Plain-language answer: Polar cap absorption is prolonged HF attenuation over polar regions caused mainly by energetic solar protons increasing lower-ionosphere ionization.
Why operators care
It can disrupt polar aviation, military, and amateur paths for hours or days even when mid-latitude paths remain usable.
What to compare on air
Check proton-event and polar absorption products, then consider lower-latitude routing or nonpolar paths and verify on air.
The common mistake
Treating PCA as a brief sunlit-side flare blackout.
Forecast boundary
Space-weather indices and propagation models describe probabilities and regional conditions. A usable contact still depends on the exact path, time, frequency, antenna pattern, noise, power, and current observations.
