skumanich

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Author name code: skumanich
ADS astronomy entries on 2022-09-14
author:"Skumanich, Andrew"

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Title: The discovery path of the inverse square root of age relations
for solar-type stars
Authors: Skumanich, A.
2022fysr.confE...1S Altcode:
This talk will describe the discovery path of the inverse square root
dependance of a star's age for main sequence stars. It will highlight
the analysis of the circumstantial evidence that led to the relation
that rotational spin down and magnetic activity decay by the square root
law. The temporal history of observed changes in key stellar properties
may be measured by various types of 'clocks'. The first clock is the
stellar nuclear reaction rate, which gives rise to the luminosity - or
power - radiated by a star. Given the mass of a star and its nuclear
composition the physics dictates the progression of its luminosity,
as well as it's radius (or equivalently the surface temperature or
color). These two parameters can be located on the famous Hertzsprung
-Russell diagram that relates luminosity with color for any observed
star. As the reaction rate begins to deplete the nuclear fuel the
star begins to migrate across the HR diagram. The second clock deals
with stellar kinematics. The turbulence within interstellar clouds
decays with time so that stars formed at an earlier epoch will have
larger velocities than those formed later. In addition, as Spitzer,
Schwarzschild and Osterbrock have shown, subsequent collisions of
stars with high velocity clouds increase their velocities. Hence lower
velocities indicate younger ages. The third clock deals with the level
of magnetic energy activity that decays by dissipation. Such processes
produce stellar coronae, with their associated magnetized stellar winds
that lead to angular momentum loss. Magnetic activity also leads to
chromospheres that age with time. Another associated age indicator is
the gradual enrichment of the 'metallic', eg Carbon, Calcium, etc.,
content of the interstellar clouds due to supernova explosions.

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Title: Constraints Imposed by the Square-root Spin-down Law on
Coronal Wind Parameters
Authors: Skumanich, Andrew
2019ApJ...878...35S Altcode:
We derive an explicit expression for the coronal wind mass-loss rate in
terms of either the surface field or the surface rotation rate. This
result is based on a currently used generalized torque interpolation
formula (per unit rotational velocity) expressed as a product of
known monomials, separately for the magnetic field and the mass-loss
rate. The latter is assumed here to be given by an unknown monomial
power of the magnetic field. This yields a torque dependent solely on
the magnetic field that must yield the square-root spin-down law. A
monomial field-rotation relation, defining the rotational power index
for the field, is derived from the congruity of the decay with age of
the observed Ca II emission luminosity (with an arbitrary magnetic
field power dependence) with the decay with age of the rotational
velocity. This allows one to express the mass-loss rate power index in
terms of the rotational index. We apply this constraining equation to a
variety of observations. We find that even though there is considerable
scatter in the data, there is sufficient observational evidence for
the reality of a linear field-rotation relation associated with a
linear temporal decay of the mass-loss rate.

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Title: Preface
Authors: Engvold, Oddbjørn; Vial, Jean-Claude; Skumanich, Andrew
2019sgsp.bookD..17E Altcode:
No abstract at ADS

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Title: Chapter 14 - Challenges and Prospects for the Future
Authors: Vial, Jean-Claude; Skumanich, Andrew
2019sgsp.book..463V Altcode:
The 14 chapters (and five subchapters) of this book have addressed
only a few aspects of the many works that have been produced during the
latest decades in the domain of solar physics and that may be relevant
to stellar physics and possibly other fields in astrophysics. The
achievements result not only from the proximity of our star, which
allows for detailed and comprehensive observations but also from the
many questions that have arisen and have been answered through the
implications of physics and sometimes have led to the stimulation of a
"new" physics. We do not include the summaries and conclusions of
these 14 chapters here. We simply focus on a few points (challenges and
prospects) that may be of interest for future progress and discoveries
in solar and stellar physics. As is shown here, we also allow ourselves
to point out that some authors of the book, including ourselves, may
have some different views on a few topics, a fact of life in science. We
address the various issues with the two following questions in mind:
Where and how is solar progress foreseen? What are the mutual benefits
in the solar-stellar connection?

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Title: The Sun as a Guide to Stellar Physics
Authors: Engvold, Oddbjørn; Vial, Jean-Claude; Skumanich, Andrew
2019sgsp.book.....E Altcode:
No abstract at ADS

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Title: Pulsation Modes with High Azimuthal Orders in Stellar Models
Based on the Self-Consistent Field Method
Authors: Reese, D. R.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
Metcalfe, T. S.
2009ASPC..416..395R Altcode:
We investigate pulsation modes with high azimuthal orders in a
uniformly and differentially rotating stellar model based on the
SCF method. Our results show a more complicated m dependence of the
pulsation frequencies than what was proposed in previous studies. These
results allow a more precise determination of the effects of rapid
uniform or differential rotation on stellar pulsations thereby
providing a better understanding of the underlying physics.

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Title: Mode identification in rapidly rotating stars
Authors: Reese, D. R.; Thompson, M. J.; MacGregor, K. B.; Jackson,
S.; Skumanich, A.; Metcalfe, T. S.
2009A&A...506..183R Altcode: 2009arXiv0905.4889R
Context: Recent calculations of pulsation modes in rapidly rotating
polytropic models and models based on the Self-Consistent Field method
have shown that the frequency spectrum of low degree pulsation modes can
be described by an empirical formula similar to Tassoul's asymptotic
formula, provided that the underlying rotation profile is not too
differential. Aims: Given the simplicity of this asymptotic
formula, we investigate whether it can provide a means by which to
identify pulsation modes in rapidly rotating stars. Methods:
We develop a new mode identification scheme which consists in scanning
a multidimensional parameter space for the formula coefficients which
yield the best-fitting asymptotic spectra. This mode identification
scheme is then tested on artificial spectra based on the asymptotic
formula, on random frequencies and on spectra based on full numerical
eigenmode calculations for which the mode identification is known
beforehand. We also investigate the effects of adding random frequencies
to mimic the effects of chaotic modes which are also expected to show
up in such stars. Results: In the absence of chaotic modes,
it is possible to accurately find a correct mode identification
for most of the observed frequencies provided these frequencies are
sufficiently close to their asymptotic values. The addition of random
frequencies can very quickly become problematic and hinder correct mode
identification. Modifying the mode identification scheme to reject the
worst fitting modes can bring some improvement but the results still
remain poorer than in the case without chaotic modes.

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Title: Pulsation modes in rapidly rotating stellar models based on
the self-consistent field method
Authors: Reese, D. R.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
Metcalfe, T. S.
2009A&A...506..189R Altcode: 2009arXiv0903.4854R
Context: New observational means such as the space missions CoRoT and
Kepler and ground-based networks are and will be collecting stellar
pulsation data with unprecedented accuracy. A significant fraction of
the stars in which pulsations are observed are rotating rapidly. Aims: Our aim is to characterise pulsation modes in rapidly rotating
stellar models so as to be able to interpret asteroseismic data from
such stars. Methods: A new pulsation code is applied to stellar
models based on the self-consistent field (SCF) method. Results:
Pulsation modes in SCF models follow a similar behaviour to those
in uniformly rotating polytropic models, provided that the rotation
profile is not too differential. Pulsation modes fall into different
categories, the three main ones being island, chaotic, and whispering
gallery modes, which are rotating counterparts to modes with low,
medium, and high l-|m| values, respectively. The frequencies of the
island modes follow an asymptotic pattern quite similar to what was
found for polytropic models. Extending this asymptotic formula to higher
azimuthal orders reveals more subtle behaviour as a function of m and
provides a first estimate of the average advection of pulsation modes
by rotation. Further calculations based on a variational principle
confirm this estimate and provide rotation kernels that could be
used in inversion methods. When the rotation profile becomes highly
differential, it becomes more and more difficult to find island and
whispering gallery modes at low azimuthal orders. At high azimuthal
orders, whispering gallery modes, and in some cases island modes,
reappear. Conclusions: The asymptotic formula found for
frequencies of island modes can potentially serve as the basis of a
mode identification scheme in rapidly rotating stars when the rotation
profile is not too differential.

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Title: Comparison of pulsation modes in rapidly rotating polytropic
and SCF models
Authors: Reese, D.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
Metcalfe, T. S.
2009CoAst.158..264R Altcode:
In this talk, I will show numerical calculations of pulsation
modes in rapidly differentially rotating stellar models based on the
self-consistent field (SCF) method {(e.g. MacGregor et al. 2007)}. The
pulsation calculations are based on the numerical method presented in
{Lignières et al. (2006)} and {Reese et al. (2006)}. I will compare
these results with previous calculations based on simpler polytropic
models, and discuss how the structure of the frequency spectrum is
affected by differential rotation and other stellar parameters. In
particular, differential rotation can lead to a breakdown of
regularities in the frequency spectrum in favour of a more chaotic
behaviour.

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Title: Pulsations of rapidly rotating stellar models based on the
Self-Consistent-Field method: numerical aspects and accuracy
Authors: Reese, D.; MacGregor, K. B.; Jackson, S.; Skumanich, A.;
Metcalfe, T. S.
2008sf2a.conf..531R Altcode:
We use the numerical method developed in tet{Lignieres2006} and
tet{Reese2006} to calculate pulsation modes in stellar models generated
by the Self-Consistent-Field method described in tet{Jackson2005}
and tet{MacGregor2007}. A discussion on the numerical method and its
accuracy is given, followed by a very brief description of some of
the results.

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Title: Absorption Line Profiles for Differentially Rotating Stellar
Models
Authors: MacGregor, Keith B.; Casini, R.; Flanagan, W.; Jackson, S.;
Skumanich, A.
2007AAS...21110310M Altcode: 2007BAAS...39..921M
We have computed absorption line profiles for rapidly rotating 2
solar-mass stars, using structural models of chemically homogeneous,
uniformly and differentially rotating stars of this mass to specify
photospheric physical conditions. The models were constructed
with a recent reformulation of the self-consistent field method, an
iterative procedure yielding two-dimensional, axisymmetric, equilibrium
configurations that are consistent solutions to the stellar structure
equations and Poisson's equation for the gravitational potential,
for a specified conservative angular velocity distribution. The
simulated line profiles display a variety of shapes, with morphological
characteristics that depend on the properties of the surface rotational
velocity distribution, the angle of inclination of the line of sight
relative to the rotation axis, and on rotational modifications to the
structure of the star, including the difference between the surface
temperatures of the poles and the equator and deviations of the stellar
shape from sphericity. Some features of the Doppler-broadened profiles
reflect details of the distribution of the projected rotation speed
over the visible surface of the star, and may thus provide the means
for distinguishing between uniform and differential rotation of the
stellar photosphere. For example, differentially rotating models can
exhibit absorption profiles with flat or even convex-upward bottoms, as
opposed to the rounded, concave-upward profiles that are indicative of
uniform rotation. We present preliminary results from an ongoing effort
to use simulated line profiles in conjunction with the techniques of
Principal Component Analysis to infer stellar rotational properties
from observations.

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Title: On the Structure and Properties of Differentially Rotating,
Main-Sequence Stars in the 1-2 Msolar Range
Authors: MacGregor, K. B.; Jackson, Stephen; Skumanich, Andrew;
Metcalfe, Travis S.
2007ApJ...663..560M Altcode: 2007arXiv0704.1275M
We present models for chemically homogeneous, differentially rotating,
main-sequence stars with masses in the range 1-2 Msolar. The
models were constructed using a code based on a reformulation of the
self-consistent field method of computing the equilibrium stellar
structure for a specified conservative internal rotation law. Relative
to nonrotating stars of the same mass, these models all have reduced
luminosities and effective temperatures, and flattened photospheric
shapes (i.e., decreased polar radii) with equatorial radii that
can be larger or smaller, depending on the degree of differential
rotation. For a fixed ratio of the axial rotation rate to the surface
equatorial rotation rate, increasingly rapid rotation generally deepens
convective envelopes, shrinks convective cores, and can lead to the
presence of a convective core (envelope) in a 1 (2) Msolar
model, a feature that is absent in a nonrotating star of the same
mass. The positions of differentially rotating models for a given mass
in the H-R diagram can be shifted in such a way as to approximate the
nonrotating ZAMS for lower mass stars. Implications of these results
include (1) possible ambiguities arising from similarities between
the properties of rotating and nonrotating models of different masses,
(2) a reduced radiative luminosity for a young, rapidly rotating Sun,
(3) modified rates of lithium destruction by nuclear processes in the
layers beneath an outer convective envelope, and (4) the excitation of
solar-like oscillations and the operation of a solar-like hydromagnetic
dynamo in some 1.5-2 Msolar stars.

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Title: On the Use of the Self-consistent-Field Method in the
Construction of Models for Rapidly Rotating Main-Sequence Stars
Authors: Jackson, Stephen; MacGregor, Keith B.; Skumanich, Andrew
2005ApJS..156..245J Altcode:
A new formulation of the self-consistent-field (SCF) method for
computing models of rapidly, differentially rotating stars is
described. The angular velocity is assumed to depend only on the
distance from the axis of rotation. In the modified SCF iterative
scheme, normalized distributions of two thermodynamic variables-pressure
and temperature-are used as trial functions, while the central values
of the pressure and temperature are adjusted by a Newton-Raphson
iteration. A two-dimensional trial density distribution, which is
needed to compute the gravitational potential, is readily obtained
from the pressure and temperature through the equation of state in
conjunction with a third trial function specifying the two-dimensional
shape of the constant-density surfaces. Rotating models of chemically
homogeneous main-sequence stars have been computed as necessary
in order to illustrate the algorithm and to make comparisons with
existing models. Unlike previous implementations of the SCF method,
the method described here is not limited to the upper main sequence:
it converges for all main-sequence masses, including those well below
9 Msolar. Moreover, the method converges for values of
the parameter t=T/|W| (the ratio of rotational kinetic energy to
gravitational potential energy) that are at least as high as those
obtained by Clement's relaxation technique. The method is also capable
of producing models with deep concavities about the poles as well
as models with extreme oblateness (far greater than that possible
in uniformly rotating stars). For cases with moderate degrees of
differential rotation (say for Ω0/Ωe&lt;10,
where Ω0 and Ωe denote the angular velocity at
the center and at the equator, respectively), the method has been found
to be remarkably robust. For higher degrees of differential rotation,
models are restricted to a portion of parameter space away from two
regions of nonconvergence, inside which some of the models evidently
develop toroidal level surfaces.

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Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Jackson, Stephen; MacGregor, Keith B.; Skumanich, Andrew
2004ApJ...606.1196J Altcode:
We present models for the Be star Achernar (α Eri), which recent
interferometric observations have shown has a photospheric shape that
is significantly distorted by the effects of rotation. The models are
two-dimensional, axisymmetric configurations, constructed using a new
version of the self-consistent field (SCF) method for computing the
structure of a rapidly, differentially rotating star. Our revised SCF
technique does not suffer from the computational difficulties that
affected previous implementations of the method, yielding converged
stellar models regardless of mass. For models with masses like those of
main sequence stars of mid- to early-B spectral type, it is possible to
reproduce Achernar's inferred equatorial and polar dimensions through a
combination of rotational flattening/distension and suitable inclination
of the rotation axis. However, while matching Achernar's apparent shape,
these models are discrepant in other respects, being (on average)
cooler and more rapidly rotating than observations indicate.

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Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Skumanich, A.; Macgregor, K. B.; Jackson, S.
2004AAS...204.0704S Altcode: 2004BAAS...36..785S
We present models for the Be star Achernar (α Eridani), which recent
interferometric observations have shown has a photospheric shape that
is significantly distorted by the effects of rotation. The models are
two-dimensional, axisymmetric configurations, constructed using a new
version of the self-consistent-field (SCF) method for computing the
the structure of a rapidly, differentially rotating star, Jackson
et al. (2004). Our revised SCF technique does not suffer from the
computational difficulties that affected previous implementations of
the method, yielding converged stellar models regardless of mass. For
models with masses like those of main sequence stars of mid to early
B spectral type, it is possible to reproduce Achernar's inferred
equatorial and polar dimensions through a combination of rotational
flattening/distension and suitable inclination of the rotation
axis. However, while matching Achernar's apparent shape, these
models are discrepant in other respects, being (on average) cooler
and more rapidly rotating than observations indicate. Jackson,
S., Macgregor, K. B. &amp; Skumanich, A., in preparation The NCAR
(National Center for Atmospheric Research) is sponsored by the National
Science Foundation.

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Title: Sunspot Modeling and Scaling Laws
Authors: Skumanich, A.
2003JKAS...36S...1S Altcode:
No abstract at ADS

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Title: Quick-Look Polarization Inversion Using PCA Decomposition. The
Effect of Telluric Lines
Authors: Skumanich, A.; Toledano, O.; López Ariste, A.
2003ASPC..286...53S Altcode: 2003ctmf.conf...53S
No abstract at ADS

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Title: On the Invariance of PCA Quick-Look Polarization Analysis to
Different Magnetic Activity Samples
Authors: Skumanich, A.; López Ariste, A.
2003ASPC..307..109S Altcode:
No abstract at ADS

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Title: Converging Flows in the Penumbra of a δ Sunspot
Authors: Lites, B. W.; Socas-Navarro, H.; Skumanich, A.; Shimizu, T.
2002ApJ...575.1131L Altcode:
Doppler velocities in the penumbra of a δ-configuration sunspot
observed near the limb indicate flows that converge upon the
line separating locally positive and negative polarity magnetic
field (the polarity inversion line). These flows persist for many
hours. Observations of this region with the Advanced Stokes Polarimeter
(ASP) reveal a convex vector field geometry with magnetic lines of
force arching upward from positive polarity, then downward to negative
polarity. The straightforward interpretation of the combined Doppler
velocity and vector field information leads to an untenable physical
situation: were flows directed from both footpoints toward the tops
of arched magnetic lines of force, mass would rapidly load the tops of
the arches. However, there is no observational evidence of the dynamics
that such a loading would require. To better understand this apparent
contradiction, we perform two-component analyses of the observed Stokes
spectral profiles in the vicinity of the polarity inversion line, in
order to extract information about unresolved structure of the magnetic
field and its associated flows. Fits to the observed profiles, obtained
by use of two different inversion techniques, suggest strongly that, as
in penumbrae of simple sunspots, the field geometry in the convergence
zone is “fluted.” However, unlike in simple sunspots, which have only
an outward-directed Evershed flow in the more horizontal of the field
components, at each spatial point our analysis reveals flows in the two
components that are oppositely directed. We interpret these observations
as indicative of an interleaved system of field lines in the vicinity
of the polarity reversal, whereby the convergent streams are able to
slip past one another and return downward into the solar interior.

