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  • 1
    Electronic Resource
    Electronic Resource
    Measurement of Kodaikanal White-Light Images – II. Rotation Comparison and Merging with Mount Wilson Data (1999)
    Springer
    Solar physics 186 (1999), S. 25-41 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Sunspot umbral positions and areas were measured for 82 years (1906–1987) of daily, full-disk photoheliogram observations at the Kodaikanal station of the Indian Institute of Astrophysics. The measurement technique and reduction procedures used were nearly identical to those used earlier for the reduction of Mount Wilson daily full-disk photoheliograms, covering an overlapping interval of 69 years. In this paper we compare the differential rotation of the Sun from the analysis of the Kodaikanal data with the Mount Wilson results. In addition, we analyze the data set formed by combining the data from the two sites for differential rotation. While doing this, it has become apparent to us that small, subtle optical effects at both sites produce systematic errors that have an influence on rotation (and other) results from these data. These optical effects are analyzed here, and corrections are made to the positional data of the sunspots from both sites. A data set containing the combined positional data of sunspots from both sites, corrected for these optical aberrations, has been constructed. Results for both sunspot groups and individual sunspots are presented. It is pointed out that optical aberrations similar to those found in the Kodaikanal data may also exist in the Greenwich photoheliograph data, because these two sets of solar images were made with similar telescopes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005153925209
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Measurement of Kodaikanal white-light images – IV. Axial Tilt Angles of Sunspot Groups (1999)
    Springer
    Solar physics 189 (1999), S. 69-83 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Kodaikanal sunspot data set, covering the interval 1906–1987, is used in conjunction with the similar Mount Wilson sunspot data set, covering the interval 1917–1985, to examine characteristics of sunspot group axial tilt angles. Good agreement is demonstrated between various results derived from the two independent data sets. In particular, the tendency for sunspot groups near the average tilt angle to be larger than those far from the average tilt angle is confirmed. Similarly the faster residual rotation rate for groups near the average tilt angle is also confirmed. Other confirmations are made for the relationships between latitude drift of sunspot groups and tilt angle, polarity separations, and axial expansion. Evidence is presented that tilt angles averaged over these long time intervals differ between the north and south hemispheres by about 1.4 deg. It is suggested that residual tilt angles show a slight systematic variation with phase in the activity cycle.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005277515551
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    MEASUREMENT OF KODAIKANAL WHITE-LIGHT IMAGES – V. Tilt-Angle and Size Variations of Sunspot Groups (2000)
    Springer
    Solar physics 196 (2000), S. 333-348 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We examine here the variations of tilt angle and polarity separation (as defined in this paper) of multi-spot sunspot groups from the Kodaikanal and Mount Wilson data sets covering many decades. We confirm the tilt-angle change vs tilt-angle result found earlier from the Mount Wilson data alone. Sunspot groups tend on average to rotate their axes toward the average tilt angle. We point out that if we separate groups into those with tilt angles greater than and less than the average value, they show tilt-angle variations that vary systematically with the growth or decay rates of the groups. This result emphasizes again the finding that growing (presumably younger) sunspot groups rotate their magnetic axes more rapidly than do decaying (presumably older) groups. The tilt-angle variation as a function of tilt angle differs for those groups whose leading spots have greater area than their following spots and vice versa. Tilt-angle changes and polarity separation changes show a clear relationship, which has the correct direction and magnitude predicted by the Coriolis force, and this strongly suggests that the Coriolis force is largely responsible for the axial tilts observed in sunspot groups. The distribution of polarity separations shows a double peak. These peaks are perhaps related to super- and meso-granulation dimensions. Groups with polarity separations less than 43 Mm expand on average, while those groups with separations more than this value contract on average. We present evidence that the rotation of the magnetic axes of sunspot groups is about a location closer to the following than to the leading sunspots.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005288227384
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Measurement of Kodaikanal White-Light Images – III. Rotation Rates and Activity Cycle Variations (1999)
    Springer
    Solar physics 188 (1999), S. 225-236 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Kodaikanal sunspot data set covering the interval 1906–1987 is analyzed for differential rotation of sunspots of different sizes. As is known, smaller sunspots rotate faster than larger sunspots, and this result is verified in the analysis of this data set. These results agree well with the Mount Wilson sunspot results published earlier. The activity cycle dependence of sunspot rotation is studied. An increase in this rate at the minimum phase is seen, which has been reported earlier. It is demonstrated that this cycle variation is seen for sunspots in all size categories, which suggests that it is not a relative increase in the number of the faster-rotating small sunspots that causes the cycle dependence. These results are discussed as they may relate to subsurface dynamic properties of the Sun.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005229124554
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    Paper (German National Licenses)
    Fulltext
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