An examination of radio-auroral aspect sensitivity

Abstract

An attempt has been made to account for the experimental observations of scattering at angles away from perpendicularity to the earth's magnetic field (aspect sensitivity). First, it was necessary to develop a scattering model appropriate for the plasma waves generally assumed to be responsible for the scattering, consisting of an assembly of irregularities, each one a wave with a Gaussoidal envelope. Then, effects were included for off-perpendicular propagation of plasma waves, the perturbation of the magnetic field direction owing to the presence of auroral currents, and refraction of the radar wave in the E region. Even when possible effects from anomalous resistivity are included, many of the experimental observations require scattering models consisting of irregularities that are elongated along the direction of the earth's magnetic field by only a few plasma wavelengths (in some cases no more than one or two wavelengths). This is physically unreasonable and suggests that our understanding of auroral E-region irregularities and (or) the scattering processes responsible for auroral echoes is still incomplete.