Night light polarization: modeling and observations of light pollution
in the presence of aerosols and background skylight or airglow
Abstract
Aurorae and nightglow are faint atmospheric emissions visible during
night-time at several wavelengths. These emissions have been extensively
studied but their polarization remains controversial. A great challenge
is that light pollution from cities and scattering in the lower
atmosphere interfere with polarization measurements. We introduce a new
polarized radiative transfer model able to compute the polarization
measured by a virtual instrument in a given nocturnal environment
recreating real world conditions (atmospheric and aerosol profiles,
light sources with complex geometries, terrain obstructions). The model,
based on single scattering equations in the atmosphere, is tested on a
few simple configurations to assess the effect of several key parameters
in controlled environments. {Our model constitutes a proof of concept
for polarization measurements in nocturnal conditions, that calls for
further investigations. In particular, we discuss how
multiple-scattering (neglected in the present study) {could} impact
our observations and their interpretation, and the future need for
inter-calibrating the source and the polarimeter in order to optimally
extract the information contained in this kind of measurements. The
model outputs are compared to field measurements in five wavelengths. A
convincing fit between the model predictions and observations is found
in the three most constrained wavelengths despite the single scattering
approximation. Several applications of our model are discussed that
concern the polarization of aurorae, the impact of light pollution, or
aerosols and air pollution measurements.}