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  • 1
    Electronic Resource
    Electronic Resource
    Center-limb observations of inhomogeneities in the solar atmosphere (1970)
    Springer
    Solar physics 14 (1970), S. 29-41 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Center-limb observations of line-center intensity and velocity fluctuations in the Magnesium b lines are described. Autocorrelation and power spectral analyses indicate small scale brightness structures having periodicities of 3000 km and 8000 km and large scale structures of 22000 km. Corresponding velocity structures are 6000 km and 30000 km. The relative rms fluctuation amplitude for the small scale bright features is found to be of order 12% and for the large scale features 8%. The variation of these rms values with heliocentric angle is also shown. At disk center some weak correlation is found between bright features and downward velocities in the large scale structures. Towards the limb there is a strong correlation in all three lines between line of sight motions and bright features. This indicates that the large scale bright features are closely associated with the supergranule motions. By inspecting the actual brightness and velocity fluctuation tracings it can be seen that, in some regions, the small scale structures show a significant negative correlation over a range of about 25000 km. Beyond this characteristic length, however, the correlation may decrease abruptly or even become positive for a similar distance. There is some evidence which suggests that this behaviour may also be related to the supergranule motions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00240158
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  • 2
    Electronic Resource
    Electronic Resource
    Calculations of two-dimensional models of the lower solar chromosphere (1971)
    Springer
    Solar physics 16 (1971), S. 314-327 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Several two-dimensional models of the lower solar chromosphere are computed using relative RMS line centre intensity variations and mean limb darkening curves observed in Mg b and NaD. These calculations indicate that the small scale fluctuations observed at line centre in these lines may result from density and electron temperature fluctuations in the lower chromosphere, while the large scale fluctuations may result from either fluctuations in the deeper lying continuum or horizontal and vertical differential velocity fields.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00162473
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  • 3
    Electronic Resource
    Electronic Resource
    The formation of Mg i 4571 Å in the solar atmosphere (1973)
    Springer
    Solar physics 29 (1973), S. 275-286 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract An analysis of the 4571 Å line of neutral magnesium is presented in which one-dimensional macroscopic velocity fields are included. It is shown that gradients over restricted heights in the vertical and horizontal components of the velocity field of order -0.005 s−1 and -0.004 s−1 (such that velocity towards the observer decreases as height increases), respectively, result in asymmetries in the computed line profile similar to those observed. The heights in the solar atmosphere at which these velocity gradients exist are shown to be very critical in reproducing the observations. It was found that the best results were obtained when the gradients existed in the height range from ∼ 200 km to ∼ 300 km below the temperature minimum. The results indicate that for the Mg i 4571 Å line model calculations that do not include one-dimensional flow velocities may safely be compared with frequency-averaged observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00150805
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  • 4
    Electronic Resource
    Electronic Resource
    The formation of Mg I 4571 Å in the solar atmosphere (1973)
    Springer
    Solar physics 30 (1973), S. 31-33 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00156169
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  • 5
    Electronic Resource
    Electronic Resource
    Center limb observations of inhomogeneities in the solar atmosphere (1971)
    Springer
    Solar physics 17 (1971), S. 288-303 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Center-limb observations of line-center intensity fluctuations in the Na D and Na 5688 doublets and the Mg 4571 line are described. For small scale structures the rms distributions for the photospheric lines show maxima of 20% at a heliocentric angle of 40° (Mg4571) and 9% at 20° (Na 5688). The rms values for Na D range between 10 and 13% but show no significant maximum. Values for large scale structures are somewhat less. Auto-correlation curves show a range of structures from 4000 km to 30000 km. Brightness-velocity cross-correlation curves and the actual brightness and velocity tracings from a region near disk-center are also studied. A new method of classifying cross-correlation coefficients calculated over many different ranges within the sample enables one to distinguish the existence and extent of regions of significant cross-correlation within the sample and this is applied both to the results for the Na D lines and for the Mg b lines given in Paper I. It is found that regions of significant correlation may extend for lengths of order 25000 km after which the correlation may abruptly change sign or become negligible. The suggestion made in Paper I that these abrupt changes may occur at the supergranule boundaries is further supported. The relation between these small scale structures and the 5 min oscillations is investigated. The question of the reproducibility of these statistical data from day to day and at different positions of the Sun at the same heliocentric angle is discussed and the need for further independent observations is stressed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00150033
