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.

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

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.
Aims: Given the simplicity of this asymptotic formula, we investigate whether it can provide a means by which to identify pulsation modes in rapidly rotating stars.
Methods: We develop a new mode identification scheme which consists in scanning a multidimensional parameter space for the formula coefficients which yield the best-fitting asymptotic spectra. This mode identification scheme is then tested on artificial spectra based on the asymptotic formula, on random frequencies and on spectra based on full numerical eigenmode calculations for which the mode identification is known beforehand. We also investigate the effects of adding random frequencies to mimic the effects of chaotic modes which are also expected to show up in such stars.
Results: In the absence of chaotic modes, it is possible to accurately find a correct mode identification for most of the observed frequencies provided these frequencies are sufficiently close to their asymptotic values. The addition of random frequencies can very quickly become problematic and hinder correct mode identification. Modifying the mode identification scheme to reject the worst fitting modes can bring some improvement but the results still remain poorer than in the case without chaotic modes. 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.
Aims: Our aim is to characterise pulsation modes in rapidly rotating stellar models so as to be able to interpret asteroseismic data from such stars.
Methods: A new pulsation code is applied to stellar models based on the self-consistent field (SCF) method.
Results: Pulsation modes in SCF models follow a similar behaviour to those in uniformly rotating polytropic models, provided that the rotation profile is not too differential. Pulsation modes fall into different categories, the three main ones being island, chaotic, and whispering gallery modes, which are rotating counterparts to modes with low, medium, and high l-|m| values, respectively. The frequencies of the island modes follow an asymptotic pattern quite similar to what was found for polytropic models. Extending this asymptotic formula to higher azimuthal orders reveals more subtle behaviour as a function of m and provides a first estimate of the average advection of pulsation modes by rotation. Further calculations based on a variational principle confirm this estimate and provide rotation kernels that could be used in inversion methods. When the rotation profile becomes highly differential, it becomes more and more difficult to find island and whispering gallery modes at low azimuthal orders. At high azimuthal orders, whispering gallery modes, and in some cases island modes, reappear.
Conclusions: The asymptotic formula found for frequencies of island modes can potentially serve as the basis of a mode identification scheme in rapidly rotating stars when the rotation profile is not too differential. 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 Msolar 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 Msolar. The models were constructed using a code based on a reformulation of the self-consistent field method of computing the equilibrium stellar structure for a specified conservative internal rotation law. Relative to nonrotating stars of the same mass, these models all have reduced luminosities and effective temperatures, and flattened photospheric shapes (i.e., decreased polar radii) with equatorial radii that can be larger or smaller, depending on the degree of differential rotation. For a fixed ratio of the axial rotation rate to the surface equatorial rotation rate, increasingly rapid rotation generally deepens convective envelopes, shrinks convective cores, and can lead to the presence of a convective core (envelope) in a 1 (2) Msolar model, a feature that is absent in a nonrotating star of the same mass. The positions of differentially rotating models for a given mass in the H-R diagram can be shifted in such a way as to approximate the nonrotating ZAMS for lower mass stars. Implications of these results include (1) possible ambiguities arising from similarities between the properties of rotating and nonrotating models of different masses, (2) a reduced radiative luminosity for a young, rapidly rotating Sun, (3) modified rates of lithium destruction by nuclear processes in the layers beneath an outer convective envelope, and (4) the excitation of solar-like oscillations and the operation of a solar-like hydromagnetic dynamo in some 1.5-2 Msolar stars. 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 Msolar. Moreover, the method converges for values of the parameter t=T/|W| (the ratio of rotational kinetic energy to gravitational potential energy) that are at least as high as those obtained by Clement's relaxation technique. The method is also capable of producing models with deep concavities about the poles as well as models with extreme oblateness (far greater than that possible in uniformly rotating stars). For cases with moderate degrees of differential rotation (say for Ω0/Ωe<10, where Ω0 and Ωe denote the angular velocity at the center and at the equator, respectively), the method has been found to be remarkably robust. For higher degrees of differential rotation, models are restricted to a portion of parameter space away from two regions of nonconvergence, inside which some of the models evidently develop toroidal level surfaces. 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.

