PLIP: An imaging Polarimeter for the Auroral line Emissions

The measurements of the polarization of auroral emission lines in the Earth’s atmosphere is of particular interest for the understanding of the upper atmosphere but also for potential space weather applications. Emissions from the oxygen red line at 630 nm has been observed polarized since 2008 and the origin of the polarization is likely due to the imbalance of Zeeman sublevels, which comes from the magnetospheric electrons precipitating with a pitch angle distribution more or less aligned with the local magnetic field. The polarization of the blue line at 427.8 nm from N₂⁺, and the green light at 557.7 nm from the atomic oxygen have been also observed but their origin remains unknown. Those observations were carried out using multi-wavelength sensitive photo-polarimeters with a narrow field of view, of about 2°. Here we will present a new instrument, the Polar Lights Imaging Polarimeter (PLIP), using 4 high-resolution monochrome cooled CMOS cameras with very low read-out noise, and a FOV of approximately 44° x 30°. Those cameras are designed for faint deep sky objects, and paired with some 24mm lenses opened at F/2.8. We added up some linear polarization filters oriented at 0°, 45°, 90° and 135° to infer the DoLP and AoLP. Filter wheels have been added with narrow interference filters (with a FWHM of about 3 nm) centered on the blue (427.8 nm), green (557.7 nm) and red (630.0 nm) emission lines. Since the polarization can also be induced from Mie and Rayleigh scattering of the light pollution from nearby sources, a radiative transfer code POMEROL is used to infer the polarization from the auroral spectral lines only. Some preliminary results will be presented from an observation campaign in Norway perfomed in November 2022 and January 2023.