Author name code: uitenbroek ADS astronomy entries on 2022-09-14 author:"Uitenbroek, Han" ------------------------------------------------------------------------ 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 Bibcode: 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. 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 Bibcode: 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 × 1011 erg s-1 cm-2) and ambient electron densities (1 - 60 × 1013 cm-3) 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. 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. Bibcode: 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. Title: Estimation of projection effects in the solar polar magnetic flux measurements from an ecliptic view. Authors: Gosain, Sanjay; Uitenbroek, Han Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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.
Aims: In this work we discuss the application of convolutional neural networks (CNNs) as a tool to advantageously initialize Stokes profile inversions.
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.
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.
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. 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 Bibcode: 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.
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.
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.
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. 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 Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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 2 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 2 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. 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. Bibcode: 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 × 1014 cm-3 and a temperature of ∼1.4 × 104 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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 (BH) 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 BH = 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.

This work, presented in an oral contribution at this Workshop, has been published on The Astrophysical Journal (Ishikawa et al. 2017). Title: Inferring micro-turbulent magnetic fields via the Hanle effect Authors: Kulkarni, Neeraj; Uitenbroek, Han Bibcode: 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.

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. 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. Bibcode: 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. 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. Bibcode: 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 b1, b2, and b4 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 b2 line as b1 forms at similar heights and always shows weaker polarization signals, while b4 is severely blended with photospheric lines. We also compare Mg I b2 with the K I D1 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 b2 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. Bibcode: 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 Bibcode: 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∘ instead of 90∘ 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. Bibcode: 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. Bibcode: 2017MNRAS.470.1453Q Altcode: 2017arXiv170510002Q We characterize the K I D1 & D2 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. Bibcode: 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}}}-1 (2.1+/- 0.4 \deg {{{s}}}-1, 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-1 (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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
Methods: The V-moments inversion (VMI) method uses shifts SV derived from moments of V-profiles and integrals of Q2, U2, and V2 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.
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.
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. Bibcode: 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 Bibcode: 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. Bibcode: 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 ≈104K 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 ≤1011ergcm−2s−1 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 1013ergcm−2s−1. The simulation produces bright near-ultraviolet and optical continuum emission from a dense (n >1015cm−3), 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 ≈104K 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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-1, respectively. There are two eruption components separated by ∼200 km s-1 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. Bibcode: 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 ≈ 102 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 D1 Line Authors: del Pino Alemán, T.; Trujillo Bueno, J.; Uitenbroek, H. Bibcode: 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 D1 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. Bibcode: 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. Bibcode: 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−1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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- 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2012A&A...543A.109L Altcode: 2012arXiv1205.5110L
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.
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.
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 D1 589.6 nm Spectral Line Authors: Leka, K. D.; Mickey, Donald L.; Uitenbroek, Han; Wagner, Eric L.; Metcalf, Thomas R. Bibcode: 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 D1 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 D1 Stokes spectra derived for a variety of solar-appropriate atmospheric and magnetic configurations. While the Na I D1 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. Bibcode: 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 A1 and A2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 (H2) exists in the sunspot umbra. The formation of H2 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 H2, and demonstrate that a significant population of H2 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. Bibcode: 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.
Aims: We propose a new inversion method, using functions derived from central moments (ICM), to determine magnetic field vectors with very few iterations.
Methods: Two quantities A1 and A2 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 AD can be used to interpolate between expansions that correspond to two basic models. Exponents β1 and β2 in the moment expressions are adjusted to minimize the sensitivity to data noise.
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.
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 Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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.

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. Bibcode: 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.

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. Bibcode: 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 D2 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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.

Appendix A is only available in electronic form at http://www.aanda.org Title: Magnetic field measurements at the photosphere and coronal base Authors: Judge, Philip; Centeno, R.; Tritschler, A.; Uitenbroek, H.; Jaeggli, S.; Lin, H. Bibcode: 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. Bibcode: 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.

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 Bibcode: 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. Bibcode: 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).

