Author name code: zwaan ADS astronomy entries on 2022-09-14 author:"Zwaan, Cornelis" OR author:"Zwaan, Kees" ------------------------------------------------------------------------ Title: Solar and Stellar Magnetic Activity Authors: Schrijver, C. J.; Zwaan, C. Bibcode: 2008ssma.book.....S Altcode: 1. Introduction: solar features and terminology; 2. Stellar structure; 3. Solar rotation and meridional flow; 4. Solar magnetic structure; 5. Solar magnetic configurations; 6. Global properties of the solar magnetic field; 7. The solar dynamo; 8. The solar outer atmosphere; 9. Stellar outer atmospheres; 10. Mechanisms of outer-atmospheric heating; 11. Activity and stellar properties; 12. Stellar magnetic phenomena; 13. Activity and rotation on evolutionary time scales; 14. Activity in binary stars; 15. Propositions on stellar dynamos; Appendix I: unit conversions; Index. Title: VizieR Online Data Catalog: Ca II H and K Measurements Made at MWO (Duncan+ 1991) Authors: Duncan, D. K.; Vaughan, A. H.; Wilson, O. C.; Preston, G. W.; Frazer, J.; Lanning, H. H.; Misch, A.; Mueller, J.; Soyumer, D.; Woodard, L.; Baliunas, S. L.; Noyes, R. W.; Hartmann, L. W.; Porter, A.; Zwaan, K.; Middelkoop, F.; Rutter, R.; Mihalas, D. Bibcode: 2005yCat.3159....0D Altcode: Summaries are presented of the photoelectric measurements of stellar CaII H and K line intensity made at Mount Wilson Observatory during the years 1966-1983. These results are derived from 65,263 individual observations of 1296 stars. For each star, for each observing season, the maximum, minimum, mean, and variation of the instrumental H and K index "S" are given, as well as a measurement of the accuracy of observation. A total of 3110 seasonal summaries are reported.

These observations were obtained with two instruments, HKP-1 and HKP-2. The HKP-2 instrument is a four-channel chopping spectrometer which records counts in 1.09{AA} FWHM triangular bandpasses centered in the H and K lines as well as in two 20{AA} reference bandpasses centered on 3901.067 and 4001.067{AA}. The stellar activity is expressed by the index S defined as

S = {alpha} (Nh+Nk)/(Nr+Nv)

where Nh and Nk are the counts (corrected from background) in the H and K lines, Nr and Nv those in the reference continuum bandpasses, and {alpha} is a constant of proportionality used to correct for night-to-night instrumental variations. Higher values of S generally correspond to higher levels of chromospehric activities.

Factors which effect the ability to detect stellar activity variations and accurately measure their amplitudes such as the accuracy of the H and K measurements and scattered light contamination are discussed. Relations are given which facilitate intercomparison of "S" values with residual intensities from ordinary spectrophotometry, and for converting measurements to absolute fluxes.

(1 data file). Title: Origin and Evolution of Filament-Prominence Systems Authors: Martens, Petrus C.; Zwaan, Cornelis Bibcode: 2001ApJ...558..872M Altcode: We present a ``head-to-tail'' linkage model for the formation, evolution, and eruption of solar filaments. The magnetic field structure of our model is based on the observation that filaments form exclusively in filament channels with no apparent magnetic connections above the polarity inversion line. The formation of a filament in this configuration is driven by flux convergence and cancellation, which produces looplike filament segments with a half-turn. Filament segments of like chirality may connect and form long quiescent filaments. Such filaments are stabilized through footpoint anchoring until further cancellation at the footpoints causes their eruption. The eruption restores the original filament channel so that filament formation may resume immediately. We then demonstrate that the combined workings of Hale's polarity law, Joy's law, and differential rotation introduce a strong hemispheric preference in the chirality of filaments formed poleward of the sunspot belt, which is in agreement with observations. We analyze the magnetic fine structure of filaments formed through our model and find consistency with the observed hemispheric preference for barb orientation and a simple explanation for barb formation. Finally, we consider the flux tubes retracted below the surface in the process of filament formation. We show that every cancellation event that generates a filament obeying the hemispheric chirality preference injects a flux tube below the surface with a poloidal field opposite that of the ongoing cycle. We suggest that this pattern of submergence of flux represents the specific mechanism for the reversal of the poloidal flux in a Babcock-Leighton-Durney-type model for the solar dynamo. Title: Solar and stellar magnetic activity Authors: Schrijver, Carollus J.; Zwaan, Cornelis; Priest, Eric R. Bibcode: 2001PhT....54i..54S Altcode: No abstract at ADS Title: Sources of the Slowly-Varying Component of Solar Microwave Emission and their Relationship with their Host Active Regions Authors: Tapping, K. F.; Zwaan, C. Bibcode: 2001SoPh..199..317T Altcode: Daily surveys of the solar disc made at 2.8 cm wavelength over the period 1-13 November 1981, complemented by magnetograms and Hα filtergrams, are used to examine the relationship between sources of the slowly varying component of solar radio emission and properties of their host regions. Two classes of source are noted: diffuse and compact. Sources are designated compact when smaller than 40''. The diffuse sources may be explained in terms of free - free thermal emission from trapped plasma in loops overlying the active region. The great majority of compact sources can be accounted for in terms of gyroresonance from thermal electrons in the strong magnetic fields overlying sunspots. A small minority are less amenable to this explanation. They are associated with magnetic complexity and dynamism, lie close to magnetic polarity reversals, and could be non-thermal. Microwave sources are an evolutionary feature common to all but the smallest active regions. Title: Solar and Stellar Magnetic Activity Authors: Schrijver, Carolus J.; Zwaan, Cornelis Bibcode: 2000ssma.book.....S Altcode: Magnetic activity in the sun and similar stars results in a wealth of phenomena--including starspots, nonradiatively heated outer atmospheres, activity cycles, deceleration of rotation rates, and even, in close binaries, stellar cannibalism. This volume uniquely combines studies of the sun with those of other stars to provide a comprehensive picture of stellar magnetic activity. Coverage brings together recent results in solar and stellar studies to provide an illuminating, new view of the subject. Key topics include radiative transfer, convective simulations, dynamo theory, outer-atmospheric heating, stellar winds and angular momentum loss. Researchers are provided with a state-of-the-art review of this exciting field. Graduate students in astrophysics and plasma physics will find the volume an ideal introduction to the subject. The book will also interest researchers studying the connection of solar Sctivity with the Earth's climate change, such as geologists, planetary scientists and atmospheric scientists. Title: Solar and stellar magnetic activity. Authors: Schrijver, C. J.; Zwaan, C. Bibcode: 2000CAS....34.....S Altcode: This timely volume provides the first comprehensive review and synthesis of the current understanding of the origin, evolution, and effects of magnetic fields in the Sun and other cool stars. Magnetic activity results in a wealth of phenomena - including starspots, nonradiatively heated outer atmospheres, activity cycles, deceleration of rotation rates, and even, in close binaries, stellar cannibalism - all of which are covered clearly and authoritatively. This book brings together for the first time recent results in solar studies, with their wealth of observational detail, and stellar studies, which allow the study of how activity evolves and depends on the mass, age, and chemical composition of stars. The result is an illuminating and comprehensive view of stellar magnetic activity. Observational data are interpreted by using the latest models in convective simulations, dynamo theory, outer-atmospheric heating, stellar winds, and angular momentum loss. Title: Phenomena in an Emerging Active Region. II. Properties of the Dynamic Small-Scale Structure Authors: Strous, Louis H.; Zwaan, Cornelis Bibcode: 1999ApJ...527..435S Altcode: The magnetic flux emergence in growing active region NOAA 5617, when it is about 8 hr old, shows an intricate fine structure. The small-scale emergence events are characterized by a coincident upflow and transient darkening (of about 2 Mm and 10 minutes) in the continuum and line-center intensity followed by the appearance of one, or in some cases two, new bright grains flanking the line-center darkening. The bright grains (faculae) coincide with magnetic flux concentrations and downflows. The footpoints move apart at on average 1.4 km s-1. Flux emergence happens recurrently in a number of locations widely distributed over the active region, which appear to form a pattern with a wavelength of about 8 Mm. A preferred orientation that fits Hale's polarity law is displayed by the spatial pattern in the emergence