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 two complete solar cycles. We find that the apparent disc diameter is slightly 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.