BRIGHTNESS OF THE SOLAR F-CORONA

We discuss our present knowledge about the brightness of the solar F-c orona in the wavelength range from the visible to the middle infrared. From the general trend of the observational data, the F-corona is reg arded as the continuous extension of the zodiacal light at smaller elo ngation of the line of sight. A contribution of thermal emission from dust is indicated by the increasing F-coronal brightness in comparison to the solar spectrum towards longer wavelength. As compared with the F-coronal brightness, the polarization and color in the visible regim e are not well determined due to the high sensitivity of these quantit ies to the observational accuracy. Aside from observational problems, our present interpretation of the F-coronal brightness is also limited due to ambiguities in the inversion of the line of sight integral. Ne vertheless, the measurements and model calculations of the brightness can be used to deduce some physical properties of dust grains. We show that the hump of the near-infrared brightness at 4 solar radii, which was sometimes observed in the corona, is related rather to the physic al properties of dust grains along the line of sight than to the exist ence of a dust ring as previously discussed. We also show that the app earance or disappearance of the near-infrared peak in the coronal brig htness cannot be described in any periodic cycle for each wavelength r ange.