Spectropolarimetric Studies of the Sun, the Moon and the Daytime Sky

Basurah, Hassan Mohammed (1991) Spectropolarimetric Studies of the Sun, the Moon and the Daytime Sky. PhD thesis, University of Glasgow.

Abstract

Probably the earliest recorded application of polarimetry to astronomy was by Arago in 1809. He found that the light of the daytime sky and also that of the Moon is partially polarized. Since that time, polarimetric studies have grown enormously and now support many areas of astronomical research. Most of the radiation from the sky can be assigned to single scattering based on Rayleigh theory. In addition, other components are required to describe the situation fully, e.g. secondary scattering, Mie scattering from small particles, etc. As well as these linear mechanisms, Raman scattering is present, altering the spectral distribution of the radiation. Effects of luminescence/fluorescence may also be important in redistributing the scattered energies to other wavebands. These latter non-linear effects have bearing on the interpretation of the Ring effect - the filling-in of the Fraunhofer spectrum line profiles. The Moon also is thought to display a line filling-in phenomenon which has been modelled in terms of luminescence, Rayleigh-Brillouin scattering etc. These effects have been discovered and explored by spectrophotometry. Polarimetric studies are somewhat sparse but as such measurements give good insight to the understanding of scattering processes in general, they have good potential for investigating any proposed model. In this thesis, an account is given of the making of some polarimetric observations which hopefully will lead to a better understanding of certain atmospheric and lunar surface phenomena. The experiments have involved the use of instruments capable of performing high precision polarimetry with spectral resolution ~2A at the Halpha and Hbeta Fraunhofer spectral lines. Photometric and the polarimetric observations are reported for the Ring effect at the zenith (daytime sky) covering many physical conditions which might be considered as controlling its behaviour. This result enabled the discovery that the polarization within the Fraunhofer lines is enhanced to a degree (when the Sun is high in the sky) which is not explainable by the Raman scattering, one of mechanisms that have been proposed in relation to line filling-in. From the photometric observations with different ground albedos, a strong clue was found that the reflected light from the Earth is not the source of the Ring effect. 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 Halpha and Hbeta lines with different strengths. The cores of these lines were filled in while the near continua were unaffected. The lunar spectropolarimetric observations reported here are the first measurements of individual spectral lines with extremely high polarimetric accuracies, even better than any previous broad band measurements. In the literature it had been considered that the lunar luminescence is unpolarized and, if that were the case, its effect would be to reduce the observed degree of polarization at line centres with respect to the continuum. Results presented here show however that there is no such consistent pattern of spectral line polarimetric emerges. So, if lunar luminescence truly exists, the added radiation must be polarized, a result contrary to previous assumptions. Recently there has been much experimental activity to record the behaviour of the whole disk of the Sun so that direct comparisons can be made with stellar observations. Such studies are sometimes referred to as the "solar-stellar connection". Both photometry and spectrometry of the integrated light have been undertaken by various observers - but very little has been attempted using polarimetry. In the stellar field, polarimetry has provided details of rotation periods and basic geometries (e.g. inclination of rotation axis) for both early- and late-type stars. With improved instruments, efforts are now being made to explore the polarimetric behaviour of stars closer to the solar spectral type. For reference (and as an experimental challenge) it is of great interest to explore the spectro-polarimetric behaviour of the Sun. Part of this thesis gives an account of some experiments in this direction. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Astronomy
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78266
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:35
• Last Modified: 30 Jan 2020 15:35
• URI: https://theses.gla.ac.uk/id/eprint/78266