Polarization characteristics of 42-MHz auroral backscatter

Abstract

During April and May 1964, a time-sharing polarimeter at Saskatoon made rapid recordings of the complete polarization state of 42-MHz oblique backscatter from some half-dozen strong breakup auroras. Two distinctly different types of polarization are presented here. The first type is characterized by a high polarization ratio m that increases with increasing backscatter intensity, differs significantly from the transmitted polarization, and has great diversity in the ellipticity and orientation of the received ellipses. This type appears to be associated with a type of fading that is quasi-sinusoidal with dominant frequencies in the 4- to 25-Hz range and that is associated with the early phase of breakup events. The second type of scatter is characterized by a lower polarization ratio m, which decreases as the backscatter intensity increases. The elliptically polarized portion of the second scatter is much closer to the transmitted horizontal linear polarization than in the first type. This second type displays more irregular fading and so shows higher frequency components in its power spectrum than the first type. It is argued that the first type arises from a limited number of strong scatterers having complex anisotropic reflection coefficients. The second type seems to arise from a large number of scatterers that are weaker and that produce largely incoherent scatter.