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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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 Bibcode: 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. Bibcode: 2010MmSAI..81..533T Altcode: 2010MmSAI..81..533R No abstract at ADS Title: Detection of chromospheric magnetic fields: a forward modeling approach . Authors: Uitenbroek, H. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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.
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.
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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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. Bibcode: 2009astro2010S...9A Altcode: No abstract at ADS Title: Evidence for a Current Sheet above a Sunspot Umbra Authors: Tritschler, A.; Uitenbroek, H.; Reardon, K. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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 D3 Multiplets Authors: Centeno, R.; Trujillo Bueno, J.; Uitenbroek, H.; Collados, M. Bibcode: 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 D3) 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 D3 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. Bibcode: 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.
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.
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.
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.
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.

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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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. Bibcode: 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.
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.
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 Bibcode: 2006ASPC..354.....L Altcode: No abstract at ADS Title: Chromospheric Diagnostics Authors: Uitenbroek, H. Bibcode: 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. Bibcode: 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. Bibcode: 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-1, 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. Bibcode: 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.
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.
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.
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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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. Bibcode: 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. Bibcode: 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. Bibcode: 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 A2Δ-X2Π 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. Bibcode: 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. Bibcode: 2004IAUS..219..103U Altcode: 2003IAUS..219E.112U No abstract at ADS Title: Observational Aspects of Waves in the Chromosphere Authors: Uitenbroek, H. Bibcode: 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. Bibcode: 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. Bibcode: 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.

This work is supported by NASA SR&T. Title: Observing MHD Oscillations in Sunspot Authors: Norton, A. A.; Uitenbroek, H. Bibcode: 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. Bibcode: 2003ASPC..286..227B Altcode: 2003ctmf.conf..227B No abstract at ADS Title: Multi-level Accelerated Lambda Iteration with Partial Redistribution Authors: Uitenbroek, H. Bibcode: 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.

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 Bibcode: 2003ASPC..286.....P Altcode: 2003ctmf.conf.....P No abstract at ADS Title: Radiative Transfer Modeling of Magnetic Fluxtubes (Invited review) Authors: Uitenbroek, H. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Teff=4475 K, logg=2.25, [Fe/H]=-0.37, vsini=17.6 km s-1, and a non-LTE logɛ(7Li)=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 Rsolar 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 Teff=4400 K and revised Hipparcos-based gravity of logg=2.30 lead to a non-LTE logɛ(7Li)=3.75. The Li abundances in both stars are supermeteoritic. By the inclusion and exclusion of 6Li in the syntheses, we show that consistent 7Li abundances are obtained only when 6Li is absent in the synthetic fit. This provides evidence for fresh 7Li 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 7Li production is caused by the same mixing mechanism that later results in CN processing and lowers the 12C/13C 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.

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 ALi = 0.0 to 3.3.

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- 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1992A&A...265..268U Altcode: This paper investigates the formation of the Na I D2, 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 D2 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. Bibcode: 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. Bibcode: 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. Bibcode: 1992sccw.conf...69U Altcode: No abstract at ADS Title: Ca uc(ii) H2v and K2v cell grains Authors: Rutten, Robert J.; Uitenbroek, Han Bibcode: 1991SoPh..134...15R Altcode: The bright Ca II H2v and K2v 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. Bibcode: 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. Bibcode: 1991BAAS...23Q1050R Altcode: No abstract at ADS Title: K2V Cell Grains and Chromospheric Heating (With 1 Figure) Authors: Rutten, R. J.; Uitenbroek, H. Bibcode: 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 Bibcode: 1990PhDT........14U Altcode: No abstract at ADS Title: The Solar CAII Lines Authors: Uitenbroek, Han Bibcode: 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 Bibcode: 1990prmc.book.....U Altcode: No abstract at ADS Title: An efficient method for the evaluation of general redistribution integration weights Authors: Uitenbroek, H. Bibcode: 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. Bibcode: 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. Bibcode: 1989ASIC..263..311B Altcode: 1989ssg..conf..311B No abstract at ADS