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We review the last advances in the study of upper atmospheric emissions polarisation. Since 2008, observations and modeling initiatives aimed at detecting and understanding the auroral emission polarisation. In recent years, this field saw major advances, which confirm the ionospheric origin of the polarisation. Polarisation has been observed in all four main auroral visible emissions lines (the red (630 nm), green (557.7 nm), blue (427.8) and purple (391.4 nm)), in several geomagnetic and auroral activity levels and has been confirmed for the N2+ lines through laboratory experiments. However, the origin of this polarisation is still debated. Several points show that it cannot be due to atmospheric scattering, and must originate from ionosphere. The link between the polarisation state of the emission and the local ionospheric conditions is still uncertain and raises a number of questions, such as: Can ground based measurements of the polarisation with light instruments track ionospheric currents? What is the origin of the green line polarisation? Our international collaboration gathers several instruments dedicated to the observations of the auroral emission polarisation, mainly located at the Skibotn observatory in Norway. The CRU series instruments, which are spectro-photo polarimeters able to measure faint polarized signals in a 2 FOV, are coupled with PLIP, for Polar Lights Imaging Polarimeter, able to measure polarization of the three main auroral emissions on a large FOV (~44 30) on the sky. Some results will be presented from our last observation campaigns in Skibotn, Norway. Combining the CRU instruments with the PLIP imager opens a new chapter of investigation.
Cessateur et al. (Mon,) studied this question.