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Title: The Physical Content of the Leading Orders of Principal
Component Analysis of Spectral Profiles
Authors: Skumanich, A.; López Ariste, A.
2002ApJ...570..379S Altcode:
We consider the principal component analysis (PCA) method of
expanding Stokes intensity and net polarization profiles in terms of
eigenfunctions (or principal components) of the spectral covariance
matrix. The expansion is ordered by the magnitude of the relevant
eigenvalue from largest to smallest. We find that the ordering
represents a perturbation expansion. This allows us to examine the
physical content of the first few orders of the basis set for 40,000
profiles for each Stokes parameter for a solar active region. For the
intensity profile, we find that the expansion represents a Taylor
series with the highest ranked, or first, eigenfunction being the
zeroth order term, the second as the (scaled) first derivative of
the zeroth term, and the third as the (scaled) second derivative
term. Thus, we can derive a velocity from the coefficients of the
first derivative term and a magnetic splitting parameter from those
of the second using the standard velocity perturbation and weak-field
expansion. For the net polarization profiles, we find that the zeroth
order terms yield, using the weak-field expansion, the vector magnetic
field. A comparison with a Stokes profile fitting inversion shows
that the thus-estimated velocity and magnetic parameters are in good
agreement with the more time-consuming profile fitting values, but
do show a roll-off, or saturation, for sufficiently large values. We
also find that the bright quiet-Sun points have an upflow signature,
while the dark regions have a downflow-one in good agreement with that
derived by traditional analysis.

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Title: On the Universality of the Leading Orders of PCA. Spectral
Eigenprofiles for the Sun.
Authors: Skumanich, A.; López Ariste, A.
2002AAS...200.5511S Altcode: 2002BAAS...34..733S
In a previous Principal Component Analysis (PCA) of the spectral
region containing the intensity profiles of the FeI λ630 .15 &amp;
λ630 .25 nm lines for a solar active region, Ref(1), the issue of the
universality or invariance of the eigenprofiles (principal components)
to changes in the magnetic composition of the studied region was
raised. To address this issue we have performed a series of PCA studies
in which we incrementally increased the threshold Zeeman polarization
for the included profiles for a given region. This reduces the relative
roles of the various components such as the quiet sun and elements of
the active sun, viz. network, pores, plage and spot. We find that the
Taylor series representation of the highest ranked eigenprofiles in the
expansion of the intensity, which yield an estimate of the velocity and
magnetic splitting (first and second derivative terms respectively,
Ref(1)), continues to hold. However increased eigenprofile 'mixing'
occurs. Consequently to obtain eigenprofiles which represent the
derivatives of the zeroth order eigenprofile one requires significant
rotations of the basis in the PCA space. The implication for derived
velocity and magnetic parameters will be discussed. - Skumanich,
A. &amp; López Ariste, A. 2002, "The Physical Content of the Leading
Orders of Principal Component Analysis of Spectral Profiles", ApJ
570. In press

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Title: The Physical Content of the Leading Orders of Principle
Component Analysis of Spectral Profiles
Authors: Skumanich, A.; Lopez Ariste, A.
2001AAS...198.0303S Altcode: 2001BAAS...33..786S
We consider the PCA (Principle Component Analysis) method of profile
analysis introduced by Ref (1). In this method one constructs a
covariance matrix Cij given by the product of the profile
amplitudes at wavelengths λ i and λ j averaged
over all profile samples (either over space or time). The matrix
is 'diagonalized' and ordered by Singular Value Decomposition. The
resulting orthonormal eigenfunctions over wavelength space are then
used as a basis for the expansion of the observed profiles at each
space(time) point. The eigenvalue ordering is by magnitude of the
mean square over space(time) of the expansion coefficients and is
given by the product of the frequency of occurrence of the particular
eigenfunction and its intrinsic mean square amplitude. An error based
truncation scheme yields a coefficient set which is a compression of
the original data set. We have applied the PCA method to the 40,000
profiles for each Stokes component for a Solar active region. In the
case of the intensity profile we find that the expansion appears to
be similar to a Taylor series with the 0th efunction being the zero
order term, the 1st efunction as the first derivative of the zero term
and the 2nd efunction as the second derivative term. Thus we derive a
line-of-sight velocity from the coefficients of the first derivative
term and a magnetic signature, using the weak field Milne-Eddington
approximation, from the second. A comparison with a Stokes profile
inversion shows that the thus estimated velocity and magnetic parameters
are in good agreement with the more time consuming profile fitting
values but do show a "roll-off" for sufficiently large values. One also
finds that the bright 'quiet' Sun points have an upflow while the dark
have a downflow similar to that derived by other analysis. 1) Rees,
D., López Ariste, A., Thatcher, J. &amp; Semel, M. 2000, A &amp; A,
355, 759 The National Center for Atmospheric Research is sponsored by
the National Science Foundation.

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Title: Optical Tomography of a Sunspot. II. Vector Magnetic Field
and Temperature Stratification
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
2001ApJ...547.1130W Altcode:
An observational determination of the three-dimensional magnetic
and thermal structure of a sunspot is presented. It has been
obtained through the application of the SIR inversion technique
(Stokes Inversion based on Response functions) on a low-noise, full
Stokes profile two-dimensional map of the sunspot as observed with
the Advanced Stokes Polarimeter. As a result of the inversion, maps
of the magnetic field strength, B, zenith angle, γ, azimuth, χ, and
temperature, T, over 25 layers at given optical depths (i.e., an optical
tomography) are obtained, of which those between logτ5=0 and
logτ5=-2.8 are considered to provide accurate information on
the physical parameters. All over the penumbra γ increases with depth,
while B is larger at the bottom layers of the inner penumbra (as in
the umbra) but larger at the top layers of the outer penumbra (as in
the canopy). The corrugation of the penumbral magnetic field already
observed by other authors has been confirmed by our different inversion
technique. Such a corrugation is especially evident in the zenith angle
maps of the intermediate layers, featuring the presence of the so-called
spines that we further characterize: spines are warmer and have a less
inclined magnetic field than the spaces between them and tend to have a
smaller gradient of γ with optical depth over the entire penumbra, but
with a field strength which is locally stronger in the middle penumbra
and locally weaker in the outer penumbra and beyond in the canopy. In
the lower layers of these external parts of the sunspot, most of the
field lines are seen to return to the solar surface, a result that is
closely connected with the Evershed effect (e.g., Westendorp et al.,
the third paper in this series). The Stokes V net area asymmetry map
as well as the average B, γ, and T radial distributions (and that
of the line-of-sight velocities; see the third paper in this series)
show a border between an inner and an outer penumbra with different
three-dimensional structure. We suggest that it is in this middle zone
where most of a new family of penumbral flux tubes (some of them with
Evershed flow) emerge interlaced (both horizontally and vertically)
among themselves and with the “background” magnetic field of the
penumbra. The interlacing along the line of sight is witnessed by
the indication of many points in the outer penumbra showing rapid
transitions with height between two structures, one with very weak
and inclined magnetic field at the bottom of the photosphere and the
other with a stronger and less inclined magnetic field. Over the whole
penumbra, and at all optical layers, a constant but weak deviation from
radiality of some 5° is detected for the azimuth of the vector magnetic
field, which may be in agreement with former detections but which is
not significantly higher than the size of the errors for this parameter.

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Title: Converging Flows in the Penumbra of a δ-Sunspot
Authors: Lites, B. W.; Socas-Navarro, H.; Skumanich, A.; Shimizu, T.
2001ASPC..248..143L Altcode: 2001mfah.conf..143L
No abstract at ADS

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Title: Velocity and Field Gradient Corrections to the Rachkovsky
Inversion for Magnetic Atmospheres
Authors: Skumanich, A.
2001ASPC..236..543S Altcode: 2001aspt.conf..543S
No abstract at ADS

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Title: Stokes Asymmetries &amp; Gradient Corrections to
Milne-Eddington Atmospheres
Authors: Skumanich, A.
2000SPD....31.0125S Altcode: 2000BAAS...32..805S
The presence of Stokes asymmetries imply the presence of line of sight
velocity gradients in a magnetic atmosphere. Such gradients, as well
as magnetic field gradients, prevents one from obtaining closed form
analytic expressions for the emergent Stokes profiles. This is due to
the fact that the absorption matrix is non-commutative at neighboring
points. However closed form expresions are still possible if one uses
a quasi-linearization technique to bootstrap from the constant matrix
case to the variable case. We use such a bootstrap approach to derive
analytic expressions for the gradients of velocity and field for the
Milne-Eddinton atmosphere. Our expressions are more general than those
derived by Sanchez Almeida &amp; Lites (Ap J 398, 359, 1992) using the
response matrix approach. The National Center for Atmospheric Research
(NCAR) is sponsored by the National Science Foundation.

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Title: On the value of `αAR' from vector magnetograph data -
I. Methods and Caveats
Authors: Leka, K. D.; Skumanich, A.
1999SoPh..188....3L Altcode:
This investigation centers upon the quantifying
magnetic twist by the parameter α, commonly defined as
(∇×Bh)z/Bz=μ0Jz/Bz,
and its derivation from vector magnetograph data. This parameter can
be evaluated at each spatial point where the vector B is measured,
but one may also calculate a single value of α to describe the
active region as a whole, here called 'αAR'. We test
three methods to calculate such a parameter, examine the influence
of data noise on the results, and discuss the limitations associated
with assigning such a quantity. The three methods discussed are
(1) to parameterize the distribution of α(x,y) using moments
of its distribution, (2) to determine the slope of the function
Jz(x,y)=αARBz(x,y) using a
least-squares fit and (3) to determine the value of α for which
the horizontal field from a constant-α force-free solution most
closely matches the observed horizontal magnetic field. The results
are qualitatively encouraging: between methods, the resulting value
of the αARparameter is often consistent to within the
uncertainties, even though the resulting αARcan differ in
magnitude, and in some cases in sign as well. The worst discrepancies
occur when a minimal noise threshold is adopted for the data. When the
calculations are restricted to detections of 3σ or better, there is,
in fact, fair quantitative agreement between the three methods. Still,
direct comparison of different active regions using disparate methods
must be carried out with caution. The discrepancies, agreements, and
overall robustness of the different methods are discussed. The effects
of instrumental limitations (spatial resolution and a restricted
field-of-view) on an active-region αAR, and quantifying
the validity of αAR, are addressed in Paper II (Leka, 1999).

---------------------------------------------------------
Title: The Evolution of Photospheric Magnetic Structures in Terms
of the Size-Flux Relationship
Authors: Skumanich, A.
1999AAS...194.9403S Altcode: 1999BAAS...31R.991S
We find that the dark regions of pores and spots (which include their
penumbrae) have a nearly universal average flux density of 800 G +/- 32
while normal contrast Azimuth Centers (ACs) and pore sheaths, i.e. the
associated magnetic envelope about the dark region, also show a similar
constancy but at 334 G +/- 22, while spot sheaths are characterized
by 107 G +/- 22. Using these flux density values and the observed dark
and magnetic (dark + sheath) radii we derive a magnetic flux-dark flux
relation which yields radius-flux relations for pores and spots. The
magnetic radius vs flux relation for spots is distinct, with larger
radii at one and the same flux than those for pores, while the curve for
the dark radius is continuous with that for pores. Spots differ from
pores only in the nature of their sheaths. The AC structure, a `pure'
sheath, has a radius-flux relation different from, but bridging, those
of pores and spots. We suggest these relations to be universal. The
temporal magnetic evolution for two developing pores and a decaying
spot in the radius-flux plane traces the universal dark radius curve
for both types of features. The evolution of an AC into a pore, perhaps
via a bifurcation, follows the universal curves for dark and magnetic
radii. Any bifurcation in the transition from pores to spots must
appear via a transition in the magnetic sheath. The National Center for
Atmospheric Research is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: The Evolution of Photospheric Magnetic Structures in Terms
of the Size-Flux Relationship
Authors: Skumanich, A.
1999ApJ...512..975S Altcode:
We find that the dark regions of pores and spots (which include their
penumbrae) have a nearly universal average flux density of 800+/-32
G while normal contrast azimuth centers (ACs) and pore sheaths, i.e.,
the associated magnetic envelopes about the dark regions, also show a
similar constancy but at 334+/-22 G, and spot sheaths are characterized
by 107+/-22 G. Using these flux density values and the observed dark
and magnetic (dark+sheath) radii, we derive a magnetic flux-dark flux
relation that yields radius-flux relations for pores and spots. The
magnetic radius versus flux relation for spots is distinct, with larger
radii at one and the same flux than those for pores, while the curve
for the dark radius is continuous with that for pores. Spots differ
from pores only in the nature of their sheaths. The AC structure,
a “pure” sheath, has a radius-flux relation different from, but
bridging, those of pores and spots. We suggest these relations to be
universal. The temporal magnetic evolution for two developing pores
and a decaying spot in the radius-flux plane traces the universal
dark radius curve for both types of features. The evolution of an AC
into a pore, perhaps via a bifurcation, follows the universal curves
for dark and magnetic radii. Any bifurcation in the transition from
a pore to a spot must appear via a transition in the magnetic sheath.

---------------------------------------------------------
Title: The Evolution of Pores and the Development of Penumbrae
Authors: Leka, K. D.; Skumanich, A.
1998ApJ...507..454L Altcode:
We investigate the evolution of the magnetic field in an emerging
active region near disk center using data from the Advanced Stokes
Polarimeter. Specifically, we follow the formation of a pore from
a radiatively undisturbed region and the formation of a protospot,
i.e., the appearance of a rudimentary penumbral sector for a mature
pore. Our approach is to use the temporal evolution of bivariate
distribution functions correlating continuum intensity, Ic,
magnetic field magnitude, | B |, magnetic fill fraction, f, local
inclination, γ, local azimuth, φ, and line-of-sight Doppler velocity,
vD. The highlights of our results include, (1) in the
pore-forming region, a preference for the relatively strong vertical
field points (1000-1500 G) to be redshifted (downflowing) and the
appearance of dark pore points to be at the expense of radiatively
undisturbed points; (2) an onset flux of ~2 × 1019 Mx
in a dark pore that appears in an area in which the flux increases
by ~1 × 1020 Mx prior to the pore's appearance and that
previously contained an azimuth center, i.e., a magnetic concentration
that otherwise has no continuum-intensity signature; (3) a threshold of
(1-1.5) × 1020 Mx above which a partial penumbra forms; (4)
the appearance of penumbral elements that at once having the appropriate
penumbral range of field strengths, intensities, and inclination
angles, i.e., no obvious gradual increase in the magnetic fields'
inclination with increasing flux during the formation of penumbrae;
(5) no delay between the appearance of inclined penumbral magnetic
fields and the Evershed flow; and (6) a self-similarity between the
pore, protospot, and a small mature sunspot with respect to the (|
B |, Ic), (| B |, γ), (f, γ), and the (vD,
Ic), (vD, γ) distributions.

---------------------------------------------------------
Title: Vector magnetic fields of emerging solar flux. I. Properties
at the site of emergence
Authors: Lites, B. W.; Skumanich, A.; Martinez Pillet, V.
1998A&A...333.1053L Altcode:
Several small emerging bipolar regions have been observed with
the Advanced Stokes Polarimeter (ASP), including extensive time
series measurements of one small region. Both new and previously
recognized properties of the actual site of first emergence, where
the magnetic field is nearly horizontal to the surface, are revealed
by these observations. They provide the most complete and accurate
observational description to date of newly emerging vector magnetic
fields. We find that: 1) the strength of the magnetic field at the site
of the emergence (where the vector field is nearly parallel to the
solar surface) ranges from about 200 to 600 G, 2) as individual flux
elements migrate rapidly away from the emergence zone, they attain
kiloGauss strengths only after becoming oriented nearly vertically,
3) the emergence zone is dotted by small, transient, upward rising ( ~
1 km s(-1) ) horizontal magnetic elements as indicated by the Doppler
shift of the polarized spectral profiles, 4) the leading polarity flux
coalesces immediately into a compact region which forms a pore, but the
emerging following polarity flux is spatially much less compact, 5) some
“moving magnetic features” having the same magnetic polarity as the
growing pore, but on the opposite side of the pore from the emergence
zone, coalesce with the pore during the observation period, and 6)
the observations suggest a low canopy of weak horizontal magnetic
fields arches over the emergence zone. These observations support a
widely accepted picture of emerging bipolar flux: the buoyantly rising
flux transports mass from the photosphere into the chromosphere, where
it then may drain downward along arched magnetic loops. The observed
formation of a pore suggests that emergence of subsurface structure,
not organized flows near the surface, is largely responsible for the
apparent coalescence of sunspots from more diffuse fields viewed at
the solar surface. These observations neither confirm nor refute the
operation of convective collapse of flux tubes.

---------------------------------------------------------
Title: An ambiguity-free determination of J_Z in solar active regions
Authors: Semel, M.; Skumanich, A.
1998A&A...331..383S Altcode:
We propose a way to derive a radial electric current density from
vector magnetograms that is free of any particular choice of the two
possible azimuths for the plane of the sky field. The method is applied
to active region data obtained with the Advanced Stokes Polarimeter
(ASP). A comparison of this current density with that derived for
two particular sets of possible azimuth assignments allows one to
evaluate the appropriateness of each set. In addition one may verify
the detection of currents in general and the occurrence of current
sheets, i.e. field discontinuities, in particular.

