Science & history · An active star
What an X-class flare means
A flare's class is a measurement of soft X-rays from one satellite instrument, not of how bright the Sun looks from your garden.
A, B, C, M, X
NOAA classifies flares by their peak X-ray flux between 0.1 and 0.8 nanometres, measured by the X-ray sensor on GOES satellites. Each letter is ten times the one before: A from 10⁻⁸ W/m², then B, C, M and X at 10⁻⁴ W/m². An M5 is five times an M1.
What reaches Earth, and when
A flare's light arrives at the speed of light, so its effect on the sunlit side of the upper atmosphere happens as the flare is seen. NOAA's radio blackout scale starts at R1 for an M1 flare and reaches R5 at X20. A flare does not by itself mean a storm is heading for Earth: that would be a separate coronal mass ejection, with its own forecast.
Sources
- NOAA Space Weather Prediction Center Solar Flares (Radio Blackouts) (opens in a new tab) for A/B/C/M/X classes from GOES XRS 0.1–0.8 nm peak flux; R1 = M1 … R5 = X20; sunlit side affected · checked 2 October 2026