The Observed Spatial Non-Uniformity and Temporal Variability of the Solar Limb-Darkening Function

Abstract

The solar radiation intensity was measured at selected heliographic latitudes by observing the continuum radiation at 550 nm over the outermost 32 arcseconds of the limb. At each observed limb position, solar edge locations are determined from the intensity profiles using the Finite Fourier Transform Definition with a series of scan amplitudes. The difference between edge locations for a fixed pair of scan amplitudes, the differential radius, provides information about the slope of the limb-darkening function. The results of the observations made from 1990 through 1992 indicate that a global change in the limb-darkening function occurred and that, for certain years, the limb-darkening function was not uniform over solar latitude. Since the energy flux through the photosphere determines the temperature gradient and hence the limb-darkening function, these observations suggest time variations in solar luminosity originating in the quiet photosphere. The latitudinal nonuniformity of the solar limb darkening is shown to be compatible with perturbations to the temperature gradient confined to a relatively thin sub-photospheric layer. Additionally, limb-darkening variations should bear directly on the interpretation of solar diameter observations.