Representation of the main physical processes at play in the following article. The instrument (lower left corner) observes two main light sources: the aurora (solid black arrows) and the scattering of another source of light in the lower atmosphere (dashed black arrows). The latter is represented by the center circle representing Mie and Rayleigh scattering on aerosol and air molecules. Its origin is shown here as street lamps but could be due to other sources like snow reflection or aurorae. The auroral polarisation model is uncertain at this point and could follow any of the three models represented in the top right numbered 1, 2, and 3. In those, the electron’s motion direction is the thin dotted lines, in 1, the magnetic field is the orange line (B), and in 2, the electric field is the purple line (E). Auroral emission is the thin black line with a polarisation direction represented by the red and green waves.