High-latitude off-great circle propagation associated with the solar terminator

Large-scale ionospheric gradients associated with the solar terminator can deflect high frequency (HF) radio waves to off-great circle paths during the morning and evening, negatively impacting technologies reliant on HF radio wave propagation. For example, geolocation algorithms used by scientific and military over-the-horizon radars (OTHRs) generally assume on-great circle propagation, and thus lateral deviations from the great-circle path can lead to positioning errors. In this study, radio wave propagation is simulated via 3D numerical ray traces though an empirical, high-latitude model ionosphere initialized for a variety of times of the day and year to explore and quantify high-latitude off-great circle propagation associated with the solar terminator. Analysis of these simulations show large scale east-west ionospheric gradients due to the solar terminator can cause lateral deviations in north-directed propagation paths exceeding 20° at sunrise and sunset depending on radio wave frequency, though the largest portion of received signal power tends to experience maximum deflections of 5°. An exploration of the dependence of propagation direction on deflection shows that propagation paths parallel to the solar terminator tend to experience the largest deflections. Since the solar terminator at high latitudes is at an angle with respect to north in the winter and summer, propagation paths oriented west or east of north can experience larger deflections than north oriented paths at sunrise and sunset during these times of year. Impacts of these diurnal deflections on the operation of OTHR and scientific radar are discussed, as well as possible strategies for mitigating them.