Polarization of auroral backscatter at 50 MHz

Abstract

Auroral backscatter from the lower E region was studied using two 50 MHz bistatic, continuous wave radar links which shared a common polarimetric receiver. The polarization parameters were defined in terms of the polarization ellipse, which is described by the ellipticity angle χ orientation angle ψ, and polarization ratio m. Spectral analysis was applied to the intensity measurements and its corresponding polarization parameters. Observations of typical auroral spectral types 1, 2, and 3 indicated that the scattering of a linearly polarized incident wave produced an essentially linear and highly polarized scattered wave. These results imply a small scattering volume and/or a small number of discrete scatterers located close to one another, “scatterer” referring to a volume where radar waves are scattered according to weak coherent scattering theory, and also reaffirms that the scattering process is a weak coherent one. These results are typical of most observations, but not all; an otherwise typical intensity spectrum may also exhibit variable polarization parameters with an appreciable reduction in the polarization ratio and/or signals of significant ellipticity. These anomalous properties can be explained as scatter coming from a number of individual scattering volumes within the scattering region (i.e., the effective radar viewing region), which are each influenced differently by Faraday rotation to and from the scattering region.