R-Scale (Radio Blackout Scale)

The official 1-to-5 scale NOAA's Space Weather Prediction Center uses to rate the severity of solar radio blackouts, caused by intense X-ray bursts from solar flares ionizing the sunlit side of Earth's upper atmosphere. R1 blackouts cause minor, brief degradation of high-frequency radio on the sunlit side; R5 extreme blackouts can knock out high-frequency radio communication across the entire sunlit side of Earth for an hour or more.