Satellite Imagery Reveals Near‐Horizon Urban Light Emissions Impacting Ambient Environment

Abstract A novel method has been developed to characterize ground‐based light sources from nighttime satellite imagery. The fraction of light directed toward low elevation angles is determined relative to the total emission into the upper hemisphere. This approach aims to identify locations that exhibit an increased amount of light emission at angles near the horizontal, thereby contributing to the extensive propagation of light pollution into the surrounding environment. The method is based on analysis of the halo of diffuse light around cities, specifically the radiance values and , measured in the direction of an artificially lit urban area, before and beyond its nearest and farthest boundaries, as observed from satellite. Unlike traditional approaches, the method avoids the exhaustive process of directly monitoring all city lights but relies on and embraces relative radiance patterns. Although the method is not based solely on the ratio, values exceeding unity are typically associated with a high fraction of light flux at low elevation angles. The method presented here offers an experimentally simple approach for systematically identifying sources with substantial potential to transport light pollution over long distances ultimately contributing to skyglow even in remote areas. The results reveal the significant dependence of the method on aerosol optical depth and aerosol stratification around cities.