The Polarisation of 50 MHZ Auroral Backscatter

This experiment consisted of two 50 MHz bistatic CW (Continuous Wave) radar links which viewed an approximately common scattering region in the lower E-region. The radars measured Doppler velocities along the bisectors of the links, which were approximately orthogonal to one another; the Gillam-Saskatoon link viewed approximately along the auroral electrojet while the La Crete-Saskatoon link viewed approximately perpendicular to it. The polarimetric receiver recorded vector information defined in terms of the polarisation ellipse, which is described by the ellipticity angle chi, orientation angle psi, and polarisation ratio m. Unlike previous experiments, this one provides polarisation parameters of the Doppler spectrum. Typical 50 MHz radar auroral spectral Types I, II, and III were observed. These events have uniform polarisation parameters and the polarisation is high, indicating well -defined scattering areas. Ellipticity angle values indicate linearly polarised waves are received; weak coherent scatter is thus confirmed. Results indicate the orientation angle is strongly dependent upon the experiment geometry and Faraday rotation. A model was constructed which takes into account these factors with respect to the scattering process. This model predicts, to first order, the received orientation angles. The ellipticity angle results reveal the occurrence of signals of significant ellipticity which are explained by overlapping spectra. The measurement procedure is unable to differentiate independent polarisation states of the same frequency from different areas of the scattering region; as such, a mean polarisation state consisting of the superposition of the independent polarisation states results. Although the transmitted waves are completely polarised, the received waves are only partially polarised. A reduction in the polarisation ratio of a received wave is expected as a result of the spatial extent of the scattering area and differential Faraday rotation. A simple model describing the depolarisation of single spectrum Type I and II waves is developed.