Space Weather Impact
Aviation HF Communication Outage Control Center
The same X-ray flux that drives the official NOAA R-scale is the real, direct cause of HF radio outages on polar flight paths.
GOES X-ray flux (live)
NOAA GOES
Current R-scale (official NOAA)
NOAA SWPC
Commercial aircraft on long-haul cross-polar routes rely on high-frequency (HF) radio for communication in regions beyond satellite and VHF ground-station coverage. During a strong solar flare, a burst of X-rays reaches Earth's dayside atmosphere within about 8 minutes — the time it takes light to travel from the Sun — and ionizes the D-region, the lowest layer of the ionosphere. That extra ionization absorbs HF radio signals passing through it, degrading or completely blocking communication on the sunlit side of Earth for the duration of the event.
How it works
This is the direct, physical basis for D-Region Absorption Predictions (D-RAP): the stronger and harder (shorter-wavelength) the X-ray flux, the deeper into the atmosphere the ionization reaches, and the more severe the resulting HF absorption. This is also exactly what drives NOAA's official R-scale (radio blackout) classification — see the Radio Blackout Tracker for the current official level.