CORONAL HOLES AGAINST THE BACKGROUND OF THE SUN - OBSERVATIONS ON THELIMB WITH RATAN-600

Radio observations of some coronal holes (CH) on the disc during the l ow level of solar activity have been analyzed by authors, and consiste nt with each other models of undisturbed solar atmosphere and CH have been constructed. The observational effects connected with emergence o f some CH on the limb are considered here. Using one-dimensional scans with the subsequent reconstruction of the radial distribution of radi o brightness, it is shown that the radio radius and the limb brightnes s enhancement are decreased when large CH appear on the limb. For exam ple, the radio radius on the wavelength of 31,6 cm changes from 1,17 t o 1,07 R. and the limb brightness enhancement decreased by (55 + 5) % when CH goes behind the limb. The three-dimensional modelling is made on the base of recently proposed models, and the radio brightness dist ributions have been calculated for different locations of CH on the di sc. This results are in good agreement with observational effects at w avelengths 8-32 cm; there is evidence that the disc model can be used for interpretation of the limb observations. Some discrepancies of the calculations with the observations are related to the use of homogene ous models. Decimetric observations give some evidence for the arch st ructure of the lowest coronal layers. Possible variations of radio emi ssion of the quiet Sun and CH with phase of solar cycle are briefly di scussed.