Jackson, S., Macgregor, K. B. & Skumanich, A., in preparation

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 & 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. & 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 Cij given by the product of the profile amplitudes at wavelengths λ i and λ j averaged over all profile samples (either over space or time). The matrix is 'diagonalized' and ordered by Singular Value Decomposition. The resulting orthonormal eigenfunctions over wavelength space are then used as a basis for the expansion of the observed profiles at each space(time) point. The eigenvalue ordering is by magnitude of the mean square over space(time) of the expansion coefficients and is given by the product of the frequency of occurrence of the particular eigenfunction and its intrinsic mean square amplitude. An error based truncation scheme yields a coefficient set which is a compression of the original data set. We have applied the PCA method to the 40,000 profiles for each Stokes component for a Solar active region. In the case of the intensity profile we find that the expansion appears to be similar to a Taylor series with the 0th efunction being the zero order term, the 1st efunction as the first derivative of the zero term and the 2nd efunction as the second derivative term. Thus we derive a line-of-sight velocity from the coefficients of the first derivative term and a magnetic signature, using the weak field Milne-Eddington approximation, from the second. A comparison with a Stokes profile inversion shows that the thus estimated velocity and magnetic parameters are in good agreement with the more time consuming profile fitting values but do show a "roll-off" for sufficiently large values. One also finds that the bright 'quiet' Sun points have an upflow while the dark have a downflow similar to that derived by other analysis. 1) Rees, D., López Ariste, A., Thatcher, J. & Semel, M. 2000, A & 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 logτ5=0 and logτ5=-2.8 are considered to provide accurate information on the physical parameters. All over the penumbra γ increases with depth, while B is larger at the bottom layers of the inner penumbra (as in the umbra) but larger at the top layers of the outer penumbra (as in the canopy). The corrugation of the penumbral magnetic field already observed by other authors has been confirmed by our different inversion technique. Such a corrugation is especially evident in the zenith angle maps of the intermediate layers, featuring the presence of the so-called spines that we further characterize: spines are warmer and have a less inclined magnetic field than the spaces between them and tend to have a smaller gradient of γ with optical depth over the entire penumbra, but with a field strength which is locally stronger in the middle penumbra and locally weaker in the outer penumbra and beyond in the canopy. In the lower layers of these external parts of the sunspot, most of the field lines are seen to return to the solar surface, a result that is closely connected with the Evershed effect (e.g., Westendorp et al., the third paper in this series). The Stokes V net area asymmetry map as well as the average B, γ, and T radial distributions (and that of the line-of-sight velocities; see the third paper in this series) show a border between an inner and an outer penumbra with different three-dimensional structure. We suggest that it is in this middle zone where most of a new family of penumbral flux tubes (some of them with Evershed flow) emerge interlaced (both horizontally and vertically) among themselves and with the ``background'' magnetic field of the penumbra. The interlacing along the line of sight is witnessed by the indication of many points in the outer penumbra showing rapid transitions with height between two structures, one with very weak and inclined magnetic field at the bottom of the photosphere and the other with a stronger and less inclined magnetic field. Over the whole penumbra, and at all optical layers, a constant but weak deviation from radiality of some 5° is detected for the azimuth of the vector magnetic field, which may be in agreement with former detections but which is not significantly higher than the size of the errors for this parameter. 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 & 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 & 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 (∇×Bh)z/Bz=μ0Jz/Bz, and its derivation from vector magnetograph data. This parameter can be evaluated at each spatial point where the vector B is measured, but one may also calculate a single value of α to describe the active region as a whole, here called 'αAR'. We test three methods to calculate such a parameter, examine the influence of data noise on the results, and discuss the limitations associated with assigning such a quantity. The three methods discussed are (1) to parameterize the distribution of α(x,y) using moments of its distribution, (2) to determine the slope of the function Jz(x,y)=αARBz(x,y) using a least-squares fit and (3) to determine the value of α for which the horizontal field from a constant-α force-free solution most