Author name code: avrett ADS astronomy entries on 2022-09-14 author:"Avrett, Eugene Hinton" ------------------------------------------------------------------------ Title: Erratum: "Modeling the Chromosphere of a Sunspot and the Quiet Sun" (2015, ApJ, 811, 87) Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wuelser, J. -P. Bibcode: 2016ApJ...821...70A Altcode: No abstract at ADS Title: Chromospheric Models and the Oxygen Abundance in Giant Stars Authors: Dupree, A. K.; Avrett, E. H.; Kurucz, R. L. Bibcode: 2016ApJ...821L...7D Altcode: 2016arXiv160307381D Realistic stellar atmospheric models of two typical metal-poor giant stars in Omega Centauri, which include a chromosphere (CHR), influence the formation of optical lines of O I: the forbidden lines (λ6300, λ6363) and the infrared triplet (λλ7771-7775). One-dimensional semi-empirical non-local thermodynamic equilibrium (LTE) models are constructed based on observed Balmer lines. A full non-LTE formulation is applied for evaluating the line strengths of O I, including photoionization by the Lyman continuum and photoexcitation by Lyα and Lyβ. Chromospheric models (CHR) yield forbidden oxygen transitions that are stronger than those in radiative/convective equilibrium (RCE) models. The triplet oxygen lines from high levels also appear stronger than those produced in an RCE model. The inferred oxygen abundance from realistic CHR models for these two stars is decreased by factors of ∼3 as compared to values derived from RCE models. A lower oxygen abundance suggests that intermediate-mass AGB stars contribute to the observed abundance pattern in globular clusters. A change in the oxygen abundance of metal-poor field giants could affect models of deep mixing episodes on the red giant branch. Changes in the oxygen abundance can impact other abundance determinations that are critical to astrophysics, including chemical tagging techniques and galactic chemical evolution. Title: Modeling the Chromosphere of a Sunspot and the Quiet Sun Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wülser, J. -P. Bibcode: 2015ApJ...811...87A Altcode: Semiempirical atmospheric modeling attempts to match an observed spectrum by finding the temperature distribution and other physical parameters along the line of sight through the emitting region such that the calculated spectrum agrees with the observed one. In this paper we take the observed spectrum of a sunspot and the quiet Sun in the EUV wavelength range 668-1475 Å from the 2001 SUMER atlas of Curdt et al. to determine models of the two atmospheric regions, extending from the photosphere through the overlying chromosphere into the transition region. We solve the coupled statistical equilibrium and optically thick radiative transfer equations for a set of 32 atoms and ions. The atoms that are part of molecules are treated separately, and are excluded from the atomic abundances and atomic opacities. We compare the Mg ii k line profile observations from the Interface Region Imaging Spectrograph with the profiles calculated from the two models. The calculated profiles for the sunspot are substantially lower than the observed ones, based on the SUMER models. The only way we have found to raise the calculated Mg ii lines to agree with the observations is to introduce illumination of the sunspot from the surrounding active region. Title: Sleuthing Helium in Cluster Red Giants Authors: Dupree, Andrea K.; Avrett, Eugene; Smith, Graeme; Strader, Jay Bibcode: 2015IAUGA..2257108D Altcode: A prediction is tested that a second generation of stars in globular clusters contains an enhanced abundance of helium. We report a near-infrared study of helium in red giants of the globular cluster Omega Centauri. In this cluster, the presence of helium is found to be associated with increased abundances of [Na/Fe] and [Al/Fe]. A realistic quantitative calculation of the helium line profile formed in the chromosphere of the red giants indicates that the element helium is enhanced by ≧0.17 dex in Omega Cen, signaling that high temperature H-burning has occurred in a previous generation of stars. The Helium transition at 1.08μ also offers a sensitive probe of the presence of stellar winds. Title: Chromospheres of Luminous Cool Stars Authors: Dupree, Andrea K.; Avrett, Eugene Bibcode: 2015IAUGA..2254979D Altcode: Ultraviolet imaging of Alpha Orionis (Betelgeuse) reveals a complex variable chromospheric structure. Such atmospheres in luminous cool stars can affect features in the optical spectrum. Constructing semi-empiricalmodel atmospheres of luminous stars including the temperature rise due to a chromosphere allows us to predict potential effects on optical transitions. The radiative transfer code, PANDORA, calculates line strengths in a LTE or non-LTE formulation, spherical symmetry, and includes velocity fields when present. Various aspects of the line calculations and their impact on equivalent widths will be discussed including developing appropriate chromospheric models, comparison to a pure radiative equilibrium model, transitions sensitive to non-LTE and the effects of a realistic spherical non-LTE approximation as compared to a plane-parallel approximation. We discuss the extent to which a chromosphere can impact the determination of stellar abundances. Title: Calculated Sunspot and Quiet-Sun Mg II Profiles Compared With IRIS DataEugene Avrett and Hui TianHarvard-Smithsonian Center for Astrophysics60 Garden Street, Cambridge, MA 02138 Authors: Avrett, Eugene H. Bibcode: 2014AAS...22442202A Altcode: A new sunpsot model has been derived, consistent with the SUMER atlas data of Curdt, et al. and Mg II profile data from IRIS. Comparisons are made with Quiet-Sun results from both sources. It is necessary to include molecules in the sunspot model not only account for the low brightness temperatures near 1850 /AA but also for the density variations higher in the atmosphere. The minimum temperature is roughly 2500 K in the sunspot model and 4500 K for the quiet Sun. The Mg II H line profile is centrally reversed in both cases, with the peak intensity originating where the temperature rises abruptly from the minimum value. The line center is formed at the top of the chromosphere where the temperature rises abruptly from 10,000 K into the chromosphere-corona transition region. The calculated Mg II line center intensity is much smaller than observed, for models constrained by the EUV continuum data. Title: Calculated Resonance Line Profiles of [Mg II], [C II], and [Si IV] in the Solar Atmosphere Authors: Avrett, E.; Landi, E.; McKillop, S. Bibcode: 2013ApJ...779..155A Altcode: NASA's Interface Region Imaging Spectrograph space mission, launched 2013 June 27, is intended to study the structure of the solar chromosphere and the transition region between the chromosphere and corona. The spectral lines to be observed include the Mg II k line at 2796.5 Å, the C II 1334.5 Å line, and the Si IV line at 1393.8 Å, which are formed in the middle chromosphere, the upper chromosphere, and the lower transition region, respectively. Here we calculate the profiles of these lines from four models of the solar atmosphere, intended to represent the faint and mean internetwork, a network lane, and bright network. We show how the profiles change from the center of the solar disk toward the limb of the Sun and in response to outflows and inflows. These results are intended to cover the range of expected quiet-Sun observations and assist in their interpretation. We expect that the observations will lead to improvements in the models, which can then be used to estimate the required non-radiative heating in the different regions. Title: Direct Evaluation of the Helium Abundances in Omega Centauri Authors: Dupree, A. K.; Avrett, E. H. Bibcode: 2013ApJ...773L..28D Altcode: 2013arXiv1307.5860D A direct measure of the helium abundances from the near-infrared transition of He I at 1.08 μm is obtained for two nearly identical red giant stars in the globular cluster Omega Centauri. One star exhibits the He I line; the line is weak or absent in the other star. Detailed non-local thermal equilibrium semi-empirical models including expansion in spherical geometry are developed to match the chromospheric Hα, Hβ, and Ca II K lines, in order to predict the helium profile and derive a helium abundance. The red giant spectra suggest a helium abundance of Y <= 0.22 (LEID 54064) and Y = 0.39-0.44 (LEID 54084) corresponding to a difference in the abundance ΔY >= 0.17. Helium is enhanced in the giant star (LEID 54084) that also contains enhanced aluminum and magnesium. This direct evaluation of the helium abundances gives observational support to the theoretical conjecture that multiple populations harbor enhanced helium in addition to light elements that are products of high-temperature hydrogen burning. We demonstrate that the 1.08 μm He I line can yield a helium abundance in cool stars when constraints on the semi-empirical chromospheric model are provided by other spectroscopic features. Title: SDO/EVE spectra of solar flares Authors: Heinzel, Petr; Avrett, Eugene; Dzifcakova, Elena; Hudson, Hugh S. Bibcode: 2012cosp...39..743H Altcode: 2012cosp.meet..743H For selected flare events we present the SDO/EVE spectra of hydrogen and helium resonance continua and compare them with the results of non-LTE transfer computations based on various flare models. We discuss the formation of these continua and their diagnostic potential for determination of the temperature structure of the flaring atmosphere. Under the optically-thin conditions, we synthesize these resonance continua using the CHIANTI database where we added the relevant non-thermal atomic rates. This shows how the electron or proton beams may affect the continua during an impulsive phase of solar flares. As a benchmark we present our comparison of the quiet-Sun EVE spectra with the flux synthesized from the model C6 of Avrett and Loeser (2008). Our study has a more general relevance to the physics of stellar flares. Title: Optical-to-Radio Continua in Solar Flares Authors: Heinzel, P.; Avrett, E. H. Bibcode: 2012SoPh..277...31H Altcode: Spectral continua observed during solar flares may contain information about both thermal and non-thermal heating mechanisms. Using two semi-empirical flare models F2 and FLA, we synthesize the thermal continua from optical to mm-radio domains and compare their intensities with quiet-Sun values computed from a recent model C7. In this way, the far-infrared and sub-mm/mm continua are studied for the first time, and we present our results as a benchmark for further modeling and for planning new observations, especially with the ALMA instrument. Finally, we demonstrate how these continua are formed and show a close correspondence between their brightness temperature and the kinetic-temperature structure of the flaring atmosphere. Title: The nature of hydrogen and helium continua in SDO/EVE spectra of solar flares Authors: Heinzel, P.; Avrett, E. H. Bibcode: 2012decs.confE.109H Altcode: For selected flare events we present the SDO/EVE spectra of hydrogen and helium resonance continua and compare them with the results of the non-LTE transfer computations based on semi-empirical flare models. We discuss the formation of these continua and their diagnostic potential for determination of the temperature structure of the flaring atmosphere. Non-thermal processes are also considered. As a benchmark we present our comparison of the quiet-Sun EVE spectra with the flux synthesized from the model C6 of Avrett and Loeser (2008). Title: Photometric analysis of Ellerman bombs . Authors: Berlicki, A.; Heinzel, P.; Avrett, E. H. Bibcode: 2010MmSAI..81..646B Altcode: Observations of Ellerman bombs (EBs) show them as short-lived, compact, and spatially localized emissions that are well observable in the wings of the Halpha hydrogen line. The Halpha line profiles of EBs are characterized by deep absorption at the line center and enhanced emission in the wings with maximum around ± 1 Å from the line center, fading beyond ± 5 Å. EBs may also be observed in the chromospheric Ca II lines and in the UV as bright points often located within active regions. Previous work suggests that EBs may be considered as micro-flares and may contribute significantly to the heating of the lower chromosphere in newly emerging magnetic flux regions. However, it is still not clear at what height in the solar atmosphere the emission of EBs originates. In our analysis we used observations of EBs obtained in the Halpha line with the Dutch Open Telescope (DOT) and in the UV range with the TRACE 1600 Å channel. These one-hour long simultaneous sequences obtained with high temporal and spatial resolution were used to analyze the relation between the emission in the Halpha line and at 1600 Å. The observations show fast variations of EB emission in both channels. Comparison between the observed emission in Halpha and at 1600 Å and theoretical calculations allowed us to draw conclusions about the vertical structure of EBs. Title: Particle diffusion, flows, and NLTE calculations Authors: Fontenla, J. M.; Avrett, E. Bibcode: 2009AIPC.1171...61F Altcode: Particle diffusion plays an important role in the transition-region between the chromosphere and the corona of the Sun and solar-type stars. Also, mass flows are often observed and these in combination with diffusion make it necessary to perform physically consistent calculations to determine the elemental ionization and atom/ion level populations. Such calculations require the solution of NLTE radiative transfer equations to determine the ionization and level populations, and need to include the effects of particle transport processes. This paper discusses the basis and implementation of the method used in the Solar Radiation Physical Modeling (SRPM) project, also implemented for H and He in the PANDORA code. Title: Mass Outflow from Red Giant Stars in M13, M15, and M92 Authors: Mészáros, Sz.; Avrett, E. H.; Dupree, A. K. Bibcode: 2009AJ....138..615M Altcode: 2009arXiv0906.3420M Chromospheric model calculations of the Hα line for selected red giant branch and asymptotic giant branch (AGB) stars in the globular clusters M13, M15, and M92 are constructed to derive mass loss rates (MLRs). The model spectra are compared to the observations obtained with the Hectochelle on the MMT telescope. These stars show strong Hα emissions and blueshifted Hα cores signaling that mass outflow is present in all stars. Outflow velocities of 3-19 km s-1, larger than indicated by Hα profiles, are needed in the upper chromosphere to achieve good agreement between the model spectra and the observations. The resulting MLRs range from 0.6 × 10-9 to 5 × 10-9 M sun yr-1, which are about an order of magnitude lower than predicted from "Reimers' law" or inferred from the infrared excess of similar stars. The MLR increases slightly with luminosity and with decreasing effective temperature. Stars in the more metal-rich M13 have higher MLRs by a factor of ~2 than in the metal-poor clusters M15 and M92. A fit to the MLRs is given by \dot{M} (M sun yr-1) = 0.092 ×L 0.16 × T -2.02 eff × A 0.37, where A=10^{[Fe/H]}. Multiple observations of stars revealed one object in M15, K757, in which the mass outflow increased by a factor of 6 between two observations separated by 18 months. Other stars showed changes in MLR by a factor of 1.5 or less. Title: Iterative Solution of Multilevel Transfer Problems Authors: Avrett, Eugene H.; Loeser, Rudolf Bibcode: 2009nrt..book..135A Altcode: No abstract at ADS Title: Rapidly-Converging Methods for Solving Multilevel Transfer Problems Authors: Avrett, Eugene H. Bibcode: 2009nmmr.conf..217A Altcode: It is well known that lambda iterations can be used to solve multilevel non-LTE transfer equations in a reasonable number of iterations when the lambda operator is preconditioned, e.g., when the diagonal part of the operator is combined with other terms analytically. This approach is currently used successfully for the solution of model atoms with many line transitions, but sometimes a very large number of iterations is needed. Lambda iteration consists of alternate solutions of the separate transfer and rate equations. For any given line transition the transfer and rate equations can be combined so that a solution can be obtained directly for that transition with no iterations needed between the transfer and rate equations. However, iterations are needed to determine the coupling between transitions. This can be time-consuming for model atoms with a large number of transitions that are treated in this way. Here we show that 1) a hybrid approach involving such a direct solution for a few of the strongest transitions, and lambda iterations for the rest, gives rapid convergence, often with oscillations that need to be damped, and 2) this approach should include preconditioning of the lambda operator that occurs in the radiative coupling terms. We illustrate these results with a simple three-level hydrogen atom and a finite, plane-parallel, symmetric atmosphere resembling a solar prominence, with a temperature of 8,000 K at the center, rising to very large values at each boundary (so that hydrogen is only partly ionized at the center and fully ionized at each boundary). Lambda iterations essentially fail to give a solution for this problem, while the hydrid solution converges in 5 to 10 iterations. Title: Absolute Physical Calibration in the Infrared Authors: Rieke, G. H.; Blaylock, M.; Decin, L.; Engelbracht, C.; Ogle, P.; Avrett, E.; Carpenter, J.; Cutri, R. M.; Armus, L.; Gordon, K.; Gray, R. O.; Hinz, J.; Su, K.; Willmer, Christopher N. A. Bibcode: 2008AJ....135.2245R Altcode: 2008arXiv0806.1910R We determine an absolute calibration for the Multiband Imaging Photometer for Spitzer 24 μm band and recommend adjustments to the published calibrations for Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRAS photometry to put them on the same scale. We show that consistent results are obtained by basing the calibration on either an average A0V star spectral energy distribution (SED), or by using the absolutely calibrated SED of the Sun in comparison with solar-type stellar photometry (the solar analog method). After the rejection of a small number of stars with anomalous SEDs (or