---------------------------------------------------------
Title: Optical Tomography of a Sunspot. I. Comparison between Two
Inversion Techniques
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
1998ApJ...494..453W Altcode:
A quantitative comparison between the Milne-Eddington (ME) inversion
technique implemented by Skumanich &amp; Lites and the SIR (Stokes
Inversion based on Response Functions) proposed by Ruiz Cobo &amp;
del Toro Iniesta is presented. Numerical experiments are carried
out to explore the capabilities and limitations of both diagnostic
techniques. Such experiments consist of inversions of Stokes profiles
previously synthesized in “realistic” solar atmospheric models. The
results show that the ME inversion provides accurate, line-of-sight
(LOS) averaged values for the input stratification of the vector
magnetic field. Its greater speed compared to SIR makes it useful for
quick analysis of large quantities of data (such as those currently
provided by modern spectropolarimeters) if one is only interested
in LOS-averaged quantities. However, the higher order description
of the atmosphere used by SIR (which acknowledges variation of the
thermal, dynamic, and magnetic parameters through the photosphere)
allows retrieval of the stratification of all these parameters to good
accuracy. This is so even in the presence of discontinuities such
as those foreseen in magnetic canopies of sunspots. The trade-offs
between thermodynamic and magnetic parameters observed in some ME
inversions are reduced considerably in the case of SIR inversions
because of the more realistic treatment of the thermodynamics in this
analysis. Notably, both allow one to extract quantitative inferences of
fairly weak magnetic fields (below 500 G), even when they are applied
to Zeeman-sensitive lines in the visible spectrum; i.e., well below
the commonly accepted limit of 500 G. The thermodynamic parameters
resulting from the ME inversion are understood theoretically in terms
of the generalized response functions introduced by Ruiz Cobo &amp;
del Toro Iniesta and through the concept of height of formation
for inferred values proposed by Sánchez Almeida, Ruiz Cobo, &amp;
del Toro Iniesta. The present comparison and verification of
the reliability of inversion methods is a natural first step toward
the ongoing analysis of the three-dimensional magnetic structure of
a sunspot. By using SIR (with ME results for initialization) on maps
of a whole sunspot observed by the Advanced Stokes Polarimeter, we
obtain maps at different optical layers (i.e., an optical tomography)
of the temperature, vector magnetic field, and LOS velocity. Such a
tomography will appear in subsequent papers of the present series. To
illustrate fits to the observed Stokes profiles, we show here actual
inversion results for three points observed within a sunspot: one
within the umbra, another from the outermost parts of the penumbra,
and a third from the magnetic canopy surrounding the sunspot.

---------------------------------------------------------
Title: Vector Magnetic Fields of Emerging Solar Flux
Authors: Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
1998ESASP.417..259M Altcode: 1998cesh.conf..259M
No abstract at ADS

---------------------------------------------------------
Title: Evidence for a downward mass flux in the penumbral region of
a sunspot
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997Natur.389...47W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Calibration of the Advanced Stokes Polarimeter
Authors: Skumanich, A.; Lites, B. W.; Martínez Pillet, V.;
Seagraves, P.
1997ApJS..110..357S Altcode:
We describe and apply the methods that have been developed to calibrate
the Advanced Stokes Polarimeter and to compensate for the polarization
effects introduced by the Vacuum Tower Telescope at the National Solar
Observatory/Sunspot. A seven-parameter model of the telescope is fitted
to data obtained at a variety of mirror angles using observations
of both the center of the solar disk and that point within a sunspot
umbra at which the magnetic field is oriented as close to the line of
sight as possible. The response matrix of the polarimeter itself is
determined by the use of polarizing calibration optics that modify
the polarization state of the beam exiting the telescope but before
entering the polarimeter. A global least-squares solution is obtained
simultaneously for the response matrix and the telescope parameters. A
detailed gain-correction procedure is described that reduces the
multiplicative gain errors in the spectral images to typically less
than 1%. We have successfully recovered net-linear polarization
profiles with peak amplitudes of 1 × 10-3Ic
against an instrumentally produced background polarization of ~=1-5 ×
10-2Ic. Net-polarization signals smaller than
~=3 × 10-4Ic are lost, even with sufficient
averaging, in a background due to photometric and other calibration
errors.

---------------------------------------------------------
Title: On The Detection of Currents and Parker's Objection
Authors: Skumanich, A.; Semel, M.
1997SPD....28.1605S Altcode: 1997BAAS...29..920S
We propose a way to derive a radial electric current density
(hereafter current) from vector magnetograms that is free of any
particular choice of the two possible azimuths for the plane of
the sky (POS) field. The method is applied to active region data
obtained with the Advanced Stokes Polarimeter. A comparison of this
current density with that derived for two particular sets of possible
azimuth assignments allows one to evaluate the appropriateness of
each set and to detect the occurrence of current sheets, i.e. field
discontinuities. A minimum current constructed from ambiguity-free
POS and line-of-sight currents allows one to verify the detection of
currents in general. Parker's scenario leads to a current distribution
that is statistically distributed about zero, looks like noise and is
also discontinuous. We observe such noise as well as continuous and
significant currents. We conclude that the unresolved magnetic field
is not necessarily always in the form of individual flux tubes.

---------------------------------------------------------
Title: On the Development of a Sunspot Penumbra
Authors: Leka, K. D.; Skumanich, A.
1997SPD....28.1702L Altcode: 1997BAAS...29Q.921L
Using data from the Advanced Stokes Polarimeter, we follow the
development of a rudimentary penumbra around a small pore. A sub-hourly
cadence coupled with the full vector spectro-polarimetry and non-linear
least-squares inversion algorithm allows us to determine not only
the vector magnetic field around the pore, but also thermodynamic
parameters and Doppler shifts. We find that a penumbra forms not by a
gradual “tilting” of the field lines, but by the initial appearance
of weaker fields at all inclinations. The distribution of these fields
is sparse, but grows with time to where a fully-populated range of
field strengths and inclination angles is present (as is common for
fully-developed sunspots). At the same time, the range of Doppler
velocities increases in the lower-field-strength areas. This implies
that any difference between the start of the Evershed flow and the
appearance of more inclined fields is small (less than half-hour). We
will discuss these results and their relation to the flux history of
this pore, comparing it to a nearby region in which a penumbra-less
pore develops from apparent quiet-sun.

---------------------------------------------------------
Title: Active Region Magnetic Fields. I. Plage Fields
Authors: Martínez Pillet, V.; Lites, B. W.; Skumanich, A.
1997ApJ...474..810M Altcode:
We present observations taken with the Advanced Stokes Polarimeter
(ASP) in active-region plages and study the frequency distribution of
the magnetic field strength (B), inclination with respect to vertical
(γ), azimuthal orientation (χ), and filling factor (f). The
most common values at disk center are B = 1400 G, γ &lt; 10°,
no preferred east-west orientation, and f = 15%. At disk center,
there is a component of weak (&lt;1000 G), more horizontal fields
that corresponds to arching field lines connecting footpoints of
different polarities. The center-to-limb variation (CLV) of the
field strength shows that, close to the limb (μ = 0.3), the field
strength is reduced to 800 G from its disk-center value. This can be
interpreted as a gradient of B with height in solar plages of around
-3 G km-1. From this CLV study, we also deduce that magnetic
field lines remain vertical for the entire range of heights involved. A
similar analysis is performed for structures found in active regions
that show a continuous distribution of azimuths (resembling sunspots)
but that do not have a darkening in continuum. These “azimuth centers”
show slightly larger values of B than normal plages, in particular
at their magnetic center. Filling factors are also larger on average
for these structures. The velocities in the magnetic component
of active regions have been studied for both averaged Stokes profiles
over the entire active region and for the spatially resolved data. The
averaged profiles (more representative of high filling factor regions)
do not show any significant mean velocities. However, the spatial
average of Doppler velocities derived from the spatially resolved
profiles (i.e., unweighted by filling factor) show a net redshift at
disk center of 200 m s-1. The spatially resolved velocities
show a strong dependence on filling factor. Both mean velocities and
standard deviations are reduced when the filling factor increases. This
is interpreted as a reduction of the p-mode amplitude within the
magnetic component. Strong evidence for velocities transverse to the
magnetic field lines has been found. Typical rms values are between
200 and 300 m s-1, depending on the filling factor. The
possible importance of these transverse motions for the dynamics of
the upper atmospheric layers is discussed. The asymmetries of the
Stokes profiles and their CLV have been studied. The averaged Stokes
V profiles show amplitude and area asymmetries that are positive
at disk center and become negative at the limb. Both asymmetries,
and for the two Fe I lines, are maximized away from disk center. The
spatially resolved amplitude asymmetries show a clear dependence on
filling factor: the larger the filling factor, the smaller the amplitude
asymmetry. On the other hand, the area asymmetry is almost independent
of the filling factor. The only observed dependence is the existence
of negative area-asymmetry profiles at disk center for filling factors
smaller than 0.2. Around 20% of the observed points in a given plage
have negative area asymmetry. The amplitude asymmetry of Stokes V is,
on the other hand, always positive. The amplitude asymmetries of the
linear polarization profiles are observed to have the same sign as
the Stokes V profiles. Similarly, the same CLV variation of the linear
polarization amplitude asymmetries as for Stokes V has been found. The
scenarios in which this similarity can exist are studied in some detail.

---------------------------------------------------------
Title: Inversion Techniques Applied to Sunspot Spectropolarimetric
Data
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997ASPC..118..197W Altcode: 1997fasp.conf..197W
Two inversion techniques are compared: the Unno-Rachkov\-sky fitting
method (UR) and the Stokes Inversion based on Response functions
(SIR). Results with synthetic profiles in sunspot model atmospheres and
real data show that whilst UR is well suited for recovering a constant
vec B, SIR enables us to know the run with depth of vec B and the line
of sight velocity together with the temperature stratification.

---------------------------------------------------------
Title: Optical Tomography of a Sunspot: Preliminary Results
Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo,
B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A.
1997ASPC..118..202W Altcode: 1997fasp.conf..202W
Preliminary results of the inversion of spectropolarimetric maps
of a sunspot observed with the Advanced Stokes Polarimeter (ASP) are
presented. The method used, Stokes Inversion based on Response functions
(SIR), does not assume constancy of the different parameters with
depth, thus enabling us to embark on an analysis of the information at
different layers in continuum optical depth (i.e. optical tomography),
of a sunspot's photosphere. Maps of the vector magnetic field and
other physical quantities like temperature or line-of-sight velocity at
several optical depths show a new and promising view of the structure
of a sunspot, casting light on long standing debates as those over
penumbral `corrugated' fields (spines), superpenumbral canopies,
return flux, or the nature of the Evershed effect.

---------------------------------------------------------
Title: Small-Scale Horizontal Magnetic Fields in the Solar Photosphere
Authors: Lites, B. W.; Leka, K. D.; Skumanich, A.; Martinez Pillet,
V.; Shimizu, T.
1996ApJ...460.1019L Altcode:
We present recent observations of quiet regions near the center
of the solar disk using the Advanced Stokes Polarimeter. These
observations reveal a component of the solar magnetic field heretofore
unobserved: isolated, small-scale (typically 1"-2" or smaller),
predominantly horizontal magnetic flux structures in the solar
photosphere. These features occur in isolation of the well-known,
nearly vertical flux concentrations usually seen in the photospheric
"network." Hence we ascribe this horizontal flux to the photospheric
"internetwork." They reveal themselves by the distinct signature
of the Stokes Q and U polarization profiles, which are symmetric
about the line center. The polarization signals are weak, with peak
amplitudes typically ∼0.1%-0.2% of the continuum intensity in the
resolved spectral profiles, but they are well above the noise level
of these observations (≍0.05%). Such magnetic fields are weak
(significantly less than 1000 G) and largely horizontal owing to
the absence, or near absence, of accompanying Stokes V polarization
when observed at the center of the solar disk. These horizontal field
elements are often associated with blueshifted Stokes line profiles,
and they often occur between regions of opposite polarity (but weak)
Stokes V profiles. The horizontal elements are short-lived, typically
lasting ∼5 minutes. Our observations suggest that we are viewing the
emergence of small, concentrated loops of flux, carried upward either
by granular convection or magnetic buoyancy. Even though these entities
show weak field strengths, they also seem to be fairly common, implying
that they could carry the order of 1024 Mx of magnetic flux
to the surface on a daily basis. However, further observational study
is needed to identify the specific nature of this phenomenon.

---------------------------------------------------------
Title: Field Azimuth Disambiguation Using Ambiguity-Free Currents
Authors: Skumanich, A.; Semel, M.
1996SoPh..164..291S Altcode:
Using the ambiguity-free vertical current defined by Semel and Skumanich
(1995) we derive a minimum-current azimuth disambiguation for the
observed magnetic field in the active region NOAA 7201. A comparison of
such a minimum-current azimuth resolution with those from other extant
methods indicates that the resulting resolution, even though found
to be limited by noise, is a useful first approximation. A comparison
of our minimum current distribution with the currents we derive from
an extant disambiguation (Lites et al., 1995) indicates the presence
of current discontinuities in the form of linear features near the
magnetic neutral line of the associated δ-spot.

---------------------------------------------------------
Title: The Possible Ascent of a Closed Magnetic System through
the Photosphere
Authors: Lites, B. W.; Low, B. C.; Martinez Pillet, V.; Seagraves,
P.; Skumanich, A.; Frank, Z. A.; Shine, R. A.; Tsuneta, S.
1995ApJ...446..877L Altcode:
We present a comprehensive interpretation of the evolution of a small
magnetic region observed during its entire disk passage. The vector
magnetic field measurements from the Advanced Stokes Polarimeter,
along with Hα and magnetogram measurements from the Lockheed SOUP
instrument operating at the Swedish Solar Observatory on La Palma,
and soft X-ray images from the Yohkoh satellite support the hypothesis
that we have observed the passage of a nearly closed magnetic system
through the photosphere into the corona. The observations suggest that
as the magnetic flux begins to emerge into the photosphere it shows a
rather simple geometry, but it subsequently develops a small δ-sunspot
configuration with a highly sheared vector field along the polarity
inversion line running through it. At that stage, the vector field is
consistent with a concave upward magnetic topology, indicative of strong
electric currents above the photosphere. An Hα prominence is found
above this inversion line when the δ-sunspot is fully formed. These
observed features and the sequence of events are interpreted in terms
of a nearly closed magnetic system that rises through the photosphere
into the corona as a result of magnetic buoyancy. The magnetic system
persists in the corona well after the dark δ-sunspot has disappeared
in the photosphere We suggest that this coronal structure is in
quasi-static equilibrium with its buoyancy partially countered by
the weight of the plasma trapped at the bottom of closed magnetic
loops. The plausibility of such a scenario is demonstrated by a
three-dimensional magnetostatic model of the emergence of a closed,
spheroidal magnetic system in the corona, in which the Lorentz force
arising from cross-field currents is balanced by the gravitational
and pressure forces. This theoretical model carries many features in
common with the observed morphology of our active region.

---------------------------------------------------------
Title: Small scale horizontal magnetic fields in the solar photosphere
Authors: Leka, K. D.; Lites, B. W.; Skumanich, A.; Martínez Pillet,
V.; Shimizu, T.
1995IAUS..176P.120L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Quantitative Comparison of Vector Magnetic Field Measurement
and Analysis Techniques
Authors: Lites, B. W.; Martinez Pillet, V.; Skumanich, A.
1994SoPh..155....1L Altcode:
We make a quantitative comparison between spectral vs filter measurement
and analysis techniques for extraction of solar vector magnetic fields
from polarimetric data using as a basis the accurately calibrated,
high angular resolution Stokes profile data from the Advanced Stokes
Polarimeter. It is shown that filter-based measurements deliver
qualitative images of the field alignment for sunspots that are visually
similar to images derived from the more detailed analysis of the Stokes
profiles. However, quantitative comparison with least-squares fits
to the full Stokes profiles show that both the strength of the field
predicted by the filter-based analysis and its orientation contain
substantial errors. These errors are largest for plage regions
outside of sunspots, where the field strengths are inferred to be
only a fraction of their true values, and errors in the orientation of
40-50° are common. Within sunspots, errors of 20° are commonplace. The
greatest source of these errors is the inability of the filter-based
measurements to account for the small fill fraction of magnetic fields
or, equivalently, scattered light in the instrument, which reduce the
degree of polarization. The uncertainties of the full profile fitting
methods are also discussed, along with the errors introduced by coarser
wavelength sampling of the observed Stokes profiles. The least-squares
fitting procedure operates best when the profiles are sampled at least
as frequently as one Doppler width of the line.

---------------------------------------------------------
Title: Evidence for Supersonic Downflows in the Photosphere of a
Delta Sunspot
Authors: Martinez Pillet, V.; Lites, B. W.; Skumanich, A.; Degenhardt,
D.
1994ApJ...425L.113M Altcode: 1994ApJ...425L.113P
We present polarization profiles observed with the High Altitude
Observatory/National Solar Observatory (HAO/NSO) Advanced Stokes
Polarimeter (ASP) that demonstrate, in a model-independent way,
the presence of strong downflows close to the neutral line of a
delta sunspot (a spot with both polarities contained within the
same penumbra). The flows are as large as 14 km/s, a velocity that,
at photospheric levels, strongly suggests the presence of supersonic
compressive fluid flows in a region only 100-200 km above the visible
surface. These velocities are probably the largest ever reported at
photospheric levels. The region containing the downflows is large enough
(about 2 sec on a side) to be resolved, although it is likely to contain
fine structure at or below our spatial resolution. The origin of these
flows is discussed in terms of the funneling of material through an
isolated magnetic nozzle in an otherwise closed magnetic system which
is rising through the surface.

---------------------------------------------------------
Title: H alpha Variability on HK Aqr: Coronal Condensations or Plages?
Authors: Byrne, P. B.; Mathioudakis, M.; Young, A.; Skumanich, A.
1994ASPC...64..375B Altcode: 1994csss....8..375B
No abstract at ADS

---------------------------------------------------------
Title: Vector spectropolarimetry with the Advanced Stokes Polarimeter
(ASP) for quantitative solar magnetometry
Authors: Skumanich, A.; Lites, B. W.; Martínez Pillet, V.
1994ASIC..433...99S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Configuration of a Short-Lived Delta SPOT
Authors: Martinez Pillet, V.; Lites, B. W.; Skumanich, A. P.;
Seagraves, P.
1994ASPC...68..244M Altcode: 1994sare.conf..244M
No abstract at ADS

---------------------------------------------------------
Title: Physical conditions in magnetic elements of different
polarities surrounding sunspots
Authors: Martínez Pillet, V.; Lites, B. W.; Skumanich, A. P.; Elmore,
D. F.; Seagraves, P.
1994smf..conf..219M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Vector field structure of a small sunspot
Authors: Skumanich, A.; Lites, B. W.
1994smf..conf..200S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Estimating Solar Forcing of Climate Change during the Maunder
Minimum
Authors: Lean, J.; Skumanich, A.; White, O. B.; Rind, D.
1994svsp.coll..236L Altcode: 1994IAUCo.143P.236L
No abstract at ADS

---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic Fields. VI. Fine
Scale Structure of a Sunspot
Authors: Lites, B. W.; Elmore, D. F.; Seagraves, P.; Skumanich, A. P.
1993ApJ...418..928L Altcode:
The vector magnetic field structure of a small, symmetric sunspot
observed very close to disk center has been explored using data
from the High Altitude Observatory/National Solar Observatory
Advanced Stokes Polarimeter (ASP). This instrument provides, for
the first time, quantitative information on sunspot photospheric
vector magnetic fields with high angular resolution, as derived from
full Stokes profiles of the Zeeman-sensitive Fe I line pair at 630
nm. We find the following. (1) The penumbra contains narrow, radial
"spines" of more intense magnetic field more vertically oriented than
their surroundings; (2) the magnetic field diverges azimuthally away
from the spine axes, as it should for a more intense intrusion of
field embedded in a gravitational stratified atmosphere; (3) there
are dark protrusions out from the umbra which traverse much of the
penumbra and which are characterized by more vertical magnetic field;
(4) the observed structure of the magnetic field in both the umbra
and penumbra is stable over the 30 minute duration of our observing
sequence; (5) we do not find a tight correlation of field inclination
with intensity in the penumbra; (6) at the outer edge of the penumbra,
the field penetrates the photosphere at an average angle of about 70°
from the vertical; (7) outside of the penumbra the sunspot field forms
a canopy which is directly observed by our technique; (8) outside of,
but close to the sunspot, there are small magnetic features of mixed
polarity that are rather highly inclined to the vertical, presumably
as influenced by the sunspot canopy; (9) away from the sunspot, the
isolated magnetic elements have nearly vertical orientation. These
observations suggest that magnetic buoyancy, in addition to thermal
buoyancy, plays an important role in maintaining the observed structure
in sunspot penumbra.