locations, the emergence events themselves, subsequent footpoint motion, and the Hα arch filament system. We find long (~15 Mm) alignments of unipolar faculae of each magnetic polarity that also follow the preferred orientation. We adapt the model for flux emergence to accommodate the observed dynamic fine structure. Essential new features are (1) the emerging bundle of flux tubes is frayed in two systems, in vertical stacks, arranged in slightly curved, nearly parallel sheets; and (2) many flux tubes emerge in multiple locations. Title: Studies of changing patterns of solar activity using the DRAO Synthesis Radio Telescope. Authors: Tapping, K.; Burke, I.; Cameron, H.; Harvey, K.; Zwaan, C. Bibcode: 1999JRASC..93Q.186T Altcode: No abstract at ADS Title: Properties of Small-Scale Flux Emergence in a Young Active Region Authors: Strous, Louis; Zwaan, Cornelis Bibcode: 1999soho....9E..82S Altcode: The magnetic flux emergence in growing active region NOAA 5617, when it is about 8 hours old, shows an intricate fine structure. The small-scale emergence events are characterized by a coincident upflow and transient darkening (of about 2 Mm and 10 minutes) in the continuum and line-center intensity, followed by the appearance of one, or in some cases two, new bright grains flanking the line-center darkening. The bright grains (faculae) coincide with magnetic flux concentrations and downflows. The footpoints move apart at on average 1.4 km/s. Flux emergence happens recurrently in a number of locations widely distributed over the active region, which appear to form a pattern with a wavelength of about 8 Mm. A preferred orientation which fits Hale's polarity law is displayed by the spatial pattern in the emergence locations, the emergence events themselves, subsequent footpoint motion, and the H-alpha arch filament system. We find long (~ 15 Mm) alignments of unipolar faculae of each magnetic polarity that also follow the preferred orientation. We adapt the model for flux emergence to accommodate the observed dynamic fine structure. Essential new features are: (1) the emerging bundle of flux tubes is frayed in two systems; in vertical stacks, arranged in slightly curved, nearly parallel sheets, and (2) many flux tubes emerge in multiple locations. Title: Coronal and chromospheric emission from cool stars in near-simultaneous ROSAT all-sky survey and Mount Wilson data. Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.; Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C. Bibcode: 1997A&A...325.1115P Altcode: Mt. Wilson Ca II H&K line-core emission fluxes for 215 F-, G- and K-type stars were obtained within at most a few days of the corresponding ROSAT All-Sky Survey observations. These stars cover wide ranges of stellar activity, spectral type and luminosity class. In this paper we study the well-known relationship between the Ca II H&K line-core emission in excess of the minimum emission and the soft X-ray emission. We find that flux densities normalised with the bolometric flux densities are the best quantity in which to express activity when comparing radiative emission in different temperature regimes. We find a power-law relationship, in which the X-ray normalised emission varies approximately quadratically with the normalised excess Ca II H&K line-core emission. This relationship does not depend on luminosity class at least up to luminosity class III, and it does not depend on effective temperature. The scatter around this relationship is consistent with the measurement errors. The X-ray spectral hardness ratios of main-sequence stars increase with the X-ray flux densities; a similar trend, but with substantially larger scatter, is also present for evolved stars. A comparison between values from different passbands of the Mt. Wilson HK spectrophotometer shows that relatively hot stars ((B-V)<=0.50) appear to have a Ca II line core emission peak about a factor 2 to 3 wider than cooler stars. Title: VizieR Online Data Catalog: Near-simultaneous ROSAT and Mt Wilson data (Piters+ 1997) Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.; Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C. Bibcode: 1997yCat..33251115P Altcode: Table 1 lists near-simultaneous X-ray data and Ca II H&K line-core emission data from the ROSAT All-Sky Survey and from the Mt. Wilson H&K spectrometer, respectively. The stars in this sample are 215 bright F-, G-, and K-type stars. Table 2 lists the derived excess Ca II H&K line-core and the X-ray flux densities for the same stars. (2 data files). Title: Why and Where do Filaments Form in Active Regions? Authors: Gaizauskas, V.; Zwaan, C. Bibcode: 1997SPD....28.0249G Altcode: 1997BAAS...29..902G New prominence models based on recent observations depend upon magnetic reconnection between small-scale magnetic elements converging at a polarity inversion (PI). How then to explain active-region filaments where magnetic flux diverges over much of the lifetime of the region? A partial answer is that still-growing active regions containing filaments are not simple bipolar entities. They are instead multipolar activity complexes (`sunspot nests') wherein magnetic flux can be compressed along a meandering PI wherever new bipolar units emerge near old ones. A complete answer requires particulars about the distribution and motions of magnetic fields internal and external to the sunspot nests. We therefore surveyed over 150 active regions photographed on a large spatial scale at ORSO during 5 successive solar rotations in 1979, an epoch of rapid emergence and decay. Of the total number of regions: - 5% are simple decaying bipolar plages with filaments on the PI; - 5% are ambiguous cases with sometimes a filament and field transition arches (FTA) sharing adjacent parts of a PI in a bipolar plage; -70% have boundary filaments exterior to the concentrations of magnetic flux around sunspots; - 61% are single bipoles of which 84% have no internal filament on their PI; - 52% are activity complexes (on at least one day, otherwise they are single bipoles) of which 60% have one or more filaments inside the complex. We find that filaments inside sunspot nests mark off bipolar entities from one another, thus fulfilling the role of boundary filaments on the inside of the nests. We conclude that the boundary filament is the quintessential active- region filament. Examination of specific cases leads to the further conclusion that force-free fields together with cancelling flux play a critical role in forming boundary filaments. Title: A Dynamo Scenario|Observational Constraints on Dynamo Theory Authors: Zwaan, Cornelis Bibcode: 1996SoPh..169..265Z Altcode: In this paper, the term dynamo refers to the complex of physical mechanisms that cause solar magnetic activity in all its manifestations. Properties of that dynamo are inferred from observational indications to fit them into a scenario. Properties and models of the manifestations of strong magnetic field are briefly summarized, together with their formation during the emergence of ω-shaped loops from the toroidal flux system in the interface below the convection zone. The evolution of magnetic concentrations and the flux removal from the atmosphere, with indications for flux retraction, are considered. Then the weak (INF) fields are discussed, together with the role of upward floating LI- shaped loops in the removal of toroidal flux. Finally features of strong and weak fields are fitted into a scenario for a cyclic dynamo, in which the regeneration of the poloidal field of proper sign relies on the cancellation of magnetic flux in the surface transport interpreted as reconnection, followed by retraction of reconnected loops. Title: Book reviews Authors: Humphreys, R. M.; Kemp, S.; Savonije, G.; van der Hucht, K. A.; van der Kruit, P. C.; Miley, G.; Bumba, V.; van Nieuwkoop, J.; van Hoolst, T.; Cox, A.; Rutten, R. J.; Kleczek, J.; de Jager, Cornelis; Jerzykiewicz, M.; Zwaan, C.; Poedts, S.; Sakai, Jun-Ichi; Pecker, J. -C.; Heikkila, W.; de Jong, T.; Wilson, P. R.; Müller, E. A.; Hoyng, P.; Icke, V.; Shore, S. N.; Achterberg, A.; Lucchin, F.; Butcher, H.; Ne'Eman, Y.; Heidmann, J.; Belton, M. J. S.; de Graauw, Th.; Waters, L. B. F. M.; Pacini, F.; Hultqvist, B.; Akasofu, S. -I.; Vial, J. -C.; Schatzman, E.; van der Laan, H.; Cole, K. D.; Vanbeveren, D.; Southwood, D.; van der Klis, M.; Katgert, Peter Bibcode: 1996SSRv...76..339H Altcode: No abstract at ADS Title: Phenomena in an emerging active region. I. Horizontal dynamics. Authors: Strous, L. H.; Scharmer, G.; Tarbell, T. D.; Title, A. M.; Zwaan, C. Bibcode: 1996A&A...306..947S Altcode: Horizontal dynamics in observations of NOAA AR 5617 are studied by tracking individual elements through the field of view. Small magnetic elements of both magnetic polarities occur everywhere in the active region, and define unipolar thread-like concentrations of magnetic field of up to 15Mm length. The horizontal granular flow field in the active region is divergent (e-time scale 2.1hours) and clockwise (time scale 32hours). Facular elements are tracers of (clumps of) fluxtubes. A hierarchy of movement of magnetic elements appears: Facular elements everywhere in the active region move obliquely toward the edges of the active region of the same polarity