bad measurements), upper limits of ~1.5% root mean square (rms) are placed on the intrinsic infrared (IR) SED variations in both A-dwarf and solar-type stars. These types of stars are therefore suitable as general-purpose standard stars in the IR. We provide absolutely calibrated SEDs for a standard zero magnitude A star and for the Sun to allow extending this work to any other IR photometric system. They allow the recommended calibration to be applied from 1 to 25 μm with an accuracy of ~2%, and with even higher accuracy at specific wavelengths such as 2.2, 10.6, and 24 μm, near which there are direct measurements. However, we confirm earlier indications that Vega does not behave as a typical A0V star between the visible and the IR, making it problematic as the defining star for photometric systems. The integration of measurements of the Sun with those of solar-type stars also provides an accurate estimate of the solar SED from 1 through 30 μm, which we show agrees with theoretical models. Title: Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme-Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen Authors: Avrett, Eugene H.; Loeser, Rudolf Bibcode: 2008ApJS..175..229A Altcode: We present the results of optically thick non-LTE radiative transfer calculations of lines and continua of H, C I-IV, and O I-VI and other elements using a new one-dimensional, time-independent model corresponding to the average quiet-Sun chromosphere and transition region. The model is based principally on the Curdt et al. SUMER atlas of the extreme ultraviolet spectrum. Our model of the chromosphere is a semiempirical one, with the temperature distribution adjusted to obtain optimum agreement between calculated and observed continuum intensities, line intensities, and line profiles. Our model of the transition region is determined theoretically from a balance between (a) radiative losses and (b) the downward energy flow from the corona due to thermal conduction and particle diffusion, and using boundary conditions at the base of the transition region established at the top of the chromosphere from the semiempirical model. The quiet-Sun model presented here should be considered as a replacement of the earlier model C of Vernazza et al., since our new model is based on an energy-balance transition region, a better underlying photospheric model, a more extensive set of chromospheric observations, and improved calculations. The photospheric structure of the model given here is the same as in Table 3 of Fontenla, Avrett, Thuiller, & Harder. We show comparisons between calculated and observed continua, and between the calculated and observed profiles of all significant lines of H, C I-IV, and O I-VI in the wavelength range 67-173 nm. While some of the calculated lines are not in emission as observed, we find reasonable general agreement, given the uncertainties in atomic rates and cross sections, and we document the sources of the rates and cross sections used in the calculation. We anticipate that future improvements in the atomic data will give improved agreement with the observations. Title: Log-normal intensity distribution of the quiet-Sun FUV continuum observed by SUMER Authors: Fontenla, J. M.; Curdt, W.; Avrett, E. H.; Harder, J. Bibcode: 2007A&A...468..695F Altcode: We analyse observations of the quiet-Sun far ultraviolet (FUV) continuum at various wavelengths near 1430 Å obtained by the SUMER instrument on SOHO. According to semi-empirical atmospheric models this continuum originates from the layers in the chromosphere where the temperature rises from low values at near-radiative equilibrium to a plateau of about 6000 K. We study raster images and intensity distribution histograms and find that a single log-normal distribution matches these observations very well, and that the spatial structure observed corresponds to a mixture of features at supergranular and smaller scales that probably correspond to granular clusters. Also, a log-normal distribution was found in the literature to correspond to other chromospheric features and we compare here with histograms obtained from a H i Ly-α quiet-Sun image. Because the continuum around 1430 Å is mainly produced by Si i recombination it is expected to respond well to deep chromospheric heating and not be directly affected by velocities. The data suggest that chromospheric heating originates through dissipation of magnetic free-energy fields of small size and magnitude in underlying photospheric intergranular lanes. It has been suggested that such fields can be produced by photospheric dynamos at the intergranular scale and/or by complex fields emerging in a “magnetic carpet”. Such fields are expected to have sufficient free-energy to power the chromospheric heating. Plasma instabilities, such as the Farley-Buneman instability, would allow this free-energy to be dissipated in the chromosphere. Title: New Models of the Solar Chromosphere and Transition Region from SUMER Observations Authors: Avrett, E. H. Bibcode: 2007ASPC..368...81A Altcode: The SUMER atlas of the disk-center spectrum of the average quiet Sun between 67 and 161 nm, together with HRTS spectra to 173 nm, provide valuable observational data that can be used to determine the structure of the chromosphere, transition region, and low corona. The Pandora atmospheric modeling program is being used to determine atmospheric models and calculated spectra consistent with these observations, resulting in a new model of the average quiet chromosphere and transition region in better agreement with observations than before. We find that temperature variations of 400 K cause the computed chromospheric intensities, both in lines and the continuum, to change by a factor of about 4. This is greater than the observed intensity variations at chromospheric wavelengths 1) with time in quiet regions, and 2) with position from cell centers to bright network. These results appear to conflict with dynamical models that predict time variations of 1000 K or more in the chromosphere. Also, we demonstrate that the optically-thin or coronal approximation that is often used to analyze observed line intensities from high stages of ionization is reasonably accurate only: 1) for C IV and higher carbon ions, and C III for T > 50,000 K, and 2) for O V and higher oxygen ions, and O IV for T > 150,000 K.. Title: A Contribution to the Understanding of Chromospheric Oscillations Authors: Baudin, F.; Ibarra, E.; Avrett, E. H.; Vial, J. -C.; Bocchialini, K.; Costa, A.; Lemaire, P.; Rovira, M. Bibcode: 2007SoPh..241...39B Altcode: We present SUMER/SOHO UV measurements of chromospheric oscillations of intensity, velocity, and linewidth observed in C I, S I, O I, and C II lines, which are formed in the altitude range from 1000 km to 2000 km above τ500=1. Oscillations in lines originating at similar altitudes exhibit different behaviors which we discuss in terms of the formation of the lines. Title: The Structured Chromosphere and Wind of TW Hya Authors: Dupree, A. K.; Avrett, E. H.; Brickhouse, N. S.; Cranmer, S. R.; Szalai, T. Bibcode: 2007astro.ph..2395D Altcode: A continuous set of echelle spectra of TW Hya, taken with the MIKE spectrograph on the Magellan2/Clay telescope at Las Campanas Observatory in April 2006 reveals systematic variations in the flux, velocity, and profile of the H-alpha emission line which appear to be consistent with the photometric period of 2.8 days. Absorption features recur at high outflow velocities in the wind. This behavior suggests that: (a) accretion is not uniformly distributed over the stellar hemisphere in view; (b) stable structures are present in the chromosphere, most likely due to the stellar magnetic field configuration. Semi-empirical models of the atmosphere are constructed to reproduce line profiles of H-alpha and He I, 10830A and to define the wind structure. These preliminary calculations suggest the mass loss rate is variable and comparable to H-alpha mass accretion rates in the literature, requiring a very efficient mechanism if the wind is powered only by accretion. Title: New Models of the Solar Chromosphere and Transition Region Determined from SUMER Observations Authors: Avrett, E. H. Bibcode: 2006ESASP.617E...9A Altcode: 2006soho...17E...9A No abstract at ADS Title: Semi-empiric Radiative Transfer Modeling of FUSE Stellar Spectra Authors: Lobel, A.; Avrett, E. H.; Aufdenberg, J. P. Bibcode: 2006ASPC..348..171L Altcode: 2005astro.ph..4525L We present an overview of radiative transfer modeling efforts to interpret spectra of a variety of stellar objects observed with FUSE. Detailed radiative transfer modeling of high ion emission line profiles of C III and O VI observed in the far-UV spectrum, provides a powerful means to probe the thermal and dynamic properties of high-temperature plasmas in the atmospheres of stars. We model asymmetric emission lines of C III λ977 (and Mg II h & k) observed in spectra of luminous cool stars such as α Aqr, to infer the wind- and microturbulence velocity structures of the upper chromosphere. Semi-empiric radiative transfer models that include transition region temperature conditions, are further developed based on detailed fits to O VI resonance emission lines in the supergiant α Aqr, the classical Cepheid variable β Dor, and to self-absorbed O VI emission lines in the cataclysmic variable SW UMa.

We observe that the C III resonance line profile of α Aqr assumes a remarkable asymmetric shape, reminiscent of P Cygni type profiles observed in hot luminous supergiants. The model calculations indicate outflow velocities above ∼140 km s-1 at kinetic temperatures of 65 kK and higher. Based on detailed model fits to the narrow red-shifted and self-absorbed O VI emission lines of SW UMa we compute that the gas- and electron density exceed the density conditions of the upper solar transition region by about three orders of magnitude. We propose that the large gas density of ρ≃1.4 10-11 g cm-3 favors a region of warm dense plasma of 100 kK ≤Tgas≤ 300 kK that collapses onto the white dwarf with a mass accretion rate of 1-2 1015 g s-1 above or between the accretion disk. We discuss how detailed semi-empiric fits to emission lines observed with the high spectral resolution of FUSE can provide reliable constraints on the mass loss or mass accretion rates in these objects. Title: New Models Of The Solar Chromosphere And Transition Region Determined From Sumer Observations Authors: Avrett, Eugene H. Bibcode: 2006SPD....37.0201A Altcode: 2006BAAS...38..221A The SUMER atlas of Curdt et al.(2001) giving the solar disk-center spectrum between 67 and 161 nm, together with HRTS spectra to 173 nm, is an extremely valuable source of observational data that can be used to determine the structure of the upper photosphere, chromosphere, transition region, and low corona. The Pandora atmospheric modeling program is being used to determine atmospheric models and calculated spectra consistent with these observations, resulting in new models of the solar atmosphere in much better agreement with observations than before. We find that temperature variations of 400 K cause the computed chromospheric intensities to change by a factor of 4. This is greater than the intensity variations at chromospheric wavelengths 1) with time in quiet regions, and 2) with position from cell centers to bright network. These results appear to conflict with dynamical models that have time variations of 1000 K or more in the low chromosphere. Title: Identification of the broad solar emission features near 117 nm Authors: Avrett, E. H.; Kurucz, R. L.; Loeser, R. Bibcode: 2006A&A...452..651A Altcode: 2006astro.ph..3361A Wilhelm et al. have recently called attention to the unidentified broad emission features near 117 nm in the solar spectrum. They discuss the observed properties of these features in detail but do not identify the source of this emission. We show that the broad autoionizing transitions of neutral sulfur are responsible for these emission features. Autoionizing lines of ion{S}{i} occur throughout the spectrum between Lyman alpha and the Lyman limit. Sulfur is a normal contributor to stellar spectra. We use non-LTE chromospheric model calculations with line data from the Kurucz 2004 ion{S}{i} line list to simulate the solar spectrum in the range 116 to 118 nm. We compare the results with SUMER disk-center observations from Curdt et al. and limb observations from Wilhelm et al. Our calculations generally agree with the SUMER observations of the broad autoionizing ion{S}{i} emission features, the narrow ion{S}{i} emission lines, and the continuum in this wavelength region, and agree with basic characteristics of the center-to-limb observations. In addition to modeling the average spectrum, we show that a change of ±200 K in the temperature distribution causes the intensity to change by a factor of 4. This exceeds the observed intensity variations 1) with time in quiet regions at these wavelengths, and 2) with position from cell centers to bright network. These results do not seem compatible with current dynamical models that have temporal variations of 1000 K or more in the low chromosphere. Title: Semiempirical Models of the Solar Atmosphere. I. The Quiet- and Active Sun Photosphere at Moderate Resolution Authors: Fontenla, J. M.; Avrett, E.; Thuillier, G.; Harder, J. Bibcode: 2006ApJ...639..441F Altcode: In this paper we study and modify previous semiempirical models of the solar photosphere as observed at moderate spatial and temporal resolution (~3" and ~30 minutes, respectively) in the main quiet- and active Sun component features. Our present models are constructed to match the relevant available observations at this resolution for which a one-dimensional and time-independent stratification is reasonable. The models do not describe the fine structure and temporal variability observed in high-resolution images but correspond to a ``radiation averaging'' over the fine-structure and p-mode variations. We use the observed limb darkening in the range 0.3-2.4 μm, as well as the absolute intensities and details of the spectral continua and lines in this range, to validate and adjust the models. Using the method described in a previous paper, we compute the emergent radiation from our models in full detail for the visible and IR continuum and the lines in the interval 0.3-5 μm for which we have atomic data from NIST (~13,000 lines used) and molecular data from HITRAN and Gray & Corbally (~480,000 molecular lines used). The observations, abundances, and atomic/molecular data are improved over previous work and yield models that better fit the observations. In addition, we construct a new penumbra model. The visible and IR detailed spectra computed from these models provide insight for understanding the effects of magnetic fields on the solar irradiance and are useful tools for computing synthetic spectral irradiances in different solar activity configurations. Title: Detailed Modeling of Fast Hot Winds from T Tauri Stars Authors: Dupree, A. K.; Avrett, E. H.; Cranmer, S. R. Bibcode: 2005AAS...207.7413D Altcode: 2005BAAS...37.1286D Infrared and ultraviolet spectra recently revealed the presence of hot (300,000 K), fast (450 km s-1) winds from 2 classical (accreting) T Tauri stars: TW Hya and T Tau. The mass loss rate attributed to these outflows may be related to the mass accretion rate, but this and other possible explanations can only be tested by determining the mass loss rates, wind speeds, and temperature variations in the outer atmospheres of these stars. In addition, a sufficiently robust wind may lead to optical jets, could remove accreted angular momentum from the star, might contribute to the opacity needed for X-ray absorption, and may influence the diminution of dust in accretion disks. We are constructing detailed models of wind-sensitive line profiles for expanding and rotating atmospheres using both: (1) the Avrett/Loeser PANDORA code for full non-LTE effects, and (2) a simpler line-specific code that will allow complex three-dimensional mass flows to be varied with minimal computational expense. These models will constrain both the atmospheric structure and the mass loss rate indicated by the observed P Cygni line profiles.

This research is supported in part by NASA and the Smithsonian Institution. Title: Radiative Transfer Modeling of Warm Transition Region Winds in F- and G-type Supergiants Authors: Lobel, A.; Avrett, E. H.; Aufdenberg, J. P. Bibcode: 2004AAS...205.5207L Altcode: 2004BAAS...36.1423L We present FUSE spectra of upper transition region emission lines of O VI in the dynamic atmosphere of the short-period classic Cepheid Beta Dor (F-G Ia). The far-UV O VI 1032 & 1037 Å lines indicate a heating mechanism in the outer atmospheres of strongly pulsating F- and G-type supergiants sustaining hot plasmas at kinetic gas temperatures between 100 kK and 300 kK. Our observation of prominent upper transition region emission lines in Beta Dor contrasts with the very low X-ray luminosities of Cepheid variables that signal only weak coronal plasmas.

On the other hand, FUSE and HST-STIS observations of the non-variable yellow (hybrid) supergiants Alpha Aqr (G2 Ib) and Beta Aqr (G0 Ib), having large X-ray fluxes, reveal supersonic warm wind velocities of 140 km/s and 90 km/s, respectively, in lower transition region emission lines of C III 977 Å and Si III 1206 Å. Our semi-empiric radiative transfer models show that these optically thick winds occur at kinetic gas temperatures well above 70 kK, much larger than assumed for the chromospheres of cool supergiants. Remarkably, these emission lines reveal peculiar shapes reminiscent of P-Cygni type line profiles observed in UV spectra of hot supergiants. Both hybrid supergiants lack the strongly oscillating photospheres of Cepheids, suggesting that their transition region wind acceleration and heating do not result from a pure mechanical driving mechanism due to atmospheric pulsations.

We present detailed semi-empiric radiative transfer models of the thermal and dynamic structures of the outer atmospheres of these luminous F- and G-type supergiants based on the FUSE and HST-STIS spectra. We investigate if warm accelerating winds observed in high ions of cool supergiants can (partly) be driven by radiation pressure.