---------------------------------------------------------
Title: Early Results from HAO/NSO Advanced Stokes Polarimeter
Authors: Lites, B. W.; Elmore, D. F.; Tomczyk, S.; Seagraves, P.;
Skumanich, A.; Streander, K. V.
1993ASPC...46..173L Altcode: 1993mvfs.conf..173L; 1993IAUCo.141..173L
No abstract at ADS

---------------------------------------------------------
Title: The Advanced Stokes Polarimeter - A new instrument for solar
magnetic field research
Authors: Elmore, D. F.; Lites, B. W.; Tomczyk, S.; Skumanich,
A. P.; Dunn, R. B.; Schuenke, J. A.; Streander, K. V.; Leach, T. W.;
Chambellan, C. W.; Hull, H. K.
1992SPIE.1746...22E Altcode:
A new Stokes polarimeter for high spatial resolution quantitative
measurement of magnetic fields at multiple heights in the solar
atmosphere has been constructed by the National Center for Atmospheric
Research and the National Solar Observatory. The instrument uses
the Vacuum Tower Telescope at Sunspot, New Mexico, and its existing
horizontal spectrograph, universal birefringment filter, and image
motion stabilization system. The polarimeter uses a rotating retarder
polarization modulator with polarization calibration optics. Multiple
paired CCDs are used for detection followed by video processing to
produce spatial maps of the full state of polarization in restricted
regions of the solar spectrum. Two spectral regions encompassing
lines sensitive to the Zeeman effect, which form in the photosphere
and low chromosphere, are recorded simultaneously. Significant
developments include: construction of the new telescope post
focus optical arrangement, creation of a polarization model for the
telescope, construction of high-speed, low-noise solid state cameras,
and construction of computer hardware for receiving and processing
high-rate 12-bit digital data.

---------------------------------------------------------
Title: The Sun in a Non-Cycling State
Authors: White, O. R.; Skumanich, A.; Lean, J.; Livingston, W. C.;
Keil, S. L.
1992PASP..104.1139W Altcode:
Using the Baliunas and Jastrow (1990) study of cyclic variability in
solar-type stars, we transform existing solar data to the stellar HK
irradiance scale and examine the state of the solar chromosphere when a
solar-type star shows little cyclic variability and surface magnestis
m. To reduce the chromospheric emission to levels for G-type stars
showing no chromospheric activity cycles, no only must the sun be
free of plages and network; the brightness of the quiet chromosphere
in the K line must be reduced to levels seen only in 15% of the quiet
Sun area today. In contrast, the present day level of K emission from
the sun places it in the class of most active solar-type stars, far
removed from a non-cycling state. (SECTION: Stars)

---------------------------------------------------------
Title: Estimating the Sun's radiative output during the Maunder
Minimum
Authors: Lean, Judith; Skumanich, Andrew; White, Oran
1992GeoRL..19.1591L Altcode:
The coincidence between the Maunder Minimum of solar magnetic activity
from 1645 to 1715 and the coldest temperatures of the Little Ice
Age raises the question of possible solar forcing of the Earth's
climate. Using a correlation which we find between measured total solar
irradiance (corrected for sunspot effects) and a Ca II surrogate for
bright magnetic features, we estimate the Sun's radiative output in
the absence of such features to be 1365.43 w/m2, or 0.15%
below its mean value of 1367.54 W/m2 measured during the
period 1980 to 1986 by the ACRIM experiment. Observations of extant
solar-type stars suggest that the Ca II surrogate vas darker during
the Maunder Minimum. Allowing for this, we estimate the total solar
irradiance to be 1364.28 W/m2 or 0.24% below its mean value
for the 1980 to 1986 period. The decrease in the global equilibrium
temperature of the Earth due to a decrease of 0.24% in total solar
irradiance lies in the range from 0.2° C to 0.6° C, which can be
compared with the approximately 1° C cooling experienced during the
Little Ice Age, relative to the present.

---------------------------------------------------------
Title: Estimating the sun's radiative output during the Maunder
Minimum
Authors: Lean, Judith; Skumanich, Andrew; White, Oran
1992GeoRL..19.1595L Altcode:
The current understanding of the origin of the variations in total
solar irradiance and in the Ca II emission from the sun and stars is
used to obtain quantitative estimates of the radiative output from
the sun corresponding to scenarios that might have characterized the
Maunder Minimum. The estimates indicate an irradiance reduction of as
much as 0.3 percent below the recent maximum activity (0.24 percent
below the mean for 1980-1986). The corresponding decrease in the global
equilibrium temperature of the earth is 0.2-0.6 C, which is comparable
with the approximately 1 C cooling during the Little Ice Age.

---------------------------------------------------------
Title: The Advanced Stokes Polarimeter: A New Instrument for Solar
Magnetic Field Research
Authors: Tomczyk, S.; Elmore, D. F.; Lites, B. W.; Dunn, R. B.;
Skumanich, A.; Schuenke, J. A.; Streander, K. V.; Leach, T. W.;
Chambellan, C. W.; Lacey, L. B.
1992AAS...180.5108T Altcode: 1992BAAS...24..814T
A new Stokes polarimeter for high spatial resolution quantitative
measurement of magnetic fields at multiple heights in the solar
atmosphere has been constructed by the National Center for Atmospheric
Research and the National Solar Observatory. The instrument has become
operational at the Vacuum Tower Telescope at Sunspot, New Mexico
and uses its existing horizontal spectrograph, universal birefringent
filter, and image motion stabilization system. To these optical systems
we have added a rotating wave plate polarimeter and polarization
calibration optics. Multiple CCDs are used for detection followed by
video processing in order to produce polarization maps of solar regions
from several spectrum lines simultaneously. Significant developments
include: a) construction of the new VTT post focus optical arrangement,
b) creation of a polarization model for the VTT, c) construction of
high speed, low noise CCD cameras, and d) construction of computer
hardware for receiving and processing high rate 12-bit digital data.

---------------------------------------------------------
Title: Stokes Inversion in the Presence of Canopy-Like Structures
and Unresolved Flux-Tubes
Authors: Skumanich, A.; Grossmann-Doerth, U.; Lites, B. W.
1992AAS...180.1203S Altcode: 1992BAAS...24Q.748S
The issue of the Stokes polarization signature of canopy-like structures
and the inversion of the resulting Stokes profiles is considered for
a self-consistent flux tube (sheet) model. It is found that when the
`canopy' and its attendant current sheet have an optical thickness at
line center tau_0 &lt;= 5, the intensity and net polarization profiles
become decoupled. An effective inversion strategy is to reduce or
eliminate the role of the intensity parameter. The Milne-Eddington
(ME) analytic inversion in current use is easily adapted for this
purpose. It is found that the use of a magnetic fill factor allows for
an approximate compensation for the presence of a field discontinuity
along the line of sight but a more accurate procedure is to restrict the
inversion to the net polarization profiles. In the case of sufficiently
thin canopies the ME inversion, when reduced to the Sears form, yields
fields within ~ 100 G of the true field. When the Stokes profiles
are averaged over the flux sheet to simulate the effects of limited
angular resolution, the fill factor strategy yields an accurate measure
of both the axial field and the width of the flux sheet at tau_0 =~
1 for observing angles &lt;= 35(deg) to the normal. For larger angles,
opacity shielding occurs and reduced fields are derived.

---------------------------------------------------------
Title: First Results from the Advanced Stokes Polarimeter
Authors: Lites, B. W.; Dunn, R. B.; Elmore, D. F.; Tomczyk, S.;
Skumanich, A.; Streander, K. V.
1992AAS...180.1201L Altcode: 1992BAAS...24..747L
The Advanced Stokes Polarimeter (ASP) is a collaborative program between
the High Altitude Observatory (HAO) and the National Solar Observatory
(NSO) to investigate the physics of solar active regions though
quantitative measurements of vector magnetic fields. First scientific
results from the ASP were obtained during an observing run in March,
1992, when high resolution Stokes profile maps of active regions were
obtained under good seeing conditions. The ASP measures simultaneously
the full Stokes profiles in photospheric Fe I lines near 630 nm and in
the temperature minimum/low chromospheric Mg I b-lines at 517 nm. We
present scans of an isolated small sunspot near disk center, and we
discuss the fine structure of the vector field within this sunspot
and in the magnetic elements surrounding it. Observations of a complex
active region near the east limb will also be presented. This active
region produced a flare during the observational sequence.

---------------------------------------------------------
Title: Physical Models of Solar and Stellar Spots (Invited)
Authors: Skumanich, Andrew
1992LNP...397...94S Altcode: 1992sils.conf...94S
No abstract at ADS

---------------------------------------------------------
Title: Fluorescene-induced Surface Inhomogeneities and Ejection and
Trapping of Plasma by the K Dwarf Component of V471 Tauri
Authors: Young, A.; Rottler, L.; Skumanich, A.
1992LNP...397...75Y Altcode: 1992sils.conf...75Y
No abstract at ADS

---------------------------------------------------------
Title: Observations of the Mesoscale Magnetic Structure of Sunspots
Authors: Skumanich, A.
1992ASIC..375..121S Altcode: 1992sto..work..121S
Recent inferences of the mesoscale structure of spot magnetic fields
based on high spectral resolution Stokes vector polarization profiles
for both visual and infrared lines are reviewed and partly considered
from the point of view of self-similarity. The Evershed flow is briefly
examined with regard to the nonuniqueness of the inferred intrinsic
flow field and its relation to the inclination of the penumbral
magnetic field.

---------------------------------------------------------
Title: Evidence for External Plasma around the K Dwarf Component of
the Eclipsing Binary V471 Tauri
Authors: Young, Arthur; Rottler, Lee; Skumanich, Andrew
1991ApJ...378L..25Y Altcode:
New, higher resolution, time-resolved spectra of the H-alpha profile
of V471 Tauri are presented in which transient appearances of emission
components are detected at the level of 5 sigma. Measurements of the
radial velocity of these components relative to the rest center of
the K dwarf reveal unambiguously that they cannot originate on the
surface of that star. The same measurements also rule out the white
dwarf, or an accretion disk around the white dwarf, as a source of the
emission. Furthermore, these external emission components are greatly
enhanced immediately following the detection of a flare on the surface
of the K dwarf, leading to an inference of causality. The behavior of
the external emission components suggests that they arise from plasma
that is held in a ring or disk orbiting the K dwarf, and that such a
structure is unstable since it decays in a few days. The fate of the
extruded plasma is unknown.

---------------------------------------------------------
Title: On the Scaling of the Axial Magnetic Field Gradient for
Twisted Sunspots
Authors: Skumanich, A.
1991BAAS...23.1051S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Velocity gradients across a flaring neutral line from Stokes
II measurements.
Authors: Skumanich, A.; Lites, B.
1991sopo.work..307S Altcode:
The pattern of profile reversals formed due to incomplete
cancellation of the Stokes net circular polarization parameter, V,
when the resolution element contains the magnetic neutral line (in the
observer's frame) represents a sensitive diagnostic of the line-of-sight
velocity difference between the opposite polarity regions. Typical V
profile patterns found in such circumstances with the HAO/SPO Stokes
II polarimeter are illustrated. Resulting velocity differences are
estimated both by analytic decomposition of a specific neutral line
profile and by composition of profiles from elements on opposite sides
of the neutral line. Pre- and post-flare Stokes II observations of
Hale Active Region ≠16604 are under such analysis.

---------------------------------------------------------
Title: Preliminary results from the HAO/NSO Advanced Stokes
Polarimeter prototype observing run.
Authors: Lites, B. W.; Elmore, D.; Murphy, G.; Skumanich, A.; Tomczyk,
S.; Dunn, R. B.
1991sopo.work....3L Altcode:
A prototype version of the Advanced Stokes Polarimeter was operated at
the Sunspot Vacuum Tower Telescope of the National Solar Observatory
in May und June, 1990. Although the prototype instrument does not
have the capability of the final instrument to be deployed in 1991,
it nonetheless provided high spatial resolution Stokes profiles
observations of a small symmetric sunspot. Analysis of these
observations shows that the magnetic field at the outer edge of
the penumbra of this sunspot is highly inclined with respect to the
local normal (γ = 70 - 80°), in agreement with previous Stokes II
measurements of larger sunspots. In addition, the axial field and flux
distribution was found to be nearly identical to that of the previous
Stokes II measurement.

---------------------------------------------------------
Title: Pressure-Magnetic Field Scaling Law for Sunspots and Starspots
Authors: Skumanich, A.
1990BAAS...22Q1234S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High-Resolution Spectra of Solar Magnetic Features. I. Analysis
of Penumbral Fine Structure
Authors: Lites, B. W.; Scharmer, G. B.; Skumanich, A.
1990ApJ...355..329L Altcode:
The Swedish Vacuum Telescope on La Palma was used to obtain spectra
of the magnetic-sensitive Fe I 630.25 nm line under conditions
of exceptional angular resolution (0.32 arcsec) and high spectral
resolution (FWHM 2.5 pm). Simultaneous 0.02 s CCD exposures of both the
spectrum and the slit-jaw image effectively 'freeze' the atmospheric
seeing motions and permit unambiguous identification of the spectra
of the various penumbral structures. These spectra reveal the magnetic
field strength in penumbral filaments through an intensity fit of the
Zeeman splitting of this line. The observations show that: (1) the field
strength varies from about 2100 G near the umbra-penumbra boundary
to about 900 G at the outer edge of the penumbra, (2) the observed
fluctuation of penumbral magnetic field is much less dramatic than the
fluctuation in intensity, (3) there is a suggestion of a rapid change
in field inclination between some light and dark filaments near the
edge of the penumbra, and (4) there is no obvious correlation between
Doppler shift (in part due to the Evershed flow) and filament intensity.

---------------------------------------------------------
Title: Development of Kernels in a Two-Ribbon Subflare
Authors: Gaizauskas, V.; Proulx, M.; Skumanich, A. P.
1990BAAS...22..890G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: OSO-8 Measurements of Velocity Signature of Flare Kernels
Authors: Skumanich, A.; Gaizauskas, V.
1990BAAS...22..891S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High-Latitude SPOT and Plage Activity on the Rapidly Rotating
M Dwarf Star Gliese 890
Authors: Young, Arthur; Skumanich, Andrew; MacGregor, Keith B.;
Temple, Scott
1990ApJ...349..608Y Altcode:
Simultaneous time-resolved observations in broadband photometry and
in H-alpha and near-UV spectroscopy are presented of Gliese 890, a
single M dwarf of the BY Draconis type with the shortest known period
of rotation among field stars of its type. The continuum light curves
exhibit modulations characteristic of the presence of localized dark
spot regions, and the spectra show corresponding modulations of the
intensity of H-alpha emission, with the latter being substantially in
phase with the former. Secular changes in the continuum light curve
indicate that the geometry of the spots changes appreciably over time
scales of the order of 60 rotation periods. The overall chromospheric
activity is not significantly greater than that found in similar stars
which rotate much more slowly. The principal active region present on
the visible disk of the star during the observations appear to be at
a remarkably high latitude compared to the occurrence of such regions
on the sun.

---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic Fields. V. The
Magnetic Field Structure of Large Sunspots Observed with Stokes II
Authors: Lites, B. W.; Skumanich, A.
1990ApJ...348..747L Altcode:
Results are presented on the analysis of Stokes II polarimeter data
on four large sunspots during 1980, yielding the vector magnetic
field structure and thermodynamic parameters of these sunspots. It
is found that the magnetic field occupies significant fractions of
the area in both light and dark penumbral filaments, and that the
intrinsic field does not fall below 600 G, either in the sunspot
or in the surrounding plage areas, and is typically 700-800 G at
the outer edge of the penumbra. The radial variation of the vector
field in these sunspots does not agree with the return-flux models of
Fla et al. (1982). The variation of the poloidal field strength and
inclination with distance from the center of the symmetric sunspots
exhibits little nonpotential character and is well represented by the
potential field of a buried dipole.

---------------------------------------------------------
Title: A Study of Excess H alpha Emission in Chromospherically Active
M Dwarf Stars
Authors: Young, Arthur; Skumanich, Andrew; Stauffer, John R.; Bopp,
Bernard W.; Harlan, Eugene
1989ApJ...344..427Y Altcode:
Spectroscopic observations from three observatories are combined to
study the properties of the excess H-alpha emission which characterizes
the most chromospherically active subset of the M dwarf stars, known
as the dMe stars. It is demonstrated that the excess H-alpha luminosity
from these stars is a monotonically decreasing function of their (R-I)
color, and evidence is presented which suggests that the product of
the mean surface brightness and the mean filling factor of the emissive
regions is essentially constant with color. Another significant result
of the study is a linear correlation between the excess luminosity in
H-alpha and the coronal X-ray luminosity.

---------------------------------------------------------
Title: The Advanced Stokes Polarimeter
Authors: Lites, B. W.; Skumanich, A.
1989hsrs.conf..389L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Transfer of Line Radiation in a Magnetic Field
Authors: Jefferies, John; Lites, Bruce W.; Skumanich, A.
1989ApJ...343..920J Altcode:
Using a classical approach, the transfer equations are derived for
spectral-line radiation in a medium which is permeated by a magnetic
field. Consideration is given to solutions of these equations for
the 'weak-field' case, when the Zeeman splitting is a fraction of
the Doppler width, and the range of validity of such solutions. Some
approximate expressions allow a simple inference of the vector-field
characteristics directly from the line profiles.

