Natural variation of the colour and spectrum of the night sky observed at a potential european reference site for dark skies

Abstract

A critical measure of the extent of light pollution is the amount of artificial light in the night sky. Some recommendations for light pollution regulation use the ratio of artificial-to-natural night sky radiance or luminance. However, the spectral radiance of the natural sky changes constantly due to changes in the airglow. In addition, the radiances of oxygen, sodium and hydroxyl lines can vary between different locations since Earths magnetic field, and the atmospheric structure is not homogenous. While it has long been known that the night sky radiance is constantly changing, it is still challenging to make a general prediction about the exact night sky background radiance at a given location. To fill this knowledge gap, measurements at sites unaffected by light pollution are urgently needed. However, it is hard to find places not affected by significant light pollution in continental Europe to perform such measurements. Our 2021 Central-European survey found a promising candidate for an easily reachable dark reference location. Based on recent measurements, we propose the Sȵlktler Naturpark in the Austrian Alps as a long-term monitoring site to obtain information on the spectral variation of the natural night sky. The spectroradiometric and all-sky radiometric measurements can provide a reference database for interpreting data at other locations.Here we show the first spectral and imaging data from the proposed reference site. The pure zenith radiance data obtained from our measurements do not describe the quality of the night sky well, as it gives brighter values than expected for a dark location. However, the excess in sky radiance has resulted from natural sources. We prove that spectroradiometric observations best estimate the artificial-to-natural sky radiance ratio. Long-term measurements at the reference site will help in better understanding the variation of sky radiance at other locations with low levels of light pollution.