This research is based on data obtained with the NASA/ESA Hubble Space Telescope, collected at the STScI, operated by AURA Inc., under contract NAS5-26555. Financial support has been provided by STScI grant HST-GO-10212.01-A and NASA FUSE grant GI-D107. Â Title: An atmospheric model for UZ Librae from mean Hα-line profiles Authors: Zboril, M.; Strassmeier, K. G.; Avrett, E. H. Bibcode: 2004A&A...421..295Z Altcode: We present the results from fitting a semi-empirical atmospheric model including a chromosphere and a transition region to the mean (seasonal) Balmer Hα line profiles of the RS CVn-type K0-giant UZ Librae. As a first step, a static 1D spherical model was applied to the mean component of the Hα-emission core and its profile. The main result of the fitting is that the transition region begins at a log mass depth of -1.8 g cm-2 at a temperature of 7400 K (approximately 3000 K warmer than the photosphere) and then has a steep increase to the peak temperature of ≈106 K. A stellar model in plane-parallel mode with ``partial-frequency redistribution'' option in the line transfer gave roughly the best fit. Subsequently, two-Gaussian fitting of the phase-dependent Hα -line profiles yields a complex velocity field. The radial velocities, from both the absorption reversal and the main emission component, display rotational and/or orbital modulation. The largest differences between consecutive line profiles occur mostly in the red line wings, suggesting the existence of both an inward-pointed velocity field and sporadic radiation events possibly related to flares. The total Hα emission, as derived from the equivalent width, possibly also displays rotational modulation. Indirect evidence is presented for the existence of circumstellar matter in form of an Hα absorption shell. Finally, a preliminary elemental abundance analysis suggests sub-solar metallicity of [M/H]≈ -0.5 dex based on ATLAS models, or -0.7 dex based on PHOENIX models. Title: The center-to-limb behavior of solar active regions at ultraviolet wavelengths Authors: Crane, P. C.; Floyd, L. E.; Cook, J. W.; Herring, L. C.; Avrett, E. H.; Prinz, D. K. Bibcode: 2004A&A...419..735C Altcode: The time series of solar ultraviolet irradiances measured by the Solar Ultraviolet Spectral Irradiance Monitor on the Upper Atmosphere Research Satellite have been analyzed to describe the center-to-limb behavior of the excess surface brightness of solar active regions over the wavelength range 142-265 nm. Comparison of these results with the analysis by Worden, Woods, and Bowman (2001, ApJ, 560, 1020) of the time series produced by the Solar-Stellar Irradiance Comparison Experiment on the same spacecraft over the wavelength range 120-170 nm shows excellent agreement. For 170-265 nm, we present new results on the center-to-limb behavior of the surface brightness of solar active regions. Comparisons with previous results for the quiet disk show roughly similar behaviors at wavelengths below 168 nm and above 210 nm, where both exhibit weak limb brightening and darkening, respectively. At intermediate wavelengths, 168-210 nm, active regions exhibit much stronger limb darkening than does the quiet disk. Our Fourier analysis and the multi-component modeling of Worden et al. (2001, ApJ, 560, 1020) are found to be complementary and could be productively combined in future work. We also compare our results with a similar analysis based upon the semi-empirical model atmospheres of Fontenla et al. (1999, ApJ, 518, 480), further improved by Avrett. We compare the measurement- and model-based analyses and suggest the direction of improvements needed in the model atmospheres. Title: Physical Modeling of the Solar Radiation, Current Status and Prospects Authors: Fontenla, J. M.; Avrett, E. H.; Goodman, M.; White, O. W.; Rottman, G.; Fox, P.; Harder, J. Bibcode: 2003SPD....34.0301F Altcode: 2003BAAS...35..808F Physical models that include full NLTE radiative transfer as well as particle transport and MHD processes are the key to understanding the solar radiative output and also are essential to our understanding of heating and the dynamics of the solar atmosphere, in particular for chromospheric layers. SOHO observations show that chromospheric emission lines do not vary dramatically in time and that chromospheric heating, even in the quiet Sun, is not simply due to, p-modes induced, strong shock waves passing through the chromosphere. The physics of the chromospheric heating is more complicated and remains elusive. The chromospheric and coronal heating are likely closely related to the dynamics in these regions as well as in the thin chromosphere-corona transition region since they are a coupled system. Solar atmospheric heating and dynamics are strongly affected by the magnetic fields and MHD mechanisms must be considered. Models for the upper photosphere and chromosphere should also consider NLTE radiative transfer and radiative losses as well as particle transport processes including tensor electric resistivity with magnetic field. Models for the transition region and coronal layers must also consider particle diffusion. In this paper we show schematically: 1) the current state of our research on modeling observed features of the solar structure and their radiative signatures; 2) the application of this modeling to the Earth solar irradiance and comparisons with observations; 3) the key achievements and the needed improvements of the modeling; 4) our plans for future research starting from ab initio semi-empirical models based on observations, and, while maintaining the agreement with relevant observations, moving towards physically consistent models that include key MHD processes thereby replacing empirical constraints by physically consistent processes and boundary conditions. Title: Solar and Stellar Atmospheric Modeling Using the Pandora Computer Program Authors: Avrett, E. H.; Loeser, R. Bibcode: 2003IAUS..210P.A21A Altcode: No abstract at ADS Title: The Solar Temperature Minimum and Chromosphere Authors: Avrett, E. H. Bibcode: 2003ASPC..286..419A Altcode: 2003ctmf.conf..419A No abstract at ADS Title: Influence of Mass Flows on the Energy Balance and Structure of the Solar Transition Region Authors: Avrett, E. H.; Fontenla, J. M. Bibcode: 2003ASPC..288..279A Altcode: 2003sam..conf..279A We have extended our previous modeling of energy balance in the chromosphere-corona transition region to include the effects of particle and mass flows. We consider quasi-steady cases satisfying the momentum and energy balance equations throughout the transition region and low corona. We include particle diffusion as well as flows in the non-LTE equations for H, HeI, and HeII. Mass flows substantially affect the ionization and radiative losses of H and He thereby affecting the structure and extent of the transition region. We find that the H and He line profiles are greatly affected by flows, and that line shifts are much less important than the changes in line intensity and central reversal due to the effects of flows on atmospheric structure. The profiles we compute can generally explain the range of observed high spectral and spatial resolution Lyman alpha profies from the quiet Sun. A full account of this work appears in a paper by Fontenla, Avrett, and Loeser submitted to The Astrophysical Journal. Title: Energy Balance in the Solar Transition Region. IV. Hydrogen and Helium Mass Flows with Diffusion Authors: Fontenla, J. M.; Avrett, E. H.; Loeser, R. Bibcode: 2002ApJ...572..636F Altcode: 2001astro.ph..9416F In this paper we extend our previous modeling of energy balance in the chromosphere-corona transition region to cases with particle and mass flows. The cases considered here are quasi-steady and satisfy the momentum and energy balance equations in the transition region. We assume one-dimensional geometry and include the flow velocity terms in all equations, but we neglect the partial derivatives with respect to time. We present a complete and physically consistent formulation and method for solving the non-LTE and energy balance equations in these situations, including both particle diffusion and flows of H and He. Our calculations include partial frequency redistribution in the Lyα and Lyβ lines. Our results show quantitatively how mass flows affect the ionization and radiative losses of H and He, thereby affecting the structure and extent of the transition region. Furthermore, our computations show that the H and He line profiles are greatly affected by flows. We find that line shifts are much less important than the changes in line intensity and central reversal as a result of the influence of flows on the excitation and ionization. In this paper we use fixed conditions at the base of the transition region and in the underlying chromosphere. Our intent is to show the physical effects of flows on the transition region, not to match any particular observations. However, our computed Lyα profiles can account for the range of observed high spectral and spatial resolution from the quiet Sun. We suggest that dedicated modeling of specific sequences of observations based on physically consistent methods like those presented here will substantially improve our understanding of the energy balance in the chromosphere and corona. Title: Intensity Fluctuations in the Solar Chromosphere Authors: Kalkofen, W.; Warren, H. P.; Winebarger, A. R.; van Ballegooijen, A.; Avrett, E. H. Bibcode: 2001AGUSM..SP41C05K Altcode: We examine two very different empirical models of the solar chromosphere. The first model implies steady heating, is hot at the top of the chromosphere and has temperature fluctuations of relatively low amplitude; the second model is heated intermittently, is cold most of the time and undergoes large temperature variations. Estimates of intensity fluctuations of chromospheric radiation are very different for the two models. We compare the model predictions with observations made by Skylab and by SUMER. Title: Solar Atmosphere: Empirical Models Authors: Avrett, E. Bibcode: 2000eaa..bookE2252A Altcode: The basic atmospheric stratification of the Sun can be described as having (1) an outwardly decreasing temperature with height in the photospheric layers which emit radiation at visible wavelengths, (2) a temperature minimum region where the temperature no longer decreases but begins to increase with height, (3) an extended chromospheric region where the temperature increases to values at which h... Title: Basic Modeling of the Solar Atmosphere and Spectrum Authors: Avrett, Eugene H. Bibcode: 2000STIN...0099713A Altcode: During the last three years we have continued the development of extensive computer programs for constructing realistic models of the solar atmosphere and for calculating detailed spectra to use in the interpretation of solar observations. This research involves two major interrelated efforts: work by Avrett and Loeser on the Pandora computer program for optically thick non-LTE modeling of the solar atmosphere including a wide range of physical processes, and work by Kurucz on the detailed high-resolution synthesis of the solar spectrum using data for over 58 million atomic and molecular lines. Our objective is to construct atmospheric models from which the calculated spectra agree as well as possible with high-and low-resolution observations over a wide wavelength range. Such modeling leads to an improved understanding of the physical processes responsible for the structure and behavior of the atmosphere. Title: Time-averaged empirical models of chromospheric features Authors: Avrett, E. H. Bibcode: 2000SPD....31.1101A Altcode: 2000BAAS...32Q.843A Vernazza, Avrett, and Loeser (1981) used Skylab observations of the inhomogeneous structure of the quiet Sun in the EUV wavelength range 40-140 nm to determine separate chromospheric models for six observed brightness components, ranging from dark cell centers to very bright network elements. The intensity at the head of the C I continuum at 110 nm varies by a factor of 4 between cell center and bright network, but the corresponding temperature in the middle chromosphere responsible for this emission varies from about 6000 K to 7000 K. These observations and recent ones from the SUMER instrument on SOHO show variations with time, but the time variations appear to be smaller than the spatial variations. If so, time-averaged models for each separate brightness component seem to be justified as a first approximation. Using separate models for each component seems a reasonable approximation since the cell and network features are large compared with the thickness of the chromosphere. Large excursions of the chromospheric temperature on short time scales appear to be ruled out given the moderate temporal variations of EUV continuum and line intensities observed to date. The EUV observations seem to indicate a chromospheric temperature rise at all locations and at all times. Title: Combined Effects Of Mass Flow And Particle Diffusion On The Ionization Structure Of The Solar Transition Region Authors: Avrett, Eugene H. Bibcode: 1999ESASP.446..141A Altcode: 1999soho....8..141A The transition region between the top of the chromosphere (T < 10,000 K) and the lowest part of the corona (T > 100,000 K) is known to have a thickness of only a few kilometers. This is the temperature range in which there is maximum radiative cooling in the strong resonance lines of H, HeI, and HeII. The heat input that balances this radiative cooling appears to come from the low corona rather than from local mechanical heating. Detailed models of the transition region without the effects of mass flow have been published by Fontenla, Avrett, and Loeser (1993). They solved the complete set of radiative transfer, statistical equilibrium, and energy balance equations for H, HeI, and HeII, balancing the cooling due to optically thick, non-LTE radiative transitions with the heating due to the transport of energy from higher temperatures by thermal conduction and particle diffusion. The resulting theoretical models showed better agreement with observations than earlier empirical models based on local statistical equilibrium without particle diffusion. Shown here are our first results of the more complicated calculations that include the additional effects of mass flows. We consider mass-conserving outflow and inflow velocities having values up to 8 km/s in the upper transition region. The hydrogen Lyman lines are not highly sensitive to such flows, but the resonance lines of HeI and HeII show pronounced effects as a result of ionization balance changes. Outflow causes greater amounts of HeI at higher temperatures and inflow causes greater amounts of HeIII at lower temperatures compared with zero-flow ionization equilibrium. As before, these calculations include particle diffusion, i.e., ions penetrate into nearby neutral regions and neutrals into nearby ionized regions. We find that particle diffusion remains important even for the highest flow velocities we have considered. Title: Does the Sun Have a Full-Time Chromosphere? Authors: Kalkofen, Wolfgang; Ulmschneider, Peter; Avrett, Eugene H. Bibcode: 1999ApJ...521L.141K Altcode: The successful modeling of the dynamics of H2v bright points in the nonmagnetic chromosphere by Carlsson & Stein gave as a by-product a part-time chromosphere lacking the persistent outward temperature increase of time-average empirical models, which is needed to explain observations of UV emission lines and continua. We discuss the failure of the dynamical model to account for most of the observed chromospheric emission, arguing that their model uses only about 1% of the acoustic energy supplied to the medium. Chromospheric heating requires an additional source of energy in the form of acoustic waves of short period (P<2 minutes), which form shocks and produce the persistent outward temperature increase that can account for the UV emission lines and continua. Title: Calculation of Solar Irradiances. I. Synthesis of the Solar Spectrum Authors: Fontenla, Juan; White, Oran R.; Fox, Peter A.; Avrett, Eugene H.; Kurucz, Robert L. Bibcode: 1999ApJ...518..480F Altcode: Variations in the total radiative output of the Sun as well as the detailed spectral irradiance are of interest to terrestrial and solar-stellar atmosphere studies. Recent observations provide measurements of spectral irradiance variations at wavelengths in the range 1100-8650 Å with improved accuracy, and correlative studies give procedures for estimating the spectral irradiance changes from solar activity records using indicators such as those derived from Ca II K and Mg II indices. Here we describe our approach to physical modeling of irradiance variations using seven semiempirical models to represent sunspots, plage, network, and quiet atmosphere. This paper gives methods and details, and some preliminary results of our synthesis of the variations of the entire irradiance spectrum. Our calculation uses object-oriented programming techniques that are very efficient and flexible. We compute at high spectral resolution the intensity as a function of wavelength and position on the disk for each of the structure types corresponding to our models. These calculations include three different approximations for the line source function: one suited for the very strong resonance lines where partial redistribution (PRD) is important, another for the most important nonresonance lines, and another approximation for the many narrow lines that are provided in Kurucz's listings. The image analysis and calculations of the irradiance variation as a function of time will be described in a later paper. This work provides an understanding of the sources of variability arising from solar-activity surface structures. We compute the Lyα irradiance to within 3% of the observed values. The difference between our computations and the Neckel & Labs data is 3% or less in the near-IR wavelengths at 8650 Å, and less than 1% in the red at 6080 Å. Near 4100 Å we overestimate the irradiance by 9%-19% because of opacity sources missing in our calculations. We also compute a solar cycle variability of 49% in the Lyα irradiance, which is very close to observed values. At wavelengths between 4100 Å and 1.6 μm, we obtain spectral irradiance variations ranging from -0.06% to 0.46% in the visible--the higher values correspond to the presence of strong lines. The variability in the IR between 1.3 and 2.2 μm is ~-0.15%. 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: The Calculation of Theoretical Chromospheric Models and the Interpretation of the Solar Spectrum Authors: Avrett, Eugene H. Bibcode: 1998nasa.reptT....A Altcode: Since the early 1970s we have been developing the extensive computer programs needed to construct models of the solar atmosphere and to calculate detailed spectra for use in the interpretation of solar observations. This research involves two major related efforts: work by Avrett and Loeser on the Pandora computer program for non-LTE modeling of the solar atmosphere including a wide range of physical processes, and work by Rurucz on the detailed synthesis of the solar spectrum based on opacity data or over 58 million atomic and molecular lines. our goals are: to determine models of the various features observed on the Sun (sunspots, different components of quiet and active regions, and flares) by means of physically realistic models, and to calculate detailed spectra at all wavelengths that match observations of those features. These two goals are interrelated: discrepancies between calculated and observed spectra are used to determine improvements in the structure of the models, and in the detailed physical processes used in both the model calculations and the spectrum calculations. The atmospheric models obtained in this way provide not only the depth variation of various atmospheric parameters, but also a description of the internal physical processes that are responsible for non-radiative heating, and for solar activity in general. Title: Modeling Solar Variability - Synthetic Models Authors: Avrett, Eugene H. Bibcode: 1998sers.conf..449A Altcode: No abstract at ADS Title: A Stochastic Model of the Solar Atmosphere Authors: Gu, Yeming; Jefferies, John T.; Lindsey, Charles; Avrett, E. H. Bibcode: 1997ApJ...484..960G Altcode: We present a model for the lower solar atmosphere based on continuum observations of the Sun spanning the 2-1200 μm wavelength range. We have shown that the data, in particular the center-to-limb brightness profiles at 50-350 μm, cannot be accounted for by any model which is plane-parallel and homogeneous in the height range in which this radiation is formed. We accordingly set out to develop a two-component model as the natural generalization. Making use of a theory for radiation transfer in a stochastic multi-component atmosphere, we find that one can indeed obtain an inhomogeneous model which satisfies center-to-limb data over the 2-1200 μm range. This composite model is made up of hot ``flux tubes'' randomly embedded in a cool medium, the flux tubes expanding to occupy an increasing proportion of the atmosphere as we move up in height.

The cool ambient component shows a monotonic decrease in temperature in the range defined by the data. The temperature in the hot component is constant at about 6500 K up to about 400 km and increases monotonically above that height. The center-to-limb observations demand that the gas in the interiors of the flux tubes be recessed downward with respect to a hydrostatic equilibrium distribution of density. This appears to constitute a chromospheric Wilson depression consistent with a magnetic field of about 120 G in the flux-tube interior at a height of about 600 km.

The new model is shown to be consistent with other spectral measurements independent of those used to define it. It gives a very good fit to the 0.5 μm continuum intensities across the disk, and provides an excellent accounting for the disk-center brightness temperature in the center of the 3-2 R14 CO line at 4.667 μm. A boundary temperature of less than about 3000 K in the cold component is suggested from the limb-darkening data available for this line.

