Measurements of the limb darkening of faculae near the solar limb
Measurements of the contrast at 525 nm of solar faculae are presented. These measurements were obtained with an extreme limb photometer (ELP) in 19775 and 1979 at the San Fernando Observatory. The mean contrast of active regions was determined from the limb inward to 54'', averaged over an annulus determined by the slits of the ELP. We find that the contrast of faculae increases with heliocentric angle theta, from ..mu..roughly-equal0.25 to ..mu..roughly-equal0.065, where ..mu.. = cos theta. The average value of the mean contrast of facular regions is 2.1% with a possible upper limit of about 5% at ..mu..roughly-equal0.1, determined from regions with the greatest filling factor. This mean contrast, when corrected for a filling factor of 10-15% implies a contrast for individual faculae of 30-50%. The results from the two observing seasons, when combined, are not inconsistent with the ..mu..-dependence of the excess solar oblateness signal measured at Princeton in 1966. The 1979 facular data would have produced an apparent excess oblateness of 66% of the Dicke and Goldenberg value. Because the contrast of faculae appears to increase to within a few arcsec of the limb, the excess facular intensity is probably due not only to ''hot wall'' effects but also to local heating in the upper levels of facular flux tubes.
- Research Organization:
- San Fernando Observatory and Department of Physis and Astronomy, California State University, Northridge
- OSTI ID:
- 6511944
- Journal Information:
- Astrophys. J.; (United States), Vol. 261:1
- Country of Publication:
- United States
- Language:
- English
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