---------------------------------------------------------
Title: The solar-stellar connection: the relationship between flaring
rates, flare power and quiescent X-ray background.
Authors: Harrison, R. A.; Pearce, G.; Skumanich, A.
1989sasf.confP..13H Altcode: 1988sasf.conf...13H; 1989IAUCo.104P..13H
The aim of this study is to extend the dMe analysis to the sun, to
explore relationships between the flaring-rate, flare power-loss and
quiescent X-ray luminosity for different solar active regions.

---------------------------------------------------------
Title: Fluorescence-induced Chromospheric H alpha Emission from the
K Dwarf Component of V471 Tauri. I. The 1983 Epoch
Authors: Young, Arthur; Skumanich, Andrew; Paylor, Victoria
1988ApJ...334..397Y Altcode:
The authors present time-resolved CCD spectroscopic observations of the
Hα region of V471 Tau covering all phases of the orbit, extending over
six consecutive nights (12 orbital cycles) and including simultaneous
broadband continuum photometry. The time-dependent profile of the Hα
feature changes from a definitive absorption line to a moderately
strong emission line systematically with phase, reaching maximum
emission strength at the phase of secondary eclipse. This suggests that
the inner hemisphere of the chromosphere of the K dwarf component is
fluorescing in response to the incident ultraviolet radiation field
from the hot white dwarf primary star. The data show also evidence of
an active magnetic region on the K dwarf.

---------------------------------------------------------
Title: The Solar-Stellar Connection: The Relationship between Flaring
Rates, Flare Power, and Quiescent X-Ray Background
Authors: Harrison, Richard A.; Pearce, Gillian; Skumanich, Andrew
1988ApJ...332.1058H Altcode:
The flaring rates, flare powers, and quiescent X-ray luminosities of dMe
(red dwarf) stars are compared with those of solar active regions. In
dMe stars, these properties are found to be closely related, and this
may have a significant influence on the understanding of the flare
process and coronal heating. For example, a correlation between flare
rate and quiescent X-ray luminosity suggests that both may be driven
by similar processes on a differing scale. It is a natural extension
of this work to investigate similar relationships for the sun. The
results are mixed. The relationships between the various parameters
are certainly not as clear for the sun as for the dMe stars. Some solar
properties appear to vary in a manner similar to the dMe analysis, and
some contradict the dMe case. The implications of this are discussed.

---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic
Fields. IV. Synthesis and Inversion of the Chromospheric MG i B Lines
Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.
1988ApJ...330..493L Altcode:
An analytic solution to the transfer equations for polarized radiation
which allows for departures from LTE in the overall excitation of a
chromospheric line is discussed, and a method of least-squares inversion
is applied to profiles of Mg I b lines at 5172.7 and 5183.6 A observed
in December 1978. An analytic form expressing the source function
dependence on line-center optical depth leads to a generalization of
an existing least-squares procedure which is found to be effective in
recovering the magnetic field from simulated Stokes profiles under
specified conditions. Reasonable magnetic field values are obtained
if a macroturbulent profile smearing of 1-2 km/s is used and if the
profiles are corrected for scattered light.

---------------------------------------------------------
Title: A Model for the Penetration of Lyman Alpha in the Solar
Chromosphere
Authors: Faurobert, M.; Frisch, H.; Skumanich, A.
1988ApJ...328..856F Altcode:
A multilevel calculation of the hydrogen spectrum by Skumanich and
Lites, followed by a two-level representation of each transition, shows
that all the solar Lyα photons are created in an optically thick layer
in the high chromosphere above τ0 ≈ 102. The
authors give a simplified model that demonstrates the penetration of
Lyα into the lower chromosphere below the creation region, i.e., for
τ0 &gt; 102. A second-order escape probability
approximation yields a simple analytical expression which accurately
reproduces the behavior of the Lyα source function for 102
⪉ τ0 ⪉ 5×106 provided the destruction
probability, ɛ is set close to 10-6.

---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic
Fields. I. Inversion of Photospheric Lines
Authors: Skumanich, A.; Lites, B. W.
1987ApJ...322..473S Altcode:
The authors consider improvements to the Auer, Heasley, and House
method for the analytic inversion of Stokes profiles via nonlinear
least squares. In the application of this method to actual sunspot
observations, the authors have found that its simplifications often
yield erroneous solutions or nonconvergent behavior. By including
damping wings and magneto-optical birefringence and by decoupling the
intensity profile from the three-vector polarization profile in the
analysis, the authors develop a more robust inversion method that
provides a more reliable and accurate estimate of sunspot vector
magnetic fields without significant loss of economy.

---------------------------------------------------------
Title: The Polarization Properties of Model Sunspots: The Broad-Band
Polarization Signature of the Schlueter-Temesvary Representation
Authors: Skumanich, A.; Lites, B. W.
1987ApJ...322..483S Altcode:
The properties and diagnostic content of integrated-over-wavelength
Zeeman-induced polarization in spectral lines formed in sunspots
are investigated. The net circular polarization (NCP) generated
by a continuously varying velocity field parallel to a constant
magnetic field with arbitrary inclination is shown to follow
the algebraic expression for the step-velocity model of Auer and
Heasley (1978). Although application of the analysis to parameters
representative of sunspots fails to yield the observed broadband
NCP radius pattern, the introduction of magnetic field gradients and
magnetooptical birefringence is found to result in a pattern similar
to that observed.

---------------------------------------------------------
Title: Stokes Profile Analysis and Vector Magnetic
Fields. III. Extended Temperature Minima of Sunspot Umbrae as Inferred
from Stokes Profiles of MG i lambda 4571
Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.;
Carlsson, M.
1987ApJ...318..930L Altcode:
Observed Stokes profiles of Mg I 4571 A are analyzed as a diagnostic
of the magnetic field and thermal structure at the temperature minimum
of sunspot umbrae. Multilevel non-LTE transfer calculations of the Mg
I-II-III excitation and ionization balance in model umbral atmospheres
show: (1) Mg I to be far less ionized in sunspot umbrae than in the
quiet sun, leading to greatly enhanced opacity in 4571 A, and (2) LTE
excitation of 4571 A. Existing umbral models predict emission cores of
the Stokes I profile due to the chromospheric temperature rise. This
feature is not present in observed umbral profiles. Moreover, such
an emission reversal causes similar anomalous features in the Stokes
Q, U, V profiles, which are also not observed. Umbral atmospheres
with extended temperature minima are suggested. Implications for
chromospheric heating mechanisms and the utility of this line for
solar vector magnetic field measurements are discussed.

---------------------------------------------------------
Title: "Underluminous" Lyman-alpha Emission Associated with
Active-Region Magnetic Canopy
Authors: Jones, H. P.; Skumanich, A.; Lemaire, P.
1987BAAS...19..930J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Synthesis and Inversion of the Chromospheric Mg I ????-Line
Stokes Profiles
Authors: Murphy, G. A.; Rees, D. E.; Lites, B. W.; Skumanich, A.
1987BAAS...19..938M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Vector Magnetic Field Structure of Large Sunspots Observed
with Stokes II
Authors: Lites, B. W.; Skumanich, A.; Lundgren, S.
1987BAAS...19..945L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Lyman α line in the solar chromosphere: penetrative
model andapproximations.
Authors: Faurobert, M.; Frisch, H.; Skumanich, A.
1987PAICz..66..205F Altcode: 1987eram....1..205F
The authors present a model for the formation of Lyman α in the
chromosphere below the creation region. The hydrogen atom is treated
as a two-level atom and the chromosphere as a semi-infinite medium
free of primary sources and illuminated by an isotropic and frequency
independent radiation field at r0 ≅ 102.

---------------------------------------------------------
Title: Radiative Transfer Diagnostics: Understanding Multilevel
Transfer Calculations. I. Analysis of the Full Statistical Equilibrium
Equations
Authors: Skumanich, Andrew; Lites, Bruce W.
1986ApJ...310..419S Altcode:
The sensitivity analysis method of Skumanich and Lites (1985), which
makes it possible to decompose the equivalent two-level parameters of
a non-LTE transition problem into their most significant excitation
(source) and deexcitation (sink) terms, is reviewed and extended. In
the method, the statistical equilibrium (SE) equations are solved
numerically for the explicit upper and lower level occupations of a
particular transition under various combinations of perturbations
of atomic rates, both collisional and radiative, about an exact
numerical solution. The sensitivity analysis is applied to the
formation of the hydrogen spectrum in a representative model of
the solar atmosphere. Although the numerical method is not a means
of avoiding the direct algebraic solution of the SE equations, it
reduces the burden of identifying the most significant terms along the
(N-1)-factorial terms that occur in such a solution.

---------------------------------------------------------
Title: Some Evidence on the Evolution of the Flare Mechanism in
Dwarf Stars
Authors: Skumanich, Andrew
1986ApJ...309..858S Altcode:
White-light flare parameters are estimated for the sun as a star. It
is found that these parameters fall in the same domain as those for the
dMe flare stars. In particular, it is found that the time-averaged flare
power loss and quiescent coronal soft X-ray power loss at solar maximum
satisfies the recently proposed flare power-coronal X-ray relation for
dMe stars (Doyle and Butler; Skumanich). In addition, one finds that dM
stars, which are believed to be magnetically evolved dMe stars, also
satisfy the same relation. On this basis, an evolutionary scenario
is suggested for the flare mechanism in which the total flare rate
remains, more or less, constant but the mean flare yield decreases
linearly with coronal X-ray strength. It is also suggested that the
flare mechanism is universal in all magnetically active dwarfs.

---------------------------------------------------------
Title: Fluorescence-Induced Chromospheric Emission in the Binary
V471 TAU
Authors: Young, A.; Skumanich, A.; Paylor, V.
1986BAAS...18..978Y Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Extended Temperature Minima of Sunspot Umbrae
Authors: Lites, B. W.; Skumanich, A.; Rees, D. E.; Murphy, G. A.;
Carlsson, M.
1986BAAS...18..662L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Effect of Extremely Rapid Rotation on Stellar Activity:
The Case of Gliese 890
Authors: Young, Arthur; Skumanich, Andrew; MacGregor, Keith; Temple,
Scott
1986LNP...254..127Y Altcode: 1986csss....4..127Y
We present an analysis of broad-band photometry and of the H-alpha
emission line profile of the active, rapidly rotating (P = 0.43
days) single M-dwarf star Gleise 890. Our analysis suggests that the
(presumed dark) spots and the bright emission regions are confined
to high latitudes on the star. We combine our results with published
work on other rapidly rotating single stars which indicate that such
activity is generally confined to the polar regions of such stars,
and we contrast that to rapidly rotating active components in binary
systems where the active regions do not appear to be confined to
the polar latitudes. Noting that solar activity is confined to lower
latitudes and seems unable to affect polar latitudes, we suggest that
extremely rapid rotation may ater the convective turnover time in the
equatorial latitudes, and that strong tidal coupling may restore it to
normal values in spite of rapid rotation. The evidence suggests that
even though radiative power losses due to activity may be comparable in
single and binary stars, the driving mechanisms may differ appreciably.

---------------------------------------------------------
Title: Some evidence on the evolution of the flare mechanism in
dwarf stars.
Authors: Skumanich, A.
1986lasf.conf..414S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evolution of activity signatures during the main sequence phase
Authors: Skumanich, A.; MacGregor, K.
1986AdSpR...6h.151S Altcode: 1986AdSpR...6Q.151S
We review recent work on the decay of magnetic activity signatures,
such as chromospheric/transition region/coronal emission as well as mean
flare emission, with age for solar and later type stars. Both implicit
and explicit evolutionary representations are discussed. In terms of
magnetic flux, as measured by excess chromospheric CaII luminosity,
we show that a simple dynamo-rotation relation that incorporates both
a saturated state with its characteristic critical rotation (or scale
factor) as well as an asymptotic linear power law, i.e. a scale free
relation, fits the extant data that includes the dMe stars. Introducing
the saturated dynamo state, as exemplified by the dMe stars, into
activity power-power diagrams, allows us to not only specify the
saturated state, but to define evolutionary tracks that represent
the decay from the saturated state. Using the quiescent coronal X-ray
power (luminosity) as a basic measure of magnetic activity, we find
simple monomial relations for both the saturated state (linear) and
for the evolutionary tracks governing both quiescent activity and mean
flare activity. In particular, the coronal power loss is found to vary
quadratically with the chromospheric power loss, hence with magnetic
flux. NCAR is sponsored by the National Science Foundation.

---------------------------------------------------------
Title: The effect of line damping, magneto-optics and parasitic
light on the derivation of sunspot vector magnetic fields
Authors: Skumanich, A.; Lites, B. W.
1985svmf.nasa..341S Altcode:
The least square fitting of Stokes observations of sunspots using a
Milne-Eddington-Unno model appears to lead, in many circumstances,
to various inconsistencies such as anomalously large doppler widths
and, hence, small magnetic fields which are significantly below those
inferred solely from the Zeeman splitting in the intensity profile. It
is found that the introduction of additional physics into the model
such as the inclusion of damping wings and magneto-optic birefrigence
significantly improves the fit to Stokes parameters. Model fits
excluding the intensity profile, i.e., of both magnitude as well
as spectral shape of the polarization parameters alone, suggest
that parasitic light in the intensity profile may also be a source
of inconsistencies. The consequences of the physical changes on the
vector properties of the field derived from the Fe I lambda 6173 line
for the 17 November 1975 spot as well as on the thermodynamic state
are discussed. A Doppler width delta lambda (D) - 25mA is bound to
be consistent with a low spot temperature and microturbulence, and a
damping constant of a = 0.2.

---------------------------------------------------------
Title: The inference of vector magnetic fields from polarization
measurements with limited spectral resolution
Authors: Lites, B. W.; Skumanich, A.
1985svmf.nasa..342L Altcode:
A method is presented for recovery of the vector magnetic field and
thermodynamic parameters from polarization measurement of photospheric
line profiles measured with filtergraphs. The method includes
magneto-optic effects and may be utilized on data sampled at arbitrary
wavelengths within the line profile. The accuracy of this method is
explored through inversion of synthetic Stokes profiles subjected to
varying levels of random noise, instrumental wave-length resolution, and
line profile sampling. The level of error introduced by the systematic
effect of profile sampling over a finite fraction of the 5 minute
oscillation cycle is also investigated. The results presented here
are intended to guide instrumental design and observational procedure.

---------------------------------------------------------
Title: Least squares inversion of Stokes profiles in the presence
of velocity gradients
Authors: Skumanich, A.; Rees, D. E.; Lites, B. W.
1985svmf.nasa..306S Altcode:
The Auer, Heasley and House Stokes inversion procedure in use at
High Altitude Observatory is based on the analytic solution of the
equation of transfer for polarized light where the representation of
the thermodynamic and magnetic structure of the atmosphere is assumed
to have a high degree of invariance, namely, a Milne-Eddington (ME)
structure with a constant magnetic field. In the presence of invariance
breaking gradients the resultant Stokes profiles are represented
only approximately, if at all, by analytic forms. The accuracy of
the inversion parameters and their significance as measures of actual
structure are explored for the ME and the Landman-Finn sunspot models
under the effects of velocity gradients. The resulting field parameters
are good to a few percent and prove to be insensitive to the errors
committed by the use of a ME-representation, but the resulting ME
parameters yield a less precise measure of thermal structure.

---------------------------------------------------------
Title: Description and operation of CHISQMO, INVRTS, VOIGEN, VOIGTV,
SMEAR, RUFF, AND POSCAR in the Stokes Analysis Program
Authors: Lites, B. W.; Skumanich, A.
1985STIN...8530991L Altcode:
Some details of the computer program used to carry out the least-squares
inversion of polarization profiles obtained with the HAO (High Altitude
Observatory) Stokes Polarimeters I and II are documented. Improvements
to the computer code originated by Auer, Heasley, and House (1977)
include: magneto-optical effects, line damping, scattered light
correction, instrumental broadening, and allowing the line center
position to be a free parameter. The entire program has not been
documented here, since various aspects of the program are still being
revised and improved.

---------------------------------------------------------
Title: On The Broad-Band Circular Polarization Signature of Sunspots
Authors: Skumanich, A.; Lites, B. W.
1985BAAS...17R.631S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Least squares inversion of Stokes profiles in the presence
of velocity gradients.
Authors: Skumanich, A.; Rees, D. E.; Lites, B. W.
1985NASCP2374..306S Altcode:
The authors study the performance of the least squares fit when
applied to theoretical profiles calculated with a realistic model of a
sunspot. The model has gradients in all physical parameters including
magnetic and velocity fields. This numerical experiment allows one
to assess the physical significance of the Milne-Eddington and mean
field parameters provided by the inversion. Observed Stokes profiles
invariably have asymmetric distortions due to velocity gradients in
the solar atmosphere. The inversion method symmetrizes the profiles
prior to fitting and it is important to know how this affects the
inference on the magnetic vector.

---------------------------------------------------------
Title: Evidence for a common magnetic driver for flares and quiescent
coronae/chromospheres
Authors: Skumanich, A.
1985AuJPh..38..971S Altcode:
The mean power and rate of optical flares in dMe stars are found to be
correlated with their quiet coronal X-ray luminosity. The mean flare
luminosity in the photometric U-band appears to scale linearly with the
X-ray luminosity with a slope of 0.04. This coincidence suggests that
quiet and flare activity in coronae/chromospheres may be related by
one and the same flaring mechanism. The author proposes that 'quiet'
activity is due to microflares - a low yield but high frequency
continuation of flares.

---------------------------------------------------------
Title: Radiative transfer diagnostics: Understanding multi-level
transfer calculations.
Authors: Skumanich, A.; Lites, B. W.
1985ASIC..152..175S Altcode: 1985pssl.proc..175S
The authors present a method of interpreting the solution to a
multi-level, multi-transition non-LTE transfer problem. The method
respresents the solutions in terms of equivalent two-level forms with
a scattering and a source term. The resulting individual quenching
probability, i.e. the difference of the scattering albedo from one,
and source term are then decomposed by a perturbation method into
their principal dependence on collisional and/or radiative rates. The
method is illustrated by considering the excitation and ionization of
hydrogen in the VAL 3C model of the quiet sun chromosphere.

