High-latitude ionospheric perturbations and gravity waves. II - Numerical simulations

We have calculated ionospheric perturbations produced by gravity waves at high latitudes. It is shown that gravity waves with reasonable parameters can produce strong ionospheric perturbations in the altitude range of 200-400 km. The ionospheric perturbations become weak above 400 km due to the presence of plasma diffusion. If the horizontal perturbation velocity of the neutrals due to gravity waves increases from 20 m/s at 200-km altitude to 30 m/s at 350-km altitude, the amplitude of the relative ionospheric density perturbations is about 30 percent below 350 km. A ray tracing program has been used to trace the ionospheric ray paths of HF rays passing through the strong periodic density perturbations resulting from the gravity waves. The paths of HF rays manifest some new and important features. Rays with small elevation angles are focused and reflected by the density undulations in the bottomside F region. These rays can undergo ground scattering at the so-called 'one-hop' range. The ray tracing results presented in this paper provide a reasonable explanation for simultaneous ground scatter and auroral scatter observations made by the Saskatoon SuperDARN HF radar. (Author)