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Author name code: avrett
ADS astronomy entries on 2022-09-14
author:"Avrett, Eugene Hinton"
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Title: Erratum: "Modeling the Chromosphere of a Sunspot and the
Quiet Sun" <A href="/abs/2015ApJ...811...87A">(2015, ApJ, 811, 87)</A>
Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wuelser, J. -P.
2016ApJ...821...70A Altcode:
No abstract at ADS
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Title: Chromospheric Models and the Oxygen Abundance in Giant Stars
Authors: Dupree, A. K.; Avrett, E. H.; Kurucz, R. L.
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.
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Title: Modeling the Chromosphere of a Sunspot and the Quiet Sun
Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wülser, J. -P.
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.
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Title: Sleuthing Helium in Cluster Red Giants
Authors: Dupree, Andrea K.; Avrett, Eugene; Smith, Graeme; Strader, Jay
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.
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Title: Chromospheres of Luminous Cool Stars
Authors: Dupree, Andrea K.; Avrett, Eugene
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.
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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.
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.
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Title: Calculated Resonance Line Profiles of [Mg II], [C II], and
[Si IV] in the Solar Atmosphere
Authors: Avrett, E.; Landi, E.; McKillop, S.
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.
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Title: Direct Evaluation of the Helium Abundances in Omega Centauri
Authors: Dupree, A. K.; Avrett, E. H.
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.
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Title: SDO/EVE spectra of solar flares
Authors: Heinzel, Petr; Avrett, Eugene; Dzifcakova, Elena; Hudson,
Hugh S.
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.
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Title: Optical-to-Radio Continua in Solar Flares
Authors: Heinzel, P.; Avrett, E. H.
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.
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Title: The nature of hydrogen and helium continua in SDO/EVE spectra
of solar flares
Authors: Heinzel, P.; Avrett, E. H.
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).
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Title: Photometric analysis of Ellerman bombs .
Authors: Berlicki, A.; Heinzel, P.; Avrett, E. H.
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.
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Title: Particle diffusion, flows, and NLTE calculations
Authors: Fontenla, J. M.; Avrett, E.
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.
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Title: Mass Outflow from Red Giant Stars in M13, M15, and M92
Authors: Mészáros, Sz.; Avrett, E. H.; Dupree, A. K.
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<SUP>-1</SUP>,
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<SUP>-9</SUP>
to 5 × 10<SUP>-9</SUP> M <SUB>sun</SUB> yr<SUP>-1</SUP>, 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 <SUB>sun</SUB> yr<SUP>-1</SUP>) =
0.092 ×L <SUP>0.16</SUP> × T <SUP>-2.02</SUP> <SUB>eff</SUB> × A
<SUP>0.37</SUP>, 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.
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Title: Iterative Solution of Multilevel Transfer Problems
Authors: Avrett, Eugene H.; Loeser, Rudolf
2009nrt..book..135A Altcode:
No abstract at ADS
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Title: Rapidly-Converging Methods for Solving Multilevel Transfer
Problems
Authors: Avrett, Eugene H.
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.
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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.
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.
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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
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.
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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.
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.
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Title: New Models of the Solar Chromosphere and Transition Region
from SUMER Observations
Authors: Avrett, E. H.
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..
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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.
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 τ<SUB>500</SUB>=1. Oscillations in lines originating
at similar altitudes exhibit different behaviors which we discuss in
terms of the formation of the lines.
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Title: The Structured Chromosphere and Wind of TW Hya
Authors: Dupree, A. K.; Avrett, E. H.; Brickhouse, N. S.; Cranmer,
S. R.; Szalai, T.
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.
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Title: New Models of the Solar Chromosphere and Transition Region
Determined from SUMER Observations
Authors: Avrett, E. H.
2006ESASP.617E...9A Altcode: 2006soho...17E...9A
No abstract at ADS
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Title: Semi-empiric Radiative Transfer Modeling of FUSE Stellar
Spectra
Authors: Lobel, A.; Avrett, E. H.; Aufdenberg, J. P.
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. <P />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<SUP>-1</SUP> 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<SUP>-11</SUP> g cm<SUP>-3</SUP>
favors a region of warm dense plasma of 100 kK ≤T<SUB>gas</SUB>≤
300 kK that collapses onto the white dwarf with a mass accretion rate
of 1-2 10<SUP>15</SUP> g s<SUP>-1</SUP> 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.
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Title: New Models Of The Solar Chromosphere And Transition Region
Determined From Sumer Observations
Authors: Avrett, Eugene H.
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.
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Title: Identification of the broad solar emission features near 117 nm
Authors: Avrett, E. H.; Kurucz, R. L.; Loeser, R.
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.
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.
2005AAS...207.7413D Altcode: 2005BAAS...37.1286D
Infrared and ultraviolet spectra recently revealed the presence of
hot (300,000 K), fast (450 km s<SUP>-1</SUP>) 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. <P />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.
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. <P />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. <P />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. <P />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.
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<SUP>-2</SUP> at a temperature of 7400 K
(approximately 3000 K warmer than the photosphere) and then has a steep
increase to the peak temperature of ≈10<SUP>6</SUP> 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.
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.
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.
2003IAUS..210P.A21A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Temperature Minimum and Chromosphere
Authors: Avrett, E. H.
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.
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.
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.
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.
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.
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.
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.
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.
1999ApJ...521L.141K Altcode:
The successful modeling of the dynamics of H<SUB>2v</SUB> 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.
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
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.
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.
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.
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. <P />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. <P />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. <P
/>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.
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.
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.
1996ASPC..109..105A Altcode: 1996csss....9..105A
No abstract at ADS
---------------------------------------------------------
Title: Next generation model atmospheres (review)
Authors: Avrett, E. H.
