Solar Flare WatchSolar flare classes explained
Flares are classified by their peak X-ray flux in the 0.1–0.8 nm band measured by GOES satellites. Each letter is ten times stronger than the previous one: A (below 10⁻⁷ W/m²), B (10⁻⁷), C (10⁻⁶), M (10⁻⁵) and X (10⁻⁴ and above). The number after the letter is a multiplier, so M5 is five times M1 and X2 is twice X1. Strong flares can disturb high-frequency radio on the sunlit side of the Earth; NOAA describes this with its radio blackout scale (R1–R5).
| Class | W/m² |
|---|---|
| A | < 10⁻⁷ |
| B | 10⁻⁷ – 10⁻⁶ |
| C | 10⁻⁶ – 10⁻⁵ |
| M | 10⁻⁵ – 10⁻⁴ |
| X | ≥ 10⁻⁴ |
NOAA radio blackout scale
| Peak X-ray class | Scale |
|---|---|
| M1 | R1 |
| M5 | R2 |
| X1 | R3 |
| X10 | R4 |
| X20 | R5 |
About this page
What the A, B, C, M and X classes of solar flares mean, how they are measured and how they relate to NOAA’s radio blackout scale.