as their own, faster than those edges (as defined by strings of pores) move apart. The pores move along the edges toward the major sunspots of their own polarity, and the major sunspots of either polarity move apart. The separation velocity of both polarities of facular elements is about 0.84km/s, of pores about 0.73km/s, and that of the major sunspots is about 0.50km/s. Title: Patterns in the solar magnetic field (Invited Review) Authors: Zwaan, C.; Harvey, K. L. Bibcode: 1994smf..conf...27Z Altcode: No abstract at ADS Title: The Sun among the stars Authors: Zwaan, C. Bibcode: 1994ASIC..433....3Z Altcode: No abstract at ADS Title: Properties and Emergence Patterns of Bipolar Active Regions - Part One Authors: Harvey, Karen L.; Zwaan, Cornelis Bibcode: 1993SoPh..148...85H Altcode: Patterns in the properties of bipolar active regions are determined throughout Cycle 21. Active regions that emerged on the visible hemisphere were identified on NSO/KP full-disk magnetograms during 29 solar rotations selected from 1975 through 1986. The bipolar active regions are included only once in this sampling; their properties are derived at the time of maximum development. In order to study an unbiased sample over the entire range of areas larger than 2.5 square degrees (or 373 Mm2), their counts are corrected for size-dependent effects that reduce the chance of their identifications. Title: Het wisselende aangezicht van zon en sterren. Authors: Schrijver, K.; Zwaan, K. Bibcode: 1993Zenit..20..292S Altcode: No abstract at ADS Title: Inzicht uit licht. (2) De spectrale streepjescode en nog wat. Authors: Zwaan, C. Bibcode: 1993Zenit..20..132Z Altcode: No abstract at ADS Title: Inzicht uit licht. 1. Kleuren en temperaturen van hemellichamen. Authors: Zwaan, C. Bibcode: 1993Zenit..20..106Z Altcode: No abstract at ADS Title: Percolation theory and the geometry of photospheric magnetic flux concentrations Authors: Balke, A. C.; Schrijver, C. J.; Zwaan, C.; Tarbell, T. D. Bibcode: 1993SoPh..143..215B Altcode: The magnetic field in solar active regions forms a highly structured pattern without an apparent length scale. We study this pattern in detail for a plage and its surroundings observed with the Swedish Solar Observatory on La Palma. The magnetogram has a resolution of about 1/3″, after image optimisation. We analysed the geometric properties of isolated patches of magnetic flux. Patches with a linear size up to 3″ appear to be statistically self-similar, with a fractal dimension ofDf = 1.54 ± 0.05 for the relation between area and linear size. This value agrees very well with the dimensionDf = 1.56 which is found in percolation theory for clusters of tracers placed randomly on a lattice with a tracer density below a critical threshold. The distribution of observed cluster areas also agrees with that of clusters on such a random lattice. The correspondence between properties of observations and of clusters on randomly filled lattices suggests that- well after emergence - the magnetic flux on the Sun is randomly distributed at least up to sizes of about 3″ and possibly larger. Title: Rotation Rates of Active Nests on the Sun Authors: van Driel-Gesztelyi, L.; van der Zalm, E. B. J.; Zwaan, C. Bibcode: 1993ASPC...46..504V Altcode: 1993mvfs.conf..504V; 1993IAUCo.141..504V No abstract at ADS Title: X-ray/Optical Survey of Late-Type Stars Authors: Piters, A. J. M.; Schmitt, J. H. M. M.; Schrijver, C. J.; Baliunas, S.; Zwaan, C.; van Paradijs, J. Bibcode: 1993ASSL..183..377P Altcode: 1993pssc.symp..377P No abstract at ADS Title: Magnetic struture in cool stars. XVIII. UV-line emission from T Tauri stars. Authors: Lemmens, A. F. P.; Rutten, R. G. M.; Zwaan, C. Bibcode: 1992A&A...257..671L Altcode: The flux densities in the UV spectral emission lines from T Tauri stars are compared with those from other cool stars. In the flux-flux diagrams for pairs of UV emission lines the data points for T Tauri stars extend the relations as defined by main-sequence and evolved stars towards larger flux densities by a factor of 40. We discuss the large emission-line flux densities and the relations between flux densities from different lines in the light of magnetic activity and recent models of accretion disks in T Tauri stars. The very large emission fluxes in chromospheric lines may be explained by a large emitting volume associated with the warm (about 10,000 K) inner disk boundary layer, but the fluxes and the line broadening in the high-excitation lines requires the presence of hotter material, with temperatures of about 200,000 K, which must be attached to the inner disks but which is not predicted by existing disk models. Title: Activity in Tidally Interacting Binaries Authors: Schrijver, C. J.; Zwaan, C. Bibcode: 1992ASPC...26..370S Altcode: 1992csss....7..370S No abstract at ADS Title: The Evolution of Sunspots Authors: Zwaan, Cornelis Bibcode: 1992ASIC..375...75Z Altcode: 1992sto..work...75Z The evolution of sunspots is described in the context of the evolution of active regions. The nesting tendency in formation of sunspots is pointed out. Birth and growth of an active region are attributed to an Omega-shaped loop emerging from a deep-seated toroidal flux strand. It is argued that the field strength in the toroidal strand exceeds the equipartition value; the top of the Omega-loop, however frayed, reaches the upper boundary of the convection zone at intrinsic field strengths close to the local equipartition value. The formation of pores is preceded by the convective collapse of flux tubes. The coalescence of the buoyant tops of flux tubes leads to the growth of large sunspots. Inclusions of bright structure in umbrae, and the formation of dark cores are discussed. Penumbrae form after the spot has reached a critical flux level. Most sunspots decay fast by fragmentation. Spots decaying gradually are surrounded by moat cells, across which magnetic features of both polarities are streaming away from the spot; the decay rate appears to be approximately constant. The removal of magnetic flux from the atmosphere, including the seeming disappearance in situ, is addressed. Finally, the complementary roles of Omega-loops and U-loops in the magnetic structure in convection zone and atmosphere are discussed. Title: Active Nests on the Sun Authors: van Driel-Gesztelyi, Lidia; van der Zalm, Ed B. J.; Zwaan, Cornelis Bibcode: 1992ASPC...27...89V Altcode: 1992socy.work...89V No abstract at ADS Title: Fractals in Magnetograms Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.; Lawrence, J. K. Bibcode: 1992ASPC...27...67S Altcode: 1992socy.work...67S No abstract at ADS Title: Patterns in the photospheric magnetic field and percolation theory Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.; Lawrence, J. K. Bibcode: 1992A&A...253L...1S Altcode: The magnetic field in solar plages forms a highly structured pattern with no apparent characteristic length scale. This pattern appears to be a fractal with a dimension between 1.45 and 1.60. Small-scale displacements of concentrations of magnetic flux in the network are consistent with a random walk on a fractal with a similar dimension. Percolation theory offers an effective explanation for observed geometric properties of small-scale flux concentrations in the solar photosphere, by demonstrating the close correspondence with clusters formed by randomly placed tracers on a 2D (irregular) lattice. Percolation theory also offers a model for the subdiffusive behavior of tracers performing a random walk on clusters formed by bonded sites. The geometry of flux concentrations and of the displacement of magnetic flux as a function of time are equivalent to situations in percolation theory below a critical value, called 'the percolation threshold'. Title: Magnetic structure in cool stars. XVII. Minimum radiative losses from theouter atmosphere. Authors: Rutten, R. G. M.; Schrijver, C. J.; Lemmens, A. F. P.; Zwaan, C. Bibcode: 1991A&A...252..203R Altcode: The emissions in several chromospheric and transition region lines and in coronal soft X-rays are analyzed for a sample of cool stars. The nature of the lower-limit flux densities is explored, and evidence is given for the possibility of a basal, nonmagnetic heating mechanism being responsible for these emission fluxes up to, and perhaps including, the upper transition region. It is argued that the excess flux density, derived by subtraction of the basal flux density from the observed stellar flux, is the proper measure of magnetic activity. The level of the basal flux density as a function of color is determined to be 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sq cm/s for K-type stars. Title: Activity in tidally interacting binaries. Authors: Schrijver, C. J.; Zwaan, C. Bibcode: 1991A&A...251..183S Altcode: A data set encompassing 23 binaries for which essential properties of components and orbits are known is presently used to study the relationship between activity, rotation, and revolution in comparatively close binaries, by comparison to the relationship defined by single stars. Confirmation is obtained for previous reports that the relationships between emissions from the outer stellar atmospheres of the binaries are indistinguishable from those defined by single stars. While the