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

Title: The discovery path of the inverse square root of age relations
    for solar-type stars
Authors: Skumanich, A.
Bibcode: 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.

Title: Constraints Imposed by the Square-root Spin-down Law on
    Coronal Wind Parameters
Authors: Skumanich, Andrew
Bibcode: 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.

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

Title: Chapter 14 - Challenges and Prospects for the Future
Authors: Vial, Jean-Claude; Skumanich, Andrew
Bibcode: 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. <P />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?

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

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.
Bibcode: 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 <P />thereby
  providing a better understanding of the underlying physics.

Title: Mode identification in rapidly rotating stars
Authors: Reese, D. R.; Thompson, M. J.; MacGregor, K. B.; Jackson,
   S.; Skumanich, A.; Metcalfe, T. S.
Bibcode: 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. <BR />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. <BR />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. <BR />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.

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.
Bibcode: 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. <BR
  />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. <BR />Methods: A new pulsation code is applied to stellar
  models based on the self-consistent field (SCF) method. <BR />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. <BR />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.

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.
Bibcode: 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.

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.
Bibcode: 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.

Title: Absorption Line Profiles for Differentially Rotating Stellar
    Models
Authors: MacGregor, Keith B.; Casini, R.; Flanagan, W.; Jackson, S.;
   Skumanich, A.
Bibcode: 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.

Title: On the Structure and Properties of Differentially Rotating,
    Main-Sequence Stars in the 1-2 M<SUB>solar</SUB> Range
Authors: MacGregor, K. B.; Jackson, Stephen; Skumanich, Andrew;
   Metcalfe, Travis S.
Bibcode: 2007ApJ...663..560M
Altcode: 2007arXiv0704.1275M
  We present models for chemically homogeneous, differentially rotating,
  main-sequence stars with masses in the range 1-2 M<SUB>solar</SUB>. 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) M<SUB>solar</SUB>
  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 M<SUB>solar</SUB> stars.

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
Bibcode: 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 M<SUB>solar</SUB>. 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 Ω<SUB>0</SUB>/Ω<SUB>e</SUB>&lt;10,
  where Ω<SUB>0</SUB> and Ω<SUB>e</SUB> 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.

Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Jackson, Stephen; MacGregor, Keith B.; Skumanich, Andrew
Bibcode: 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.

Title: Models for the Rapidly Rotating Be Star Achernar
Authors: Skumanich, A.; Macgregor, K. B.; Jackson, S.
Bibcode: 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. <P />Jackson,
  S., Macgregor, K. B. &amp; Skumanich, A., in preparation <P />The NCAR
  (National Center for Atmospheric Research) is sponsored by the National
  Science Foundation.

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

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

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

Title: Converging Flows in the Penumbra of a δ Sunspot
Authors: Lites, B. W.; Socas-Navarro, H.; Skumanich, A.; Shimizu, T.
Bibcode: 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.

Title: The Physical Content of the Leading Orders of Principal
    Component Analysis of Spectral Profiles
Authors: Skumanich, A.; López Ariste, A.
Bibcode: 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.

Title: On the Universality of the Leading Orders of PCA. Spectral
    Eigenprofiles for the Sun.
Authors: Skumanich, A.; López Ariste, A.
Bibcode: 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

Title: The Physical Content of the Leading Orders of Principle
    Component Analysis of Spectral Profiles
Authors: Skumanich, A.; Lopez Ariste, A.
Bibcode: 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 C<SUB>ij</SUB> given by the product of the profile
  amplitudes at wavelengths λ <SUB>i</SUB> and λ <SUB>j</SUB> 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.

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.
Bibcode: 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 logτ<SUB>5</SUB>=0 and
  logτ<SUB>5</SUB>=-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.

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

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

Title: Stokes Asymmetries &amp; Gradient Corrections to
    Milne-Eddington Atmospheres
Authors: Skumanich, A.
Bibcode: 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.

