Evidence on chromospheric structure from observations of solar brightness distribution at millimetre wavelengths.

Observations of the brightness distributions at millimeter wavelengths over the quiet sun are presented and compared with models of chromospheric structure. Measurements at 3 mm made during a total eclipse and by means of aperture synthesis with a two-element interferometer indicate a brightness distribution which is flat with the exception of a narrow bright ring at the limb. The model of Vernazza (1973) is used to calculate brightness profiles at 3.0 and 8.6 mm and is found to be compatible with observations at 3.0 mm, however not with previous observations at 8.6 mm. The inclusion of chromospheric spicules in the model leads to good agreement with observations in the 3 mm range, but a large center to limb brightness rise at 8.6 mm. It is suggested that a model incorporating slightly modified properties of the interspicular medium and a roughened chromosphere would be able to predict more nearly the observed brightness profiles.