The ratio of the radio and optical diameters of the sun at centimeter wavelengths

The ratios of radio to optical diameter of the solar disc at 10.7 and 3 cm wavelengths are examined. The radio observations are daily east-west scans of the solar disc, made over the period 1975-1992, which includes almost tw o complete solar cycles. We find that the apparent disc diameter is slightl y greater at solar minimum than it is at solar maximum, suggesting that the radio diameter varies over the cycle. Moreover, the ratio is smaller at 3 cm wavelength than it is at 10.7 cm. at both solar maximum and minimum.