| Abstrakt: |
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. Plain Language Summary: Over‐the‐horizon radar (OTHR) systems can detect targets at extreme ranges by reflecting high‐frequency radio waves in the 3–30 MHz band off the ionosphere, an ionized portion of the upper atmosphere, to beyond the horizon. Due to the large coverage area provided by a single radar, OTHR can be a cost‐effective solution to monitoring expansive areas; however, as OTHR relies on the ionosphere to reflect radio waves, any disturbances that may be present within the ionosphere, such as aurora, can have a profound effect on the operation of OTHR systems. This may be realized as a drastic change in available coverage area and a disruption to operational frequency, amongst other effects. To better understand the direct implications of aurora on OTHR operation, we model a hypothetical OTHR using specialized computer code to represent the ionosphere and trace the path of radio waves under different conditions. We find that principally, aurora act to increase the maximum frequency available to the radar whilst also reducing the overall range of the covered area. In addition to this, we observed aurora reflecting radio waves in two new manners, unavailable without aurora present; this included the ducting of radio waves and the introduction of lower altitude reflections. Key Points: Auroral conditions associated with Auroral Electrojet index >100 nT substantially modify the coverage of a hypothetical over‐the‐horizon radar (OTHR) in northern UKEnhanced auroral activity causes a significant variation in the coverage area of a northern UK OTHR system and an increase in overall maximum usable frequencyIn instances of low background electron density, such as solar minimum winter, aurora may be the only available mode for OTHR operation [ABSTRACT FROM AUTHOR] |
