Abstract
A polarised radiative transfer model (POMEROL) has been developed to compute the polarisation measured by a virtual instrument in a given nocturnal environment. This single-scattering model recreates real-world conditions (among them atmospheric and aerosol profiles, light sources with complex geometries at the ground and in the sky, terrain obstructions). It has been successfully tested at mid-latitudes where sky emissions are of weak intensity. We show a series of comparisons between POMEROL predictions and polarisation measurements during two field campaigns in the auroral zone, in both quiet and active conditions. These comparisons show the strength of the model to assess the aerosol characteristics in the lower atmosphere by using a mesospheric line. They also show that three main upper atmosphere emissions must be polarised: the green atomic oxygen line at 557.7 nm and the 1stN2+negative band at 391.4 nm (purple) and 427.8 nm (blue). This polarisation can be either created directly at the radiative de-excitation or may occur when the non-polarised emission crosses the ionospheric currents. We provide some of the potentialities it offers in the frame of space weather. These require refinements of the preliminary modeling approach considered in the present study.
Highlights
Since the first half of the 20th century, several attempts were made to observe the polarisation properties of upper atmospheric emissions, such as aurorae or nightglow (Harang, 1933; Duncan, 1959)
We show a series of comparisons between POMEROL predictions and polarisation measurements during two field campaigns in the auroral zone, in both quiet and active conditions
Given a set of entries for the instrumental setup, the atmospheric properties, and emissions maps, POMEROL integrates the contributions of all point sources along the line of sight to compute the flux, the Degree of Linear Polarisation (DoLP) and the Angle of Linear Polarisation (AoLP) of the light measured by the virtual instrument
Summary
Since the first half of the 20th century, several attempts were made to observe the polarisation properties of upper atmospheric emissions, such as aurorae or nightglow (Harang, 1933; Duncan, 1959). We recently developed a radiative transfer code designed to examine the nightglow polarisation (Bosse et al, 2021) This single-scattering model incorporates the contributions of light pollution from the ground, as well as the background sky glow (stars and airglow). It takes into account mechanisms such as Mie scattering by aerosols, Rayleigh scattering by air molecules, and possibly a polarisation at the emission. This paper aims to assess this possibility by modeling several polarisation sources (see Fig. 1): in the upper atmosphere or through scattering in the lower atmosphere The latter may be due either to pollution sources on ground, or to indirect illumination of the instrument by auroral emissions all over the sky, or to the reflection of the aurorae on the snowy landscape.
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