Impacts of auroral precipitation on HF propagation: a hypothetical over-the-horizon radar case study

Over-the-horizon radar (OTHR) systems operating in the high-frequency (HF) band (3–30 MHz) are unique in their ability to detect targets at extreme ranges, offering cost-effective large-area surveillance. Due to their reliance on the reflective nature of the ionosphere in this band, OTHR systems are extremely sensitive to ionospheric conditions and can expect significant variations in operational performance. At high latitudes, the presence of auroral enhancements in the E-Region electron density can substantially modify the coverage area and frequency management of OTHR systems. In this study, HF raytracing is utilized to investigate these impacts for a hypothetical radar under different auroral conditions simulated using the Empirical Canadian High Arctic Ionospheric Model. Aurora were seen to increase maximum usable frequency from 8.5 to 26 MHz whilst also reducing median available range from 2,541 to 1,226 km and changing coverage area by −50.4% to 58.6%, for the greatest differences. Target interception showed large variations in path coverage of between 33%–115% and 0%–107% for two flight paths tested with precipitation toggled. Two distinct auroral propagation modes were observed, noted as the F-E ducted and Auroral E-modes. Long-range coverage provided by the auroral F-E ducted mode was of limited capacity with low solar activity due to reduced NmF2. F-mode propagation transitioned to the dominating Auroral E-mode between Auroral Electrojet index values of 50- and 200-nT. The significant variations in both frequency and coverage observed within this study highlight some aspects of the importance of considering aurora in OTHR modeling and design.