Radio auroral scattering anisotropy inferred from 42 MHz polarization studies

Abstract

Polarization measurements of 42.1 MHz continuous-wave (CW) backscatter made on two occasions during 1973–1974 are reported. When the orientation (ψ), ellipticity (χ), and polarization ratio ratio (m) are plotted vs. intensity, they exhibit a more well-defined behaviour at high intensities than at low intensities, so only the former case is discussed. The most striking feature of the results is that the received orientation is opposite to that expected on the basis of Faraday rotation, which implies that the scattering process must be an important factor in modifying the polarization state. To explain the results, a simple three-stage model of auroral backscatter is presented involving Faraday rotation [Formula: see text] on the way to the scatterer, anisotropic scattering, and equal Faraday rotation [Formula: see text] on the return path from the scatterer. Using the above model, one must invoke an anisotropic scattering process which introduces a phase anisotropy of 180° between the electric field components parallel and perpendicular to the earth's field, and an amplitude anisotropy ratio in the range 2:1 to 4:1 for these components respectively. There is a received polarization ratio of about 0.95, which can be explained readily by introducing into the model some spread, both in the amplitude anisotropy of the scatterers and in the Faraday rotation, the latter spread being due to spatial separation of the scatterers.