No abstract

We present here observations of the polarisation of four auroral lines in the auroral oval and in the polar cusp using a new ground polarimeter called Petit Cru . Our results confirm the already known polarisation of the red line, and show for the first time that the three other lines observed here (namely 557.7 nm, 391.4 nm and 427.8 nm) are polarised as well up to a few percent. We show that in several circumstances, this polarisation is linked to the local magnetic activity and to the state of the ionosphere through the electron density measured with EISCAT. However, we also show that the contribution of light pollution from nearby cities via Rayleigh scattering can not be ignored and can play an important role in polarisation measurements. This series of observations questions the geophysical origin of the polarisation. It also leaves open its relation to the magnetic field orientation and to the state of both the upper atmosphere and the troposphere.

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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.