rotation-activity relationship for single stars can be described in terms of fundamental stellar properties, properties of the secondary are important in determining the activity of close binaries. Title: CA II H and K Measurements Made at Mount Wilson Observatory, 1966--1983 Authors: Duncan, Douglas K.; Vaughan, Arthur H.; Wilson, Olin C.; Preston, George W.; Frazer, James; Lanning, Howard; Misch, Anthony; Mueller, Jean; Soyumer, David; Woodard, L.; Baliunas, Sallie L.; Noyes, Robert W.; Hartmann, Lee W.; Porter, Alain; Zwaan, Cornelis; Middelkoop, Frans; Rutten, Rene G. M.; Mihalas, Dimitri Bibcode: 1991ApJS...76..383D Altcode: Summaries are presented of the photoelectric measurements of stellar Ca II H and K line intensity made at Mount Wilson Observatory during the years 1966-1983. These results are derived from 65,263 individual observations of 1296 stars. For each star, for each observing season, the maximum, minimum, mean, and variation of the instrumental H and K index 'S' are given, as well as a measurement of the accuracy of observation. A total of 3110 seasonal summaries are reported. Factors which affect the ability to detect stellar activity variations and accurately measure their amplitudes, such as the accuracy of the H and K measurements and scattered light contamination, are discussed. Relations are given which facilitate intercomparison of 'S' values with residual intensities derived from ordinary spectrophotometry, and for converting measurements to absolute fluxes. Title: Magnetic Activity Across the Hertzsprung-Russell Diagram (With 8 Figures) Authors: Zwaan, C. Bibcode: 1991mcch.conf..241Z Altcode: No abstract at ADS Title: Non-Steady Global Magnetic Fields in Kinematic Theory Authors: van Geffen, J. H. G. M.; Hoyng, P.; Zwaan, C. Bibcode: 1991LNP...380..129V Altcode: 1991IAUCo.130..129V; 1991sacs.coll..129V The dynamo equation for the mean field (B) contains a random forcing term of unknown magnitude, which is therefore always omitted. The influence of this term is potentially large. To evaluate its effect, we employ ensemble averaging. If an ensemble average is used, there is no random forcing term in the dynamo equation. The effect of fluctuations is that the ensemble members get out of phase, so that (B) 0. The damping time of (B) can be found by requiring that the mean energy (BB) remains finite. The eigenvalues of the dynamo equation then all have negative real parts. Im determines the period, and -Re/Im the relative period stability of the dynamo. We have developed a code to solve the equation for (BB) in a spherical shell (the convection zone), assuming axisymmetry. We report our first results, which do not yet include differential rotation. Using spherically symmetric boundary conditions, we reproduce the well known 2-dynamo, whose behaviour is known analytically. For instance, for an 2-dynamo located in a shell with inner boundary at R/2, we find that (BB) remains finite for R2/ = 1.48, where 3 represents turbulent diffusion and .y turbulent vorticity. Taking = 1/4(βy)1/2 — a factor of four below maximum helicity — implies that we have a dynamo number Ca ≡ Rα/β = 0.30. Using this value we find a damping time of 6 X 10-2 R 2/β for (B), which is a measure for the coherence time of B in a single ensemble member. This result implies that the large-scale field of this particular α2-dynamo reorganizes its structure completely on a time scale of only about one year (for solar values of R and β), and it shows the enormous influence of random forcing in general. Title: Activity of Relatively Close Binaries Authors: Schrijver, Carolus J.; Zwaan, Cornelis Bibcode: 1991LNP...380..435S Altcode: 1991sacs.coll..435S; 1991IAUCo.130..435S Whereas the rotation-activity relationship for single cool stars can be described in terms of fundamental stellar properties, the activity in relatively close binaries is enhanced with respect to that of single stars by an amount which depends strongly on the properties of the companion. We consider mechanisms which could cause an excess in activity of binaries, and conclude that it is likely that the presence of a companion affects the interior structure of the stars in such a way that either the efficiency of the dynamo or of the atmospheric heating is enhanced. Title: Sunspot Nests as Traced by a Cluster Analysis Authors: Brouwer, M. P.; Zwaan, C. Bibcode: 1990SoPh..129..221B Altcode: The appearance of sunspot groups between August 1959 and December 1964 is investigated in search of sunspot nests. A sunspot nest is a relatively small space on the surface within which a succession of spot groups appears. A single-linkage clustering technique is used to trace clusters in the three dimensions longitude, latitude and time. The number of genuine sunspot nests is estimated and their properties are determined. Title: Patterns of Activity Authors: Zwaan, C. Bibcode: 1990PDHO....7...67Z Altcode: 1990dysu.conf...67Z; 1990ESPM....6...67Z Patterns in the occurrence of bipolar active regions in the solar atmosphere are discussed. This review focuses on the patterns in the flux emergences, i.e., on patterns seen in young active regions or in sunspots. The reason is that these may yield the most direct information on the dynamo action in the convection zone. Particular attention is paid to studies that try to describe quantitative aspects of the patterns of activity. Title: Unresolved magnetic features: possibilities and limitations of (I,V) diagnostics Authors: Zwaan, C. Bibcode: 1989hsrs.conf..420Z Altcode: No abstract at ADS Title: Magnetic structure in cool stars. XVI. Emissions from the outer atmospheres of M-type dwarfs. Authors: Rutten, R. G. M.; Schrijver, C. J.; Zwaan, C.; Duncan, D. K.; Mewe, R. Bibcode: 1989A&A...219..239R Altcode: Consideration is given to emission from the outer atmospheres of M-type dwarfs in several spectral lines originating from the chromosphere, the transition-region, and the soft X-ray emission from the corona. It is shown that M-type dwarfs systematically deviate from relations between flux densities in soft X-rays and chromospheric and transition-region emission lines. The quantitative relation between the equivalent width of H-alpha and the Ca II, H, and K emission index is determined. It is suggested that the emission in the Balmer spectrum may result from back heating by coronal soft X-rays. Title: Relations between the Photospheric Magnetic Field and the Emission from the Outer Atmospheres of Cool Stars. I. The Solar CA II K Line Core Emission Authors: Schrijver, C. J.; Cote, J.; Zwaan, C.; Saar, S. H. Bibcode: 1989ApJ...337..964S Altcode: Observations of a solar active region complex and its surroundings are used to establish a quantitative relation between the Ca II K line core intensity and magnetic flux density. The Ca II K line core intensity is transformed to a Ca II H + K line core flux density to facilitate a comparison of solar and stellar data. A new absolute calibration for the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The minimum Ca II K flux, found in the centers of supergranulation cells in quiet regions on the sun, is identical to the minimum flux that is observed for solar-type stars. An expression is presented for the nonlinear trend between the Ca II H + K line core excess flux density and the absolute value of the magnetic flux density. Models that explain the nonlinearity of the mean trend and the large intrinsic scatter about it are discussed. The solar data define a relation that is similar to the relation between stellar hemisphere-average magnetic flux densities and Ca II H + K excess flux densities. Title: Asynchronous rotation in close binary systems with circular orbits. Authors: Habets, G. M. H. J.; Zwaan, C. Bibcode: 1989A&A...211...56H Altcode: The origin of asynchronism observed in some binary systems in which the components rotate much more rapidly or much more slowly than the orbital rate was investigated using Pylyser's (1988) evolutionary models of stars of 1.0, 1.5, and 2.0 solar masses. Under the assumption of rigid-body rotation, the evolution of the angular rotation velocity from the main sequence up to the red-giant branch is derived; the orbital circularity of these systems is explained by strong tidal interaction, either during the early premain-sequence phase, or, for systems containing a white dwarf, during the red-giant phase of its progenitor. It is argued that, at the zero-age main sequence (ZAMS), the binary components rotate much faster than the orbital rate. During the evolution off the ZAMS, the angular rotation rate of the components decreases, mainly because of the increase of the moment of inertia, and, possibly, because of magnetic braking. Title: SYSTEMATIC OBSERVATIONS OF THE SUN (In honour of Helen Dodson Prince): Future work Authors: Livingston, W.; Zwaan, K.; Hiei, E.; Paterno, L. Bibcode: 1989HiA.....8..677L Altcode: No abstract at ADS Title: Book Review: Small-scale magnetic flux concentrations in the solar photosphere. / Vandenhoeck & Ruprecht, Gottingen, 1986 Authors: Zwaan, C. Bibcode: 1988SSRv...48..187Z Altcode: 1988SSRv...48..187D No abstract at ADS Title: Joint Discussion on Topics of Sessions 5 and 6 Authors: Zwaan, C.; Maltby, P. Bibcode: 1987rfsm.conf..165Z Altcode: No abstract at