Title: On the value of `αAR' from vector magnetograph data -
    I. Methods and Caveats
Authors: Leka, K. D.; Skumanich, A.
Bibcode: 1999SoPh..188....3L
Altcode:
  This investigation centers upon the quantifying
  magnetic twist by the parameter α, commonly defined as
  (∇×B<SUB>h</SUB>)<SUB>z</SUB>/B<SUB>z</SUB>=μ<SUB>0</SUB>J<SUB>z</SUB>/B<SUB>z</SUB>,
  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 'α<SUB>AR</SUB>'. 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
  J<SUB>z</SUB>(x,y)=α<SUB>AR</SUB>B<SUB>z</SUB>(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 α<SUB>AR</SUB>parameter is often consistent to within the
  uncertainties, even though the resulting α<SUB>AR</SUB>can 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 α<SUB>AR</SUB>, and quantifying
  the validity of α<SUB>AR</SUB>, are addressed in Paper II (Leka, 1999).

Title: The Evolution of Photospheric Magnetic Structures in Terms
    of the Size-Flux Relationship
Authors: Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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, I<SUB>c</SUB>,
  magnetic field magnitude, | B |, magnetic fill fraction, f, local
  inclination, γ, local azimuth, φ, and line-of-sight Doppler velocity,
  v<SUB>D</SUB>. 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 × 10<SUP>19</SUP> Mx
  in a dark pore that appears in an area in which the flux increases
  by ~1 × 10<SUP>20</SUP> 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) × 10<SUP>20</SUP> 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 |, I<SUB>c</SUB>), (| B |, γ), (f, γ), and the (v<SUB>D</SUB>,
  I<SUB>c</SUB>), (v<SUB>D</SUB>, 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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. <P />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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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<SUP>-3</SUP>I<SUB>c</SUB>
  against an instrumentally produced background polarization of ~=1-5 ×
  10<SUP>-2</SUP>I<SUB>c</SUB>. Net-polarization signals smaller than
  ~=3 × 10<SUP>-4</SUP>I<SUB>c</SUB> 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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<SUP>-1</SUP>. 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. <P />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<SUP>-1</SUP>. 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<SUP>-1</SUP>, depending on the filling factor. The
  possible importance of these transverse motions for the dynamics of
  the upper atmospheric layers is discussed. <P />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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 10<SUP>24</SUP> 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1994smf..conf..219M
Altcode:
  No abstract at ADS

Title: Vector field structure of a small sunspot
Authors: Skumanich, A.; Lites, B. W.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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/m<SUP>2</SUP>, or 0.15%
  below its mean value of 1367.54 W/m<SUP>2</SUP> 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/m<SUP>2</SUP> 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 1992LNP...397...75Y
Altcode: 1992sils.conf...75Y
  No abstract at ADS

Title: Observations of the Mesoscale Magnetic Structure of Sunspots
Authors: Skumanich, A.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1990BAAS...22..890G
Altcode:
  No abstract at ADS

Title: OSO-8 Measurements of Velocity Signature of Flare Kernels
Authors: Skumanich, A.; Gaizauskas, V.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 τ<SUB>0</SUB> ≈ 10<SUP>2</SUP>. The
  authors give a simplified model that demonstrates the penetration of
  Lyα into the lower chromosphere below the creation region, i.e., for
  τ<SUB>0</SUB> &gt; 10<SUP>2</SUP>. A second-order escape probability
  approximation yields a simple analytical expression which accurately
  reproduces the behavior of the Lyα source function for 10<SUP>2</SUP>
  ⪉ τ<SUB>0</SUB> ⪉ 5×10<SUP>6</SUP> provided the destruction
  probability, ɛ is set close to 10<SUP>-6</SUP>.

Title: Stokes Profile Analysis and Vector Magnetic
    Fields. I. Inversion of Photospheric Lines
Authors: Skumanich, A.; Lites, B. W.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 r<SUB>0</SUB> ≅ 10<SUP>2</SUP>.

Title: Radiative Transfer Diagnostics: Understanding Multilevel
    Transfer Calculations. I. Analysis of the Full Statistical Equilibrium
    Equations
Authors: Skumanich, Andrew; Lites, Bruce W.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 1986lasf.conf..414S
Altcode:
  No abstract at ADS

Title: Evolution of activity signatures during the main sequence phase
Authors: Skumanich, A.; MacGregor, K.
Bibcode: 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. <P />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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 inference of vector magnetic fields from polarization
    measurements with limited spectral resolution.
Authors: Lites, B. W.; Skumanich, A.
Bibcode: 1985NASCP2374..342L
Altcode:
  The authors present a method for recovery of the vector magnetic
  field and thermodynamic parameters from polarization measurements of
  photospheric line profiles measured with filtergraphs.

