Research Article

At the source of the polarisation of auroral emissions: experiments and modeling

¹ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) CNRS UGA, 3800 Grenoble, France
² Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, UGE, ISTerre, 38000 Grenoble, France
³ Univ. Lille, CNRS, UMR 8518 – LOA Laboratoire d’Optique Atmosphérique, 59000 Lille, France
⁴ Tromsø Geophysical Observatory, UiT – The Arctic University of Norway, 9019 Tromsø, Norway
⁵ Honorary Astronomer at Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium

^* Corresponding author: jean.lilensten@univ-grenoble-alpes.fr

Received: 29 July 2021
Accepted: 17 February 2022

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 1^st N₂⁺ 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.