Change in the radio radius of the sun upon the emergence of a coronal hole at the limb

A careful review of the daily radio scans obtained on the Pulkovo Large Radio Telescope (PLRT) in 1973 and in 1974--1976 in the centimeter range permitted a study of effects connected with coronal holes against the background of the quiet sun. Against this background holes are low-contrast formations. The fact that a decrease by 3--6% in the radio radius at a wavelength of 9.0 cm is observed upon the emergence of a coronal hole at the limb was obtained by two independent methods: By absolute determinations of the distance from the center of the optical disk to the radio limb and by measurements of the distance from the limb to a local radio source as it moves over the solar disk. The effect is confirmed by a statistical study of the widths of radio scans for 1974--1976. The pronounced effect near the limb indicates that the observed increase in radio radius with a change from 3 to 9 cm is connected with the influence of high coronal loops, which exist not only in the active but also in the undisturbed corona