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.
1996AAS...187.3806B Altcode: 1996BAAS...28..755B
No abstract at ADS
---------------------------------------------------------
Title: Next generation model atmospheres
Authors: Avrett, Eugene H.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
10<SUP>4</SUP> to 10<SUP>5</SUP>K. 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 = 10<SUP>4</SUP>K)
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.
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.
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.
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.
1992ASPC...26..525D Altcode: 1992csss....7..525D
No abstract at ADS
---------------------------------------------------------
Title: The PANDORA Atmosphere Program (Invited Review)
Authors: Avrett, E. H.; Loeser, R.
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.
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.
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.
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.
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.
1991BAAS...23.1029C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New Models of the Chromosphere and Transition Region (With
2 Figures)
Authors: Avrett, E. H.
1991mcch.conf...97A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The photosphere as a radiative boundary.
Authors: Anderson, Lawrence S.; Avrett, Eugene H.
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.
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 eta<SUB>HeI</SUB>
for T greater than 35,000 K and decreases eta<SUB>HeI</SUB> (while
increasing eta(sub(HeII)) between 9,000 and 25,000 K. Including the
effects of diffusion also increases eta<SUB>HeIII</SUB> 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.
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.
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
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.
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.
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.
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.
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
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
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
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.
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<SUP>-</SUP> 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.
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.
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.
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.
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
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
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 b<SUB>2</SUB> 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.
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.
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.
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.
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.
1987BAAS...19Q.931F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mass Loss from FU Orionis Objects
Authors: Croswell, Ken; Hartmann, L.; Avrett, E. H.
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
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
1987nrt..book..135A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The solar chromosphere
Authors: Noyes, R. W.; Avrett, E. H.
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.
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.
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.
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.
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<SUP>-</SUP>, but H<SUP>-</SUP> 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.
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.
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.
1985mlrg.proc...29D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Recent thermal models of the chromosphere.
Authors: Avrett, E. H.
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.
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.
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<SUP>-6</SUP>M_sun;yr<SUP>-1</SUP> 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.
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.
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<SUP>-9</SUP>M_sun; yr<SUP>-1</SUP>. 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.
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.
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.
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.
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.
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.
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.
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.
1983ITABO..59...43A Altcode:
Kurucz has recently compiled data for a list of 1.7×10<SUP>7</SUP>
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.
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.
1982SAOSR.392B.191E Altcode: 1982csss....2..191E
No abstract at ADS
---------------------------------------------------------
Title: Predictions of wave-driven wind models.
Authors: Hartmann, L.; Avrett, E.
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.
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.
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.
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.
1981BAAS...13..788A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Reference model atmosphere calculation - The Sunspot sunspot
model
Authors: Avrett, E. H.
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.
1981ssss.book.....K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Energy balance in solar and stellar chromospheres.
Authors: Avrett, E. H.
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.
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.
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.
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.
1979BAAS...11..448B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Frontiers of astrophysics.
Authors: Avrett, E. H.
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.
1978BAAS...10..461B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Review of Publications (Frontiers of Astrophysics)
Authors: Avrett, Eugene H.
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.
1977BAAS....9..568Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The calculation of theoretical chromospheric models and
predicted OSO I spectra
Authors: Avrett, E. H.
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.
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.
1977BAAS....9Q.432A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Models of the Solar Atmosphere
Authors: Avrett, E. H.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
1973NASSP.317...27A Altcode: 1973IAUCo..19...27A; 1973stch.coll...27A
No abstract at ADS
---------------------------------------------------------
Title: Stellar Chromospheres
Authors: Jordan, S. D.; Avrett, Eugene H.
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
1972PASP...84..633A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Radiative transfer in two-component stellar atmospheres.
Authors: Avrett, E. H.; Loeser, R.
1971JQSRT..11..559A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solution of non-LTE transfer problems.
Authors: Avrett, E. H.
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.
1970BAAS....2..292A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar H and K Lines
Authors: Linsky, Jeffrey L.; Avrett, Eugene H.
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.
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.
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.
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.
1968rla..conf...27A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Source-Function Equality in Multiplets
Authors: Avrett, E. H.
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.
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) ∫<SUP>T</SUP><SUB>0</SUB>K(|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.
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.
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.
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.
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.
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.
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.
1964SAOSR.167..198A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evaluation of Flux and Intensity Integrals
Authors: Avrett, E. H.
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.
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<SUP>+</SUP><SUB>2</SUB> 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 10<SUP>4</SUP>°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.
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.
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.
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.
1964AnAp...27..781A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Calculation of Early-Type Model Stellar Atmospheres.
Authors: Strom, S. E.; Avrett, E. H.
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.
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Title: Departures from LTE in a Model Atmosphere.
Authors: Kalkofen, W.; Avrett, E. H.
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.
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Title: A rapidly convergent iterative procedure for the calculation
of the temperature-pressure relation in a stellar atmosphere
Authors: Avrett, E.
1963JQSRT...3..107A Altcode:
No abstract at ADS
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Title: The Temperature Distribution in a Stellar Atmosphere.
Authors: Avrett, E. H.; Krook, Max
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.
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Title: Solutions of the Equation of Radiative Transfer.
Authors: Avrett, Eugene Hinton
1962PhDT.........4A Altcode:
No abstract at ADS
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Title: Particle Motion in the Equatorial Plane of a Dipole Magnetic
Field
Authors: Avrett, E. H.
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.
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Title: A Non-Gray Radiative-Transfer Problem.
Authors: Carrier, C. F.; Avrett, E. H.
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.