Title: The effect of line damping, magneto-optics and parasitic
    light on thederivation of sunspot vector magnetic fields.
Authors: Skumanich, A.; Lites, B. W.
Bibcode: 1985NASCP2374..341S
Altcode:
  No abstract at ADS

Title: The Extremely Rapidly Rotating M-Dwarf Star Gliese 890
Authors: Young, A.; Skumanich, A.; MacGregor, K.; Temple, S.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 1983BAAS...15..917Y
Altcode:
  No abstract at ADS

Title: Variability of the Lyman alpha flux with solar activity
Authors: Lean, J. L.; Skumanich, A.
Bibcode: 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 2.32×10<SUP>11</SUP> photons/cm<SUP>2</SUP>/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 5×10<SUP>11</SUP> photons/cm<SUP>2</SUP>/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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1978BAAS...10..441S
Altcode:
  No abstract at ADS

Title: Non-LTE diagnostics
Authors: Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1976BAAS....8R.331L
Altcode:
  No abstract at ADS

Title: Evolution of sunspot and active region morphology as observed
    on the resonance lines of Ca<SUP>+</SUP> (K, H), Mg<SUP>+</SUP>
    (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.
Bibcode: 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.
Bibcode: 1976BAAS....8..397V
Altcode:
  No abstract at ADS

Title: Spatial variations of solar profiles of Ca<SUP>+</SUP> H and
    K, Mg<SUP>+</SUP> 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.
Bibcode: 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.
Bibcode: 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<SUP>+</SUP>
    emission network.
Authors: Skumanich, A.; Smythe, C.; Frazier, E. N.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1972BAAS....4W.391S
Altcode:
  No abstract at ADS

Title: Time Scales for Ca II Emission Decay, Rotational Braking,
    and Lithium Depletion
Authors: Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1970sfss.coll..295V
Altcode: 1970IAUCo...2..295V
  No abstract at ADS

Title: Line Formation in Multidimensional Media
Authors: Jones, H. P.; Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1968SoPh....4..176A
Altcode:
  Calculations are made for the center-limb variations of the
  K<SUB>2</SUB> and K<SUB>3</SUB> 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 Δλ<SUB>D</SUB> has increased by a factor e. Good
  agreement with observations is found when, and only when, the increase
  in Δλ<SUB>D</SUB> occurs nearly simultaneously with the increase in
  chromospheric temperature.

Title: Non-Radial Oscillations of Slowly Rotating Polytropes
Authors: Durney, B.; Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 r<SUB>0</SUB> are prescribed
  as functions of τ from a density model of the atmosphere. By comparing
  computed and observed profiles of the K<SUB>3</SUB>, K<SUB>2</SUB>
  and inner K<SUB>1</SUB> components of the lines we determine both
  the approximate depth variation of Δλ<SUB>D</SUB> 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 τ<SUB>0</SUB> ≈ 10<SUP>−2</SUP>. 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 τ<SUB>0</SUB>(λ 2800) ≈
  10<SUP>−4</SUP>-10<SUP>−5</SUP> with a value T<SUB>min</SUB>≲
  4200 ° and which has a temperatu near 7000 ° at τ<SUB>0</SUB> =
  10<SUP>−6</SUP> where K<SUB>2</SUB> is formed. The intensity in
  K<SUB>3</SUB> is determined largely by dΔλ<SUB>D</SUB>/dτ in the
  K<SUB>2</SUB> and K<SUB>3</SUB> regions.

Title: On the Empirical Determination of Doppler Widths.
Authors: Skumanich, A.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 1966AJ.....71R.871S
Altcode:
  No abstract at ADS

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

Title: Population Differences among Bright G Dwarfs and CA II
    Emission Reversals.
Authors: Skumanich, A.
Bibcode: 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.

Title: Dependence of Chromospheric Activity upon Age in Main-Sequence
Stars: Additional Evidence.
Authors: Wilson, O. C.; Skumanich, Andrew
Bibcode: 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.

Title: On Bright-Dark Symmetry of Solar Granulation.
Authors: Skumanich, Andrew
Bibcode: 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
Bibcode: 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
Bibcode: 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.
Bibcode: 1955AJ.....60...95V
Altcode:
  No abstract at ADS

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

Title: Concerning population groups among G and K stars
Authors: Vyssotsky, A. N.; Skumanich, A.
Bibcode: 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
Bibcode: 1952ApJ...116..452S
Altcode:
  No abstract at ADS