In an appendix we mention a procedure for an analogous study based on the intensities of multiplet lines, which may hold promise for modeling over a wider range of heights that can be spanned by the IR data. Title: The Pandora Atmosphere Program Authors: Avrett, E. H.; Loeser, R. Bibcode: 1996AAS...188.3903A Altcode: 1996BAAS...28Q.882A Pandora is a general-purpose non-LTE computer program for calculating stellar atmospheric models and spectra. We continue to improve and expand the capabilities of this program as we seek better agreement with observations. We also provide extensive documentation and explanation of the computational procedures so that the results are reasonably self explanatory. The Pandora program is available on request to anyone willing to invest the effort needed to become a successful user. Current applications include extensive modeling of the outer atmospheres of the Sun and cooler stars, including time-dependent effects, and the atmospheric response to external ionizing radiation. The program also can be used for simple illustrative studies of optically thick non-LTE spectral line formation for different multilevel atoms and atmospheric models. Title: Formation of the Solar Helium Spectrum Authors: Avrett, Eugene H.; Sasselov, Dimitar D. Bibcode: 1996sao..rept.....A Altcode: The formation of the solar helium spectrum is addressed. The concept of the one-dimensional complexity in the contribution function of subordinate helium transitions, and a quantitative confirmation of interspecies effects are discussed. 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: Next generation model atmospheres (review) Authors: Avrett, E. H. Bibcode: 1996IAUS..176..503A Altcode: No abstract at ADS Title: What Happens to My Electronic Manuscript Submission to the AAS-affiliated Journals? Authors: Barnes, J.; Abt, H. A.; Avrett, E. H.; Dalgarno, A.; Hodge, P. Bibcode: 1996AAS...187.3806B Altcode: 1996BAAS...28..755B No abstract at ADS Title: Next generation model atmospheres Authors: Avrett, Eugene H. Bibcode: 1996stss.book..503A Altcode: Continued progress in the physical interpretation of solar and stellar observations, the increased speed of computing now available, and new highly efficient numerical procedures have all led to significant advances in stellar atmospheric modeling. This review attempts to summarize recent progress in the field and to describe the many interrelated physical processes that should be taken into account in next-generation modeling programs. Title: The Solar LY alpha Line Profile Authors: Woods, Thomas N.; Rottman, Gary J.; White, O. R.; Fontenla, Juan; Avrett, E. H. Bibcode: 1995ApJ...442..898W Altcode: Solar Ly-alpha irradiance measurements from the SOLar STellar Irradiance Comparison Experiment (SOLSTICE) on the Upper Atmosphere Research Satellite (UARS) have been made since 1991 October with a spectral resolution of 0.1 nm. The uniqueness of the small molecular oxygen cross section near Ly-alpha permits the Ly-alpha radiation to penetrate much deeper into the atmosphere than the other emissions near Ly-alpha. We have taken advantage of this phenomenon by performing solar occultation experiments near the Ly-alpha to evaluate precisely the instrument scattered light contribution. After correcting for scattered light, the broad wings of the solar Ly-alpha line can be extracted out to 5 nm from line center with a typical accuracy of +/-20%. The variability in the Ly-alpha wings near 2 nm from line center is about one-half that of the Ly-alpha core emission, defined within 0.1 nm from line center. These Ly-alpha profile measurements are found to be consistent with the Skylab radiance measurements and theoretical models of the Ly-alpha line profiles computed using partial redistribution of photons in the source function. Title: Outer Layers of a Carbon Star: The View from the Hubble Space Telescope Authors: Johnson, Hollis R.; Ensman, Lisa M.; Alexander, David R.; Avrett, Eugene H.; Brown, Alexander; Carpenter, Kenneth G.; Eriksson, Kjell; Gustafsson, Bengt; Jorgensen, Uffe G.; Judge, Philip D.; Linsky, Jeffrey L.; Luttermoser, Donald G.; Querci, Francois; Querci, Monique; Robinson, Richard D.; Wing, Robert F. Bibcode: 1995ApJ...443..281J Altcode: To advance our understanding of the relationship between stellar chromospheres and mass loss, which is a common property of carbon stars and other asymptotic giant branch stars, we have obtained ultraviolet spectra of the nearby N-type carbon star UU Aur using the Hubble Space Telescope (HST). In this paper we describe the HST observations, identify spectral features in both absorption and emission, and attempt to infer the velocity field in the chromosphere, upper troposphere, and circumstellar envelope from spectral line shifts. A mechanism for producing fluoresced emission to explain a previously unobserved emission line is proposed. Some related ground-based observations are also described. Title: Calculation of Absolute Solar Irradiances and Their Variation with Solar Activity Authors: White, O. R.; Fontenla, J.; Fox, P.; Avrett, E. H.; Harvey, K. Bibcode: 1995SPD....26..308W Altcode: 1995BAAS...27..954W No abstract at ADS Title: Two-Component Modeling of the Solar IR CO Lines Authors: Avrett, E. H. Bibcode: 1995itsa.conf..303A Altcode: One-dimensional hydrostatic models of quiet and active solar regions can be constructed that generally account for the observed intensities of lines and continua throughout the spectrum, except for the infrared CO lines. There is an apparent conflict between a) observations of the strongest infrared CO lines formed in LTE at low-chromospheric heights but at temperatures much cooler than the average chromospheric values, and b) observations of Ca II, UV, and microwave intensities that originate from the same chromospheric heights but at the much higher temperatures characteristic of the average chromosphere. A model M_CO has been constructed which gives a good fit to the full range of mean CO line profiles (averaged over the central area of the solar disk and over time) but this model conflicts with other observations of average quiet regions. A model L_CO which is approximately 100 K cooler than M_CO combined with a very bright network model F in the proportions 0.6L_CO+0.4F is found to be generally consistent with the CO, Ca II, UV, and microwave observations. Ayres, Testerman, and Brault found that models COOLC and FLUXT in the proportions 0.925 and 0.075 account for the CO and Ca II lines, but these combined models give an average UV intensity at 140 nm about 20 times larger than observed. The 0.6L_CO+0.4F result may give a better description of the cool and hot components that produce the space- and time-averaged spectra. Recent observations carried out by Uitenbroek, Noyes, and Rabin with high spatial and temporal resolution indicate that the faintest intensities in the strong CO lines measured at given locations usually become much brighter within 1 to 3 minutes. The cool regions thus seem to be mostly the low- temperature portions of oscillatory waves rather than cool structures that are stationary. Title: The calculation of theoretical chromospheric models and the interpretation of the solar spectrum Authors: Avrett, Eugene H. Bibcode: 1994sao..reptR....A Altcode: Since the early 1970s we have been developing the extensive computer programs needed to construct models of the solar atmosphere and to calculate detailed spectra for use in the interpretation of solar observations. This research involves two major related efforts: work by Avrett and Loeser on the Pandora computer program for non-LTE modeling of the solar atmosphere including a wide range of physical processes, and work by Kurucz on the detailed synthesis of the solar spectrum based on opacity data for over 58 million atomic and molecular lines. Our goals are to determine models of the various features observed on the sun (sunspots, different components of quiet and active regions, and flares) by means of physically realistic models, and to calculate detailed spectra at all wavelengths that match observations of those features. These two goals are interrelated: discrepancies between calculated and observed spectra are used to determine improvements in the structure of the models, and in the detailed physical processes used in both the model calculations and the spectrum calculations. The atmospheric models obtained in this way provide not only the depth variation of various atmospheric parameters, but also a description of the internal physical processes that are responsible for nonradiative heating, and for solar activity in general. Title: Modeling the Infrared Magnesium and Hydrogen Lines from Quiet and Active Solar Regions Authors: Avrett, E. H.; Chang, E. S.; Loeser, R. Bibcode: 1994IAUS..154..323A Altcode: No abstract at ADS Title: Formation of the Solar 10830 Angstrom Line Authors: Avrett, E. H.; Fontenla, J. M.; Loeser, R. Bibcode: 1994IAUS..154...35A Altcode: No abstract at ADS Title: The resonance lines of MG2 as diagnostics of the upper solar chromosphere Authors: Avrett, Eugene H. Bibcode: 1994sao..reptQ....A Altcode: The resonance lines of singly ionized magnesium, the MgII h&k lines at about 280 nm, are two of the small number of lines in the solar spectrum that are optically thick in the chromospheric part of the solar atmosphere. Potentially these lines contain information on the initial temperature rise that occurs at the top of the photosphere. Unfortunately, few good observations of the lines exist due to their wavelength near 280 nm the ultraviolet. However, a fair number of observations (on the order of 200) are available from the data base of the UltraViolet Polarimeter and Spectrometer (UVSP) instrument that flew on board of NASA's Solar Maximum Mission (SMM) satellite. In addition, this data base contains a number of spectra that include the Mg I resonance line at (lambda)285.2nm, just longward of the h&k lines. The neutral magnesium line is not as strong as its ionic counterparts and samples slightly lower parts of the atmosphere. Its width is a sensitive diagnostic of the ionization balance between neutral and singly ionized magnesium, which determines the opacity scale (and formation height) of other diagnostically important MgI lines like the 457.1 nm intercombination line, the magnesium b lines and the infrared MgI emission lines near 12 microns. Analysis of the observed line profiles shows that it is necessary to include the effects of partial frequency redistribution (PRD) in the formation of the line as in the case of the h&k lines. This implies that the core of the line is very sensitive to the way scattering is treated in the modeling of the line, and in turn this allows us to separate the uncertain effects in the atomic data (viz. the Van der Waals broadening) from the uncertainties in the underlying atmospheric model. The main objective of this research was to compare observed spectra of the magnesium resonance lines against theoretical line profiles calculated from recent models of the solar atmosphere by Fontenla et al., hereafter called FAL. These models extend earlier solar models by Avrett et al. and provide yet a better match between observed and calculated spectra, in particular of the hydrogen Lyman line, by including the effects of ambipolar diffusion of hydrogen and helium in the mass and energy transport equations for the chromosphere-corona transition region. Although the transition region models can be built self-consistently (without additional observational input) once the proper boundary conditions are given at the top of the chromosphere, there are uncertainties in the models precisely because these boundary conditions are not well known. With the analysis of UVSP magnesium observations we strive to constrain these uncertainties. Title: Formation of the solar 10830 A line Authors: Avrett, E. H.; Fontenla, J. M.; Loeser, R. Bibcode: 1994isp..book...35A Altcode: One-dimensional hydrostatic-equilibrium models are shown here for faint, average, and bright components of the quiet Sun, and for a plage region, describing in each case how the atmosphere is stratified through the photosphere, chromosphere, and transition region up to a temperature of 10^5 K. The observed coronal line radiation is assumed to be the inward incident radiation at the 10^5 K boundary. This coronal radiation penetrates into the upper chromosphere causing sufficient helium ionization to populate the lower level of the He I 10830 A line, producing optically-thin absorption of the photospheric continuum at 10830 A. The amount of absorption, which is proportional to the optical thickness of the upper chromosphere in the 10830 line, depends on 1) the strength of the coronal lines at wavelengths in the He I 504 A ionizing continuum, and 2) the density and geometrical thickness of the upper chromosphere. The computed 10830 A line is shown for the four atmospheric models and for three values of the coronal illumination. The calculated off-limb 10830 intensity distribution shows a minimum in the low chromosphere and a maximum at roughly 2000 km above the photosphere, in general agreement with observations, indicating that this is the predominant height of the transition region over most of the solar surface. Title: Modeling the infrared magnesium and hydrogen lines from quiet and active solar regions Authors: Avrett, E. H.; Chang, E. S.; Loeser, R. Bibcode: 1994isp..book..323A Altcode: The emissions of Mg I at 7.4, 12.2, and 12.3 mu m are now known to be formed in the upper photosphere; the line emission is due to collisional coupling of higher levels with the continuum together with radiative depopulation of lower levels. These combined effects cause the line source functions of high-lying transitions to exceed the corresponding Planck functions. However, there are uncertainties in a) the relevant atomic data, particularly the collisional rates and ultraviolet photoionization rates, and b) the sensitivity of the calculated results to changes in atmospheric temperature and density. These uncertainties are examined by comparing twelve calculated Mg I line profiles in the range 2.1-12.3 mu m with ATMOS satellite observations. We show results based on different rates and using different atmospheric models representing a range of dark and bright spatial features. The calculated Mg profiles are found to be relatively insensitive to atmospheric model changes, and to depend critically on the choice of collisional and photoionization rates. We find better agreement with the observations using collision rates from van Regemorter (1962) rather than from Seaton (1962). We also compare twelve calculated hydrogen profiles in the range 2.2-12.4 mu m with ATMOS observations. The available rates and cross sections for hydrogen seem adequate to account for the observed profiles, while the calculated lines are highly sensitive to atmospheric model changes. These lines are perhaps the best available diagnostics of the temperature and density structure of the photosphere and low chromosphere. Further calculations based on these infrared hydrogen lines should lead to greatly improved models of the solar atmosphere. Title: The calculation of theoretical chromospheric models and the interpretation of solar spectra from rockets and spacecraft Authors: Avrett, Eugene H. Bibcode: 1993sao..rept.....A Altcode: Since the early 1970s we have been developing the extensive computer programs needed to construct models of the solar atmosphere and to calculate detailed spectra for use in the interpretation of solar observations. This research involves two major related efforts: work by Avrett and Loeser on the Pandora computer program for non-LTE modeling of the solar atmosphere including a wide range of physical processes, and work by Kurucz on the detailed synthesis of the solar spectrum based on opacity data for over 58 million atomic and molecular lines. Our goals are to determine models of the various features observed on the Sun (sunspots, different components of quiet and active regions, and flares) by means of physically realistic models, and to calculate detailed spectra at all wavelengths that match observations of those features. These two goals are interrelated: discrepancies between calculated and observed spectra are used to determine improvements in the structure of the models, and in the detailed physical processes used in both the model calculations and the spectrum calculations. The atmospheric models obtained in this way provide not only the depth variation of various atmospheric parameters, but also a description of the internal physical processes that are responsible for non-radiative heating, and for solar activity in general. Title: Measurement of solar limb extension at CO fundamental band wavelengths in a total eclipse: comparison with atmospheric model predictions. Authors: Clark, T. A.; Naylor, D. A.; Tompkins, G. J.; Avrett, E. H. Bibcode: 1993JRASC..87S.179C Altcode: No abstract at ADS Title: Imaging Solar Bolometric and Spectral Intensity Using Thermal Detector Arrays Authors: Deming, D.; Glenar, D.; Kostiuk, T.; Bly, V.; Forrest, K.; Nadler, D.; Hudson, H.; Lindsey, C.; Kopp, G.; Avrett, E.; Terrill, C. W. Bibcode: 1993BAAS...25R1221D Altcode: No abstract at ADS Title: Energy Balance in the Solar Transition Region. III. Helium Emission in Hydrostatic, Constant-Abundance Models with Diffusion Authors: Fontenla, J. M.; Avrett, E. H.; Loeser, R. Bibcode: 1993ApJ...406..319F Altcode: In our previous papers we described the mathematical formalism and the computed results for energy-balance hydrostatic models of the solar transition region. In this paper we discuss in some detail the limitations of the hydrostatic and one-dimensional assumptions used. Then we analyze the determination of helium emission when diffusion is included. We use transport coefficients estimated from kinetic theory to determine the helium departures from local ionization balance. We calculate the helium spectra for each of our models and evaluate the role of helium in the energy transport. Also, we investigate the effects of coronal illumination on the structure of the transition region and upper chromosphere, and show how coronal illumination affects various EUV lines and the He I 10830 A line. Comparing with both absolute intensities and detailed line profiles, we show that our models are consistent not only with the observed hydrogen spectra but also with the available helium spectra. Title: Effects of hydrogen and electron diffusion on trace species diffusion in the solar atmosphere. Authors: Fontenla, Juan M.; Avrett, E. H. Bibcode: 1992ESASP.348..335F Altcode: 1992cscl.work..335F The authors study the effects of the diffusion of neutral hydrogen, protons, and electrons, on the diffusion of heavy species in the lower transition-region. This layer has large temperature and hydrogen-ionization gradients as the temperature changes from 104 to 105K. Hydrogen diffusion induces an abundance gradient opposed to the hydrogen ionization gradient. Thermal diffusion counteracts this effect for ionized particles. The low FIP species (fully ionized in the lower transition region) experience both effects. The high FIP species (neutral at T = 104K) are practically affected by only the hydrogen-diffusion effect. The result is the enhancement of the abundance of the low-FIP elements with respect to the high-FIP elements at the top of the models. However, the rate at which this process occurs is critical and large departures from the equilibrium abundances are expected because of the slow relaxation rates at the top