Variations in photospheric limb darkening as a diagnostic of changes in solar luminosity
We report an photospheric limb-darkening measurements obtained at the McMath Solar Telescope in 1980 July, September, and October as part of a continuing program to study possible long-term variations in the photospheric emergent flux. A total of 243 usable full-diameter scans was recorded over 7 days in the clean continuum window at 4451.25 A. Individual scans were fitted to a fifth order polynomial in xi = ln..mu.., and a mean limb-darkening curve was derived for each day. The standard deviation of the daily mean of intensities ranged between 0.08% and 0.12% for all ..mu..>0.15. We note that the limb darkening decreased significantly between September 25 and 26. This change of the limb-darkening curve is not readily understandable in terms of variations in scattered light, seeing, or nonlinear photometric gain. We suggest that it is caused by a decrease of the temperature gradient in the upper photosphere, in the region above approximately tau/sub 5000/ = 0.5. The small increase in effective temperature that might accompany this limb-darkening variation is estimated using a standard radiative equilibrium photospheric model. We note that the increase of effective temperature corresponds in sign with an increase in the solar irradiance reported between September 25 and 26 from both the Nimbus 7 ERB and the SMM ACRIM radiometers.
- Research Organization:
- Vassar College, Poughkeepsie, New York
- OSTI ID:
- 5169889
- Journal Information:
- Astrophys. J., Lett. Ed.; (United States), Vol. 253:2
- Country of Publication:
- United States
- Language:
- English
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NUMERICAL DATA
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640104* - Astrophysics & Cosmology- Solar Phenomena