, Han
1990PhDT........14U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar CAII Lines
Authors: Uitenbroek, Han
1990ASPC....9..103U Altcode: 1990csss....6..103U
The formation of the 854.2 nm IR line and the K line are calculated
by using the VAL3C model to manage multilevel non-LTE line transfer
with partial frequency redistribution. A partial redistribution
(PR) transfer equation and transfer operator are used to correct the
population numbers, and an extra error term drives the redistribution
of frequency over the emission-line profile. The scattering integral
introduces nonlinearities in the line formation, but PR effects of many
lines can be handled without significant increases in computer time
and memory. Calculations made using the plane parallel model show that
cross redistribution between the K line and the 854.2 nm IR line is
not significant. Ca II K(2V) bright points are also examined, and the
asymmetric profiles of the bright points suggest that their formation
involves a distinct mechanism from that of Ca II network enhancements.
---------------------------------------------------------
Title: Partial redistribution modeling of the CaII K line
Authors: Uitenbroek, Han
1990prmc.book.....U Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An efficient method for the evaluation of general
redistribution integration weights
Authors: Uitenbroek, H.
1989A&A...216..310U Altcode:
The approximations of Gouttebroze (1986) for the frequency
redistribution function RII(nu1,nu2) have been generalized to obtain a
redistribution function PII(nu1,nu2) which models the coherent (in the
atom's frame) scattering between two different spectral lines. Three
regimes of the emission frequency are treated, a core region (where the
distance, x2, from the line center in units of Doppler width is less
than 2), a wing region (where x2 is greater than 4), and a transition
region (where x2 is between 2 and 4, inclusively). An efficient method
for evaluating the associated scattering integral is proposed which
is based on these approximations, the spline representation method of
Adams et al. (1971), and a Gauss-Hermite quadrature for the integration
of the exponentially decaying parts of PII.
---------------------------------------------------------
Title: Operator perturbation method for multi-level line transfer
with partial redistribution
Authors: Uitenbroek, H.
1989A&A...213..360U Altcode:
A numerical method for solving radiative transfer problems with
partial frequency redistribution (PR) is presented. The method permits
full multilevel non-LTE solutions in plane parallel atmospheres,
and is based on the operator perturbation method of Scharmer and
Carlsson (1985). In this paper the formalism of the method, including
generalized redistribution functions is presented. As an example, line
source functions and line profiles of the Ca II K and 8542 A infrared
line are computed for various solar atmospheric models. A comparison
is made between PR profiles obtained with and without accounting for
cross-redistribution between these two lines. The method appears to
be reliable and to have all the favorable properties of the CR method
of Scharmer and Carlsson.
---------------------------------------------------------
Title: The Granulation Sensitivity of Neutral Metal Lines
Authors: Bruls, J. H. M. J.; Uitenbroek, H.; Rutten, R. J.
1989ASIC..263..311B Altcode: 1989ssg..conf..311B
No abstract at ADS
