Statistical Analysis of Off-Great Circle Radio Wave Propagation in the Polar Cap

High latitude ionospheric density structures such as polar cap patches and arcs are capable of deflecting high frequency (HF) radio waves to off-great circle paths, and are likely detrimental to technologies dependent on HF radio propagation. In this study, nearly 2.5 years of 4.6–14.4 MHz data from a multi-frequency HF radio link between Qaanaaq, Greenland and Alert, Canada are used to investigate high-latitude off-great circle propagation in the polar cap. After an example of HF radio propagation affected by polar cap patches is shown in detail, a statistical analysis of the occurrence and impacts of off-great circle deflections in the polar cap is presented. Off-great circle propagation is shown to be increasingly common with increasing frequency up to 11.1 MHz, such that averaged over 1 year, received 11.1 MHz signals experienced deflections >30° from the great circle direction 65.6% of the time. The occurrence of these deflections across the year is shown to be at a maximum in the winter, while occurrence across the day varies with season. Trends across both time of day and time of year for 11.1 and 14.4 MHz deflections are consistent with polar cap patch occurrence trends. Off-great circle deflections are shown to be associated with increased time-of-flights, a larger range of positive and negative Doppler shifts, increased Doppler spreads, and lower signal-to-noise ratios. These results are discussed in the context of ionospheric phenomena in the polar cap, and implications for over-the-horizon radars operating at high latitudes.