---------------------------------------------------------
Title: The Extremely Rapidly Rotating M-Dwarf Star Gliese 890
Authors: Young, A.; Skumanich, A.; MacGregor, K.; Temple, S.
1984BAAS...16.1014Y Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Behavior of Excess Chromospheric Hα Emission in Late-Type
Stars &amp;Correlation with Coronal X-ray Emission
Authors: Skumanich, A.; Young, A.; Stauffer, J.; Bopp, B. W.
1984BAAS...16..940S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Rapid rotation and H alpha emission anomalies among some
members of the population of low-mass stars.
Authors: Young, A.; Skumanich, A.; Harlan, E.
1984ApJ...282..683Y Altcode:
The paper reports on some important anomalous phenomena which were
discovered during a systematic investigation of the properties of
low-mass (M dwarf) stars. It is found that the single M dwarfs GLS 410
and GLS 179 have a diluted H-alpha absorption line. It is suggested
that this is due to weak plage activity and that these stars are
in a transition state between the chromospherically active dMe and
the inactive dM state. It is found that the dMe star GLS 890 is a
rapidly rotating single M dwarf with v sin i of approximately 70 km/s
and predicted that it should have a photometric wave with about 8 hr
period. It is found that the Joy and Abt 'dMe' assignment is in error
not only for GLS 179 but also for GLS 176, GLS 611.3, and GLS 908. The
'dMe' assignment for BD + 12.2201 deg is problematic and may be due
to misidentification.

---------------------------------------------------------
Title: The sun as a star - Three-component analysis of chromospheric
variability in the calcium K line
Authors: Skumanich, A.; Lean, J. L.; Livingston, W. C.; White, O. R.
1984ApJ...282..776S Altcode:
A three-component model of the solar cycle variability of the Ca II
K emission is developed using extant contrast and fractional area
parameters for (1) cell, (2) network, and (3) plage components that
are resolution-consistent. A fit has been achieved for the quiet-sun
Ca II K emission (at solar minimum) as observed by White and Livingston
(1981) with cell and network features alone using extant limb-darkening
laws. The occurrence of plages during the growth of the solar cycle
was found to be insufficient to account for the cycle increase in
the K emission and required the introduction of additional network in
excess of the quiet-sun value. The implications of, and evidence for,
such an active network are discussed.

---------------------------------------------------------
Title: Properties of a Rapidly Rotating DME Star &amp; of H-Alpha
Anomalous Low-Mass Stars
Authors: Skumanich, Andrew
1984iue..prop.1878S Altcode:
The issue of the effect of ultra-high rotation without attendant tidal
effects on chromospheric and transition region signatures of dynamo
activity in cool stars is to be examined by the observation of a
recently discovered (Young, Skumanich and Harlan (1983), hereafter
YSH83) rapidly rotating single dMe field star. In addition the
chromospheric and transition region properties of stars found to be in
a transition H-alpha emission state, i.e. between dMe and dM. YSH83 but
with strong Ca+ chromospheres, will be determined. These observations
will allow further study of the rotationactivity laws governing dynamo
behaviour in solar-like stars as well as supplying needed constraints
on chromospheric models for these stars.

---------------------------------------------------------
Title: The K2-dwarf V471 Tau: a stellar version of solar variability.
Authors: Skumanich, A.; Young, A.
1984NASCP2310..185S Altcode: 1984siva.work..185S
Simultaneous observations of the rotational modulation with a 0.5 day
period of chromospheric Hα emission and of broadband irradiance for
the K2-dwarf in V471 Tau are presented. The observations cover eight
rotation periods but do not cover the full surface of the dwarf because
of timing constraints. Preliminary results show a phase relation between
enhanced chromospheric emission and continuum darkening similar to that
observed on the sun. A comparison with chromospheric Mg II resonance
emission modulation observed about 2.25 years earlier by Guinan and
Sion shows that the same active longitude is involved.

---------------------------------------------------------
Title: Modelling solar spectral irradiance variations at ultraviolet
wavelengths.
Authors: Lean, J. L.; Livingston, W. C.; White, O. R.; Skumanich, A.
1984NASCP2310..253L Altcode: 1984siva.work..253L
The authors examine solar ultraviolet irradiance variations with solar
activity by using a three component model of the Ca II K chromospheric
emission. This model, developed from ground based observations of the
location, area and relative intensity of Ca II K plage, in conjunction
with measurements throughout solar cycle 21 of the full disc Ca II
K emission, includes the contributions to the ultraviolet flux from
both plage and active network emission. Evolution and rotation of
the plage regions on the solar disc produce a 27-day modulation of
the UV flux. Over longer time scales, such as the eleven year solar
cycle, changes in the active network are an important source of UV
flux variability.

---------------------------------------------------------
Title: The Rapidly-Rotating Field M-Dwarf Gliese 890
Authors: Young, Arthur; Skumanich, Andrew; Heller, Clayton; Temple,
Scott
1984LNP...193..112Y Altcode: 1984csss....3..112Y
No abstract at ADS

---------------------------------------------------------
Title: Plage Structure and Activity on the Cool Star Component of
the Eclipsing Binary V471 Tau
Authors: Young, A.; Skumanich, A.
1983BAAS...15..917Y Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Variability of the Lyman alpha flux with solar activity
Authors: Lean, J. L.; Skumanich, A.
1983JGR....88.5751L Altcode:
A three-component model of the solar chromosphere, developed from ground
based observations of the Ca II K chromospheric emission, is used to
calculate the variability of the Lyman alpha flux between 1969 and
1980. The Lyman alpha flux at solar minimum is required in the model
and is taken as 2.32×1011 photons/cm2/s. This
value occurred during 1975 as well as in 1976 near the commencement of
solar cycle 21. The model predicts that the Lyman alpha flux increases
to as much as 5×1011 photons/cm2/s at the maximum
of the solar cycle. The ratio of the average fluxes for December 1979
(cycle maximum) and July 1976 (cycle minimum) is 1.9. During solar
maximum the 27-day solar rotation is shown to cause the Lyman alpha
flux to vary by as much as 40% or as little as 5%. The model also shows
that the Lyman alpha flux varies over intermediate time periods of
2 to 3 years, as well as over the 11-year sunspot cycle. We conclude
that, unlike the sunspot number and the 10.7-cm radio flux, the Lyman
alpha flux had a variability that was approximately the same during
each of the past three cycles. Lyman alpha fluxes calculated by the
model are consistent with measurements of the Lyman alpha flux made
by 11 of a total of 14 rocket experiments conducted during the period
1969-1980. The model explains satisfactorily the absolute magnitude,
long-term trends, and the cycle variability seen in the Lyman alpha
irradiances by the OSO 5 satellite experiment. The 27-day variability
observed by the AE-E satellite experiment is well reproduced. However,
the magntidue of the AE-E 1 Lyman alpha irradiances are higher than
the model calculations by between 40% and 80%. We suggest that the
assumed calibration of the AE-E irradiances is in error.

---------------------------------------------------------
Title: Rapid rotation and stellar activity in the triple system
HD 165590.
Authors: Stern, R. A.; Skumanich, A.
1983ApJ...267..232S Altcode:
X-ray and ultraviolet observations of the spectroscopic and visual
binary HD 165590 are presented. Results show that the spectroscopic
binary component has a period of 0.88 day and a near-solar spectral
type. A substantial soft X-ray flux for HD 165590 is also found with
an X-ray luminosity of 10 to the 30.6th ergs/sec for the system. No
obvious periodicities of less than 0.5 days are detected in the X-ray
data, although stochastic variability is present. Observations of the
transition region emission lines using the International Ultraviolet
Explorer satellites are shown to be consistent with the X-ray data
and reveal an approximately 50% variability on the time scale of
days. It is concluded that comparisons of the results for HD 165590
with X-ray observations of other late F and G stars demonstrate that
rotation-activity scaling relations are useful as gross predictors of
X-ray emission.

---------------------------------------------------------
Title: A three-component model of the variability of the solar
ultraviolet flux: 145-200 nM
Authors: Lean, J. L.; White, O. R.; Livingston, W. C.; Heath, D. F.;
Donnelly, R. F.; Skumanich, A.
1982JGR....8710307L Altcode:
A three-component model has been developed to examine the variation
with solar activity of the far ultraviolet irradiance between 145 and
200 nm. This model is based on spatially resolved observations of
the Call K chromosphere and includes the contributions to the full
disk flux from both plage and active network emission. The 27-day
modulation of the ultraviolet flux is explained by the evolution and
rotation of the plage regions on the solar disc. Over the longer time
scale of the eleven-year cycle it is essential that changes in the
active network arising from the decay of plage regions also be solar
flux is it possible to simultaneously reproduce the 27-day variability
observed by the solar backscatter ultraviolet experiment on the Nimbus 7
satellite and the changes from the minimum to the maximum of the solar
activity cycle observed by the rocket experiments of the Laboratory
for Atmospheric and Space Physics and by the extreme ultraviolet
spectrometer on the Atmospheric Explorer E satellite. It is shown that
the AE-E experiment measured a smaller solar cycle variability for the
ultraviolet irradiances than is predicted by the model calculations
because of the spatially restricted field of view of this instrument.

---------------------------------------------------------
Title: On the magnetic and thermodynamic consequences of the
return-flux sunspot model
Authors: Fla, T.; Skumanich, A.; Osherovich, V. A.
1982ApJ...261..700F Altcode:
The magnetic structure of sunspots from photosphere to low corona
is considered on the basis of a new magnetohydrostatic theory
of sunspots. The first numerical solutions of the basic nonlinear
force-balance equation which takes into account returning magnetic flux
are obtained using observational data for the maximum field, umbral,
and penumbral size and for the vertical distribution of pressure on
the sunspot axis and in the quiet sun. Both vertical and horizontal
distributions for the magnetic field were obtained as well as its
vertical gradient. Further, the pressure and temperature distributions
consistent with this field were also obtained. The dependence of the
model on the Wilson depression, the maximum magnetic field of the
sunspot, and the amount of return magnetic flux is investigated. A
'global' force-balance relation is found to hold which relates the
magnetic field gradient on the spot axis with the maximum magnetic field
and the pressure deficit, or Wilson depression, in the spot photosphere.

---------------------------------------------------------
Title: A model of a sunspot chromosphere based on OSO 8 observations
Authors: Lites, B. W.; Skumanich, A.
1982ApJS...49..293L Altcode:
OSO 8 spectrometer observations of the H I, Mg II, and Ca II resonance
lines of a large quiet sunspot during November 16-17, 1975, along with
a C IV line of that event obtained by a ground-based spectrometer,
are analyzed together with near-simultaneous ground-based Stokes
measurements to yield an umbral chromosphere and transition region
model. Features of this model include a chromosphere that is effectively
thin in the resonance lines of H I and Mg II, while being saturated
in Ca II, and an upper chromospheric structure similar to that of
quiet-sun models. The similarity of the upper chromosphere of the
sunspot umbra to the quiet-sun chromosphere suggests that the intense
magnetic field plays only a passive role in the chromospheric heating
mechanism, and the observations cited indicate that solar-type stars
with large areas of ordered magnetic flux would not necessarily exhibit
extremely active chromosphere.

---------------------------------------------------------
Title: X-Ray and UV Observations of the Rapidly Rotating Triple
System HD 165590
Authors: Stern, R. A.; Skumanich, A.
1981BAAS...13..812S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Temperature-Field Relation and Current Distribution in the
Return-Flux Model
Authors: Skumanich, A.; Osherovich, V. A.; Flaa, T.
1981BAAS...13..910S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the magnetic field topology in the return-flux sunspot model
Authors: Skumanich, A.; Osherovich, V. A.
1981phss.conf..104S Altcode:
The Return-Flux Sunspot Model of Osherovich (1981) is discussed and
applied to the determination of the magnetic structure for a long-lived
sunspot from an observationally derived pressure deficit. Analysis
yields the identification of the magnetic surface which separates
the set of 'open' field lines from the set that return to the surface
with the umbral-penumbral boundary. The basic non-linear differential
equation, which represents a special case of the exact force-balance
equation, for the return-flux model is solved. The choice of the depth
of the tau-5 = 1 level relative to the quiet sun photosphere, i.e., the
Wilson depression, was determined from a self-consistency requirement.

---------------------------------------------------------
Title: Aspects of long-term variability in sun and stars.
Authors: Skumanich, A.; Eddy, J. A.
1981ASIC...68..349S Altcode: 1981spss.conf..349S
The present review is most specifically concerned with the variability
of solar and stellar dynamos, and the observable manifestation of
these phenomena on time scales longer than about 25 years. The
evidence for the long-term variability of the sun is examined,
taking into account the telescopic sunspot record, auroral data,
pretelescopic sunspot reports, tree-ring radiocarbon, the question of
the 'Gleissberg Cycle', and pretelescopic evidence of the 11-year cycle
of the solar activity. Evidence for secular and evolutionary changes
in the dynamo for solar-like stars is considered, giving attention
to magnetic activity (mean)-age relationship, the magnetic activity
(mean) - rotation relationship, the magnetic activity (variance)
- age relationship, and other age/dynamo related effects. In a
discussion of the conclusions and the observational implications,
aspects regarding the nature of magnetic braking are explored along
with the characteristics of a nonlinear dynamo model.

---------------------------------------------------------
Title: An empirical view of the chromospheric temperature structure
above a sunspot umbra
Authors: Lites, B. W.; Skumanich, A.
1981phss.conf..152L Altcode:
OSO-8 observations of the chromospheric and transition region emission
lines are used to develop a temperature-based model of the chromosphere
above a sunspot. The data base included observations of Lyman-alpha,
Lyman-beta, the resonance lines of singly-ionized Mg, and the resonance
lines of singly-ionized Ca. Details of the line profiles are provided,
and features of the derived plateau model of the chromosphere are
discussed. Weak reversals and broad wings in the observed Ca II lines
were accounted for by assuming a 5 km/sec broadening, thus implying
greater significance for the Ca II/Mg II, K/H, and k/h intensity
ratios than details of the line profile shapes. The L-alpha and L-beta
intensities are noted to constrain the upper chromosphere and transition
region pressure to lower values than in the average quiet solar corona.

---------------------------------------------------------
Title: OSO-8 Observations of the Sunspot Chromosphere
Authors: Skumanich, A.; Lites, B. W.
1980BAAS...12..896S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Physical Effects of Radiative Transfer in Multidimensional
Media Including Models of the Solar Atmosphere
Authors: Jones, H. P.; Skumanich, A.
1980ApJS...42..221J Altcode:
We review the astrophysical literature concerning radiative transfer
in multidimensional media where one requires the solution of the
transfer equation under scattering conditions for a medium in which
some combination of boundary configuration, external illumination,
and internal thermodynamic structure causes the radiation field to vary
with more than one spatial dimension. In constant opacity atmospheres,
the radiation field is shown to scale systematically with to a
characteristic geometric scale for a wide variety of configurations
and types of scattering. Some effects of radiative exchange between
different regions of multidimensional media are reviewed, and the
constraining influence of an exponential vertical variation of opacity
is discussed. Particular emphasis is given to recent applications of
multidimensional transfer to the interpretation of the fine spatial
structure on the Sun.

---------------------------------------------------------
Title: Chromospheric (Ca+KH) and Coronal (X-ray) Activity-Rotation
Relations In Late-Type Spectroscopic Binaries and Main Sequence Stars.
Authors: Skumanich, A.
1979BAAS...11R.624S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Comparison of the 19 April 1977 Two Ribbon Disk Flare with
X-ray Flares Observed at the Limb
Authors: Skumanich, A.; Gaizauskas, V.; Ku, W. H.
1979BAAS...11..410S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Calibrated full disk solar H I Lyman-alpha and Lyman-beta
profiles.
Authors: Lemaire, P.; Charra, J.; Jouchoux, A.; Vidal-Madjar, A.;
Artzner, G. E.; Vial, J. C.; Bonnet, R. M.; Skumanich, A.
1978ApJ...223L..55L Altcode:
Resolved solar H I Ly-alpha and -beta profiles have been recorded by the
French LPSP experiment on OSO 8. Intensity observations at the center
and the limb have been combined to obtain flux-equivalent profiles
(5.46 plus or minus 30 percent and 0.078 plus or minus 25 percent
erg/sq cm per sec for Ly-alpha and -beta, respectively). Comparison of
the flux profiles with unresolved calibration rocket profiles allows
one to obtain an absolute calibration.

---------------------------------------------------------
Title: The LPSP instrument on OSO 8. II. In-flight performance and
preliminary results.
Authors: Bonnet, R. M.; Lemaire, P.; Vial, J. C.; Artzner, G.;
Gouttebroze, P.; Jouchoux, A.; Leibacher, J. W.; Skumanich, A.;
Vidal-Madjar, A.
1978ApJ...221.1032B Altcode:
The paper describes the in-flight performance for the first 18 months of
operation of the LPSP (Laboratoire de Physique Stellaire et Planetaire)
instrument incorporated in the OSO 8 launched June 1975. By means of
the instrument, an absolute pointing accuracy of nearly one second
was achieved in orbit during real-time operations. The instrument
uses a Cassegrain telescope and a spectrometer simultaneously
observing six wavelengths. In-flight performance is discussed with
attention to angular resolution, spectral resolution, dispersion
and grating mechanism (spectral scanner) stability, scattered light
background and dark current, photometric standardization, and absolute
calibration. Real-time operation and problems are considered with
reference to pointing system problems, target acquisition, and L-alpha
modulation. Preliminary results involving the observational program,
quiet sun and chromospheric studies, quiet chromospheric oscillation
and transients, sunspots and active regions, prominences, and aeronomy
investigations are reported.