closely matches the observed horizontal magnetic field. The results are qualitatively encouraging: between methods, the resulting value of the αARparameter is often consistent to within the uncertainties, even though the resulting αARcan differ in magnitude, and in some cases in sign as well. The worst discrepancies occur when a minimal noise threshold is adopted for the data. When the calculations are restricted to detections of 3σ or better, there is, in fact, fair quantitative agreement between the three methods. Still, direct comparison of different active regions using disparate methods must be carried out with caution. The discrepancies, agreements, and overall robustness of the different methods are discussed. The effects of instrumental limitations (spatial resolution and a restricted field-of-view) on an active-region αAR, and quantifying the validity of αAR, are addressed in Paper II (Leka, 1999). Title: The Evolution of Photospheric Magnetic Structures in Terms of the Size-Flux Relationship Authors: Skumanich, A. 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, Ic, magnetic field magnitude, | B |, magnetic fill fraction, f, local inclination, γ, local azimuth, φ, and line-of-sight Doppler velocity, vD. The highlights of our results include, (1) in the pore-forming region, a preference for the relatively strong vertical field points (1000-1500 G) to be redshifted (downflowing) and the appearance of dark pore points to be at the expense of radiatively undisturbed points; (2) an onset flux of ~2 × 1019 Mx in a dark pore that appears in an area in which the flux increases by ~1 × 1020 Mx prior to the pore's appearance and that previously contained an azimuth center, i.e., a magnetic concentration that otherwise has no continuum-intensity signature; (3) a threshold of (1-1.5) × 1020 Mx above which a partial penumbra forms; (4) the appearance of penumbral elements that at once having the appropriate penumbral range of field strengths, intensities, and inclination angles, i.e., no obvious gradual increase in the magnetic fields' inclination with increasing flux during the formation of penumbrae; (5) no delay between the appearance of inclined penumbral magnetic fields and the Evershed flow; and (6) a self-similarity between the pore, protospot, and a small mature sunspot with respect to the (| B |, Ic), (| B |, γ), (f, γ), and the (vD, Ic), (vD, γ) distributions. Title: Vector magnetic fields of emerging solar flux. I. Properties at the site of emergence Authors: Lites, B. W.; Skumanich, A.; Martinez Pillet, V. 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 & Lites and the SIR (Stokes Inversion based on Response Functions) proposed by Ruiz Cobo & 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 & del Toro Iniesta and through the concept of height of formation for inferred values proposed by Sánchez Almeida, Ruiz Cobo, & del Toro Iniesta.

The present comparison and verification of the reliability of inversion methods is a natural first step toward the ongoing analysis of the three-dimensional magnetic structure of a sunspot. By using SIR (with ME results for initialization) on maps of a whole sunspot observed by the Advanced Stokes Polarimeter, we obtain maps at different optical layers (i.e., an optical tomography) of the temperature, vector magnetic field, and LOS velocity. Such a tomography will appear in subsequent papers of the present series. To illustrate fits to the observed Stokes profiles, we show here actual inversion results for three points observed within a sunspot: one within the umbra, another from the outermost parts of the penumbra, and a third from the magnetic canopy surrounding the sunspot. Title: Vector Magnetic Fields of Emerging Solar Flux Authors: Martínez Pillet, V.; Lites, B. W.; Skumanich, A. 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 × 10-3Ic against an instrumentally produced background polarization of ~=1-5 × 10-2Ic. Net-polarization signals smaller than ~=3 × 10-4Ic are lost, even with sufficient averaging, in a background due to photometric and other calibration errors. Title: On The Detection of Currents and Parker's Objection Authors: Skumanich, A.; Semel, M. 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 < 10°, no preferred east-west orientation, and f = 15%. At disk center, there is a component of weak (<1000 G), more horizontal fields that corresponds to arching field lines connecting footpoints of different polarities. The center-to-limb variation (CLV) of the field strength shows that, close to the limb (μ = 0.3), the field strength is reduced to 800 G from its disk-center value. This can be interpreted as a gradient of B with height in solar plages of around -3 G km-1. From this CLV study, we also deduce that magnetic field lines remain vertical for the entire range of heights involved. A similar analysis is performed for structures found in active regions that show a continuous distribution of azimuths (resembling sunspots) but that do not have a darkening in continuum. These ``azimuth centers'' show slightly larger values of B than normal plages, in particular at their magnetic center. Filling factors are also larger on average for these structures.

