The ring effect in the sodium D2 Fraunhofer line of day skylight over Mawson, Antarctica

A dual, scanning Fabry‐Perot spectrometer at Mawson, Antarctica (67.6°S, 62.9°E), was used to obtain simultaneous direct solar and daytime sky spectra of the sodium D2 Fraunhofer absorption line at λ589 nm. The intensity of the Ring component of skylight was determined from these spectra. Our observations indicated that the absolute intensity of this component varied diurnally, maximizing at local noon. Observations over ice and snow gave greater absolute Ring intensities than those over open water. Fractional Ring intensities were typically 6% of the adjacent day sky continuum. During full daylight, no significant variation in fractional Ring intensity was observed with changing solar zenith angle; sharp increases were observed during twilight. These observations are consistent with inelastic molecular scattering in the atmosphere being the cause of the Ring effect.