The Rayleigh scattering correction in measurements of the sodium twilight flash

Abstract

It is pointed out that sunlight at the wavelength of the D lines is attenuated by passage through the sodium layer before it is scattered by the lower atmosphere in twilight. Thus the use of the scattered intensity at another wavelength to correct for Rayleigh scattering will lead to an overcompensation. This effect increases for decreasing angles of solar depression and may lead to negative values for the twilight flash intensity. Calculations are presented for a standard detector and for a resonance detector consisting of a sodium vapour cell and a photomultiplier tube.