An analysis of full-disk observations of facular contrast in the blue and red

Abstract

Full-disk images from the Cartesian Full-Disk Telescope no. 2 (CFDT2) were used to study the center-to-limb (CLV) variation of facular contrast in two colors. The CFDT2 images, which have 2.5 arc sec pixels, were obtained during the summer months of 1993, 1994 and 1995. In order to minimize the bias in finding faint facular features in continuum images, we have used coaligned images obtained in the Ca K-line to identify faculae. Faculae were sorted into 20 annular bins of equal width. To reduce the effects of seeing, faculae were not identified closer to the limb than μ=0.2. The facular pixel contrasts were fitted to various trial functions. The contrast in the blue filter (470.6 nm) rose from 0.122% at disk center to 12.2% at μ=0.2. The contrast in the red filter (672.3 nm) rose from 0.13% at disk center to 8.16% at μ=0.2. We have also analyzed the facular contrasts multiplied by their μ-value to obtain an estimate of facular flux tube contrasts. These flux tube contrasts increased roughly linearly from μ=0.95 to 0.25. The blue flux tube contrast reached a maximum of 2.48% near μ=0.25. The red flux tube contrast reached a maximum of 1.59% at μ=0.2. These contrast values are not corrected for the filling factor. The blue curve leveled off slightly betwen μ=0.25 and 0.2 while the red curve showed no deviation from its linear trend. These results may provide some support for the hot wall model of facular flux tubes.