---------------------------------------------------------
Title: Doppler Shifts measured in 0 VI line from OSO-B observations
above and in the vicinity of plage Mc Math 13738.
Authors: Lemaire, P.; Skumanich, A.; Artzner, G.; Gouttebroze, P.;
Vial, J. C.; Bonnet, R. M.; McWhirter, P.
1978BAAS...10Q.440L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: OSO-8 Radio and X-ray observations of the 19 April 1977 flare.
Authors: Skumanich, A.; Jouchoux, A.; Castelli, J.; Lemaire, P.;
Artzner, G.; Gouttebroze, P.; Vial, J. C.; Bonnet, R. M.
1978BAAS...10..441S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE diagnostics
Authors: Skumanich, A.
1977MmSAI..48..375S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: EUV observations of class-C X-ray flare by the LPSP
(Laboratoire de Physique Stellaire et Planétaire du Centre National
de la Recherche Scientifique) spectrometer on OSO-8.
Authors: Jouchoux, A.; Skumanich, A.; Bonnet, R. M.; Lemaire, P.;
Artzner, G.; Leibacher, J.; Vial, J. C.; Vidal-Madjar, A.
1977BAAS....9..432J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Extreme-Ultraviolet Observations of New Cycle Sunspots with
the LPSP Spectrometer from OSO-8
Authors: Skumanich, A.; Jouchoux, A.; Artzner, G.; Bonnet, R. M.;
Leibacher, J.; Lemaire, P.; Vial, J. C.; Vidal-Madjar, A.
1977BAAS....9..340S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: OSO-8 Observations of the EUV Chromospheric Network
Authors: Artzner, G.; Bonnet, R. M.; Leibacher, J.; Lemaire, P.;
Skumanich, A.; Vial, J. C.; Vidal-Madjar, A.
1976BAAS....8..332A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Solar OVI Profiles as Observed by the French Experiment
Aboard OSO-8
Authors: Lemaire, P.; Artzner, G.; Bonnet, R. M.; Vial, F. C.;
Skumanich, A.; Leibacher, J.; Vidal-Madjar, A.
1976BAAS....8R.331L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Evolution of sunspot and active region morphology as observed
on the resonance lines of Ca+ (K, H), Mg+
(k, h), hydrogen (Lalpha , Lbeta ), and O VI from OSO-8 (LPSP).
Authors: Artzner, G.; Skumanich, A.; Bonnet, R. M.; Vidal-Madjar,
A.; Leibacher, J.; Lemaire, P.; Vial, J. C.
1976BAAS....8..397A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Periodic temporal phenomena in the solar chromosphere as
observed from OSO-8 (LPSP).
Authors: Vial, J. C.; Leibacher, J.; Lemaire, P.; Artzner, G.; Bonnet,
R. M.; Skumanich, A.; Vidal-Madjar, A.
1976BAAS....8..397V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spatial variations of solar profiles of Ca+ H and
K, Mg+ h and k, and Lyman alpha and beta (H I) resonance
lines as observed from OSO-8 (LPSP).
Authors: Bonnet, R. M.; Artzner, G.; Leibacher, J.; Lemaire, P.;
Skumanich, A.; Vial, J. C.; Vidal-Madjar, A.
1976BAAS....8..397B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Prominence Observations with the OSO-8 French Experiment
Authors: Vial, J. C.; Artzner, G.; Bonnet, R. M.; Lemaire, P.;
Leibacher, J.; Skumanich, A.; Vidalmadjar, A.
1976BAAS....8..344V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the statistical desription of inhomogeneities in the
quiet solar atmosphere. I. Linear regression analysis and absolute
calibration of multichannel observations of the Ca+
emission network.
Authors: Skumanich, A.; Smythe, C.; Frazier, E. N.
1975ApJ...200..747S Altcode:
Observations with a 2';4 square aperture (1968 epoch) of the Ca
+ K core brightness (1.1 A bandwidth), vertical magnetic field,
vertical velocity, and continuum brightness have been analyzed to yield
average network and nonnetwork values and dispersions as well as linear
regression relations. The statistics of all variables but the continuum
consist of a symmetric core superposed on an extended tail which is
associated with the chromospheric network. Network statistics were
derived by subtraction of a Gaussian fit to the core which was taken
to represent nonnetwork fluctuations. The velocity network was only
partially resolved so that its distribution, and that of the continuum,
was obtained by identifying network regions as those with field &gt;
10 gauss. The average network has a field of 26 gauss, independent
of polarity, a Ca + brightness 1.27 that of the average nonnetwork or
undisturbed chromosphere, a downdraft velocity of 41 m a monochromatic
continuum enhancement of 0.34 percent, and covers 39 percent of the
quiet Sun. The network brightness increases relative to its average
with a slope of 2.1 percent gauss-1 for Ca + and 2.6 percent gauss
-1 for the continuum while the downdraft velocity increases by 1.6
m 1 gauss-1 for fields between 25 and 120 gauss. Nonlinear effects
appear for larger and smaller fields. The average outward flux density
of network over nonnetwork regions is 2.4 &gt;c 10 ergs 1 forCa+ Kand
3.8 x l08ergscm-2s-1 for the continuum. Photometric calibration of the
Ca + K line observations was effected by a comparison of the observed
relative contrast statistics with a statistically equivalent absolute
brightness distribution obtained from a calibrated slit spectrum of
the Ca + K line. Subject headings:atmospheres, solar - Ca ii emission -
magnetic fields, solar - solar atmospheric motions

---------------------------------------------------------
Title: Characteristic Relaxation Times for Non-LTE Atmospheres.
Authors: Skumanich, A.
1975BAAS....7Q.257S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: An empirical interpretation for the time evolution of the Ca
ii K line
Authors: Liu, S. -Y.; Skumanich, A.
1974SoPh...38..109L Altcode:
An empirical approach to interpret the time evolution of the high
spatial resolution Ca II K line is presented. We specify the physical
parameters, such as electron temperature, hydrogen density, and
velocity (microturbulent and systematic) as functions of height. The
electron density is obtained from scaled non-LTE solutions for
hydrogen ionization. The population indices, and thus the Ca II
source functions, for a 5-level Ca II atom are computed by using the
generalized Newton-Raphson method. K line profiles are then synthesized
for different evolutionary stages and are compared with the observed
ones. The explanation of the `peculiar' type profile is also attempted.

---------------------------------------------------------
Title: An Empirical Interpretation for the Time Evolution of the Ca
II K Line
Authors: Liu, Sou-Yang; Skumanich, Andrew
1974BAAS....6T.290L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Formation of Resonance Lines in Multidimensional
Media. II. Radiation Operators and Their Numerical Representation
Authors: Jones, Harrison P.; Skumanich, A.
1973ApJ...185..167J Altcode:
A general integral-operator representation of radiative transfer
in an atmosphere whose properties vary in more than one spatial
dimension is developed together with well-posed procedures for its
discrete numerical representation. This methodology is applied to the
"non-LTE" excitation equilibrium of a two-level impurity species, i.e.,
to the formation of a resonance absorption line. The radiation operator
that appears in the excitation equation is converted into a finite
but large matrix by means of a suitably selected finite set of basis
functions. This matrix-excitation equation is inverted directly to yield
the excitation state and, hence, the impurity absorption characteristics
of the plasma. Illustrative applications to the formation of a resonance
line in a constant-density plasma with several types of two-dimensional
temperature structures are presented. Subject headings: line formation -
radiative transfer

---------------------------------------------------------
Title: A New Method for Initializing Non-LTE Statistical Equilibrium
Problems.
Authors: Barkstrom, B. R.; Skumanich, A.
1973BAAS....5..455B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the Representation of the Lambda Operator in the Integral
Equation (Green's Function) Approach to Non-LTE Spectral Line
Formation.
Authors: Barkstrom, B. R.; Skumanich, A.
1973BAAS....5..337B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnesium II Doublet Profiles of Chromospheric Inhomogeneities
at the Center of The Solar Disk
Authors: Lemaire, P.; Skumanich, A.
1973BAAS....5R.275L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnesium II Doublet Profiles of Chromospheric Inhomogeneities
at the Center of the Solar Disk
Authors: Lemaire, P.; Skumanich, A.
1973A&A....22...61L Altcode:
Summary. An analysis of a balloon spectrum of the sun obtained on
June 24 1970, with 7" angnlar resolution and 25 mA spectral resolution
respectively is presented. Average cell, network and plages profiles
near the center of the solar disk are identified and compared with
profiles computed on the basis of recent chromospheric models. Key
words: sun - spectrum - ultraviolet chromosphere

---------------------------------------------------------
Title: On the Quantitative Description of the Fluctuating Solar
Atmosphere. I. Regression Analysis and Calibration of Multi-channel
Observations
Authors: Skumanich, A.; Smythe, C.; Frazier, E. N.
1972BAAS....4W.391S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Time Scales for Ca II Emission Decay, Rotational Braking,
and Lithium Depletion
Authors: Skumanich, A.
1972ApJ...171..565S Altcode:
A comparison of the Ca+ emission luminosity-after correction for
spectral-type effects-for the Pleiades, Ursa Major, and Hyades stars
and the Sun indicate an emission decay which varies as the inverse
square root of the age. Further, the rotational decay curve is
found to satisfy the same law. It is further suggested that lithium
depletion follows the same law but only as far as the Hyades age,
after which the depletion proceeds exponentially. Since Ca+ emission
is linearly proportional to magnetic field strength at the surface,
one can predict that the surface fields are proportional to angular
velocity and decay as the inverse square root. The above results are
predicated Qi' the standard Hyades age (0.4 billion years).

---------------------------------------------------------
Title: Thermalization Lengths and Mean Numbers of Scatterings for
Line Photons
Authors: Athay, R. G.; Skumanich, A.
1972lfpm.conf..167A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Thermalization Lengths and Mean Numbers of Scatterings for
Line Photons
Authors: Athay, R. Grant; Skumanich, A.
1971ApJ...170..605A Altcode:
A simple algebraic method is given for the derivation of approximate
thermalization lengths, mean number of scatterings for line photons,
and scaling laws for the maximum value of the line-source function
in finite atmospheres. The method utilizes only the shape of the line
absorption coefficient.

---------------------------------------------------------
Title: Time Scales for CaII Emission Decay, Rotational Braking and
Lithium Depletion.
Authors: Skumanich, A.
1971BAAS....3Q.455S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrogen Ionization and n=2 Population for Model Spicules
and Prominences
Authors: Poland, A.; Skumanich, A.; Athay, R. G.; Tandberg-Hanssen, E.
1971SoPh...18..391P Altcode:
Using slab model atmospheres that are irradiated from both sides by
photospheric, chromospheric, and coronal radiation fields we have
determined the ionization and excitation equilibrium for hydrogen.

---------------------------------------------------------
Title: On the Application of the Newton-Raphson Method to the
Excitation and Transfer of Continuum Radiation
Authors: Domenico, B. A.; Poland, A. I.; Skumanich, A.
1971BAAS....3....9D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: -Non-LTE effects for He i in early B type stars
Authors: Poland, A.; Skumanich, A.
1971MNRAS.152P..18P Altcode:
The contention by Hearn and Leckrone that non-LTE affects helium
abundances derived for early main sequence stars is shown to be in
error. Although the cores of some lines exhibit non-LTE effects, their
equivalent widths, which are dominated by line wings that remain in
LTE, are unaffected and yield the same helium abundances as in LTE. The
lines A6678 and A587S have weak wings and remain affected by non-LTE.

---------------------------------------------------------
Title: On the application of the generalized Newton-Raphson method
in radiative transfer problems.
Authors: Skumanich, A.; Domenico, B. A.
1971JQSRT..11..547S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: An Application of the Generalized Newton-Raphson Method to
the Multilevel Line Transfer Problem
Authors: Domenico, B. A.; Skumanich, A.
1970BAAS....2R.309D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Ionization Equilibrium and the N = 2 Population of Hydrogen
in Quiescent Prominences
Authors: Poland, A.; Athay, R. G.; Skumanich, A.; Tandberg-Hanssen, E.
1970BAAS....2R.338P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the Existence of a Purely Radiative Temperature Rise in
the Solar Chromosphere
Authors: Skumanich, A.
1970ApJ...159.1077S Altcode:
The approzimations implicit in Cayrel's statistical-equilibrium or
non-LTE analysis of the H- bound-free continuum in the Sun are ezamined
and are shown to be consistent and independent of any assumption
regarding the nature of the energy equilibrium. It is also shown that
Jordan's contention that radiative equilibrium imposes a constraint
on the non-LTE state of H- and thus removes any non-LTE temperature
rise in the solar chromosphere is in error.

---------------------------------------------------------
Title: Line Formation in Multi-Dimensional Media
Authors: Jones, H. P.; Skumanich, A.
1970sfss.coll..138J Altcode: 1970IAUCo...2..138J
No abstract at ADS

---------------------------------------------------------
Title: Circumstellar CAII K Lines in g, K and M Giants and Supergiants
Authors: Vaughan, A. H., Jr.; Skumanich, A.
1970sfss.coll..295V Altcode: 1970IAUCo...2..295V
No abstract at ADS

---------------------------------------------------------
Title: Line Formation in Multidimensional Media
Authors: Jones, H. P.; Skumanich, A.
1969BAAS....1S.281J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radiative transport in finite homogeneous cylinders by the
Monte Carlo technique.
Authors: Avery, L. W.; House, L. L.; Skumanich, A.
1969JQSRT...9..519A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Theoretical Line Intensities. VII. Wavelength and Depth
Dependence of Line-Blanketing Effects for Pure Absorption and
Non-Coherent Scattering
Authors: Athay, R. Grant; Skumanich, A.
1969ApJ...155..273A Altcode:
The processes involved in line blanketing are investigated in detail
for a two-level atom whose spectral line is formed partially in pure
absorption and partially in non-coherent scattering (non-LTE). Line and
continuum interactions are included through absorption of line photons
by continuum processes, and vice versa. Interactions with the thermal
energy of particles are included through collisional excitations
and de-excitations. Calculations of the "local" blanketing effect,
designated by e, are made for lines of different equivalent widths, in
different spectral regions, and for different absorption profiles. We
consider an atmosphere in which B(r) is linear. In general, such an
atmosphere is not in radiative equilibrium and requires mechanical
energy to maintain the assumed temperature profile even if no lines
are present in the spectrum. If lines are added to the spectrum without
distorting B(r), the local energy requirements are changed by an amount
e and the total energy (per unit area) is changed by an amount E =
J'edr. For a Milne-Eddington (M-E) atmosphere in pure absorption (LTE)
and linear B(r), E is shown to be positive (mechanical energy must be
added to the atmosphere) and equal to the flux "blocked" out by the
equivalent widths of the lines, »=W~H~c. In the corresponding case for
a Schuster-Schwarzschild (S-S) atmosphere, just the reverse is true,
E = - ~ and energy must be removed from the atmos- phere to preserve
the linear B(r). In the limiting case of an isothermal atmosphere
in LTE, E -p 0 for the M-E atmosphere. When scattering (non-LTE) is
present for a M-E atmosphere, E is the same as for LTE for saturated
lines in all spectral regions, while it is approximately the same for
iøisaturated lines in the violet and visual spectral regions. In
the case of the S-S atmosphere, the LTE result for the integrated
blanketing remains unchanged. However, in both cases the local LTE and
non-LTE blanketing effects as given by e differ appreciably, mainly in
the degree of thermal cooling at the boundary. Equations derived to
express the self-consistent (radiative equilibrium) problem provide,
we believe, more accurate and numerically more tractable calculations
of the blanketed temperature profile

---------------------------------------------------------
Title: Line Formation in Multi-Dimensional Media
Authors: Jones, H. P.; Skumanich, A.
1968rla..conf...79J Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the Application of the Newton-Raphson Method to Non-Linear
Line Transfer
Authors: Skumanich, A.
1968rla..conf..475S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Emission Cores in H and K Lines. IV: Center-to-Limb Variation
Authors: Athay, R. Grant; Skumanich, A.
1968SoPh....4..176A Altcode:
Calculations are made for the center-limb variations of the
K2 and K3 components of the solar Ca II K line
using an optically thick model of the chromosphere. The center-limb
variations are shown to require an increase of Doppler width with
height in the chromosphere and to depend critically upon the location
of the point where ΔλD has increased by a factor e. Good
agreement with observations is found when, and only when, the increase
in ΔλD occurs nearly simultaneously with the increase in
chromospheric temperature.

---------------------------------------------------------
Title: Non-Radial Oscillations of Slowly Rotating Polytropes
Authors: Durney, B.; Skumanich, A.
1968ApJ...152..255D Altcode:
The linearized equations for non-radial adiabatic oscillations of slowly
rotating polytropes are studied for both stable and marginally stable
states. For oscillations in the stable state, the eigenfrequencies
are a continuous function of the parameter = (~ - F)/(m~)2, which
measures the ratio of buoyant to gyroscopic forces; here T = 1 + 1/n,
where n is the polytropic index, ~y is the ratio of the specific
heats, w the non-dimensional angular velocity, and m determines
the azimuthal dependence. The structures of these oscillations,
which could be called gravitational gyroscopic waves, are determined
from two coupled first-order partial differential equations. In the
marginally stable state one finds the solutions to be oscillatory,
thus indicating overstability; the parameter yi takes on discrete
negative values which indicate the stabilizing influence of rotation

---------------------------------------------------------
Title: Theoretical Line Intensities.VI. Milne-Eddington Curves of
Growth for Non-Coherent Scattering
Authors: Athay, R. Grant; Skumanich, A.
1968ApJ...152..211A Altcode:
Curves of growth are derived for different mixtures of non-coherent
scattering and pure absorption and compared with similar curves computed
by Wrubel for mixtures of coherent scattering and pure absorption. The
curves induding fractional non-coherent scattering tend to mimic the
pure coherent scattering curves on the linear portion of the curve
of growth but tend to bridge over to the pure absorp- tion curves for
very strong line

---------------------------------------------------------
Title: Emission Cores in h- and K-Lines III. The Wilson-Bappu Effect
Authors: Athay, R. Grant; Skumanich, A.
1968ApJ...152..141A Altcode:
Widths of the emission components, W2, of the H- and K-lines of Ca
ii are investigated, assuming that the lines are formed in optically
thick chromospheres with temperature increasing outward. The line
source function S is computed from an equation relating S to the
Planck function and the divergence of the line flux W2 is found to be
particularly sensitive to chromospheric opacity both in the line and
in the continuum, to the damping parameter a, and to the chromospheric
Doppler width We interpret the results to mean that the Wilson-Bappu
effect arises primarily from changes in Doppler width, and, fur- ther,
that there is a tendency among the chromospheres of late-type stars
to have approximately the same line and continuum opacities and to
have a &lt; 3 X 1O~. To a first approximation, the intensity of the
emission component is found to be uncorrelated with W

---------------------------------------------------------
Title: Wavelength and Depth Dependence of Line Blanketing Effects
for Pure Absorption and Noncoherent Scattering.
Authors: Athay, R. Grant; Skumanich, A.
1968AJS....73Q...3A Altcode:
The processes involved in line blanketing are investigated in detail
for a two-level atom whose spectral line is formed partially in pure
absorption and partia~lly in non coherent scattering. Line and continuum
interactions (back-warming) as well as interactions with the thermal
energy of particles (local-cooling) are included. Calculations of the
"local" blanketing effect are made for Milne-Eddington and Schuster-
Schwarzschild atmospheres for lines of different equivalent widths, in
different spectral regions and for different absorption profiles. Both
back-warming and local-cooling are present to some degree at all
depths. Generally, however, back-warming predominates near r c 1
and local- cooling in the higher, line-forming layers. We consider
an atmosphere in which B (T) is linear. Such an atmosphere requires
mechanical energy to maintain the assumed B (T). The presence of
lines changes the local energy requirements by an amount e and the
total energy (per unit area) by an amount E= f e dr. For the case
of pure absorption (LTE) in an AI-E atmosphere, E is shown to be
positive (the atmosphere is cooled by the lines and energy must be
added). The back-warming and local-cooling effects are each wavelength
and depth dependent, sometimes reinforcing and sometimes counteract~ng
each other. They combine to give E = ~ W, the equivalent width of the
lines, in all cases, however. When scattering (non-LTE) is present in
the ~tE case the back-warming effect remains relatively unchanged in
the violet and visual regions of the spectrum but decreases markedly
in the red. The local-cooling effectdecreasesmarkedly in all spectral
regions. The nel effect leaves e very nearly the same for lines in the
violet and visual, except in the surface layers, and diminishes e at
all depths in the red. We still find E~~W in the violet and visual
spectral regions. In the extreme case of an isothermal atmosphere,
however, we find E = -~W, i.e., the atmosphere is now heated by the
lines and requires an energy sink equal to the equivalent widths. The
S-S atmosphere gives E = -~W for lines of any strength, at any
wavelength and irrespective of whether pure absorption or scattering
dominates. The detailed behavior of e (r) depends upon all of these
parameter, however. A full treatment of line blanketing must include
a proper classification of lines according to wavelength, equivalent
width, the relative role of pure absorption versus scattering and
the depth variation of the absorption coefficient, the latter being
particularly important. Equations derived to express the self-consistent
(radiative equilibrium) problem provide, we believe, more accurate and,
numerically, more tractable calculations of the blanketed temperature
profile.