ADS Title: Elements and patterns in the solar magnetic field. Authors: Zwaan, Cornelis Bibcode: 1987ARA&A..25...83Z Altcode: This review concentrates on observational studies that may reveal the MHD processes in the solar interior and photosphere that generate and shape the magnetic field. The intrinsically strong magnetic field is contained in seemingly isolated elements, ranging from the thick boundles of flux tubes in sunspots to the hypothetical thin flux fibers. These elements are arranged in the typical patterns observed in active regions and in the magnetic network. The processes of emergence of magnetic flux into the atmosphere and removal of flux from the photosphere are directly related to the magnetic structure and dynamics in the solar convective envelope. Title: Sunspot Nests - Manifestations of Sequences in Magnetic Activity Authors: Castenmiller, M. J. M.; Zwaan, C.; van der Zalm, E. B. J. Bibcode: 1986SoPh..105..237C Altcode: For the period August 1959-December 1964 the Greenwich Photoheliographic Results were searched for sunspot nests. Such a nest is a sequence of sunspot groups that appear within a small area on the solar surface and that last for several months. The search procedure is described and data for 41 probable sunspot nests are given. At least three quarters of these nests appear to be real, and not chance clusters. Title: Relations Between Magnetic Activity and Stellar Properties Authors: Zwaan, Cornelis Bibcode: 1986LNP...254...19Z Altcode: 1986csss....4...19Z This paper discusses the basal emission found in strong chromospheric resonance lines, the activity emission, and the tight power-law relations that exist for the majority of cool stars between the flux densities measured in the various coronal, transition-region and excess chromospheric emissions (the "flux-flux relations"). These relations do not depend on effective temperature and surface gravity. Title: The Emergence of Magnetic Flux Authors: Zwaan, C. Bibcode: 1985SoPh..100..397Z Altcode: This paper first summarizes the morphology and dynamics of emerging flux regions and arch filament systems and then discusses detailed observations of a particular active region with emerging magnetic flux. Title: The Bright Pre-main Sequence Variable HR5999 Authors: The, P. S.; Tjun-A-Djie, H. R. E.; Brown, A.; Catala, C.; Doazan, V.; Linsky, J.; Mewe, R.; Praderie, F.; Talavera, A.; Zwaan, C. Bibcode: 1985IrAJ...17...79T Altcode: A meeting to review the reduced coordinated observations by EXOSAT, IUE and ground-based instruments of the bright and variable Herbig A7e star HR 5999 is summarized. The attempt was to delineate the relationship between the various observable quantities of its atmosphere (colors, emission-line fluxes, wind velocities, etc.) in order to develop a dynamical model of the extended atmosphere. The observations were made around September 11, 1983. Title: A study of ultraviolet and X-ray emissions of selected solar regions Authors: Schrijver, C. J.; Zwaan, C.; Maxson, C. W.; Noyes, R. W. Bibcode: 1985A&A...149..123S Altcode: Apollo Telescope Mount-based Skylab observations of quiet regions, coronal holes, and active regions of various sizes and in various stages of evolution are studied. Total intensities in coronal emissions are noted to increase more rapidly than linearly with total intensities in chromospheric emissions, and areas of active regions determined from chromospheric emissions are found to be systematically larger than areas determined from transition region emissions. The coronal X-ray temperature, as well as the average intensities of active regions, depend on the area of the active region, on Ca II intensity, and on relative coverage by sunspots. Title: Magnetic structure in cool stars. VIII. The MG II H and K surface fluxes in relation to the MT Wilson photometric CA II H and K measurements. Authors: Oranje, B. J.; Zwaan, C. Bibcode: 1985A&A...147..265O Altcode: Data from IUE observations of 14 F, G, and K stars of luminosity II-V in the Mg II h + k lines are presented in tables and compared with published Mt. Wilson photometry of the Ca II H + K lines, and empirical relations are derived to facilitate the use of Ca II H + K data in investigating the chromospheric structure of cool stars. The results are presented in graphs, and consideration is given to the Vaughan-Preston gap for main-sequence stars; the (B-V)-dependent flux minima for main-sequence (LC V) stars, LC IV subgiants, and LC III giants (consistent with a dynamo model with magnetic braking and tidal synchronization); the very large fluxes of FK Com stars; and the positions of the LC II bright giants and LC I supergiants on the chromospheric-flux/color diagram. Title: High Resolution Spectroscopy of Active Regions - Part One - Observing Procedures Authors: Zwaan, C.; Brants, J. J.; Cram, L. E. Bibcode: 1985SoPh...95....3Z Altcode: We describe an observing program designed to obtain spectra of sunspots, pores, and other features in active regions using the Vacuum Tower Telescope and Echelle Spectrograph at Sacramento Peak Observatory. The spectral lines used in this study have been especially chosen to allow pointed studies of fine structure in the intensity distribution, and in the velocity and magnetic fields in the photospheric levels of active regions, and to relate this structure to chromospheric observations made in the CaII H line. We demonstrate the capacities of the observing program by two examples: umbral fine structure, and an emerging active region. Although the umbral spectrograms resolve the brightness structure down to less than one arc sec we do not find clear-cut relations in the spatial variations of brightness, magnetic field strength and line-of-sight velocity across the umbral structure. In the emerging active region several processes are observed in their relationship: the rising and draining of an arch-filament system, the convective collapse of flux tubes, and the growth of a sunspot pore by coalescing fluxtubes. Title: Outer Atmospheres of Evolved Starss of Low Activity Authors: Zwaan, C. Bibcode: 1985iue..prop.2390Z Altcode: No abstract at ADS Title: Atmospheric Fine Structure as a Probe for the Solar Interior Authors: Zwaan, C. Bibcode: 1985LNP...233..263Z Altcode: 1985hrsp.proc..263Z The study of the solar atmosphere's fine structure features promises to deepen insight into the solar interior. Attention is given to the use of magnetic elements as velocity field tracers, the large scale structures of the solar interior's magnetic field, emerging flux regions, the removal of magnetic flux, and the structure and evolution of magnetic elements. The importance of efforts in site testing and high resolution instrumentation development studies is stressed. Title: The Interacting Binary HD 352 (5 CET) Authors: Zwaan, C. Bibcode: 1984iue..prop.2071Z Altcode: No abstract at ADS Title: Book reviews Authors: van der Hucht, K. A.; Pedersen, A.; Kesák, Ľ.; Zwaan, C.; Fárník, F.; Kovalevsky, J.; van Beek, H. F.; Mewe, R.; Page, D. E.; van Genderen, A. M.; Koch-Miramond, L.; de Jager, C.; Ooms, G.; Pedersen, Arne; de Jager, C.; Hultqvist, Bengt; Waters, R.; Schrijver, J. Bibcode: 1984SSRv...38..179V Altcode: No abstract at ADS Title: Chromospheric Transition Region and Coronal Diagnostics for Stellar Magnetic Activity Authors: Schrijver, C. J.; Zwaan, C. Bibcode: 1984srps.conf..291S Altcode: Using various diagnostics magnetic activity has been inferred for the vast majority of stars with convective envelopes. The authors discuss relations between emissions from different temperature regimes, and their dependence on various stellar parameters. They suggest several practical indicators for stellar magnetic structure and acitivity, and outline a few topics for which space observations would be useful. Title: Book-Review - Activity in Red Dwarf Stars Authors: Byrne, P. B.; Rodono, M.; Zwaan, C. Bibcode: 1984SSRv...38..180B Altcode: No abstract at ADS Title: Patterns of the Sun Authors: Gaizauskas, V.; Harvey, K. L.; Harvey, J. W.; Zwaan, C. Bibcode: 1983S&T....66..291G Altcode: No abstract at ADS Title: Large-scale patterns formed by solar active regions during the ascending phase of cycle 21 Authors: Gaizauskas, V.; Harvey, K. L.; Harvey, J. W.; Zwaan, C. Bibcode: 1983ApJ...265.1056G Altcode: Synoptic maps of photospheric magnetic fields prepared at the Kitt Peak National Observatory are used in investigating large-scale patterns in the spatial and temporal distribution of solar active regions for 27 solar rotations between 1977 and 1979. The active regions are found to be distributed in 'complexes of activity' (Bumba and Howard, 1965). With the working definition of a complex of activity based on continuity and proximity of the constituent active regions, the phenomenology of complexes is explored. It is found that complexes of activity form within one month and that they are typically maintained for 3 to 6 solar rotations by fresh injections of magnetic flux. During the active lifetime of a complex of activity, the total magnetic flux in the complex remains steady to within a factor of 2. The magnetic polarities are closely balanced, and each complex rotates about the sun at its own special, constant rate. In certain cases, the complexes form two