The velocities in the magnetic component of active regions have been studied for both averaged Stokes profiles over the entire active region and for the spatially resolved data. The averaged profiles (more representative of high filling factor regions) do not show any significant mean velocities. However, the spatial average of Doppler velocities derived from the spatially resolved profiles (i.e., unweighted by filling factor) show a net redshift at disk center of 200 m s-1. The spatially resolved velocities show a strong dependence on filling factor. Both mean velocities and standard deviations are reduced when the filling factor increases. This is interpreted as a reduction of the p-mode amplitude within the magnetic component. Strong evidence for velocities transverse to the magnetic field lines has been found. Typical rms values are between 200 and 300 m s-1, depending on the filling factor. The possible importance of these transverse motions for the dynamics of the upper atmospheric layers is discussed.

The asymmetries of the Stokes profiles and their CLV have been studied. The averaged Stokes V profiles show amplitude and area asymmetries that are positive at disk center and become negative at the limb. Both asymmetries, and for the two Fe I lines, are maximized away from disk center. The spatially resolved amplitude asymmetries show a clear dependence on filling factor: the larger the filling factor, the smaller the amplitude asymmetry. On the other hand, the area asymmetry is almost independent of the filling factor. The only observed dependence is the existence of negative area-asymmetry profiles at disk center for filling factors smaller than 0.2. Around 20% of the observed points in a given plage have negative area asymmetry. The amplitude asymmetry of Stokes V is, on the other hand, always positive. The amplitude asymmetries of the linear polarization profiles are observed to have the same sign as the Stokes V profiles. Similarly, the same CLV variation of the linear polarization amplitude asymmetries as for Stokes V has been found. The scenarios in which this similarity can exist are studied in some detail. Title: Inversion Techniques Applied to Sunspot Spectropolarimetric Data Authors: Westendorp Plaza, C.; del Toro Iniesta, J. C.; Ruiz Cobo, B.; Martinez Pillet, V.; Lites, B. W.; Skumanich, A. 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 1024 Mx of magnetic flux to the surface on a daily basis. However, further observational study is needed to identify the specific nature of this phenomenon. Title: Field Azimuth Disambiguation Using Ambiguity-Free Currents Authors: Skumanich, A.; Semel, M. 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/m2, or 0.15% below its mean value of 1367.54 W/m2 measured during the period 1980 to 1986 by the ACRIM experiment. Observations of extant solar-type stars suggest that the Ca II surrogate vas darker during the Maunder Minimum. Allowing for this, we estimate the total solar irradiance to be 1364.28 W/m2 or 0.24% below its mean value for the 1980 to 1986 period. The decrease in the global equilibrium temperature of the Earth due to a decrease of 0.24% in total solar irradiance lies in the range from 0.2° C to 0.6° C, which can be compared with the approximately 1° C cooling experienced during the Little Ice Age, relative to the present. Title: Estimating the sun's radiative output during the Maunder Minimum Authors: Lean, Judith; Skumanich, Andrew; White, Oran 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 <= 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 <= 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 τ0 ≈ 102. The authors give a simplified model that demonstrates the penetration of Lyα into the lower chromosphere below the creation region, i.e., for τ0 > 102. A second-order escape probability approximation yields a simple analytical expression which accurately reproduces the behavior of the Lyα source function for 102 ⪉ τ0 ⪉ 5×106 provided the destruction probability, ɛ is set close to 10-6. Title: Stokes Profile Analysis and Vector Magnetic Fields. I. Inversion of Photospheric Lines Authors: Skumanich, A.; Lites, B. W. 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 r0 ≅ 102. 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.

