From AMS Glossary
A NOAA classification scheme for radiation storm or solar proton event severity and a means of communicating to the general public possible effects to people at high latitudinal altitudes (i.e., polar-region airline flights), to astronauts, or on systems. The S-scale is logarithmic and correlates to the number of energetic protons at the >10-MeV energy level as measured at geosynchronous orbit. The scale is classified as follows: S1 (minor): 10 pfu; S2 (moderate): 100 pfu; S3 (strong): 1000 pfu; S4 (severe): 10 000 pfu; and S5 (extreme): 100 000 pfu.
Space Weather Prediction Center, 2018: Solar radiation storm. Accessed 10 September 2018. Available at https://www.swpc.noaa.gov/phenomena/solar-radiation-storm.
Term edited 10 September 2018.