Diurnal variation of the linear polarization across the Hα and Hβ Fraunhofer lines of the terrestrial atmosphere, and a detection of a ‘daylight flash’, II

Abstract

In Paper I, the ability of using the polarimetric technique to observe the filling-in of the Fraunhofer spectrum line profiles—the Ring effect—has been presented (using the Hβ Fraunhofer line only). It has also been found that the added light, which caused the filling-in, is polarized. Here, a description is given of some observations, which, hopefully, will lead to a better understanding of the Ring effect using the Hα and Hβ Fraunhofer spectral lines. The observations of the filling-in effect at the zenith have covered many conditions which might be considered as controlling its behaviour at these two spectral regions. The data reveal variations of polarization at the centres of those Fraunhofer lines as a function of solar zenith angle. The results show no uniformity for the added light intensity, its value was depending on the kind of day (clear and turbid) and/or on the solar zenith angle. Also the photometric observational studies reported here suggest that a new phenomenon behaving like airglow has been discovered. It is referred to here as a ‘daylight flash’ and was recorded (for short periods) on three days at both the Hα and Hβ Fraunhofer lines with different strengths. The cores of these lines were filled in while the near continua were unaffected.