Coronal holes against the quiet-Sun background: an analysis of RATAN-600 observations in the 2-32 cm wavelength range

Observations of coronal holes (CH) and neighbouring solar-surface regions have been carried at ten wavelengths of the cm- and dm-range at the RATAN-600 radio telescope. In 1984-1986, four large CH were investigated. Brightness -temperature spectra have been constructed for CH and for quiet-Sun regions. The holes are distinctly seen as dark patches on the quiet-Sun background at the wavelengths from 4 cm; with growing λ, the CH contrast is growing, reaching 50 per cent in brightness temperature at λ=32 cm. On the basis of the radio data obtained and using the results of EUV-lines and optical solar observations, homogeneous models have been constructed, by means of the unique method, for upper layers of the quiescent solar atmosphere and for CH