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  • 6
    Electronic Resource
    Electronic Resource
    The effect of two-dimensional macroscopic velocity fields on models of the lower solar chromosphere (1971)
    Springer
    Solar physics 21 (1971), S. 82-92 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Two-dimensional macroscopic velocity fields are featured in the calculation of two-dimensional models of the lower solar chromosphere. Relative rms line centre intensity fluctuation data and mean limb darkening data obtained in Mg b and Na D are used together with values of the cross-correlation between line centre brightness and ‘line of sight velocities’. It is found that the large scale fluctuation data can be explained by models of the lower solar chromosphere in which the inhomogeneous effects arise only from horizontal, two-dimensional macroscopic velocity fields. It is also shown, however, that the corresponding small scale fluctuation data cannot be explained in a similar manner. The cross-correlation data is found to be a powerful constraint in the computation of two-dimensional models of these regions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155776
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  • 7
    Electronic Resource
    Electronic Resource
    The formation of Mg i 4571 Å in the solar atmosphere (1972)
    Springer
    Solar physics 26 (1972), S. 21-29 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A one-dimensional analysis of the 4571 Å line of neutral magnesium is presented. The Harvard-Smithsonian Reference Atmosphere (HSRA) and the Bilderberg Continuum Atmosphere (BCA) are used to compute the emergent line profiles at various positions on the solar disc. The resultant profiles, when compared to the observations, indicate that the HSRA electron temperature distribution is a more satisfactory representation of the solar atmosphere in the region of the temperature minimum than is the BCA. A slight modification to the HSRA is suggested which reduces the minimum temperature to 4140K and enables an even more satisfactory ‘fit’ to the available data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155102
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  • 8
    Electronic Resource
    Electronic Resource
    The Formation of MgI 4571 Å in the solar atmosphere (1975)
    Springer
    Solar physics 42 (1975), S. 289-302 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The two-dimensional equation of transfer is solved for the case of locally-controlled source function (LTE) and radiationally-controlled ionization. Horizontal fluctuations in electron temperature and macroscopic velocity fields are superposed on the basic one-dimensional model (cf. Altrock and Cannon, 1972). Output intensities are compared with observed rms intensity fluctuations and spatially-averaged intensities in Mg i 4571 Å. We find that at least one model (with a height-independent temperature fluctuation ΔT/T=±0.02 in the range 0⩽h⩽450 km) can predict the magnitude of the intensity fluctuations in both the continuum and λ4571 Å. The asymmetry of the line can be explained by adding a height-independent, temperature-correlated flow of amplitude 1 to 2 km s−1. The relationship between these results and other multi-dimensional analyses is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00149912
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  • 9
    Electronic Resource
    Electronic Resource
    The structure of a sunspot (1968)
    Springer
    Solar physics 4 (1968), S. 3-17 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract White-light photographs of a fairly regular sunspot have been obtained for all but one day of its passage across the disk. From microphotometer tracings across these photographs, intensity profiles across the spot have been obtained at several heliocentric angles, θ. Apparent sunspot, umbral and penumbral widths, have been obtained from these profiles, and an examination of these reveals that the well-known Wilson effect (Wilson, 1774) is a rather complex phenomenon comprising four main features: (1) The intensity profiles become increasingly asymmetric at large θ. The penumbra remote from the limb is poorly defined while the penumbral intensity plateau nearer the limb is well defined and sometimes enhanced by an intensity maximum near the umbra-penumbra boundary. (2) A gradual decrease in the apparent width of the disk-side penumbra may occur but this effect is barely significant compared with the rms errors of the observations. (3) The apparent width of the limb-side penumbra is independent of θ for θ 〈 60° but at larger heliocentric angles it increases sharply and by a significant amount. (4) The apparent umbral diameter also shows no θ-dependence for θ 〈 60° but beyond this it decreases in an almost complementary manner. A general model for the structure of a sunspot is put forward which readily explains these results in a qualitative manner but it is emphasised that an adequate analysis of sunspot structure based on these observations requires solutions of the three-dimensional equation of radiative transfer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146994
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  • 10
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    Center limb observations of inhomogeneities in the solar atmosphere (1971)
    Springer
    Details
    Publication Date: 1971-04-01
    Print ISSN: 0038-0938
    Electronic ISSN: 1573-093X
    Topics: Physics
    Published by Springer
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