diverging branches. Title: Chromospheric and coronal indicators of stellar magnetic structure Authors: Zwaan, C. Bibcode: 1983IAUS..102...85Z Altcode: The use of observations of the H and K lines of Ca II, UV emission lines, and soft X-rays to characterize the magnetic structure of F, G, and K stars is discussed, and representative data are summarized. The solar data are reviewed, and stellar data are presented in graphs. Close relations between the soft-X-ray flux, the transition-region flux, and the chromospheric or H+K flux are identified and interpreted, and a need for simultaneous magnetographic observations is indicated. Title: Empirical NLTE analyses of solar spectral lines. IV - The Fe I curve of growth Authors: Rutten, R. J.; Zwaan, C. Bibcode: 1983A&A...117...21R Altcode: The effects of departures from local thermodynamic equilibrium on the equivalent widths of solar Fe I lines are studied as an example for the analysis of the stellar curve of growth. The solar curve of growth obtained is based on the nonlocal thermodynamic equilibrium (NLTE) modeling of the solar spectrum of Lites (1972, 1973) and the best available oscillator strengths for 991 Fe I lines. The empirical curve obtained relating the equivalent width-wavelength ratio to the oscillator strength under NLTE is shown to differ appreciably from curves neglecting the NLTE ionization departures, although these effects may be corrected by assuming a NLTE-masking model. Theoretical NLTE curves of growth are also presented, and splittings due to wavelength dependency, differences in NLTE excitation, and variation in collisional damping, which are largely hidden by noise in observed values, are discussed. A new value for the solar iron abundance of 0.000047 times the hydrogen abundance is also derived. Title: Observational Requirements - Ground-Based - Solar and Stellar Magnetic Fields Authors: Zwaan, C. Bibcode: 1983IAUS..102..503Z Altcode: No abstract at ADS Title: Coronal activity in F-, G-, and K-type stars - Empirical relations between stellar parameters Authors: Mewe, R.; Schrijver, C. J.; Gronenschild, E. H. B. M.; Zwaan, C. Bibcode: 1983IAUS..102..205M Altcode: For a sample of 58 late type stars the authors analyse the relations between the soft X-ray flux density Fx, the Ca II H and K line-core flux density FH+K, and parameters determining the global stellar structure. By analysing the soft X-ray spectra from 15 stars the authors determine the coronal temperatures T and specific emission measures per unit area ζ. They discuss the dependence of T on B-V, Fx and stellar radius R. The diagram of the specific emission measure ζ against the temperature T is interpreted in terms of a coronal model consisting of static loops. Also, a search for time variations in the X-ray flux has been performed. Title: Book reviews Authors: Ne'Eman, Y.; Zahn, J. -P.; Habing, H. J.; Wittenberg, H.; Zwaan, C.; Murray, C. A.; de Jager, Cornelis; Kresák, L. Bibcode: 1982SSRv...33..459N Altcode: No abstract at ADS Title: The Structure of Sunspots - Part Four - Magnetic Field Strengths in Small Sunspots and Pores Authors: Brants, J. J.; Zwaan, C. Bibcode: 1982SoPh...80..251B Altcode: Magnetic field strengths in small umbrae and pores are measured using the line Ti I λ6064.6 Å, which is formed purely in umbrae. We find field strengths between 1900 and 2600 G in the darkest parts of small umbrae and of well established pores; the spread is partly intrinsic. The field strength in diffuse transient `protopores' amounts to 1500 ± 250 G. Title: Magnetic structure in cool stars. V. Chronospheric and transition-region emission from giants. Authors: Oranje, B. J.; Zwaan, C.; Middelkoop, F. Bibcode: 1982A&A...110...30O Altcode: The outer atmospheres of giants at larger heights than the low chromosphere, where the Ca II H and K lines are formed, are studied. In particular, the coupling between the upper chromosphere and the transition region is investigated. G and early K-type giants of different Ca II H and K line-core flux were selected for observations with the IUE. The transition-region flux, defined as the sum of fluxes in the lines of Si IV, C IV, and N V are plotted against the chromospheric flux, defined as the sum of the fluxes in lines of O I and Si II. A surprisingly tight relation is found between transition-region and chromospheric flux which extends over two orders of magnitude in the chromospheric flux and three orders in the transition-region flux. This relation holds for all stars in the sample, including giants, main-sequence stars, and close binaries. The results are discussed in terms of discrete solar-like magnetic structure. Title: Coronal activity in F-, G-, and K-type stars; relations between parameters characterizing stellar structures and X-ray emission Authors: Schrijver, C. J.; Mewe, R.; Zwaan, C. Bibcode: 1982AdSpR...2i.243S Altcode: 1982AdSpR...2..243S A sample of 52 stars containing dwarfs and giants is subjected to a multidimensional factor analysis. The parameters used are the soft X-ray flux at the stellar surface Fx, the Ca II H+K line-core flux FH+K, the stellar radius and mass. We find a high correlation between Fx and the Ca II H+K excess flux ΔFH+K obtained by subtracting an observational lower-limit flux from FH+K. We conclude that the lower-limit Ca II flux is uncorrelated with the stellar X-ray emission. The common-factor analysis shows that, for the present sample, Fx depends only on ΔFH+K, and not on the stellar radius or mass. All stars included in our analysis follow the relation Fx ~ Δ1.4H+K over almost four decades in Fx. Title: Magnetic Structure of F, G, and K Type Stars, II Authors: Zwaan, C. Bibcode: 1982iue..prop.1251Z Altcode: No abstract at ADS Title: The MHD of sunspots. Invited reviews presented at the Joint Meeting of IAU Commissions 10, 12, and 44 during the IAU Assembly in Montreal, August 20, 1979. Authors: Zwaan, C. Bibcode: 1981SSRv...28..387Z Altcode: No abstract at ADS Title: The Magnetohydrodynamics of Sunspots Authors: Zwaan, Cornelis Bibcode: 1981SSRv...28..385Z Altcode: No abstract at ADS Title: Magnetic structure in cool stars. I. The CA II H and K emission from giants. Authors: Middelkoop, F.; Zwaan, C. Bibcode: 1981A&A...101...26M Altcode: Estimates of the intensities of the Ca II H and K emission lines of 500 subgiants, giants and supergiants are reanalyzed in an investigation of the emission mechanisms in evolved cool stars. The Ca II emission intensity classes estimated by Wilson (1976) are compared with B-V colors of the stars in the sample. The majority of K- and M-type giants and many late G-type giants and subgiants are found to constitute a band of intensity class increasing with B-V, while G-type giants show a large spread in intensity class. Enhanced H and K emission is found in two groups of evolved stars: some of the G-type giants, and short-period binaries among the giants and subgiants with orbital periods shorter than a critical value, and the emission is explained by a relatively large stellar rotation rate. Data are consistent with the hypothesis that Ca II H and K emission depends on dynamo action in the convective envelope, with dynamo efficiency decreasing with decreasing rotation rate. Title: Magnetic structure in cool stars. II - Observational evidence for transverse magnetic fields Authors: Tinbergen, J.; Zwaan, C. Bibcode: 1981A&A...101..223T Altcode: The apparently intrinsic linear polarization of the light from nearby F, G, K and M stars is attributed to transverse magnetic fields near the stellar limb in activity belts on either side of the stellar equator. The polarization results from the pi components of strong spectral lines being more saturated than sigma components (Leroy, 1962). The proposed mechanism is compatible with existing observational data. Stringent tests and prospects for further observations are indicated. Title: Magnetic braking in low-mass X-ray binaries. Authors: Verbunt, F.; Zwaan, C. Bibcode: 1981A&A...100L...7V Altcode: It is pointed out that the cool main-sequence components of cataclysmic binaries and low-mass X-ray binaries will undergo rotational braking by a magnetically coupled stellar wind, in a way similar to single main-sequence stars. Since the low-mass components of close binaries are forced by tidal forces to remain in corotation, this leads to a loss of orbital angular momentum from the system, and to an enhanced mass-transfer rate. Using the empirical braking law for cool main-sequence stars, it is found that in systems with secondary mases equal to, or greater than 0.3 to 0.5 solar masses the induced mass-transfer rate is larger than that produced by gravitational radiation losses. For a secondary mass of 0.8 solar masses, a mass transfer rate of 10 to the -8.5th is achieved, sufficient to power the brightest low mass X-ray binaries. Title: Solar magnetic structure and the solar activity cycle. Review of observational data. Authors: Zwaan, C. Bibcode: 1981NASSP.450..163Z Altcode: 1981suas.nasa..163Z Data on solar magnetism that may offer clues for understanding stellar magnetism in general were reviewed. Magnetic phenomena in the photosphere and low chromosphere, where the magnetic structure can be studied in detail, are discussed. Properties of the discrete magnetic elements