of the chromosphere. Title: Temporal variations of near UV,Visible, and Infrared Spectral Irradiance From A Theoretical Viewpoint Authors: Avrett, Eugene H. Bibcode: 1992sers.conf...20A Altcode: Solar atmospheric models can be used to calculate the spectra from various quiet and active regions on the solar disk. One-dimensional models for such regions are adequate because vertical changes occur over very short distances compared with the horizontal size of most observed features. Several quiet-Sun component models and a plage-facula model are discussed. The computed spectra can be used to interrelate the intensities at different wavelengths, so that the irradiance at UV wavelengths, for example, can be calculated given the distribution of quiet and active regions observed at visible wavelengths. Intensity and irradiance calculations based on solar atmospheric models can be used to normalize and interpolate between available observations at various wavelengths obtained at different times during the solar activity cycle. Title: Erratum - Electron Collisional Rates for Atomic Hydrogen Revisited Authors: Chang, E. S.; Avrett, E. H.; Loeser, R. Bibcode: 1992A&A...256..724C Altcode: No abstract at ADS Title: Non-LTE Effects on MG I Line Profiles in the Infrared Solar Spectrum Authors: Chang, E. S.; Avrett, E. H.; Mauas, P. J.; Noyes, R. W.; Loeser, R. Bibcode: 1992ASPC...26..521C Altcode: 1992csss....7..521C No abstract at ADS Title: Wind; Atmosphere Models for the Hybrid Supergiant Alpha Aquarii (G2 Ib) Authors: Dupree, A. K.; Whitney, B. A.; Avrett, E. H. Bibcode: 1992ASPC...26..525D Altcode: 1992csss....7..525D No abstract at ADS Title: The PANDORA Atmosphere Program (Invited Review) Authors: Avrett, E. H.; Loeser, R. Bibcode: 1992ASPC...26..489A Altcode: 1992csss....7..489A No abstract at ADS Title: Formation of the Infrared Emission Lines of MG i in the Solar Atmosphere Authors: Chang, E. S.; Avrett, E. H.; Mauas, P. J.; Noyes, R. W.; Loeser, R. Bibcode: 1991ApJ...379L..79C Altcode: A non-LTE radiative transfer investigation of the emission lines is conducted at 7 and 12 microns using a realistic atomic model for neutral magnesium. An average quiet sun atmospheric model is used to calculate emission-line profiles that resemble the observed ones, i.e., broad absorption troughs with narrow central emission, and significant limb brightening. The charge exchange rates are found to be significant, but the effects of high-n coupling between Mg and Mg(+) together with radiative low-n transitions are of greater importance. It is confirmed that the emission cores are formed no higher than the temperature minimum region, and that the emission is caused by non-LTE effects rather than by the chromospheric temperature rise. It is inferred from the model calculations that the line core is sensitive to magnetic fields located almost 400 km above those measured in ordinary magnetograms; the gas pressure decreases 20-fold between these two heights. Title: Energy Balance in the Solar Transition Region. II. Effects of Pressure and Energy Input on Hydrostatic Models Authors: Fontenla, J. M.; Avrett, E. H.; Loeser, R. Bibcode: 1991ApJ...377..712F Altcode: The radiation of energy by hydrogen lines and continua in hydrostatic energy-balance models of the transition region between the solar chromosphere and corona is studied using models which assume that mechanical or magnetic energy is dissipated in the hot corona and is then transported toward the chromosphere down the steep temperature gradient of the transition region. These models explain the average quiet sun and also the entire range of variability of the Ly-alpha lines. The relations between the downward energy flux, the pressure of the transition region, and the different hydrogen emission are described. Title: Electron collisional rates for atomic hydrogen, revisited Authors: Chang, E. S.; Avrett, E. H.; Loeser, R. Bibcode: 1991A&A...247..580C Altcode: It is shown that there are significant inconsistencies in the compilation of electron impact transition rate coefficients for atomic hydrogen by Giovanardi et al. (1987) and Giovanardi and Palla (1989). In particular, the rates for the n-n prime transitions 1-4, 2-4, 3-4, 3-5, 4-5, and 4-6 are not consistent with neighboring transitions rates, and qualitatively disagree with the rates of Johnson (1972). Title: Diffusion of Helium in the Solar Transition Region Authors: Avrett, E. H.; Fontenla, J. M. Bibcode: 1991BAAS...23.1029A Altcode: No abstract at ADS Title: Formation of the Infrared Emission Lines of Mg I in the Solar Atmosphere Authors: Chang, E. S.; Avrett, E. H.; Mauas, P. J.; Loeser, R.; Noyes, R. W. Bibcode: 1991BAAS...23.1029C Altcode: No abstract at ADS Title: New Models of the Chromosphere and Transition Region (With 2 Figures) Authors: Avrett, E. H. Bibcode: 1991mcch.conf...97A Altcode: No abstract at ADS Title: The photosphere as a radiative boundary. Authors: Anderson, Lawrence S.; Avrett, Eugene H. Bibcode: 1991sia..book..670A Altcode: The authors review the role of the photosphere as a radiative boundary for the solar interior, concentrating on semi-empirical and a priori models for the structure and emergent radiation. Contents: 1. Introduction. 2. Plane-parallel models. 3. Departures from plane-parallel structure. 4. Dynamical convection models. Title: The role of particle diffusion in the lower transition region: Revised interpretation of emission measures Authors: Avrett, Eugene H.; Fontenla, Juan M. Bibcode: 1991deas.rept.....A Altcode: Our energy-balance models of the lower transit ion region were presented in a previous paper (New Models of the Chromosphere and Transition Region). Here we show the influence of particle diffusion on the calculated hydrogen and helium number densities for a given temperature-density model (model C in the preceding paper). We have also solved the statistical equilibrium and radiative transfer equations for a 13-level He I atom (22 radiative transitions) and a 6-level He II ion (15 radiative transitions) together with He III. The resulting He I and He II level-1 number densities and He III density are shown as functions of temperature. Diffusion substantially increases etaHeI for T greater than 35,000 K and decreases etaHeI (while increasing eta(sub(HeII)) between 9,000 and 25,000 K. Including the effects of diffusion also increases etaHeIII for T less than 60,000 K. Title: The Role of Particle Diffusion in the Lower Transition Region: Revised Interpretation of Emission Measures (With 2 Figures) Authors: Avrett, E. H.; Fontenla, J. M. Bibcode: 1991mcch.conf..100A Altcode: No abstract at ADS Title: The White-Light Flare of 1982 June 15: Models Authors: Mauas, Pablo J. D.; Machado, Marcos E.; Avrett, Eugene H. Bibcode: 1990ApJ...360..715M Altcode: Models are presented for the two continuum-emitting kernels observed in the white-light flare (WLF) of June 15, 1982. They are the first semiempirical models of a WLF which are consistent not only with observations of the continuum emission level but also with a set of spectral lines having heights of formation which span the chromosphere and upper photosphere. It is shown that the models are not compatible with the hypothesis that the continuum emission is caused by enhanced Balmer and Paschen hydrogen continua, and they present strong evidence instead that the emission is of photospheric origin and that its source is due to H(-). The observation and the models derived from them show that white-light emission can occur in areas of the active region where there is no chromospheric emission and in particular no H-alpha emission. This fact seems to rule out the viability of downward transport mechanisms as the source of the energy required for the WLF. Title: On Carbon Monoxide Cooling in the Solar Atmosphere Authors: Mauas, Pablo J.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1990ApJ...357..279M Altcode: The CO cooling rate for models of the solar atmosphere using the detailed line-by-line CO opacity in the fundamental band, and carrying out a full radiative transfer calculation for each line is computed. The importance of the different assumptions that have been made to obtain the CO cooling rate and find that when detailed optical depth effects are taken into account, the calculated CO cooling rate at line optical depths near unity can be smaller than optically thin estimates by more than an order of magnitude is studied. It is found that CO cooling does not account for the missing source of radiative cooling in the temperature minimum region of the quiet sun. Title: Energy Balance in the Solar Transition Region. I. Hydrostatic Thermal Models with Ambipolar Diffusion Authors: Fontenla, J. M.; Avrett, E. H.; Loeser, R. Bibcode: 1990ApJ...355..700F Altcode: The energy balance in the lower transition region is analyzed by constructing theoretical models which satisfy the energy balance constraint. The energy balance is achieved by balancing the radiative losses and the energy flowing downward from the corona. This energy flow is mainly in two forms: conductive heat flow and hydrogen ionization energy flow due to ambipolar diffusion. Hydrostatic equilibrium is assumed, and, in a first calculation, local mechanical heating and Joule heating are ignored. In a second model, some mechanical heating compatible with chromospheric energy-balance calculations is introduced. The models are computed for a partial non-LTE approach in which radiation departs strongly from LTE but particles depart from Maxwellian distributions only to first order. The results, which apply to cases where the magnetic field is either absent, or uniform and vertical, are compared with the observed Lyman lines and continuum from the average quiet sun. The approximate agreement suggests that this type of model can roughly explain the observed intensities in a physically meaningful way, assuming only a few free parameters specified as chromospheric boundary conditions. Title: Formation of the He I Resonance Lines in the Solar Transition Region Authors: Avrett, E. H.; Fontenla, J. M. Bibcode: 1990BAAS...22..816A Altcode: No abstract at ADS Title: Flare Heating and Ionization of the Low Solar Chromosphere. I. Inversion Methods for MG i lambda lambda 4571 and 5173 Authors: Metcalf, Thomas R.; Canfield, Richard C.; Avrett, Eugene H.; Metcalf, Frederic T. Bibcode: 1990ApJ...350..463M Altcode: Various methods of inverting solar Mg I 4571 and 5173 spectral line observations are examined to find the best method of using these lines to calculate the vertical temperature and electron density structure around the temperature minimum region. Following a perturbation analysis by Mein (1971), a Fredholm integral equation of the first kind is obtained which can be inverted to yield these temperature and density structures as a function of time. Several inversion methods are tested and compared. The methods are used to test data as well as to a subset of observations of these absorption lines taken on February 3, 1986 before and during a solar flare. A small but significant increase is found in the temperature and a relatively large increase in the electron density during this flare. The observations are inconsistent with heating and ionization by an intense beam of electrons and with ionization by UV photoionization of Si I. Title: Models of the Solar Outer Photosphere Authors: Avrett, E. H. Bibcode: 1990IAUS..138....3A Altcode: No abstract at ADS Title: Winds from T Tauri Stars. I. Spherically Symmetric Models Authors: Hartmann, Lee; Calvet, Nuria; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1990ApJ...349..168H Altcode: Line fluxes and profiles are computed for a sequence of spherically symmetric T Tauri wind models. The calculations indicate that the H-alpha emission of T Tauri stars arises in an extended and probably turbulent circumstellar envelope at temperatures above about 8000 K. The models predict that Mg II resonance line emission should be strongly correlated with H-alpha fluxes; observed Mg II/H-alpha ratios are inconsistent with the models unless extinction corrections have been underestimated. The models predict that most of the Ca II resonance line and IR triplet emission arises in dense layers close to the star rather than in the wind. H-alpha emission levels suggest mass loss rates of about 10 to the -8th solar mass/yr for most T Tauri stars, in reasonable agreement with independent analysis of forbidden emission lines. These results should be useful for interpreting observed line profiles in terms of wind densities, temperatures, and velocity fields. Title: Chromospheric Structure of Cool Carbon Stars Authors: Luttermoser, Donald G.; Johnson, Hollis R.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1989ApJ...345..543L Altcode: A semiempirical chromospheric model is proposed for TX Psc which is a prototype for the N-type carbon stars. Observational data imply that the chromospheric temperature rise must begin at a low density, that the temperature gradient in the lower chromosphere must be steep, that partial redistribution must be employed in the Mg II calculation, and that the lower chromosphere is expanding away from the photosphere with a velocity of close to 50 km/s. The present model also shows that the microturbulent velocity is about 7 km/s at the temperature minimum region, dropping to 5 km/s in the chromosphere, and that the Lyman lines are optically thick in the chromosphere. Title: Computed Profiles of the Solar C i Multiplets at 1561 and 1657 Angstrom Authors: Mauas, Pablo J.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1989ApJ...345.1104M Altcode: A nine-level atomic model is presented for C I lines synthesis in the sun. All the atomic parameters are based on recent experimental and theoretical data. Profiles of the multiplets at 1561 and 1657 A are computed from a model of the solar chromosphere. The sensitivity of the results to changes in the atomic parameters is examined. Partial frequency redistribution must be included in these lines to obtain agreement between calculated and observed profiles. Title: Radiative Backwarming in White-Light Flares Authors: Machado, Marcos E.; Emslie, A. Gordon; Avrett, Eugene H. Bibcode: 1989SoPh..124..303M Altcode: We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum. Title: Winds of T Tauri Stars Authors: Calvet, N.; Hartmann, L.; Avrett, E. H.; Loeser, R. Bibcode: 1989RMxAA..18..180C Altcode: No abstract at ADS Title: On CO Cooling in the Solar Atmosphere Authors: Mauas, P. J.; Avrett, E. H.; Loeser, R. Bibcode: 1989BAAS...21..841M Altcode: No abstract at ADS Title: New Observations and Analysis of the Helium D3 Shell above the Limb Authors: Avrett, E. H.; Koutchmy, S. Bibcode: 1989BAAS...21..828A Altcode: No abstract at ADS Title: A New Collaboration in Modeling the Atmospheres of Mira-type Variables Authors: Luttermoser, D. G.; Bowen, G. H.; Willson, L. A.; Avrett, E. H.; Johnson, H. R. Bibcode: 1988BAAS...20R.996L Altcode: No abstract at ADS Title: Radiative Transfer in the Broad Emission Line Regions of Quasi-stellar Objects Authors: Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1988ApJ...331..211A Altcode: Most of the photoionization models used to account for the broad emission lines from quasi-stellar objects have adopted escape-probability approximations to avoid solving the equations of radiative transfer. The few published models based on transfer solutions have used the approximation of complete frequency redistribution for the hydrogen resonance lines. We show that both of these approximations lead to errors of at least factors of 2 in the calculated line ratios and to qualitative changes in the calculated atmospheric parameters. Here we solve the radiative transfer, statistical equilibrium, and energy-balance equations for an illuminated, plane-parallel cloud having a prescribed optical thickness and constant gas pressure. The cloud composition is assumed to be either pure hydrogen or H and He in the ratio 1 to 0.1. In addition, heavier elements are presumed to provide radiative cooling and heating and to absorb X-rays. Six energy-balance models are presented, all having the same illuminating radiation, optical thickness, and pressure. The first is a pure hydrogen model considered earlier by Hubbard and Puetter. Model 2 includes the effects of radiative cooling by heavy elements. In model 3 we add helium and X-ray opacity. Model 4 includes X-ray heating. In model 5 we replace the "on-the-spot" approximation by a solution of the Lyman continuum transfer equation. In model 6, line escape probabilities are replaced by solutions of the line transfer equations, including Lyα partial frequency redistribution. The largest changes in the calculated results are due to heavy-element cooling and to the transfer effects with partial frequency redistribution. Title: MG i as a Probe of the Solar Chromosphere: The Atomic Model Authors: Mauas, Pablo J.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1988ApJ...330.1008M Altcode: The authors present a complete atomic model for Mg I line synthesis, where all the atomic parameters are based on recent experimental and theoretical data. It is studied how the computed profiles at 4571 Å and 5173 Å are influenced by the choice of these parameters and the number of levels included in the model atom. The authors also present observed profiles of the λ5173 Å b2 line and theoretical profiles for comparison based on a recent atmospheric model for the average quiet Sun. Title: Line formation in cool carbon star chromospheres Authors: Luttermoser, Donald G.; Johnson, Hollis R.; Avrett, Eugene H. Bibcode: 1988ESASP.281a.327L Altcode: 1988IUE88...1..327L; 1988uvai....1..327L Based on adopted photospheric plus chromospheric models, synthetic fluxes of the C II (UV 0.01) and fluxes and profiles for the Mg II (UV 1) lines in complete (CRD) and partial (PRD) redistribution are generated and compared to IUE spectra of the carbon star TX Psc. Non-LTE calculations with the program PANDORA are made for H, C, Na, Mg, and Ca. For CRD and a well constrained chromospheric model, the best fit to the Mg II line profiles gives a flux in the C II lines (produced at the same depth as the Mg II lines) 2.6 times too great and produces too much emission in the Mg II line wings. The PRD in the Mg II h and k lines in an expanding chromosphere with an adjusted temperature structure produces the observed Mg II emission. The reduction in temperature produces C II fluxes that matches the IUE low resolution spectra. Title: Determination of the Temperature Structure of the Temperature Minimum Region during Solar Flares Authors: Metcalf, T. R.; Metcalf, F. T.; Canfield, R. C.; Avrett, E. H. Bibcode: 1988BAAS...20..688M Altcode: No abstract at ADS Title: Hydrogen Ionization and Energy Balance in the Solar Transition Region Authors: Avrett, E. H.; Fontenla, J. M. Bibcode: 1988BAAS...20..721A Altcode: No abstract at ADS Title: Chromospheric Structure of the Cool Carbon Star TX Psc Authors: Luttermoser, D. G.; Johnson, H. R.; Avrett, E. H.; Loeser, R. Bibcode: 1987BAAS...19.1134L Altcode: No abstract at ADS Title: The Energy Balance at the Base of the Solar Transition Region Authors: Fontenla, J. M.; Avrett, E. H.; Loeser, R. Bibcode: 1987BAAS...19Q.931F Altcode: No abstract at ADS Title: Mass Loss from FU Orionis Objects Authors: Croswell, Ken; Hartmann, L.; Avrett, E. H. Bibcode: 1987ApJ...312..227C Altcode: Estimates are presented for the mass loss rates of FU Ori, V1057 Cyg and V1515 Cyg. The approximations are made on the basis of models of the physical conditions required to generate the P Cygni profiles of the observed H-alpha and Na I lines. The spectra were obtained in 1985 using Reticons at the Whipple Observatory and Mt. Hopkins Multiple Mirror Telescope. FU Ori and V1515 Cyg each exhibited an estimate mass loss rate of 1/100,000 solar mass/yr, while that of V1057 Cyg is 10 times smaller. Magnetic wave acceleration is proposed as the driving force for the stellar winds associated with the mass losses, which are at a temperature near 5000 K when leaving the disks proposed around the T Tauri objects. Title: Synthetic Spectra for the N-Type Carbon Star TX Psc Authors: Luttermoser, Donald G.; Johnson, Hollis R.