---------------------------------------------------------
Title: Emission Cores in H and K Lines. I: The Optically Thick
Chromosphere
Authors: Athay, R. Grant; Skumanich, A.
1968SoPh....3..181A Altcode:
Profiles of the H and K lines of MgII and the K line of Ca II are
computed using a two-level atom for five model atmospheres distinguished
from each other mainly by the location of the temperature minimum. In
the five models the temperature minimum and the chromospheric
temperature are adjusted to give best agreement between computed and
observed profiles. The parameters ɛ and r0 are prescribed
as functions of τ from a density model of the atmosphere. By comparing
computed and observed profiles of the K3, K2
and inner K1 components of the lines we determine both
the approximate depth variation of ΔλD and the best of
the temperature models. We find that the Doppler width increases
rapidly with height in the chromosphere beginning from a value of
1.6 km/sec at τ0 ≈ 10−2. This latter
value corresponds closely to the thermal velocity of Mg atoms in
the upper photosphere. The preferred temperature model is one for
which the temperature minimum occurs near τ0(λ 2800) ≈
10−4-10−5 with a value Tmin≲
4200 ° and which has a temperatu near 7000 ° at τ0 =
10−6 where K2 is formed. The intensity in
K3 is determined largely by dΔλD/dτ in the
K2 and K3 regions.

---------------------------------------------------------
Title: On the Empirical Determination of Doppler Widths.
Authors: Skumanich, A.
1968AJS....73R.202S Altcode:
For the inversion of the center-limb observations of line profiles
one customarily assumes a functional form (over depth) for the
line source function 5L and solves for the Doppler width AXD and
its depth dependence. No physical arguments are used to select the
particular form. Those forms are selected (cf. Curtis, G. W., and
Jefferies, J. T., Astrophys. J. 150, 1061, 1967) which yield explicit
Laplace transforms. However, in simple cases the physical theory of
noncoherent scattering suggests certain forms for SL (cf. Skumanich,
A., Astron. J. 72, 828,1967). In the region where the Doppler core of a
line is formed 5L varies with the square root of the line-center optical
depth and leads to a ~~ darkening law. An analysis of J. H. Waddell's
observations of the NaD and Mgb multiplets (Astrophys. J. 136, 223,
1962; 137,1210,1962) indicates that the square-root darkening law
obtains within the observational errors. Furthermore, assuming that AND
iS constant one obtains AX~(NaD) =0.039 A and AN~(Mgb) =0.034 A. The
sodium value is 10% higher than that of Curtis and Jefferies (1967,
above) and implies a nonthermal velocity of 1.2 km/sec (with T=46000K),
while magnesium yields l~ km/sec. These values are in reasonable
agreement with those needed to fit the observations with a multilevel
transfer calculation (Athay and Canfield, preprint). Evidence is found
for a depth dependence. In the case of Ca+ K-H the square-root law was
also found to hold, considerably better for the limb-darkening data
(double pass) of 0. K. White and Z. Suemoto (Solar Phys. 3, 60, 1968)
than that (single pass) of J. B. Zirker (Solar Phys. 3,164, 1968). The
White-Suemoto data yields A ND (Ca+) =0.08 A or a velocity broadening
of 6 km/sec, considerably lower than that derived by Zirker (0.135 A)
but in agreement with the theoretical calculations of K. Athay and
A. Skumanich (Solar Phys. 3,181,1968).

---------------------------------------------------------
Title: Frequency-Dependent Line Source Functions.
Authors: Athay, R. G.; Skumanich, A.
1968AJS....73S...2A Altcode:
By the use of a previously described differential equation method
for solving resonance line transfer (Skumanich, Astron. J. 71,
871,1966) the frequency dependence of the line source function has
been studied with the view of explaining the discrepancy between the
derived solar minimum temperature of 42000K based on Ca II K analysis
(Athay, R. G., and Skumanich, A., Solar Phys. 2, to be published)
and the minimum temperature of 46000K derived from other observations
(Bilderberg Conference, Solar Phys., to be published). Several cases
were calculated for atmospheres with and without a chromospheric
temperature rise. These include (1) noncoherent scattering, (2)
coherent scattering, and (3) partially coherent scattenug in the
atoms rest frame (for lines with a finite dispersion width). These
calculations differ from others in that the exact scattering functions
were used. A comparison of line profiles for the non coherent case
with that for complete redistribution (frequency-independent source
function) shows, in agreement with earlier results based on iterative
solutions, that complete redistribution (or uncorrelated scattering)
is a good approximation for this case with intensity differences
amounting to only 5 to 7% of the continuum. Furthermore, our results
for isothermal atmospheres corroborates, in general, the earlier (three
frequency point) calculations of Jefferies and White (Astrophys. J. 132,
767,1960). In the chromospheric case we find that essentially complete
coherency is required to depress the source function by the necessary
factor of 2 outside the Doppler core and thus allow the "radiation"
temperature to fall below the kinetic temperature in comparison with
the complete redistribution calculations.

---------------------------------------------------------
Title: Statistical Equilibrium for a Multilevel Model of Calcium in
the Solar Chromosphere.
Authors: Jones, H. P.; Domenico, B. A.; Skumanich, A.
1968AJS....73S..66J Altcode:
The statistical equilibrium of Ca is studied for collisional and
radiative interactions a~propriate to various states of the solar
chromosphere (cf. Athay and Skumanich, 1967). A model ion with
levels representing the ground states of Ca I, II, and III is used,
including the 4p2P and 3d2D excited states (ignoring fine structure)
of Ca II. Dielectronic recombination from Ca II to Ca I is included in
such a way as to display its maximum effect. For the mean chromosphere
the ratio of continuum to line absorption coefficient, r0, for the
"H-K" line (2P-2S transition) of Ca II is found to be independent
of the radiation fields in the "H-K" and infrared "triplet" (2P-2D)
lines. For other chromospheric conditions (e.g., spicules and plages)
the above is not true and care must be exercised because line transfer
calculations must include a self-consistent treatment of the effect of
line radiation fields on line opacities. Dielectronic recombination
does not affect r0 by more than a factor of 2 to 3. For an optically
thick chromosphere in the "H-K" line ETH~K(Tmin) 3 X 104J such as
used by Athay and Skumanich (1967), Linsky (1967), and Dumont (1967),
studies of the interlocking terms in the "H-K" source function show
the two-level atom to be a useful first approximation. In such a thick
atmosphere the infrared "triplet" saturates to a condition of radiative
detailed balance well before the temperature minimum. This requires the
K1 intensities to yield a temperature minimum of the order of 42000K
regardless of the rate coefficients assumed in the model atom. To
reconcile the K1 intensities with a Tn~jn of 46000K as suggested by
the Bilderberg model (1967) requires an unsaturated infrared "triplet"
and hence a reduction in the "H-K" line optical depth by a factor of
several hundred.

---------------------------------------------------------
Title: Broadening of H and K Emission Cores and the Wilson-Bappu
Effect.
Authors: Athay, R. Grant; Skumanich, A.
1967AJ.....72..784A Altcode:
Widths of K2 components W ofIl and K lines are investigated, assuming
that they are formed in optically thick chromospheres with temperature
increasing outward. The source * function S is computed from an equation
relating S to the Planck function B and the divergence of the line
flux. At very large optical depths in the line S~B. Hence 8(r) mimics
the minimum in B (r) then rises with B (r) in the chromosphere until
the flux divergence forces S(r) to decrease to S(ro)~~B (ro). The
K2 maximum results from the maximum in 8(T) and, for the sun, lies
near the edge of the Doppler core. W is investigated as a function of
the following chromospheric parameters: line opacity TO*, continuum
opacity Tc*, electron density ne*, temperature T*, Doppler width AND*,
and damping parameter a*. W is particularly sensitive to TO*, Tc*,
a*, and AND*, but relatively insensitive to T* and fle*. Changes in
W resulting from TO*, Tc*, and a* arise from the displacement of K2
between the Doppler core and line wings and are associated with major
changes in the character of the profile. On the other hand, changes
in AND* change W through a rather uniform scaling of the K2 and K3
profile without seriously affecting the location of K2 relative to the
Doppler core. We interpret these results to mean that the Wilson-Bappu
effect arises primarily from changes in AND*, and, further, that there
is a tendency among the chromospheres of all late-type stars to have
approximately the same values of Tc*, TO*, and a*. Increases in the
K2 intensity 12 result from increases in T* and/or increases in ne*,
which leaves 12 uncorrelated with W. To a first approximation, 12 ne*B*
at T5OOO 10-~.

---------------------------------------------------------
Title: An integral equation for the line source function and its
numerical solution
Authors: Athay, R. G.; Skumanich, A.
1967AnAp...30..669A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Interpolation Laws for Chromospheric Ca II (Mg II) K or H
Line Source Function for Complete Redistribution.
Authors: Skumanich, Andrew
1967AJ.....72T.828S Altcode:
Interpolation formulas for the chromospheric contribution to the line
source function are derived from numerical calculations for Ca II
(Mg II) when formed by resonance scattering with selective absorption
(two-level approximation) for various parameters (cf. Hummer, D. G.,
and Averett, E. H., Monthly Not?ces Roy. Astron. Soc. 130, 295,1965;
and Skumanich, A., Astron. J. 71, 871, 1966). For a thin chromosphere,
the contribution is constant at EB, where E is the probability of
a collisional de-excitation and B the line Planck function. For
a thick chromosphere which is thinner than the saturation length
(cf. J. T. Jefferies, Astrophys. J. 132, 775,1960), the contribution
has a boundary value of eBt and a peak value of EBt and varies as
the square root of the depth for a Doppler "core" controlled line
(i.e., t &lt;a-') it is the chromospheric thickness at line center
and a, the damping constant. For a "wing" controlled line (i.e.,
t&gt; a-'), the surface value is EB (t/a)1 while the peak is EB (t/a)
~. Here the variation is as the square root for depths less than a-'
and as the fourth root for larger depths. For a &gt; 1, one obtains
the dispersion profile results with a =1. The location of the peak
occurs at -21t. For chromospheres thicker than the saturation length,
the expected results are obtained by setting t to the saturation
length. Using the Eddington-Barbier approximation, we find that the
intensity ratio K2/K3-t~ if the "core" controlled case applies. The
solar K2/K3 ratio (Goldberg, Mohier, and M~ller, Astrophys. J. 129,
119, 1959) yields t-4, i.e., a fairly thin chromosphere. This value
and the observed wavelength of K2 yields a broadening tur- bulence
of 12 km/sec. Since I(K2)-EBt0.04B (60000K), we have that T(chromo)
-10 0000K for an electron density of 2X10'1(e~10-3).

---------------------------------------------------------
Title: Numerical solution of the equation of transfer for a two-level
atom when treated as a two-point boundary value problem
Authors: Skumanich, A.
1966AJ.....71R.871S Altcode:
No abstract at ADS

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Title: Chromospheric Activity as a Function of Age in Main-Sequence
Stars
Authors: Wilson, O. C.; Skumanich, A.
1966IAUS...24...40W Altcode:
No abstract at ADS

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Title: Population Differences among Bright G Dwarfs and CA II
Emission Reversals.
Authors: Skumanich, A.
1965AJ.....70S.692S Altcode:
Wilson that the strength of Ca II emission reversals in main sequence
stars decreases with increasing age (Wilson. 0. C., Astrophys. J. 138,
832, 1963; Wilson, 0. C., and Skumanich, A., Astrophys. J. 140, 1401,
1964) would imply the existence of kinematic and hence population
differences among stars segregated by the presence or absence of Ca II
reversals. This criterion for population discrimination has already
been observed in M dwarfs with dMe stars characterized by smaller
than average peculiar motions (Vyssotsky, A. N., and Dyer, E. R., Jr.,
Astrophys. J. 125, 832, 1957). It is found in this paper that dG stars
with Ca II emission are also characterized by small peculiar motions
in contradistinction to dG stars whose spectra are known to be free
of any emission. In particular, 47 emission stars are found to have a
dispersion in peculiar radial velocities of 18 km/sec as opposed to 29
km/sec for 106 nonemission stars. Furthermore, for 40 emission stars
with known space motions, the vertex of the distribution of motions
in the galactic plane deviates by some 230 from the direction to the
galactic center, while no such deviation appears in the distribution
for 95 zero-emission stars. It is also found that stars with strong
reversals are characterized by the smallest peculiar motions. The above
findings indicate that bright dG stars with Ca II emission belong to
the population subgroup previously designated as population A among
the fainter G dwarfs and the bright K giant stars (Vyssotsky, A. N.,
and Skumanich, A., Astron. J. 58, 96,1953). Thus one may conclude
that the existence of a population difference corroborates the Ca II
intensity-age relationship. A similar analysis for K giants indicates
that Ca II emission is not a population indicator and hence is not
age-correlated.

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Title: Dependence of Chromospheric Activity upon Age in Main-Sequence
Stars: Additional Evidence.
Authors: Wilson, O. C.; Skumanich, Andrew
1964ApJ...140.1401W Altcode:
Spectrograms of 10 A/mm dispersion have been obtained of 142 field
stars from the catalogue of Str6mgren and Perry for those objects with
b - y &gt; 0.325. Mter elimination of known spectroscopic binaries
and stars of higher luminosity, a sample of 114 main-sequence stars
remains. In this sample seventeen stars showing H and K bright reversals
are found, and all but two of these lie close to the lower boundary
(zero-age edge) of the distribution in a Ci - (b - y) diagram. This
result is taken as evidence in support of the view that chromospheric
activity of main-sequence stars decreases with age. In particular,
it removes the possibility that stars formed in large clusters differ
significantly in respect to their chromospheric properties from those
formed in small groups or singly.

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Title: On Bright-Dark Symmetry of Solar Granulation.
Authors: Skumanich, Andrew
1955ApJ...121..404S Altcode:
The possible existence of a statistical asymmetry between bright and
dark in granulation intensities is investigated. Direct photographs
of the quiet sun, taken with the 60-foot tower telescope of the Mount
Wilson Observatory, were used. Two isophotes were determined, so that
the total bright region (i.e., brighter than the isophote) formed
20 and 80 per cent of the total area of the eld, respectively. The
frequency distribution according to size of singly connected regions
for both isophotes (bright on the 20 per cent and dark on the 80 per
cent isophote) were obtained and intercompared. Only a very slight
excess of bright regions was found, which implies a slight-if any-bright
asymmetry. Further, a similar test based on the frequency distribution
of granulation intensities did not show any appreciable asyrnmetry. Thus
a strong bright-dark asymmetry, as implied by the traditional rice-grain
picture of solar granulation, does not appear to exist, at least not
for the granulation elements resolved on the Mount Wilson plates.

---------------------------------------------------------
Title: On Thermal Convection in a Polytropic Atmosphere.
Authors: Skumanich, Andrew
1955ApJ...121..408S Altcode:
Thermally induced convection is investigated in an atmosphere with
a strong density variation from top to bottom, such as is found in
the hydrogen convection zone of the sun. The atmosphere is assumed
to be initially stratified in horizontal planes and to follow the
polytrnpe law, P pP. Viscosity and heat conduction are ignored,
the perturbations are assumed to be adiabatic, and the perturbation
equations are linearized. It is found that in a polytropic atmosphere-in
contrast to the classical case of a homogeneous speed of development
of a perturbation increases persistently with decreasing horizontal
wave length of the perturbations, even for wave lengths shorter than
the thickness of the atmosphere. Thus in the solar hydrogen convection
zone small element sizes are to be expected to predominate.

---------------------------------------------------------
Title: Thermo-convection in a polytropic atmosphere.
Authors: Skumanich, Andrew
1955AJ.....60Q.179S Altcode:
Thermally induced convection is investigated in an atmosphere with
a strong density variation from top to bottom, such as is found in
the hydrogen convection zone of the sun. The atmosphere is assumed
to be initially stratified in horizontal planes and to follow the
polytrope law, P cc ~r Viscosity and heat conduction are ignored,
the perturbations are assumed to be adiabatic, and the perturbation
equations are linearized. It is found that in a polytropic atmosphere
in constrast to the classical case of a homogeneous atmosphere the
speed of development of a perturbation increases persistently with
decreasing horizontal wave length of the perturbations, even for wave
lengths shorter than the thickness of the atmosphere. Thus in the
solar hydrogen convection zone small element sizes are to be expected
to predominate. Las Vegas, Nev.

---------------------------------------------------------
Title: The eclipsing variable GL Carinae.
Authors: van Wijk, U.; Rogerson, J. B.; Skumanich, A.
1955AJ.....60...95V Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Studies on solar granulation (Parts I and II)
Authors: Skumanich, Andrew
1954PhDT.........1S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Concerning population groups among G and K stars
Authors: Vyssotsky, A. N.; Skumanich, A.
1953AJ.....58...96V Altcode:
. Two groups of G dwarfs have been distinguished spectroscopically
both at the Yerkes Observatory among bright stars and at the McCormick
Observatory among faint stars. in one group the solar motion and the
dispersion in the peculiar motions are only two-thirds as large as
in the other. A similar spectroscopic differentiation among K giants
indicates that the first group is more closely connected with the
spiral structure of the galaxy than is the second.

---------------------------------------------------------
Title: Internal Motions Within Interstellar Clouds.
Authors: Spitzer, Lyman, Jr.; Skumanich, Andrew
1952ApJ...116..452S Altcode:
No abstract at ADS