Variability of the Solar Limb-Darkening Function with Latitude

Details Variability of the Solar Limb-Darkening Function with Latitude Variability of the Solar Limb-Darkening Function with Latitude

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.8107k&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #81.07; Bulletin of the American Astronomical Society, Vol. 24, p.1253

Astronomy and Astrophysics

Astronomy

Scientific paper

Variations in the brightness of the quiet solar photosphere, in addition to sunspots and faculae, could contribute significantly to solar total-irradiance changes. To determine the extent to which this occurs, a long-term project is underway which is capable of measuring changes in solar limb darkening near the extreme edge of the Sun by monitoring the differential radius, a quantity charac- terizing the slope of the limb-darkening function. The differential radius is the difference between two edge locations of the Sun determined by the Finite Fourier Transform Definition for two different scan amplitudes, and thus has a reduced sensitivity to atmospheric seeing. Beginning in 1988, observations were made at 24 limb positions, and at 36 positions since 1990. Each limb position includes approximately 5.7deg of the solar circumference. The observations consist of intensity profiles of the outermost 32 arc- seconds of the solar limb made in the continuum at 550 nm. This allows limb darkening to be studied as a function of both position and time. The average differential radius at each position for an observing season is obtained from the daily observations that are not affected by regions of solar activity. For the nine seasons under consideration, the average deviations from the seasonal means range from 11 to 22 milliarcseconds, corresponding to latitudinal changes in the limb-darkening function of approximately 0.8 to 1.5% at cos theta =0.28. These results suggest a mechanism that causes perturbations to the temperature gradient which are localized in latitude and persist for several months. This work was supported in part by the U.S. Department of Energy.

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