Grandmontagne and later Vaucouleurs have observed discontinuities in the slope of the zenith-sky brightness curve during twilight at solar depressions of about 6°, 9°, 10°, and 12°, which are explained to be due to separate diffusing and luminescent layers in the upper atmosphere. No such evidence is, however, found by Koomen and co-workers in their recent photoelectric measurements in the United States.
The effect of a possible discontinuity could be enhanced by observing the difference in radiance of two similar patches of the zenith sky in the sun’s meridian, this being more susceptible to changes accompanying the crossing of a layer by the earth’s shadow. Observations on very clear days with a photoelectric differential twilight photometer recording this difference through three narrow-band interference filters have not disclosed any evidence of the discontinuities, the recorded curves running perfectly smooth till the end of astronomical twilight. From estimates of the least radiant energy which the photometer could detect, the possibility of detecting a layer of scattering material at the heights of the reported discontinuities is discussed.
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