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 &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 & 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 2.32×1011 photons/cm2/s. This value occurred during 1975 as well as in 1976 near the commencement of solar cycle 21. The model predicts that the Lyman alpha flux increases to as much as 5×1011 photons/cm2/s at the maximum of the solar cycle. The ratio of the average fluxes for December 1979 (cycle maximum) and July 1976 (cycle minimum) is 1.9. During solar maximum the 27-day solar rotation is shown to cause the Lyman alpha flux to vary by as much as 40% or as little as 5%. The model also shows that the Lyman alpha flux varies over intermediate time periods of 2 to 3 years, as well as over the 11-year sunspot cycle. We conclude that, unlike the sunspot number and the 10.7-cm radio flux, the Lyman alpha flux had a variability that was approximately the same during each of the past three cycles. Lyman alpha fluxes calculated by the model are consistent with measurements of the Lyman alpha flux made by 11 of a total of 14 rocket experiments conducted during the period 1969-1980. The model explains satisfactorily the absolute magnitude, long-term trends, and the cycle variability seen in the Lyman alpha irradiances by the OSO 5 satellite experiment. The 27-day variability observed by the AE-E satellite experiment is well reproduced. However, the magntidue of the AE-E 1 Lyman alpha irradiances are higher than the model calculations by between 40% and 80%. We suggest that the assumed calibration of the AE-E irradiances is in error. Title: Rapid rotation and stellar activity in the triple system HD 165590. Authors: Stern, R. A.; Skumanich, A. 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+ (K, H), Mg+ (k, h), hydrogen (Lalpha , Lbeta ), and O VI from OSO-8 (LPSP). Authors: Artzner, G.; Skumanich, A.; Bonnet, R. M.; Vidal-Madjar, A.; Leibacher, J.; Lemaire, P.; Vial, J. C. 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+ H and K, Mg+ h and k, and Lyman alpha and beta (H I) resonance lines as observed from OSO-8 (LPSP). Authors: Bonnet, R. M.; Artzner, G.; Leibacher, J.; Lemaire, P.; Skumanich, A.; Vial, J. C.; Vidal-Madjar, A. 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+ 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 > 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 >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 K2 and K3 components of the solar Ca II K line using an optically thick model of the chromosphere. The center-limb variations are shown to require an increase of Doppler width with height in the chromosphere and to depend critically upon the location of the point where ΔλD has increased by a factor e. Good agreement with observations is found when, and only when, the increase in ΔλD occurs nearly simultaneously with the increase in chromospheric temperature. Title: Non-Radial Oscillations of Slowly Rotating Polytropes Authors: Durney, B.; Skumanich, A. 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 < 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 r0 are prescribed as functions of τ from a density model of the atmosphere. By comparing computed and observed profiles of the K3, K2 and inner K1 components of the lines we determine both the approximate depth variation of ΔλD and the best of the temperature models. We find that the Doppler width increases rapidly with height in the chromosphere beginning from a value of 1.6 km/sec at τ0 ≈ 10−2. This latter value corresponds closely to the thermal velocity of Mg atoms in the upper photosphere. The preferred temperature model is one for which the temperature minimum occurs near τ0(λ 2800) ≈ 10−4-10−5 with a value Tmin≲ 4200 ° and which has a temperatu near 7000 ° at τ0 = 10−6 where K2 is formed. The intensity in K3 is determined largely by dΔλD/dτ in the K2 and K3 regions. Title: On the Empirical Determination of Doppler Widths. Authors: Skumanich, A. 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 <a-') it is the chromospheric thickness at line center and a, the damping constant. For a "wing" controlled line (i.e., t> 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 > 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 > 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