and their extensions through the chromosphere and transition zone up to the corona are described. The structure and evolution of active regions and other structured aggregates of magnetic elements are considered. Title: Results from an extensive Einstein stellar survey. Authors: Vaiana, G. S.; Cassinelli, J. P.; Fabbiano, G.; Giacconi, R.; Golub, L.; Gorenstein, P.; Haisch, B. M.; Harnden, F. R., Jr.; Johnson, H. M.; Linsky, J. L.; Maxson, C. W.; Mewe, R.; Rosner, R.; Seward, F.; Topka, K.; Zwaan, C. Bibcode: 1981ApJ...245..163V Altcode: The preliminary results of the Einstein Observatory stellar X-ray survey are presented. To date, 143 soft X-ray sources have been identified with stellar counterparts, leaving no doubt that stars in general constitute a pervasive class of low-luminosity galactic X-ray sources. Stars along the entire main sequence, of all luminosity classes, pre-main sequence stars as well as very evolved stars have been detected. Early type OB stars have X-ray luminosities in the range 10 to the 31st to 10 to the 34th ergs/s; late type stars show a somewhat lower range of X-ray emission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late type main-sequence stars show little dependence of X-ray emission levels upon stellar effective temperature; similarly, the observations suggest weak, if any, dependence of X-ray luminosity upon effective gravity. Instead, the data show a broad range of emission levels (about three orders of magnitude) throughout the main sequence later than F0. Title: The Size Dependence of Contrasts and Numbers of Small Magnetic Flux Tubes in an Active Region Authors: Spruit, H. C.; Zwaan, C. Bibcode: 1981SoPh...70..207S Altcode: Intensity contrasts and number densities of bright points, knots and pores ranging in size between 0″.15 and 4″ are studied using high resolution pictures in Mg b1 of a young active region. On the average, the contrast in the wing of the line increases very strongly with decreasing size, while the continuum contrast increases more slowly. The ratio of contrast in the line to contrast in the continuum increases rapidly with decreasing size. The possibility is explored of using this contrast ratio as an indicator of size. The distribution of the contrast ratio in a part of the active region is used in this way to derive a size distribution of facular points. The resulting distribution has a limited accuracy, but is free from systematic distortion due to selection effects. Validity checks on the method are presented. We measure the size distribution of the pores in the same area, and combine the result with that for the facular points. The combined distribution shows that the surface area covered by magnetic elements with diameter δ has a maximum near δ = 0″.8. It increases roughly proportional to δ for δ < 0″.3 and falls off as δ for δ > 1″.5. It is inferred that elements with 0″.5 < δ < 1″.6, which show no conspicuous contrast in the line wing or in the continuum, occupy as much area as the pores, and twice as much as the facular points. We suggest that the changing appearance of a facular area with increasing height of formation reflects at least as much the increasing weight of the small elements in the contrast as a real change in intrinsic properties (such as the diameter) of individual elements. A spatial resolution better than 0″.1 may be needed to resolve the individual elements in plages and the chromospheric network. Title: Large-Scale Patterns in Solar Activity During the Ascending Phase of Cycle 21 Authors: Gaizauskas, V.; Harvey, K.; Harvey, J.; Zwaan, C. Bibcode: 1981BAAS...13R.906G Altcode: No abstract at ADS Title: Coronal Activity an F-Type G-Type and K-Type Stars Authors: Mewe, R.; Schrijver, C. J.; Zwaan, C. Bibcode: 1981SSRv...30..191M Altcode: Soft X-ray (0.3 3.5 keV) observations with the Imaging Proportional Counter (IPC) onboard Einstein Observatory are presented for a sample of some 20 cool stars of luminosity classes III V. The results are compared with the Ca II H and K emission, which had served as a selection criterion. The specific X-ray flux FX is an increasing function of the specific Ca II H and K line-core flux FH+K. This correlation can be considerably improved by replacing FH+K by the excess flux (ΔFH+K) above a certain lower limit which varies with B-V. This relation holds with little scatter over the two decades in FX in our sample. The FX-ΔFH+K relation shows no significant dependence on spectral type or luminosity class, it suits close binaries as well as single stars. However, the coronal X-ray temperature Tc strongly depends on the luminosity class: Tc≈ 3 106 K for dwarfs and ≈ 107 K for giants. The results are interpreted in the framework of magnetic activity. The X-ray emission and the excess Ca II H and K flux are attributed to magnetic structure in the corona and chromosphere, the magnetic features emerging from the stellar convective envelope, where they are generated by dynamo action. Title: Empirical sunspot models - Statement of the problem Authors: Zwaan, C. Bibcode: 1981phss.conf..123Z Altcode: The motivations for devising empirical sunspot models and possible data input to them are considered. Use of empirical models is noted to place greater constraints on the physical processes which determine the atmospheric structure than observational data does. Key factors which must be included in any sunspot model comprise the thermodynamic parameters, the magnetic field, and the velocity field. Choice of a suitable coordinate system for the key factors is discussed, with suggestions made of the vertical and radial directions, size dependence, and separate calculations for the umbra and penumbra. An assumption of hydrostatic equilibrium is recommended for the photosphere and the low chromosphere. Problems extant in deriving models of the umbral photosphere from the visible spectrum are outlined. Title: Magnetic Activity in Cool Stars Authors: Zwaan, C. Bibcode: 1981MitAG..51..139Z Altcode: No abstract at ADS Title: Magnetic Structure of F, G, and K Type Stars Authors: Zwaan, C. Bibcode: 1981iue..prop..962Z Altcode: No abstract at ADS Title: Magnetic activity in cool stars Authors: Zwaan, C. Bibcode: 1981AGAb...51..139Z Altcode: The solar magnetic structure is discussed as a framework for stellar magnetism. Ways of observing magnetic structure in cool stars are outlined with emphasis on observations of Ca 2, H, and K emission in main sequence stars. It is inferred that the chromospheric, transition-region, and coronal emissions are correlated manifestations of magnetic activity in stars with convective envelopes, and that the main level of activity depends on the rotation rate. Title: Magnetic field strengths in umbrae and pores Authors: Zwaan, C.; Brants, J. J. Bibcode: 1981phss.conf..210Z Altcode: A technique for accurately determining the magnetic field strength in small umbrae and pores through use of line spectra which are enhanced in the umbral spectrum is described. An example is provided of the field strength obtained from taking the difference in the wavelength between the medians of the line cores (yielded by an echelle spectrograph) for densitometer tracings in the direction opposite to polarization. The lines comprised Ti I at 6064.6 A, and additional readings of Fe I at 6173.3 A and Fe I at 6302.5 A. The field strength determined from the photospheric Fe lines is shown to be less than that from the umbral atmosphere Ti line, which is caused by the Fe sigma components being strongly blended into the central component, which is weak in the Ti line. Finally, it is demonstrated that the strength of the field is independent of the size of the spot. Title: An emerging active region - Some preliminary results Authors: Brants, J. J.; Zwaan, C.; Cram, L. E. Bibcode: 1981phss.conf...60B Altcode: Observations of an emerging flux region on July 24, 1979 are analyzed in terms of six spectral lines, the broadband continuum, and the Ca II K line core. The process of formation of the umbrae, including the appearance of protopores leading to fully formed pores, is described. Results are presented from spectrographic observation of the 6302 A and 5692 A lines, taken while pores were still forming. Field strengths of 1900 and 2500 gauss were detected, in line with previous measurements of pore groups. Title: Stellar magnetic structure and activity. Authors: Zwaan, C. Bibcode: 1981ASIC...68..463Z Altcode: 1981spss.conf..463Z Observational aspects of stellar magnetism are summarized. Methods of observing stellar magnetic structure and activity are discussed, including search for sigma components in spectral lines of large magnetic splitting, broadband, spectral, and line photometry. Ca II H and K emissions are discussed for main sequence stars, giants, and spectroscopic binaries. Relationships are analyzed between emissions from chromospheres, transition regions, and coronas, and evolutionary scenarios are described for single stars and short-period binaries. Long- and short-period variations of magnetic activity are discussed. At the outset, four hypotheses are stated which give structure to the following discussion and suggest empirical tests. Title: Methods for the analysis of stellar spectra veiled by lines (III). Authors: Greve, A.; Zwaan, C. Bibcode: 1980A&A....90..239G Altcode: UV spectrograms of the sun and of Alpha CMi are analyzed to investigate the effects of finite spectral resolution in the interpretation of stellar spectra veiled by lines. The recorded completeness of intrinsic spectral detail is examined by means of the ruggedness parameter of Ivanov and Salman-Zade (1976). Solar spectrograms of approximately 0.03 A resolution are a good representation of the true spectral distribution. The power of the Minnaert-Houtgast method for tracing line wings and establishing the local 'continuum' background in spectra crowded with lines is demonstrated. Title: A Polarimeter for a Fourier Transform Spectrometer and Initial Solar Observations Authors: Harvey, J.; Brault, J.; Stenflo, J.; Zwaan, C. Bibcode: 1980BAAS...12R.476H Altcode: No abstract at ADS Title: Chromospheric and Coronal Activity in F-Type G-Type and K-Type Stars Authors: Mewe, R.; Zwaan, C. Bibcode: 1980SAOSR.389..123M Altcode: 1980csss....1..123M No abstract at ADS Title: Chromospheres Authors: Zwaan, C. Bibcode: 1980iue..prop..553Z Altcode: No abstract at ADS Title: Report from the discussion group on a large optical observational facility (LEST) Authors: Zwaan, C. Bibcode: 1980fsoo.conf..292Z Altcode: No abstract at ADS Title: MHD phenomena in the photosphere. Authors: Zwaan, C. Bibcode: 1979ssms.conf...71Z Altcode: No abstract at ADS Title: Report from the discussion group on a large optical observational facility (LEST). Authors: Zwaan, C. Bibcode: 1979MmArc.106..295Z Altcode: No abstract at ADS Title: On the Appearance of Magnetic Flux in the Solar Photosphere Authors: Zwaan, Cornelis Bibcode: 1978SoPh...60..213Z Altcode: Ideas and models for the appearance of photospheric magnetic structure are confronted with observational data. Some findings are: The magnetic flux emerging in an active region consists of a bundle of flux tubes which were already concentrated before penetrating into the photosphere. A model of a rising bunch of flux tubes joining into a few strands at larger depths describes the coalescence of spots near the leading and following edges of the active region while more flux may surface near the center of the region. There is no observational evidence for appreciable helical twists in the flux bundles. Title: Report from the discussion group on a large optical observational facility (LEST) Authors: Zwaan, C. Bibcode: 1978fsoo.conf..292Z Altcode: No abstract at ADS Title: Magnetic structures in photosphere of sun and stars Authors: Zwaan, C. Bibcode: 1977MmSAI..48..525Z Altcode: Following a review of the principal observational data on the magnetic structures in the solar photosphere and low chromosphere, some simple models are examined which provide a semi-quantative interpretation of the observed structures. The interplay between the magnetic field and the velocity fields (rotation, convection), is examined, along with its role in the emergence and redistribution of magnetic flux over the solar surface. The magnetic structure to be expected in stars with convective zones is studied, along with the observational indications for stellar magnetic activity. Title: Fotograferen van zonnevlekken met een 17-cm refractor. Authors: Brückner, W.; Zwaan, C. Bibcode: 1976Zenit...3...13B Altcode: No abstract at ADS Title: On Differences in Differential Rotation Authors: Vantend, W.; Zwaan, C. Bibcode: 1976IAUS...71...45V Altcode: No abstract at ADS Title: Solar activity (Activité solaire). Authors: Newkirk, G.; Dunn, R. B.; Mehltretter, P.; MacQueen, R.; Bonnet, R. M.; White, O. R.; Fokker, A. D.; Zwaan, C.; Bruzek, A.; Durrant, C.; Grossmann-Doerth, U.; Mehltretter, J. P.; Svestka, Z.; de Feiter, L. D.; Tandberg-Hanssen, E.; Howard, R.; Stix, M.; Pneuman, G. W.; Hundhausen, A. J.; Sawyer, C.; Simon, P. Bibcode: 1976IAUTA..16b..13N Altcode: No abstract at ADS Title: Sunspots Authors: Zwaan, C. Bibcode: 1976IAUTA..16...24Z Altcode: 1976IAUT...16A..24Z No abstract at ADS Title: Structure of Sunspots. III: A Minimum-Gradient Model Atmosphere for Umbrae Authors: Zwaan, C. Bibcode: 1975SoPh...45..115Z Altcode: We present a minimum-gradient model to complement a maximum-gradient model (Zwaan, 1974). Both models are compatible with published continuum intensities and within constraints on the temperature gradient; both are adjusted to Teff=4000K. In order to explain the visual intensities the minimum-gradient model requires an additional `line-haze' opacity which is only slightly smaller than in the maximum-gradient model. Title: Proceedings of the First European Solar Meeting. Florence, February 25 - 27, 1975. Authors: Chiuderi, C.; Landini, M.; Righini, A.; Zwaan, C. Bibcode: 1975MmArc.105....1C Altcode: No abstract at ADS Title: Structure of Sunspots. II: A Continuum Model Atmosphere for Dark Umbral Cores Authors: Zwaan, Cornelis Bibcode: 1974SoPh...37...99Z Altcode: The sunspot models published so far do not reproduce the observed run of the umbral continuum intensities over the entire spectral range 0.5 < λ < 4 μm. Moreover, in several previous models is the temperature gradient smaller than both the adiabatic and the radiative equilibrium gradient. Title: Cooperation in solar Astronomy in Europe Authors: Zwaan, C. Bibcode: 1974MmSAI..45..929Z Altcode: No abstract at ADS Title: On the Relation Between Moving Magnetic Features and the Decay Rates of Sunspots Authors: Zwaan, C. Bibcode: 1974IAUS...56..233Z Altcode: No abstract at ADS Title: Magnetische velden en convectie in de zon. Authors: Spruit, H. C.; Zwaan, C. Bibcode: 1974NTNA...40.....S Altcode: No abstract at ADS Title: An Efficient Wind Shield for the Protection of Telescopes Authors: Hammerschlag, R. H.; Zwaan, C. Bibcode: 1973PASP...85..468H Altcode: A semitransparent wind shield to protect telescopes against wind is described and some measurements and experiences during a site-testing campaign are reported. Key words: instrumentation - telescope protection - seeing Title: The Structure of Sunspots. I: Observational Constraints: Current Sheet Models Authors: Gokhale, M. H.; Zwaan, C. Bibcode: 1972SoPh...26...52G Altcode: As a first step in constructing three-dimensional decaying sunspot models we select the relevant observational data. From these we conclude: sunspots, except the smallest, obey a radial and evolutionary similarity; Title: Empirical NLTE Analyses of Solar Spectral Lines. I: A Method and Some Applications to Earlier Analyses Authors: Wijbenga, Jan W.; Zwaan, Cornelis Bibcode: 1972SoPh...23..265W Altcode: A method is suggested for empirical NLTE analyses of solar spectral lines. Special depthdependent departure coefficients β(x) are introduced and the formulas are given for further application. From test calculations it is shown that the separation of the departure coefficients for the upper and the lower level from each other and from the uncertainties in several input parameters (oscillator strength times abundance, turbulent velocities and damping constant) is greatly facilitated when spectral lines are analysed on the disk, around the limb, as well as in the flash spectrum. Therefore it is necesssary that all accessible line data are used, from accurate line profiles to equivalent widths or integrated intensities. To reduce the number of independent variables the analysis should include many multiplets between a few spectroscopic terms. Title: Magnetic Fields Strengths Derived from Various Lines in the Umbral Spectrum Authors: Zwaan, C.; Buurman, J. Bibcode: 1971IAUS...43..220Z Altcode: No abstract at ADS Title: The Structure of Sunspots Authors: Zwaan, Cornelis Bibcode: 1968ARA&A...6..135Z Altcode: No abstract at ADS Title: Small-Scale Solar Magnetic Fields and 'Invisible Sunspots' Authors: Zwaan, Cornelis Bibcode: 1967SoPh....1..478Z Altcode: No abstract at ADS Title: Note on the normalization of the NBS gf-values and on the solar abundances of the alkali metals Authors: Zwaan, C. Bibcode: 1967BAN....19....1Z Altcode: No abstract at ADS Title: Sunspot Spectroscopy Authors: Zwaan, C. Bibcode: 1967sp...conf..229Z Altcode: No abstract at ADS Title: Sunspot models : a study of sunspot spectra Authors: Zwaan, Cornelis Bibcode: 1965RAOU...17.....Z Altcode: No abstract at ADS Title: The influence of scattered light on line intensity measurements Authors: Zwaan, C. Bibcode: 1965IAUS...22..277Z Altcode: No abstract at ADS Title: Sunspot models; a study of sunspot spectra Authors: Zwaan, C. Bibcode: 1965smss.book.....Z Altcode: 1965QB525.Z89...... No abstract at ADS Title: The interpretation of the equivalent widths of atomic and ionic lines in the spectra of umbrae Authors: Zwaan, C. Bibcode: 1964susp.conf..169Z Altcode: No abstract at ADS Title: The continuous absorption coefficient in the violet and ultraviolet region Authors: Zwaan, C. Bibcode: 1962BAN....16..225Z Altcode: No abstract at ADS Title: Note on partition functions Authors: Zwaan, C. Bibcode: 1960AnAp...23..811Z Altcode: No abstract at ADS Title: Curves of growth for a large sunspot Authors: Zwaan, C. Bibcode: 1959BAN....14..288Z Altcode: No abstract at ADS Title: Note on the Occurence of Technetium in the Solar Atmosphere Authors: Hubenet, H.; Zwaan, C.; de Jager C. Bibcode: 1954LIACo...5..471H Altcode: 1954MSRSL...1..471H; 1954pna..conf..471H No abstract at ADS