All-sky polarization imager deployment at Mauna Loa Observatory, Hawaii

Abstract

An all-sky imaging polarimeter was deployed in summer 2008 to the Mauna Loa Observatory in Hawaii to study clear-sky atmospheric skylight polarization. The imager, designed at Montana State University, operates in five distinct wavebands in the visible region of the spectrum and is capable of imaging the overhead skylight hemisphere with a fisheye lens. This thesis describes the Mauna Loa deployment and presents an initial comparison of these data to those observed by Coulson with a zenith-slice polarimeter in the late 1970s and early 1980s. We show how the all-sky imaging technique yields additional insight to the nature of skylight polarization beyond what is observed in a single zenith scan. It was found that the skylight polarization data collected compared well to that collected by Coulson. Furthermore, the polarization signatures obtained over the two week deployment were found to depend inherently on the underlying cloud cover at altitudes beneath the observatory. The different cloud topologies provided variable upwelling unpolarized light which entered the field of view of the instrument. As a result, the anticipated polarization signatures were reduced by variable amounts from this increased unpolarized radiation countering the strong skylight polarization band observed at 90° from the sun. Finally, to quantify the nature of the upwelling scattered radiation from the clouds to the variation in the degree of polarization, the maximum degree of polarization was fit to a decreasing exponential trend versus upwelling radiance as obtained from overhead satellites. Statistical correlation for these quantities was found to be favorable and this result can potentially yield useful prediction ability for skylight polarization signatures in clear-sky conditions.