The effect of large‐scale ionospheric gradients on backscatter ionograms

Abstract

This paper presents the results of the synthesis of a range of backscatter ionograms using ray tracing through model ionospheres. The backscatter ionograms were obtained by the Jindalee over‐the‐horizon radar facility at Alice Springs in northern Australia. Sample ionograms obtained during 1990 were used, and the study concentrated on reproducing effects due to sunrise‐sunset gradients and the equatorial anomaly. Backscatter 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 was calculated taking account of ray divergence, ionospheric absorption, and antenna patterns. Analytical ray tracing produced quite realistic results when horizontal gradients were small but did not reproduce prominent features observed during sunrise‐sunset or when propagation occurred through the equatorial anomaly region. Since the analytical ray tracing was restricted to a single vertical profile which could be tilted, this result shows that gradients in ionospheric electron density, rather than simple tilts, are most significant in determining propagation characteristics. Numerical ray tracing through ionospheric models based on the FAIM model reproduced dominant features of backscatter ionograms 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 ionograms, 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 backscatter ionogram traces can be synthesized and accurately interpreted in terms of ionospheric structure.