The auroral red line polarisation: modelling and measurements

Abstract

In this work, we model the polarisation of the auroral red line using the electron impact theory developed by Bommier et al. (2011). This theory enables the computation of the distribution of the Degree of Linear Polarisation (DoLP) as a function of height if the flux of precipitated electrons is provided as input. An electron transport code is used to infer the stationary electron flux at each altitude in the ionosphere as a function of energy and pitch angle. Using adequate cross-sections, the integral of this electron flux over energy and pitch angle provides an anisotropy parameter from which the theoretical local DoLP can be computed at each altitude. The modelled DoLP is then derived by integrating along the line-of-sight. Depending on the integration length, the modelled DoLP ranges between 0.6% for a very long integration length and 1.8% for a very short integration length localised around an altitude of 210 km. A parametric study is performed to check how the characteristics of the local DoLP (maximum value, altitude of the maximum, integrated height profile) vary. It is found that the polarisation is highly sensitive to the scattering function of the electrons, to the electron precipitation and to the geomagnetic activity. We compare these values to measured ones obtained during an observational campaign performed in February 2012 from Svalbard. The measured DoLP during the campaign was 1.9% ± 0.1%. The comparison between this value and the theoretical one is discussed. Discrepancies may be due to the poor constraint of the input parameters (thermosphere and ionosphere), to the fact that only electron precipitation is considered in this approach (and not proton precipitation for instance) and to the difficulty in constraining the exact width of the emission layer in the thermosphere.

Measurement of the polarisation in the auroral N2+ 427.8 nm band

“…This have been performed for the night of March 2nd. The DoLP in this case is 4.6% ± 0.9% with the ratio method which is in the same order of magnitude compared to previous measurements of the red line DoLP (Lilensten et al, 2008(Lilensten et al, , 2015.…”

“…Subsequent observations (e.g., Barthelemy et al, 2011) confirmed that the red line is polarized at a level of 2-3% with significant variations on short time scales. This new observable could be useful either to monitor the response of the upper atmosphere to particle precipitations (Lilensten et al, 2015) or possibly to map the variations of the direction of the local magnetic field during geomagnetic activity (Lilensten et al, 2016). Polarization is always linked to some anisotropy, either in the emission process itself or along the line-of-sight.…”

The thermospheric auroral red line Angle of Linear Polarization

“…The red line is the signal reconstructed from a fit on the filtered data as in Lilensten et al []. The black line is the signal reconstructed from the Stockes parameters deduced with a time‐dependent Mueller method as in Lilensten et al []. (top) The polarization lens orientation (with respect to the vertical axis) was 0°.…”

“…The process was later improved in Lilensten et al [2013] to take account of the instrumental polarization using the Mueller formalism [Goldstein, 2003;Stenflo, 1994;Martinez Herrero et al, 2009;Clarke, 2010]. Recently, this latter approach was again improved by taking into account the temporal variability of the polarization during the rotation of the polarizing lens [Lilensten et al, 2015]. All three data processing are still used and are compared in Figure 2 on a set of calibration tests where an additional polarization lens was placed in front of the polarized channel and oriented along different angles with respect to the vertical axis.…”

“…Indeed, the precipitating electrons enter the upper atmosphere with their center of mass following the magnetic field line. Secondary electrons, although less focused [ Porter et al , ; Lilensten et al , ], still follow the magnetic field lines. The oxygen atoms are then mostly hit by primary and secondary electrons with a well‐focused pitch angle, and it is likely that the emission due to the deexcitation vibrates globally in a homogenous direction.…”