Occurrence rate and duration of space weather impacts to high frequency radio communication used by aviation

High frequency (HF) radio wave propagation is sensitive to space weather induced ionospheric disturbances that result from enhanced photoionization and energetic particle precipitation. Recognizing the potential risk to HF radio communication systems used by the aviation industry, as well as potential impacts to GNSS navigation and the risk of elevated radiation levels, the International Civil Aviation Organization (ICAO) initiated development of a space weather advisory service. For HF systems, this service specifically identifies shortwave fadeout, auroral absorption, polar cap absorption, and post storm maximum useable frequency depression (PSD) as phenomena impacting HF radio communication, and specifies moderate and severe event thresholds to describe event severity. This paper examines the occurrence rate and duration of events crossing the moderate and severe thresholds. Shortwave fadeout was evaluated based on thresholds in the solar X-ray flux. Analysis of 40-years of solar X-ray flux data showed that moderate and severe level solar X-ray flares were observed, on average, 123 and 5 times per 11-year solar cycle, respectively. The mean event duration was 68 minutes for moderate level events and 132 minutes for severe level events. Auroral absorption events crossed the moderate threshold for 40 events per solar cycle, with a mean event duration of 5.1 hours. The severe threshold was crossed for 3 events per solar cycle with a mean event duration of 12 hours. Polar cap absorption had the longest mean duration at ~8 hours for moderate events and 1.6 days for severe events; on average, 24 moderate and 13 severe events observed per solar cycle. Moderate and severe thresholds for shortwave fadeout, auroral absorption, and polar cap absorption were used to determine the expected impacts on HF radio communication. Results for polar cap absorption and shortwave fadeout were consistent with each other, but the expected impact for auroral absorption was shown to be 2-3 times higher. Analysis of 22 years of ionosonde data showed moderate and severe PSD events occurred, on average, 200 and 56 times per 11-year solar cycle, respectively. The mean event duration was 5.5 hours for moderate level events and 8.5 hours for severe level events. During solar cycles 22 and 23, HF radio communication was expected to experience moderate or severe impacts due to the ionospheric disturbances caused by space weather a maximum of 163 and 78 days per year, respectively, due to the combined effect of absorption and PSD. The distribution of events is highly non-uniform with respect to solar cycle: 70% of moderate or severe events were observed during solar maximum compared to solar minimum.