R-scale
From Glossary of Meteorology
A NOAA classification scheme for radio blackout intensity and a means of communicating to the general public the likely effects to HF communications and navigation systems on the sunlit side of Earth. The R-scale correlates to solar flare intensity and is classified as follows: R1 (minor) M1 flare, R2 (moderate) M5 flare, R3 (strong) X1 flare, R4 (severe) X10 flare, and R5 (extreme) X20 flare.
Term edited 14 August 2018.