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

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 (atmospheric and aerosol profiles, light sources with complex geometries at ground and in the sky, terrain obstructions...). It has been successfully tested at mid latitude where sky emissions are of weak intensity. We show here 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 are polarised in the upper atmosphere, namely the green atomic oxygen line at 557.7 nm and the $1^{st}\ N^+_2$ 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.

Laboratory Experiments Confirm the Polarization of Auroral Emissions

“…We conclude that the polarization depends on the direction of the electron flux (or of the current in the upper atmosphere) since the electron path is driven in this case by the magnetic field lines. This is in agreement with the results found in the upper atmosphere by (Bosse et al., 2020, 2022) and therefore confirms that these observations are not only due to the auroral light scattered during the lower atmosphere crossing, nor to light pollution. However, we failed in determining the exact AoLP because of a too wide field of view, merging too many magnetic field lines, even if a slight effect of the rotation is seen in the blue line AoLP (but not in the purple one).…”

“…The same lines have been observed in auroras (Barthelemy et al, 2019;Bosse et al, 2020Bosse et al, , 2022. The average DoLP was evolving between 1% and 5% in the blue line, and up to 6% in the purple.…”

“…With this experiment, we aimed at reproducing in controlled and confined conditions the observations made in auroral conditions at high latitudes (Barthelemy et al., 2019; Bosse et al., 2020, 2022), in order to check whether the polarization observed from the ground is created in the upper atmosphere at the emission, or during the crossing of the light through the current ionospheric layer, or by light pollution and scattering in the lower atmosphere. The experimental setting prevents from any external pollution light, from the internal reflection on the glass chamber and from diffusion on aerosols.…”

“…A first attempt to disentangle the contribution of light scattering and ionospheric emissions was conducted by Bosse et al. (2022) by combining field measurements and polarized radiative transfer modelization. They show that scattering in the lower atmosphere cannot be fully neglected while it also cannot fully explain the observations.…”