THE EFFECT OF LARGE-SCALE IONOSPHERIC GRADIENTS ON BACKSCATTER IONOGRAMS

This paper presents the results of the synthesis of a range of backsca tter ionogams using ray tracing through model ionospheres. The backsca tter ionograms were obtained by the Jindalee over-the-horizon radar fa cility at Alice Springs in northern Australia. Sample ionograms obtain ed during 1990 were used, and the study concentrated on reproducing ef fects due to sunrise-sunset gradients and the equatorial anomaly. Back scatter ionograms were synthesized using both analytical and numerical ray tracing through ionospheric models based on FAIM (fully analytic ionospheric model). To make the synthesis realistic, signal strength w as calculated taking account of ray divergence, ionospheric absorption , and antenna patterns. Analytical ray tracing produced quite realisti c results when horizontal gradients were small but did not reproduce p rominent features observed during sunrise-sunset or when propagation o ccurred through the equatorial anomaly region. Since the analytical ra y tracing was restricted to a single vertical profile which could be t ilted, this result shows that gradients in ionospheric electron densit y, rather than simple tilts. are most significant in determining propa gation characteristics. Numerical ray through ionospheric models based on the FAIM model reproduced dominant features of backscatter ionogra ms for those situations when analytical ray tracing proved inadequate. Major seasonal variations were also reproduced. Overall, the results of this initial study show that many premier features on backscatter i onograms, including the power variation of the backscattered signals, can be realistically modeled using ray tracing and ionospheric models. Further work is required before all the detailed structure of backsca tter ionogram traces can be synthesized and accurately interpreted in terms of ionospheric structure.