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1987LNP...291..167L Altcode: 1987csss....5..167L; 1987LNP87.291..167L Initial modeling of TX Psc's chromosphere is presented. Simple LTE flux calculations can be made to reproduce the IUE spectrum. Non-LTE effects of hydrogen decrease the electron density in the outer layers as compared to LTE. Title: Iterative Solution of Multilevel Transfer Problems Authors: Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1987nrt..book..135A Altcode: No abstract at ADS Title: The solar chromosphere Authors: Noyes, R. W.; Avrett, E. H. Bibcode: 1987soap.conf..125N Altcode: 1987sap..book..125N Observational data on the solar chromosphere (SC) are compiled in tables and graphs and illustrated with sample spectra and H-alpha filtergrams. The visible structure of the SC in active and quiet regions is discussed; the mean SC structure determined from spectroscopic observations is characterized; and particular attention is given to plages and network, the role of magnetic fields, small-scale structure, and the structure and dynamics of the high SC. From the similarity in the thermal structures of magnetic and nonmagnetic SC regions it is inferred that slow-mode MHD waves are the most likely mechanism of energy transport, although high-angular-resolution observations are required to verify this hypothesis. The implications of recent stellar-chromosphere observations for SC models are briefly considered. Title: Formation of Emission Lines in the Outer Atmosphere of Arcturus (Alpha Boo K2 III) Authors: Judge, P. G.; Avrett, E. H.; Loeser, R. Bibcode: 1986BAAS...18..982J Altcode: No abstract at ADS Title: A New Sunspot Umbral Model and Its Variation with the Solar Cycle Authors: Maltby, P.; Avrett, E. H.; Carlsson, M.; Kjeldseth-Moe, O.; Kurucz, R. L.; Loeser, R. Bibcode: 1986ApJ...306..284M Altcode: Semiempirical model atmospheres are presented for the darkest parts of large sunspot umbrae, regions have called umbral cores. The approach is based on general-purpose computational procedures that are applicable to different types of stellar atmospheres. It is shown that recent umbral intensity measurements of the spectral energy distribution may be accounted for by an umbral core atmospheric model that varies with time during the solar cycle; the observed center-limb variation can be accounted for by the properties of the model. Three umbral core models are presented, corresponding to the early, middle, and late phases of the solar cycle. These three models also may be regarded as having the properties of dark, average, and bright umbral cores respectively. The effects of atomic, opacity, and abundance data uncertainties on the model calculations are briefly discussed. For comparison, a new reference model for the average quiet solar photosphere is given. Title: The calculation of theoretical chromospheric models and the interpretation of solar spectra from rockets and spacecraft Authors: Avrett, E. H. Bibcode: 1986sao..rept.....A Altcode: Calculated results based on two chromospheric flare models F1 and F2 of Machado, et al., (1980) are presented. Two additional models are included: F1*, which has enhanced temperatures relative to the weak-flare model F1 in the upper photosphere and low chromosphere, and F3 which has enhanced temperatures relative to the strong flare model F2 in the upper chromosphere. Each model is specified by means of a given variation of the temperature as a function of column mass. The corresponding variation of particle density and the geometrical height scale are determined by assuming hydrostatic equilibrium. The coupled equations of statistical equilibrium is solved as is radiative transfer for H, H-, He I-II, C I-IV, Si I-II, Mg I-II, Fe, Al, O I-II, Na, and Ca II. The overall absorption and emission of radiation by lines throughout the spectrum is determined by means of a reduced set of opacities sampled from a compilation of over 10 to the 7th power individual lines. That the white flight flare continuum may arise by extreme chromospheric overheating as well as by an enhancement of the minimum temperature region is also shown. The radiative cooling rate calculations for our brightest flare model suggest that chromospheric overheating provides enhanced radiation that could cause significant heating deep in the flare atmosphere. Title: Chromospheric flare models. Authors: Avrett, E. H.; Machado, M. E.; Kurucz, R. L. Bibcode: 1986lasf.conf..216A Altcode: The calculations presented provide insights on how the observed chromospheric flare spectrum is produced. The results also suggest numerous improvements that should be made in the model calculations in order to interpret observations in better detail. The upper chromospheric contribution to white light flare emission is due to H, not H-, but H- plays an important role deeper in the atmosphere, and there could be a temperature enhancement in the deeper layers as a result of enhanced radiation produced in the upper chromosphere. Further work is needed to account for the important radiative processes, as well as time-dependent and kinetic processes, that affect the lower atmosphere in solar flares. Title: White light flares and atmospheric modeling (Working Group report). Authors: Machado, M. E.; Avrett, E. H.; Falciani, R.; Fang, C.; Gesztelyi, L.; Henoux, J. -C.; Hiei, E.; Neidig, D. F.; Rust, D. M.; Sotirovski, P.; Svestka, Z.; Zirin, H. Bibcode: 1986lasf.conf..483M Altcode: The authors give a short summary of their discussions, and a set of recommendations which may help in the study of white light flare emission processes. Title: Chromosphere flare models Authors: Avrett, E. H.; Kurucz, R. L.; Machado, M. E. Bibcode: 1985smm..conf.....A Altcode: Further calculated results based on the F1 and F2 chromospheric models of Machado et al. (1980) are presented in addition to results from a model with enhanced temperatures relative to the weak-flare model F1 in the upper photosphere and low chromosphere, and from a model with enhanced temperatures relative to the strong flare model F2 in the upper chromosphere. The coupled equations of statistical equilibrium and radiative transfer for H, H(-), He I-II, C I-IV, Si I-II, Mg I-II, Fe, Al, O I-II, Na, and Ca II are solved, and the overall absorption and emission of radiation by lines throughout the spectrum are determined by means of a reduced set of opacities taken from a compilation of over 10 million lines. Semiempirical models show that the white light flare continuum may arise by extreme chromospheric overheating, as well as by an enhancement of the minimum temperature region. Title: Mass Loss in Metal Deficient Red Giants Authors: Dupree, A. K.; Hartmann, L.; Smith, G. H.; Avrett, E. H. Bibcode: 1985mlrg.proc...29D Altcode: No abstract at ADS Title: Recent thermal models of the chromosphere. Authors: Avrett, E. H. Bibcode: 1985cdm..proc...67A Altcode: This paper describes recent research on the calculation of solar chromospheric models. The models included here are based on the observed spectrum, and on the assumption of hydrostatic equilibrium. The calculations depend on realistic solutions of the radiative transfer and statistical equilibrium equations for optically thick lines and continua, and on including the effects of large numbers of lines throughout the spectrum. Although spectroheliograms show that the structure of the chromosphere is highly complex, one-dimensional models of particular features are reasonably successful in matching observed spectra. There has been considerable recent progress in applying such models to the interpretation of chromospheric observations. Title: The calculation of theoretical chromospheric models and the interpretation of solar spectra from rockets and spacecraft Authors: Avrett, E. H. Bibcode: 1985sao..rept.....A Altcode: Solar chromospheric models are described. The models included are based on the observed spectrum, and on the assumption of hydrostatic equilibrium. The calculations depend on realistic solutions of the radiative transfer and statistical equilibrium equations for optically thick lines and continua, and on including the effects of large numbers of lines throughout the spectrum. Although spectroheliograms show that the structure of the chromosphere is highly complex, one-dimensional models of particular features are reasonably successful in matching observed spectra. Such models were applied to the interpretation of chromospheric observations. Title: On the extended chromosphere of alpha Orionis. Authors: Hartmann, L.; Avrett, E. H. Bibcode: 1984ApJ...284..238H Altcode: The authors develop preliminary models for the extended chromosphere of α Orionis. A flux of Alfvén waves sufficient to drive an outflow of ≡10-6M_sun;yr-1 is assumed. The momentum and energy equations for the wave-driven wind are solved assuming steady, radial, spherically symmetric flow. The mechanical energy fluxes needed to heat the extended chromosphere are of sufficient magnitude that momentum deposition by waves can support an extended chromosphere. The model accounts reasonably well for the observed radio continuum emission, the observed surface brightness distributions, and the emission fluxes in a variety of spectral lines. The detailed kinematics of the solution is less satisfactory in reproducing observed line profiles. It is suggested that the observed, nearly symmetric line profiles can be understood best in terms of a "fountain" model, in which most of the ejected material falls back rather than escaping. Title: Ultraviolet and optical observations of metal deficient red giants and chromospheric models. Authors: Dupree, A. K.; Avrett, E. H.; Hartmann, L.; Smith, G. Bibcode: 1984ESASP.218..191D Altcode: 1984iue..conf..191D Three metal deficient field stars were observed in the ultraviolet and optical spectral regions: HD 165195, HD 110281, and HD 232078. High dispersion spectra near H alpha, and low dispersion, long wavelength IUE spectra were obtained. The H alpha profiles have strong asymmetric emission with absorption cores that are frequently asymmetric. The surface flux of Mg II lines is similar to that of luminous Pop I stars in spite of the lower metal abundance. Semi-empirical atmospheric models suggest that the characteristic emission in the wings of the H alpha line can arise within static chromospheres. Radial expansion gives an asymmetric, blue-shifted H alpha core accompanied by greater emission in the red line wing than the blue wing. Wind models with extended atmospheres suggest mass loss rates - 2 billion M/yr. Thus H alpha provides no evidence that steady mass loss is substantial enough to significantly affect the evolution of stars on the red giant branch of globular clusters. Title: Chromospheres and mass loss in metal-deficient giant stars. Authors: Dupree, A. K.; Hartmann, L.; Avrett, E. H. Bibcode: 1984ApJ...281L..37D Altcode: Semiempirical atmospheric models indicate that the characteristic emission in the wings of the Hα line observed in Population II giant stars can arise naturally within static chromospheres. Radial expansion gives an asymmetric, blueshifted Hα core accompanied by greater emission in the red line wing than in the blue wing. Wind models with extended atmospheres suggest mass loss rates much smaller than 2×10-9M_sun; yr-1. Thus Hα provides no evidence that steady mass loss can significantly affect the evolution of stars on the red giant branch of globular clusters. Title: New Models of the Solar Temperature Minimum Region and Low Chromosphere Authors: Avrett, E. H.; Kurucz, R. L.; Loeser, R. Bibcode: 1984BAAS...16..450A Altcode: No abstract at ADS Title: Solar Cycle Temperature Variations in Sunspots Authors: Maltby, P.; Albregtsen, F.; Moe, O. Kjeldseth; Kurucz, R.; Avrett, E. Bibcode: 1984LNP...193..176M Altcode: 1984csss....3..176M The observed umbra/photosphere intensity ratio varies from the beginning to the end of each solar cycle by ∼ 30% at 1.6 μm and by factors > 2 at visible wavelengths. We present the intensity ratios measured in 10 wavelength bands extending from 0.387 to 2.35 μ m for 22 large sunspots observed during the period 1968-82, thus covering most of solar cycles 20 and 21. These results together with new observations of umbral limb darkening, and available data on photospheric absolute intensities, are used to estimate the dependence of the relative umbral intensity, and the absolute umbral intensity, on wavelength, heliocentric angle, and phase of the solar cycle. These umbral intensities are used to determine preliminary sunspot models which show the temperature as a function of depth in early, mid-, and late phases of the solar cycle. In the model calculations we use an extensive new compilation of atomic and molecular line data, allowing us to carry out the analysis by means of a detailed synthesis of the observed spectral bands. Title: Interpretation of the Observed Mg II Emission from N-type Carbon Stars Authors: Avrett, Eugene H.; Johnson, Hollis R. Bibcode: 1984LNP...193..330A Altcode: 1984csss....3..330A No abstract at ADS Title: Line transfer in static and expanding spherical atmospheres. Authors: Avrett, E. H.; Loeser, R. Bibcode: 1984mrt..book..341A Altcode: 1984mrt..conf..341A The authors show how to solve transfer problems using simultaneous equations based on the integral dependence of the intensity I on the source function S. The method is simple in concept, easy to apply, and applicable to a wide range of practical line and continuum transfer problems. Explicit equations with numerical solutions are derived for some examples of static spherical atmospheres and for line transfer in expanding spherical atmospheres with complete frequency redistribution and with partial redistribution. Title: Heating of the chromosphere and transition region by fast electrons from the temperature minimum. Authors: Byalko, A. V.; Avrett, E. H. Bibcode: 1984BAAS...16R.730B Altcode: No abstract at ADS Title: The calculation of theoretical chromospheric models and the interpretation of solar spectra from rockets and spacecraft Authors: Avrett, E. H. Bibcode: 1984sao..rept.....A Altcode: Models and spectra of sunspots were studied, because they are important to energy balance and variability discussions. Sunspot observations in the ultraviolet region 140 to 168 nn was obtained by the NRL High Resolution Telescope and Spectrograph. Extensive photometric observations of sunspot umbrae and prenumbrae in 10 chanels covering the wavelength region 387 to 3800 nm were made. Cool star opacities and model atmospheres were computed. The Sun is the first testcase, both to check the opacity calculations against the observed solar spectrum, and to check the purely theoretical model calculation against the observed solar energy distribution. Line lists were finally completed for all the molecules that are important in computing statistical opacities for energy balance and for radiative rate calculations in the Sun (except perhaps for sunspots). Because many of these bands are incompletely analyzed in the laboratory, the energy levels are not well enough known to predict wavelengths accurately for spectrum synthesis and for detailed comparison with the observations. Title: Metal Deficient Stellar Chromospheres: Semi-Empirical Models Authors: Dupree, A. K.; Hartmann, L.; Avrett, E. H. Bibcode: 1983BAAS...15..946D Altcode: No abstract at ADS Title: Photospheric and sunspot models from high-resolution spectrum synthesis. Authors: Avrett, E. H.; Kurucz, R. L. Bibcode: 1983ITABO..59...43A Altcode: Kurucz has recently compiled data for a list of 1.7×107 atomic and molecular lines. This list includes most of the lines seen in the solar spectrum. The authors use the line list and a standard photospheric model to calculate the detailed photospheric spectrum in the wavelength band 386 - 388 nm. The calculated spectrum is shown to be in good agreement with high-resolution observations, duplicating most individual line features and matching broad-band and maximum absolute line intensities. The results suggest that the sunspot 386 - 388 nm intensities are formed in the sunspot temperature-minimum region, and that the observed ≡3350K brightness temperature in this band is close to the sunspot minimum temperature value. Title: Wave-driven winds from cool stars. II - Models for T Tauri stars Authors: Hartmann, L.; Avrett, E.; Edwards, S. Bibcode: 1982ApJ...261..279H Altcode: The Alfven wave-driven wind theory of Hartmann and MacGregor is applied to T Tauri variables, including modifications which permit the calculation of wind temperatures. It is shown that large wave fluxes generate low-temperature winds, which can radiate strongly in Balmer and other optical emission lines. If wave fluxes are restricted to be less than the stellar luminosity, mass loss rates are restricted to values less than or equal to 10 to the -8th solar mass per year. Although these mass loss rates are low in comparison to many previous estimates, it is shown that the wind models produce optical and ultraviolet emission roughly consistent with observations. The theory predicts large wave amplitudes, so that 'turbulent' velocities are generally comparable to, or larger than, local expansion velocities in the optical line-emitting regions, thus making the Sobolev approximation invalid. It is suggested that the discrepancy between the mass loss rates predicted here and previously estimated values is due to the inapplicability of the Sobolev approximation in T Tauri winds. Title: Alfvén wave-driven winds as a source of mass loss from T Tauri stars. Authors: Edwards, S.; Hartmann, L.; Avrett, E. Bibcode: 1982SAOSR.392B.191E Altcode: 1982csss....2..191E No abstract at ADS Title: Predictions of wave-driven wind models. Authors: Hartmann, L.; Avrett, E. Bibcode: 1982SAOSR.392B.127H Altcode: 1982csss....2..127H No abstract at ADS Title: Solar Spectrum Synthesis. I. A Sample Atlas from 224 to 300 nm Authors: Kurucz, Robert L.; Avrett, Eugene H. Bibcode: 1981SAOSR.391.....K Altcode: We have developed sophisticated computer programs for determining solar and stellar atmospheric structure through the analysis of spectra. These programs allow us to treat the spectrum as a whole and to draw much stronger conclusions than would be apparent from individual spectral features. For a given LTE or non-LTE model atmosphere, the programs compute the emergent flux or the specific intensity at up to 20 angles. The spectrum can be broadened by macroturbulence and rotation; it can be modified by transmission through the Earth's atmosphere; it can be convolved with the instrumental profile; and it can finally be plotted together with the observed spectrum with each line labeled. In the opacity calculation, the lines are broadened by radiative, Stark, and van der Waals damping, and they can have isotopic and hyperfine splitting, autoionization, partial redistribution, or be merged into a continuum. The departure coefficients for ions treated in non-LTE in the model atmosphere calculation can be used in the spectrum