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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 129 (1990), S. 221-246 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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. At least one third of the sunspot appearances belong to intrinsically physical clusters. Even a substantial fraction of the clusters of two-member groups is genuine. The clustering criteria are set such that the smallest scales in the clustering are brought out. The sizes of the nests turn out very small: the typical (1σ-) spread about the longitude-time regression line is about 2°, and the spread about the mean latitude is about 1°. The lifetimes range from one to seven months. The nests follow the rotation rate-latitude relation of recurrent sunspots within a small but intrinsic spread of about 15 m s−1. In the present sample the displacements in latitude are extremely small: the amplitude of any large-scale flow pattern must be less than 3 m s−1, whereas the intrinsic proper motions in latitude are less than 5 m s−1. The appearance of spot groups in a nest is intermittent. On one hand, in an active nest spots may be absent for nearly two complete rotations. On the other hand, in many nests more than one spot group has been recorded during one disk crossing. There is a strong tendency for the nests to cluster once again in larger nests: more than 1/3 of the compact nests are components in larger nests. Within such a large nest the components overlap in time, their mean latitudes differ by less than 2°.5, but the difference in longitude may extend up to 55°. The present and earlier results are either in harmony, or seemingly discordant results are readily explained by properties of the nests and differences in the search criteria.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 143 (1993), S. 215-227 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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 ofD f = 1.54 ± 0.05 for the relation between area and linear size. This value agrees very well with the dimensionD f = 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.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 70 (1981), S. 207-228 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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. The observed variation of continuum contrast of facular points with size agrees with predictions based on magnetostatic flux tube models if a field strength of about 2000 G is assumed.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 95 (1985), S. 3-14 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Space science reviews 33 (1982), S. 459-462 
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 80 (1982), S. 251-258 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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. We demonstrate that usage ofthe well-known magnetic line λ6173.3 Å and other Fe i lines yield systematically smaller magnetic field strengths than Ti i λ6064.6 Å. This is due to blending ofthe σ components with the central component due to photospheric stray light and the π component. Routine measurements are therefore unreliable for small spots 251-01
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 105 (1986), S. 237-255 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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. The nests are the same type of activity sequences as the ‘Fleckenherde’ discovered by Becker (1955) and the complexes of activity pointed out by Gaizauskas et al. (1983). The complexes of activity as defined by Bumba and Howard (1965) are different patterns, however; the relation between complexes and the nests is shown. Some properties of the nests are: (i) many nests appear as double structures; (ii) single nests and components of double nests are quite compact: the effective areas are comparable to those of medium-large sunspot groups; (iii) each nest rotates at its own steady rate about the Sun; (iv) the intrinsic scatter in the rotation rates is much larger than the trend in the differential rotation; (v) displacements in latitude are less than a few meters per second; (vi) many nests live for 6 to 15 Carrington rotation periods, the minimum lifetime is not yet determined; (vii) the fraction of the sunspot groups that are members of nests is large (at least 30%).
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 26 (1972), S. 52-75 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract As a first step in constructing three-dimensional decaying sunspot models we select the relevant observational data. From these we conclude: (i) sunspots, except the smallest, obey a radial and evolutionary similarity; (ii) sunspots may be considered as isolated, fairly well defined flux tubes, wrapped in thin current sheets; (iii) a substantial number among stable regular spots show a phase of slowest decay whose rate is independent of the spot's area. Arguments are given that the slowest rate of decay is ultimately determined by Ohmic dissipation in the inner part of the current sheet. Preliminary asymptotic models for the deep layers (deeper than 2000 km below the photosphere) are given which satisfy the above three constraints. To meet the observed rate of slowest decay the current sheet has to be very thin, about 10−5 to 10−4 times the umbral radius. Radial large-scale fluid motions are required in the current sheet to maintain the similarity of the structure. The radial motions are linked with the vertical motions which may be connected with the Evershed flow. Finally we discuss details which are less relevant in the large-scale structure of stable sunspots, such as fine structures, twists, the break-down of the similarity and the relation between sunspots and smaller magnetic structures, and the intrinsic scatter in some observed quantities.
    Type of Medium: Electronic Resource
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