Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...183.2509c&link_type=abstract
American Astronomical Society, 183rd AAS Meeting, #25.09; Bulletin of the American Astronomical Society, Vol. 25, p.1332
Astronomy and Astrophysics
Astronomy
Scientific paper
We report on photometric observations of sunspots carried out with the Cartesian Full Disk Telescope (CFDT) at the San Fernando Observatory (SFO). The pixel size is 5.1 arc-sec and the wavelength for the data discussed here is 6723 Angstroms. Fluctuations in total solar irradiance due to sunspots are often modeled using a constant value of alpha, which we are calling the bolometric contrast of a sunspot. We have defined alpha_ {eff} as DEF/(2 times PSI), where DEF is the sunspot's photometric deficit relative to the quiet photosphere, and PSI is the digitally determined Photometric Sunspot Index (Willson et al., 1981). For 40 sunspot groups, we find that alpha_ {eff} = (0.276 +/- 0.051) + (3.22 +/- 0.34) 10(-5) A_s, where A_s is the corrected area of the sunspot in micro-hemispheres. The coefficient of determination is r(2) = 0.1936, which is significant at the p = 0.005 level. We also find that alpha_ {eff} is highly correlated with the ratio of umbral to total spot area (A_u/A_s). For 86 sunspot-days we find alpha_ {eff} = (0.219 +/- 0.018) + (0.643 +/- 0.028) (A_u/A_s) with the linear coefficient of determination r(2) = 0.859. This suggests that an improved PSI can be constructed from knowledge of a sunspot's umbral to total area ratio. The use of such an improved PSI or, better still, actual photometry should reduce the statistical noise in comparisons with spacecraft measurements of the total solar irradiance. This work has been partially supported by grants from NSF and NASA.
Chapman Gary A.
Cookson Angela M.
Dobias Jan J.
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