synthesis programs for all lines of these ions, and highly ionized lines can be treated in the coronal approximation. The model atmosphere can have depth-dependent doppler shifts corresponding to large-scale motions. Using the Vernazza, Avrett, and Loeser models for the average quiet sun, we have computed theoretical solar spectra that include all available atomic and molecular line data. In this atlas we compare with the best available observed spectra in the 224- to 300-nm wavelength range, namely, the Kohl, Parkinson, and Kurucz (Harvard) center and limb rocket spectra in the range 224 to 300 nm; the Allen, McAllister, and Jefferies (Hawaii) disk center rocket spectrum for 268 to 293 nm; and the Brault and Testerman disk center spectrum taken at Kitt Peak for 294 to 300 nm. We also compare the observed spectra with each other. The existing spectra are noisy and do not have adequate resolution, so that it is difficult or impossible to identify weak features, to resolve blends, to study velocity fields, to search for variability; or to do any of the projects that can be routinely done in the visible where high quality spectra are available. Because the ultraviolet spectrum provides diagnostics for the upper photosphere, the temperature minimum, and the chromosphere, our understanding of these regions of the solar atmosphere is seriously impaired. One-half the lines in the observed spectrum are not identified. The overall level of the calculation lies considerably above the observed. The discrepancy is caused mainly by missing atomic and molecular lines and possibly by errors in the measured continuum opacities. Laboratory spectral analyses are seriously incomplete, especially for the iron group atoms and for high J and V molecular levels of CO, SiO, and the hydrides. Any model atmosphere or non-LTE rate calculation that depends directly on available ultraviolet opacities should not be trusted. Title: Structure of the solar chromosphere. III. Models of the EUV brightness components of the quiet sun. Authors: Vernazza, J. E.; Avrett, E. H.; Loeser, R. Bibcode: 1981ApJS...45..635V Altcode: The described investigation is concerned with the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semiempirical models for six components of the quiet solar chromosphere. For a given temperature-height distribution, the solution is obtained of the equations of statistical equilibrium, radiative transfer for lines and continua, and hydrostatic equilibrium to find the ionization and excitation conditions for each atomic constituent. The emergent spectrum is calculated, and a trial and error approach is used to adjust the temperature distribution so that the emergent spectrum is in best agreement with the observed one. The relationship between semiempirical models determined in this way and theoretical models based on radiative equilibrium is discussed by Avrett (1977). Harvard Skylab EUV observations are used to determine models for a number of quiet-sun regions. Title: Theoretical Models for T Tauri Mass Loss Authors: Hartmann, L.; Edwards, S.; Avrett, E. H. Bibcode: 1981BAAS...13..856H Altcode: No abstract at ADS Title: Radiative Transfer in Quasar Emission Line Clouds Authors: Avrett, E. H.; Loeser, R.; Pinto, P. A. Bibcode: 1981BAAS...13..788A Altcode: No abstract at ADS Title: Reference model atmosphere calculation - The Sunspot sunspot model Authors: Avrett, E. H. Bibcode: 1981phss.conf..235A Altcode: A composite temperature-density model of sunspots is developed from the results of five previous modeling attempts. The model is designed to describe the region extending from the deepest photospheric layers up through the chromosphere-corona transition region. Photospheric, chromospheric, and transition region temperature distributions proposed by other authors are employed to quantity the phenomena out to 2280 km. Additional calculations yield the various line profiles, the internal properties of the region above the sunspot including the chromospheric net radiative cooling rates, and the radiative flux as a function of depth in the photosphere. Observations of the hydrogen ion flux is used to account for the net cooling in the temperature minimum region. Furthermore, the integrated radiative flux is found to be constant with depth in the photosphere. Title: Solar Spectrum Synthesis I. A sample atlas from 224 to 300 NM Authors: Kurucz, Robert L.; Avrett, Eugene H. Bibcode: 1981ssss.book.....K Altcode: No abstract at ADS Title: Energy balance in solar and stellar chromospheres. Authors: Avrett, E. H. Bibcode: 1981ASIC...68..173A Altcode: 1981spss.conf..173A Net radiative cooling rates for quiet and active regions of the solar chromosphere and for two stellar chromospheres are calculated from corresponding atmospheric models. Models of chromospheric temperature and microvelocity distributions are derived from observed spectra of a dark point within a cell, the average sun and a very bright network element on the quiet sun, a solar plage and flare, and the stars Alpha Boo and Lambda And. Net radiative cooling rates due to the transitions of various atoms and ions are then calculated from the models as a function of depth. Large values of the net radiative cooling rate are found at the base of the chromosphere-corona transition region which are due primarily to Lyman alpha emission, and a temperature plateau is obtained in the transition region itself. In the chromospheric regions, the calculated cooling rate is equal to the mechanical energy input as a function of height and thus provides a direct constraint on theories of chromospheric heating. Title: Semiempirical models of chromospheric flare regions Authors: Machado, M. E.; Avrett, E. H.; Vernazza, J. E.; Noyes, R. W. Bibcode: 1980ApJ...242..336M Altcode: Homogeneous plane-parallel semiempirical flare model atmospheres which reproduce observations in lines and continua of H I, Si I, C I, Ca II, and Mg II have a thin transition zone at the top of the enhanced chromosphere, indicating a significant amount of heating from the zone to the temperature minimum level. The minimum temperature is located deeper and is higher than in the quiet-sun and active-region models. The results do not agree with the particle-heated theoretical models, and it is suggested that the models of Brown (1973) and Henoux and Nakagawa (1977, 1978) do not include an essential term for heat conduction in their energy balance equations. It is concluded that substantial Ly-alpha radiative heating occurs in the upper chromosphere resulting from the conductive energy flux in the transition zone where the Ly-alpha line cools the gas. Title: Energy Balance in the Solar Chromosphere Authors: Avrett, E. H.; Vernazza, J. E. Bibcode: 1979BAAS...11..657A Altcode: No abstract at ADS Title: High-pressure transition regions in stellar model chromospheres. Authors: Baliunas, S. L.; Avrett, E. H.; Hartmann, L.; Dupree, A. K. Bibcode: 1979ApJ...233L.129B Altcode: The implications of recent ultraviolet observations of stellar transition-region lines for calculations of the Ca II and Mg II resonance lines are investigated. It is found that the adoption of high transition-region pressures for stars with active chromospheres, such as Lambda And and Alpha Aur, can be consistent with observed Ca II fluxes, contrary to the results obtained by Kelch et al. (1978) for Alpha Aur. Furthermore, the adoption of the high-pressure models removes a long-standing difficulty in the line profile calculations, since the deep central absorption present in earlier calculations is less pronounced or absent, in closer agreement with observations. The apparent contradiction between these models and the recent density diagnostic of Doschek et al. (1978) is also discussed. Title: High-Pressure Transition Regions in Stellar Model Chromospheres Authors: Baliunas, S. L.; Avrett, E. H.; Hartmann, L. W.; Dupree, A. K. Bibcode: 1979BAAS...11..448B Altcode: No abstract at ADS Title: Frontiers of astrophysics. Authors: Avrett, E. H. Bibcode: 1979fras.book.....A Altcode: No abstract at ADS Title: Photometric and Spectroscopic Varability of Ca II H and K in G- and K-Type Giants. Authors: Baliunas, S. L.; Hartmann, L. W.; Liller, W.; Vaughan, A. H., Jr.; Avrett, E. H.; Dupree, A. K. Bibcode: 1978BAAS...10..461B Altcode: No abstract at ADS Title: Review of Publications (Frontiers of Astrophysics) Authors: Avrett, Eugene H. Bibcode: 1977JRASC..71..472A Altcode: No abstract at ADS Title: Formation of the Solar Mg I Spectrum. Authors: Zachary, A. L.; Avrett, E. H.; Durucz, R. L.; Loeser, R. K.; Vernazza, J. E. Bibcode: 1977BAAS....9..568Z Altcode: No abstract at ADS Title: The calculation of theoretical chromospheric models and predicted OSO I spectra Authors: Avrett, E. H. Bibcode: 1977sao..rept.....A Altcode: A computer program was developed which, given a line list and a model atmosphere, computes a solar ultraviolet spectrum, broadens it, plots it together with an observed spectrum, and labels each line. An iterative procedure is utilized. Several of the computed and observed spectra are presented. Title: Frontiers of Astrophysics (Book Review) Authors: Avrett, E. H. Bibcode: 1977ApL....18..137A Altcode: No abstract at ADS Title: Scattering in the Doppler core of the solar Lyman-alpha line: its effect on the Lyman continuum and on the chromospheric electron number density. Authors: Avrett, E. H.; Vernazza, J. E. Bibcode: 1977BAAS....9Q.432A Altcode: No abstract at ADS Title: Models of the Solar Atmosphere Authors: Avrett, E. H. Bibcode: 1977soiv.conf..327A Altcode: No abstract at ADS Title: Formation of the HeI and HeII Lines in the Solar Atmosphere. Authors: Avrett, E. H.; Vernazza, J. E.; Linsky, J. L. Bibcode: 1976BAAS....8..534A Altcode: No abstract at ADS Title: Excitation and ionization of helium in the solar atmosphere. Authors: Avrett, E. H.; Vernazza, J. E.; Linsky, J. L. Bibcode: 1976ApJ...207L.199A Altcode: The excitation and ionization of He I and He II is investigated for the case of a realistic solar model. The calculations are based on a simplified numerical treatment of the He I and He II continua and the He II 304-A line. The extent to which various proposed mechanisms can account for the observed line and continuum intensities is discussed. Title: Frontiers of astrophysics Authors: Avrett, Eugene H. Bibcode: 1976fras.conf.....A Altcode: 1976fras.book.....A; 1976QB461.F79...... The present volume constitutes a current intermediate-level text providing an introduction to the frontiers of astrophysical research. Topics discussed include the formation of the solar system; recent developments in solar research; star formation and the early stages of stellar evolution; endpoints of stellar evolution; neutron stars, black holes, and supernovae; and infrared astronomy. Other contributions deal with gaseous nebulae and their interstellar environment, the chemistry of the interstellar medium, radio observations of galactic masers, properties of active galaxies, galaxies and cosmology, and the contribution of intergalactic matter to the density of the Universe. Detailed attention is given to planetary accretion processes, the solar neutrino problem, solar flares, properties of young stellar objects, a scenario for pre-main-sequence evolution, properties of pulsars and binary X-ray stars, the energy balance of interstellar clouds, galactic maser models, theories on the nature of active galaxies, galaxy clustering, and the Robertson-Walker-Friedmann cosmological model. Title: Structure of the solar chromosphere. II. The underlying photosphere and temperature-minimum region. Authors: Vernazza, J. E.; Avrett, E. H.; Loeser, R. Bibcode: 1976ApJS...30....1V Altcode: The paper presents a non-LTE empirical model of the quiet solar photosphere and the temperature-minimum region. The continuous spectrum computed from this model is in good overall agreement with available disk-center observations throughout the wavelength range from 0.125 to 500 microns. It is found that (1) absolute-intensity measurements are needed in the range between 1 and 2 microns to establish the structure of the deepest observable layers; (2) absolute-intensity or flux measurements are needed in the range between 20 and 200 microns to determine whether the minimum solar temperature occurring between the photosphere and the chromosphere is as low as indicated by present observations or much higher, as recent theoretical predictions indicate; (3) studies of the far-ultraviolet spectrum based on the assumption of LTE can be substantially in error; and (4) line opacity seems to account for the 'missing opacity' in the ultraviolet. Title: The calculation of theoretical chromospheric models and predicted OSO 1 spectra Authors: Avrett, E. H. Bibcode: 1975sao..rept.....A Altcode: Theoretical solar chromospheric and photospheric models are computed for use in analyzing OSO 8 spectra. The Vernazza, Avrett, and Loeser (1976) solar model is updated and self-consistent non-LTE number densities for H I, He I, He II, C I, Mg I, Al I, Si I, and H(-) are produced. These number densities are used in the calculation of a theoretical solar spectrum from 90 to 250 nm, including approximately 7000 lines in non-LTE. More than 60,000 lines of other elements are treated with approximate source functions. Title: Formation of the Solar EUV Spectrum Authors: Avrett, Eugene H. Bibcode: 1975BAAS....7..360A Altcode: No abstract at ADS Title: Analysis of OSO data to determine the structure and energy balance of the solar chromosphere Authors: Avrett, E. H.; Kalkofen, W. Bibcode: 1975STIN...7520190A Altcode: A detailed reexamination of the temperature-density structure of the photosphere and low chromosphere shows that the middle and upper chromosphere, which directly emits most of the OSO spectrum, is sensitive to conditions in this underlying region of the atmosphere. A model of this region is based on a unified compilation of all recently published broadband flux and central intensity observations of the solar spectrum from 500 microns in the far infrared to 1220 A in the far ultraviolet. This extensive compilation includes the OSO 4 and 6 observations in the wavelength range 1400 to 1220 A. A model is presented of the quiet solar atmosphere in the height range between the temperature minimum and the upper part of the chromosphere-corona transition region. This model is based on statistical equilibrium calculations of H, He 1, He 2, Si 1, C 1, and other ions. Title: Report on the Celescope ultraviolet observations from the OAO 2 satellite and associated research at the Smithsonian Astrophysical Observatory. Authors: Avrett, E.; Davis, R.; Deutschman, W.; Haramundanis, K.; Kurucz, R.; Payne-Gaposchkin, C.; Peytremann, E.; Schild, R. Bibcode: 1974spre.conf..515A Altcode: No abstract at ADS Title: Blanketed model atmospheres for early-type stars Authors: Kurucz, Robert L.; Peytremann, Eric; Avrett, Eugene H. Bibcode: 1974bmae.book.....K Altcode: 1974QB843.E2K37.... No abstract at ADS Title: Report on the Celescope ultraviolet observations from the OAO-2 satellite and associated research at the Smithsonian Astrophysical Observatory. Authors: Avrett, E.; Davis, R.; Deutschman, W.; Haramundanis, K.; Kurucz, R.; Payne-Gaposchkin, C.; Peytremann, E.; Schild, R. Bibcode: 1974rnuo.confC...2A Altcode: No abstract at ADS Title: Structure of the Solar Chromosphere. Basic Computations and Summary of the Results Authors: Vernazza, Jorge E.; Avrett, Eugene H.; Loeser, Rudolf Bibcode: 1973ApJ...184..605V Altcode: We compute a one-component model of the solar atmosphere, including in that model the photosphere, chromosphere, and chromosphere-corona transition zone. The calculations are performed with PANDORA, a general computer program that enables us to obtain detailed solutions of the radiative-transfer and statistical-equilibrium equations for a six-level hydrogen atom and for eight-level C I and Si I atoms. All radiative transitions are treated explicitly. The program is formulated in a general way and is currently being used for the analysis of other atomic systems as well. The present calculation of the continuous absorption and emission throughout the spectrum takes into account the non-LTE contributions of H, H-, C I, and Si I. Other ions and atoms are treated in LTE. The atmosphere is assumed to be in hydrostatic equilibrium, but radiative equilibrium is not assumed. By trial-and-error adjustments, we obtained an empirical temperature-height distribution such that our computed synthetic intensities are consistent with the observed radiation from the chromosphere in the extreme-ultraviolet and microwave regions of the quiet solar spectrum. Our present determination of chromospheric structure differs from previous ones in that the Lyman lines are treated explicitly; they are not assumed to be in detailed balance. We find that small departures from detailed balance in the Lyman lines can lead to significant changes in the ionization of hydrogen in the middle chromosphere. The present paper is concerned mainly with the equations we solve in order to construct the solar model. Section XI gives a brief summary of the results. Further details will appear in subsequent papers of this series. Subject headings: atmospheres, solar - chromosphere, solar - spectra, solar Title: "stellar Chromospheric MODELS," Authors: Avrett, Eugene H. Bibcode: 1973NASSP.317...27A Altcode: 1973IAUCo..19...27A; 1973stch.coll...27A No abstract at ADS Title: Stellar Chromospheres Authors: Jordan, S. D.; Avrett, Eugene H. Bibcode: 1973NASSP.317.....J Altcode: 1973IAUCo..19.....J; 1973stch.coll.....J No abstract at ADS Title: Free-Free and Balmer-Line Emission from Optically Thick Stellar Shells Authors: Avrett, Eugene H.; Baldwin, J. Rand Bibcode: 1972PASP...84..633A Altcode: No abstract at ADS Title: Radiative transfer in two-component stellar atmospheres. Authors: Avrett, E. H.; Loeser, R. Bibcode: 1971JQSRT..11..559A Altcode: No abstract at ADS Title: Solution of non-LTE transfer problems. Authors: Avrett, E. H. Bibcode: 1971JQSRT..11..511A Altcode: No abstract at ADS Title: Radiative Interaction between the Solar Chromosphere and Corona in the Far Ultraviolet Authors: Avrett, Eugene H. Bibcode: 1970BAAS....2..292A Altcode: No abstract at ADS Title: The Solar H and K Lines Authors: Linsky, Jeffrey L.; Avrett, Eugene H. Bibcode: 1970PASP...82..169L Altcode: ASTRONOMICAL SOCIETY OF THE PACIFIC Vol. 82 April 1970 No. 485 THE SOLAR H AND K LINES JEFFREY L. LINSKY Joint Institute for Laboratory Astrophysics of the National Bureau of Standards and the University of Colorado AND EUGENE H. AVRETT Smithsonian Astrophysical Observatory Harvard College Observatory itory 22, 1970 We review our current understanding of the formation of the Ca ii H and K resonance and infi-ared triplet subordinate lines in the sun in view of the wedth of observations of these lines and the devdopment of non-LTE line formation theory. We descrihe the low- and high-spatial resolution data of these lines on the solar disk, off the limb, and in stellar spectra. We also describe obseiwations of the analogous Mg ii resonance lines. We review the various explanations proposed for the features of the lines the reversal, limb darkening, plage and spot profiles, and the anomalous line ratios. Line profiles are according to a first-order steady-state theoiy in which we assume a one-component atmosphere in hydrostatic , noncoherent scattering, and a five -level atom and 'im representation for Ca ii. The chromospheric model chosen is to be representative nither than definitive, biit it profiles of all five a microwave continuum in agreement with observations at the of the disk. We then extensions of this first-order theory. Title: A Model of the Solar Temperature-Minimum Region Authors: Avrett, Eugene H.; Linsky, Jeffrey L. Bibcode: 1970BAAS....2R.181A Altcode: No abstract at ADS Title: Formation of Line and Continuous Spectra. I. Source-Function Calculations Authors: Avrett, E. H.; Loeser, R. Bibcode: 1969SAOSR.303.....A Altcode: We present in full detail a method for solving the combined equations of statistical equilibrium and radiative transfer for line and continuum transitions. The method can be used to determine the variation of atomic number densities with depth in a stellar atmosphere. Local thermodynamic equilibrium is not assumed. In this paper, we discuss only the basic source-function calculations. In subsequent papers, we shall discuss related interative procedures and the general determination of a theoretical spectrum. Title: Calculations of Solar Hydrogen Lines: Comparative Solutions for a Standard Line Transfer Problem Authors: Athay, R. G.; Avrett, E. H.; Beebe, H. A.; Johnson, H. R.; Poland, A. I.; Cuny, Y. Bibcode: 1968rla..conf..169A Altcode: No abstract at ADS Title: Questions of Consistency and Convergence in the Solution of Multilevel Transfer Problems Authors: Avrett, E. H. Bibcode: 1968rla..conf...27A Altcode: No abstract at ADS Title: Source-Function Equality in Multiplets Authors: Avrett, E. H. Bibcode: 1966ApJ...144...59A Altcode: The line transfer equations are solved for an idealized multiplet consisting of three discrete atomic levels and no continuum. Complete frequency redistribution within each line is assumed The results permit a clarification of previous discussions by Wad dell and Athay on the necessary conditions for source-function equality. Title: Kernel Representations in the Solution of Line-Transfer Problems Authors: Avrett, E. H.; Loeser, R. Bibcode: 1966SAOSR.201.....A Altcode: In this paper we describe in detail a method of solution for a certain class of line-transfer problems. We also discuss the physical characteristics of this given type of problem, and show how the equations for particular cases are derived. The basic numerical method involves the solution of integral equations having the form S(τ) = [1-ɛ(τ) ∫T0K(|t-τ|) S(t) dt + ɛ(τ) B(τ). We deal only with those cases for which the kernel function has the form of K(|t-τ|) a restriction that involves several physical approximations. Despite this restriction, the method is applicable to a great many problems of physical interest-espeically to the transfer of line radiation by atoms with many levels. Title: The Temperature Structure of Early-Type Model Stellar Atmospheres. II. a Grid of Stellar Models Authors: Strom, S. E.; Avrett, E. H. Bibcode: 1965ApJS...12....1S Altcode: The essential numerical data are presented for a grid of accurate non-gray stellar models, covering the range 10000 K < < 20000 K and log g = 3 and 4. Details of the calculational procedures used in computing this grid are discussed. Included as sources of continuous opacity are H, H-, Hi+, He 1, He ii, electron, and Rayleigh scattering. In all cases, constancy of the flux and flux derivative is preserved to better than 0.2 per cent. Some of our models overlap, and are in good agreement, with those recently published by Mihalas. Title: Solutions of the Two-Level Line Transfer Problem with Complete Redistribution Authors: Avrett, E. H. Bibcode: 1965SAOSR.174..101A Altcode: No abstract at ADS Title: The Formation of Spectrum Lines. Proceedings Second Harvard-Smithsonian Conference on Stellar Atmospheres Authors: Avrett, E. H.; Gingerich, O. J.; Whitney, C. A. Bibcode: 1965SAOSR.174.....A Altcode: A conference on the theory of line formation was held at the Observatory on January 20-22, 1965, in conjunction with the Harvard College Observatory. The conference was convened to facilitate discussions among an increasing number of scientists engaged in the study and application of line-transfer theory. An edited transcript of the discussions is presented, together with the following papers: Session I: (1) Introductory Remarks, by C. A. Whitney; (2) Survey of the Problem, by J. T. Jefferies; (3) Theory of the Line Absorption Coefficient, by H. R. Griem; (4) The Emission Coefficient, by D. G. Hummer; (5) Collision Cross Sections, by M. J. Seaton: (6) Dielectronic Recombination, by A. Burgess; (7) The Plasma Microfield, by K. Hunger, R. W. Larenz, and K. Wilke; (8) Computation of the Line Source Function. A Review of the Physical Problem, by R. N. Thomas; (9) Solutions of the Two-Level Line Transfer Problem with Complete Redistribution, by E. H. Avrett; (10) General Noncoherent Scattering, by D. G. Hummer; (11) A New Differential Equation Approach to Transfer Problems, by G. B. Rybicki; (12) A Differential Equation for the Solution of the Non-LTE Line Transfer Problem, by E. Böhm-Vitense (13) Multilevel Problems, by J. T. Jefferies; (14) Radiative Transfer in Lines for Media in Statistical Equilibrium, by W. Kalkofen; (15) Calculations of Collisional-Radiative Decay, by M. J. Seaton; (16) On the Coupled Line-Transfer Problem for Hydrogen, by H. R. Johnson and D. A. Klinglesmith; (17) Solution of the Line and Continuum Transfer Problem for a Three-Level Atom, by W. Kalkofen and E. H. Avrett; (18) Solution of the Transfer Problem, by Y. Cuny; (19) Remarks by J. T. Jefferies Preceding the Paper by G. W. Curtis; (20) Inference of the Line Source Function for the Sodium D Lines, by G. W. Curtis; (21) Sodium Equilibrium and the Na I D Lines, by H. R. Johnson; and (22) The Profiles of the Sodium D Lines, by D. Mugglestone. Session II: (1) An Analysis of Solar Balmer Line Profiles, by O. R. White; (2) Stellar Chromosphere and Ca II H and K Emission, by O. C. Wilson; (3) Observational Requirements for Theory of Formation, of H and K Lines, by L. Goldberg; (4) The Calcium H and K Lines in Solar Plages, by J. B. Zirker; (5) Dielectronic Recombination and the Solar H and K Lines, by R. W. Noyes; (6) Total Fluxes in Strong Emission Lines, by R. G. Athay; (7) Deductions as to Accuracy of LTE from Excitation Temperature Measurements, and a Comment on the Abundance of "Trace Elements"; (8) Micromotions, Macromotions, and Non-LTE Effects, by J. -C. Pecker and F. Roddier; and (9) Non-LTE Effects on Abundance Determination, by R. Cayrel. Title: Solution of the Line and Continuum Transfer Problem for a Three-Level Atom Authors: Kalkofen, W.; Avrett, E. H. Bibcode: 1965SAOSR.174..249K Altcode: No abstract at ADS Title: Non-coherent scattering, II: Line formation with a frequency independent source function Authors: Avrett, E. H.; Hummer, D. G. Bibcode: 1965MNRAS.130..295A Altcode: The frequency-independent source function for a two-level atom in a self-excited atmosphere with no continuous absorption is encountered, for example, in theories of line formation in the solar corona and outer chromosphere. We present accurate numerical solutions to this transfer problem in finite and semi-infinite atmospheres for physically interesting values of the relevant parameters and discuss the mathematical and physical features of the source functions we obtain. We do not here apply our results to any particular astrophysical problem, but rather concentrate on understanding the physics of the transfer problem. Particular attention is given to the effects of collision broadening. Finally, the assumption of a frequency-independent source function for Doppler broadening is examined for self-consistency. Title: Comparison between model atmospheres and spectra from early-type stars Authors: Avrett, E. H.; Strom, S. E. Bibcode: 1965IAUS...23..147A Altcode: No abstract at ADS Title: Boundary Temperatures for Models with a Step-Function Absorption Coefficient Authors: Avrett, E. H.; Loeser, R. Bibcode: 1964SAOSR.167..198A Altcode: No abstract at ADS Title: Evaluation of Flux and Intensity Integrals Authors: Avrett, E. H. Bibcode: 1964SAOSR.167...63A Altcode: No abstract at ADS Title: Proceedings of the First Harvard-Smithsonian Conference on Stellar Atmospheres Authors: Avrett, E. H.; Gingerich, O. J.; Whitney, C. A. Bibcode: 1964SAOSR.167.....A Altcode: A Conference on model stellar atmospheres was held at the Observatory on January 20-21, 1964, in conjunction with Harvard College Observatory. The Proceedings principally comprise texts provided by the participants. Discussions from the floor were tape-recorded and have been summarized and paraphrased by the organizing committee for inclusion in these proceedings. Following is a list of papers and participants: Session I. Basic Data for Model Atmospheres: (1) Introductory Remarks, by E. Böhm-Vitense (2) Pressure-Dependent Partition Functions and Equilibrium Constants for Molecular Hydrogen, by M. S. Vardya; (3) Computation of Adiabatic Gradient, by M. S. Vardya; (4) Review of Opacity Calculations, by O. Gingerich; (5) H+2 Absorption as a Source of Continuous Opacity in the Stellar Atmospheres, by S. Matsushima; (6) Opacity at λ 1.65 μ in Late-Type Stars, by M. S. Vardya; (7) Computation of Some Photoionization Cross Sections of Neutral Carbon, by F. Praderie and (8) Hamming's Method for Integrating the Equation of Hydrostatic Equilibrium, by O. Gingerich, Session II. Nongray Atmospheres: (1) Evaluation of Flux and Intensity Integrals, by E. H. Avrett; (2) A Procedure for Evaluating the Flux Integral, by O. Gingerich; (3) A Procedure for Computing the Mean Intensity and the Flux, by P. Feautrier; (4) Temperature-Correction Procedures, by E. H. Avrett; (5) A Temperature-Correction Procedure, by L. B. Lucy; (6) An Integral Equation for the Temperature Correction in a Nongray Atmosphere, by E. Böhm-Vitense (7) A Method for Improving the T(x) Law, by P. Feautrier; (8) An Iterative Solution for the Source Function in the Gray Atmosphere in Radiative Equilibrium, by K. Grossman; (9) Effects of Errors in the Absorption Coefficient on the Non-gray Model Stellar Atmospheres, by S. Matsushima and Y. Terashita; (10) Comments on Blanketing, by D. Fischel; and (11) Theoretical Results on the Effect of Blanketing on T(τ), by R. Cayrel. Session III. Comparison of Specific Models: (1) Summary: Comparison of Archetype Model Atmospheres, by O. Gingerich; and (2) A Standard Model Atmosphere for a Pure Hydrogen Star of Effective Temperature 104°K, by M. Lecar. Session IV.Significance of Uncertainities in the Physical Theory: (1) Significance of Uncertainties in the Physical Theory of Radiative Transfer, by K. H. Böhm (2) On the Temperature of Radiative Equilibrium in a Layer of Optically Thin Gas in the Radiation Field of the Solar Photosphere, by R. Cayrel; (3) Departures from LTE Implied by Bound-Free and Free-Free Transitions, by W. Kalkofen; (4) Estimates of the Boundary Temperature of a Nongray Stellar Atmosphere, by M. Lecar; (5) Boundary Temperatures for Models with a Step-Function Absorption Coefficient, by E. H. Avrett and R. Loeser; (6) Convective Stellar Model Atmospheres, by D. W. Latham; (7) Convection in Late-Type Stars: A Few Random Remarks, by M. S. Vardya;(8) Solar Convection, by T. L. Swihart; and (9) Comments on Convection, by D. Fischel. Session V. Contact with Astronomical Data: (1) Contact with Astronomical Data, by A. B. Underhill; (2) The Utrecht Reference Model of the Photosphere, by J. R. W.Heintze, H. Hubenet, and C. de Jager; (3) The Center-to-Limb Variations in the Far Ultraviolet Continuum, by S. Matsushima; (4) Quantitative Interpretation of Stellar Spectra Using Model Atmospheres, by B. Baschek; (5) The Computation of Spectroscopic Data for 60 New Model Photospheres, by C. de Jager and L. Neven; (6) A Comparison between Model Atmospheres and Observations of Early-Type Stars, by S. E. Strom; (7) Abundances in Normal Stars and AM Stars Using Semiempirical Model Atmosphere Conti; (8) Comparison of Some Theoretical Line Contours of Hγ with Observations, by F. M. Stienon; and (9) Theoretical Model Atmosphere Data Needed for Interpretation of Observational Material, by L. H. Aller. Title: Temperature-Correction Procedures Authors: Avrett, E. H. Bibcode: 1964SAOSR.167...83A Altcode: No abstract at ADS Title: The Temperature Structure of Early-Type Model Stellar Atmospheres. I. Detailed Analysis of a 10000° K Model. Authors: Strom, S. E.; Avrett, E. H. Bibcode: 1964ApJ...140.1381S Altcode: Calculations are presented for five model atmospheres, all with T,11 = 10000 K and log g = 4, which illustrate the detailed effects of individual opacity sources, scattering processes, the Lyman continuum, and the blended wings of the higher Balmer and Lyman lines. Title: Detailed Examination of a Non-Grey Stellar Atmosphere. Authors: Strom, S. E.; Avrett, E. H. Bibcode: 1964AJ.....69S.559S Altcode: Results of calculations for five model atmospheres, all with TeCf= 10 0000K and log g =4, are presented. These results illustrate the detailed effects of individual opacity sources, scattering processes, the Lyman continuum, and the blended wings of the higher Balmerand Lyman lines. When electron scattering is included, it is found that the presence of frequencies in the Lyman continuum affects the temperature distribution for optical depths as large as r = 10-~. However, the only observable features influenced by incorporating these frequencies are the cores of strong Balmer lines. By means of the Kolb-Griem theory the blended opacity due to the higher members of the Balmer and Lyman series has been calculated. We exhibit the effect of the blended wings of these lines on the emergent flux for this standard model. We compare the calculated fluxes with those measured for Vega and find that the observed flux can be accurately reproduced even near the Balmer limit. Finally, we exhibit the results for three model atmospheres (kindly calculated for us by Dr. Owen Gingerich), having Te = 95000Kand logg = 4.3, for which log [H/metalsJ has the values 2.0, 3.0, and 4.0. As the metal abundance is increased, the Balmer discontinuity decreases. It is speculated that the anomalous (U- B) color for Sirius (U- B = -0.08) arises from a higher than normal metal abundance for that star. Title: Comparison between model atmospheres and spectra from early-type stars Authors: Avrett, E. H.; Strom, S. E. Bibcode: 1964AnAp...27..781A Altcode: No abstract at ADS Title: Calculation of Early-Type Model Stellar Atmospheres. Authors: Strom, S. E.; Avrett, E. H. Bibcode: 1964AJ.....69S.150S Altcode: The iterative temperature correc~ioi~ scheme proposed by Krook and Avrett (Krook, M., Astrophys. J. 137, 863, 1963; Avrett, F. H., and Krook, ~~., ibid. 137, 874,1963) is employed in calculating a number of early-type model stellar atmospheres. We have computed a grid of model atmospheres for which 100000<Te<250000K, 3<log g<5 and X=0.833, Y=0.167. We include in these models the sources of opacity: neutral hydrogen, i~eutral and singly ionized helium, H2~, TI-, and the effects of electron scattering, Rayleigh scatterii~g, and radiation pressure. The temperature and pressure distributions as well as the emergent fluxes are exhibited for several representative cases. The accuracy obtainable with this temperature correction scheme used in conjunction with a highspeed computer (the IBM 7090) is shown to he a few parts in 10~, with respect to the constancy of net flux, the flux derivative and the temperature. The importance of obtaining such accuracy is illustrated by a case in which a flux, constant to a few percent, requires temperature corrections of several hui~dred degrees. Title: Departures from LTE in a Model Atmosphere. Authors: Kalkofen, W.; Avrett, E. H. Bibcode: 1964AJ.....69Q.546K Altcode: The assumption of LTE is investigated in a model atmosphere composed of atomic hydrogen, with an effective temperature of 10 0000K and a surface gravity of g=104. Model atmospheres are constructed (a) under the assumption of local thermodynamic equilibrium and (b) with departures from LTE permitted in the populations of the two lowest bound levels. We illustrate the influence of Lyman-a radiation on the populations of these two levels and, hence, on the continuous flux. The following models are compared: (1) LTE, no lines; (2) Non-LTE in the two lowest levels, no lines; (3) LTE, with Lyman-a line; (4) Non-LTE in the two lowest levels, with Lyman-a line. The models (1), (2), and (4) have approximately the same boundary temperature, whereas the LTE model (3) with the Lyman-a line has a very low boundary temperature. The two LTE models (1) and (3) have the same emergent continuous flux. The flux of the two non-LTE models is depressed in the Lyman con- tinuum relative to the LTE models by a factor of approximately 4, and the ratio of the monochromatic emergent fluxes on the red and blue sides of the Balmer absorption edge is reduced by approximately 15%. Thus on the basis of the LTE model, the effective temperature of a star would tend to be overestimated. The two non-LTE models (2) and (4) differ in the pop~lation of the ground state which in case (2) is in LTE at TiOOO 10-6; in case (4), where the Lyman-a line is included, the ground state is underpopulated by approximately 10% r5000 = 0.1. Title: A rapidly convergent iterative procedure for the calculation of the temperature-pressure relation in a stellar atmosphere Authors: Avrett, E. Bibcode: 1963JQSRT...3..107A Altcode: No abstract at ADS Title: The Temperature Distribution in a Stellar Atmosphere. Authors: Avrett, E. H.; Krook, Max Bibcode: 1963ApJ...137..874A Altcode: We describe an iterative perturbation method for the determination of the temperature distribution in a plane-parallel pure absorption atmosphere. The method is unrestricted by the nature of the absorption coefficient and is rapidly convergent. Title: Solutions of the Equation of Radiative Transfer. Authors: Avrett, Eugene Hinton Bibcode: 1962PhDT.........4A Altcode: No abstract at ADS Title: Particle Motion in the Equatorial Plane of a Dipole Magnetic Field Authors: Avrett, E. H. Bibcode: 1962JGR....67...53A Altcode: An exact relation is derived which describes bound particle orbits in the equatorial plane of a dipole magnetic field. An exact expression is then obtained for the average angular velocity of the particle about the dipole axis. The corresponding drift velocity is compared with the usual first-order expression based on a constant local field gradient. It is shown that the first-order expression for the drift velocity can be considerably in error when the particle loops are not small compared with the mean distance from the dipole axis. Title: A Non-Gray Radiative-Transfer Problem. Authors: Carrier, C. F.; Avrett, E. H. Bibcode: 1961ApJ...134..469C Altcode: The absorption coefficient for a stellar atmosphere depends on frequency as well as depth. Often the frequency dependence is characterized by a large discontinuity at each of the series limits In order `to investigate the effect of such a discontinuity, we consider a model atmosphere with an absorption'coefficient of the simple form K(x, ) = where E is constant and small compared with unity. Using the Eddington approximation, we obtain for the temperature distribution a highly non4inear differential equation which is of boundary4ayer, type. This equation is solved by conventional methods of boundary4ayer theory. The calculated results exhibit a greatly reduced temperature in the outer layers of the atmosphere (the boundary layer) and a greatly reduced amount of ultraviolet flux (v> po) emerging from the atmosphere. The method by which the boundary4ayer equation is obtained and solved is applicable for absorption coefficients of greater complexity and for approximations of higher order than the Eddington approximation.