SUN · FIELD GUIDE
A, B, C, M, X: The Solar Flare Scale Explained
Why is a flare called 'M5' or 'X1.2' — and how much stronger is one class than the next?
Every solar flare gets a tidy label like C3.0, M1.5 or X2.8. Once you know how the scale works, you can read any flare report at a glance.
A logarithmic ladder
Flares are classified by their peak brightness in soft X-rays, on a five-rung letter scale:
A → B → C → M → X
The crucial thing: it's logarithmic. Each letter is ten times stronger than the one below it. So the jump from C to M is the same size as the jump from M to X — a full power of ten each time. An X-class flare is a hundred times brighter in X-rays than a C-class one.
The number is a fine-tune
After the letter comes a number from 1 to (almost) 10, and that part is linear:
- M1 is the bottom of the M band.
- M5 is five times M1 — halfway to the next rung up.
- X1 is ten times M1.
- X2 is twice X1.
The X class is open-ended: there's no "X10 cap." The largest flares of the modern era have reached the high X-tens, and the famous 2003 event saturated the sensors near X28.
What each rung means at Earth
- A / B — background flickering. You'd never notice.
- C — minor flares, common during an active solar cycle, little effect on the ground.
- M — moderate. Capable of short radio blackouts on the daylit half of Earth and minor satellite effects.
- X — the strongest. Wider, longer radio blackouts; possible GPS and satellite disruption; and, if a coronal mass ejection travels with the flare, a geomagnetic storm and aurora a day or two later.
Where the numbers come from
The classification isn't a guess — it's a direct measurement. NOAA's GOES satellites stare at the Sun with X-ray sensors around the clock, and the peak flux during each flare sets its class. That live feed is exactly what powers our solar-flare tracker: the class you see there is the real GOES reading, stamped to NOAA's Space Weather Prediction Center.
Frequently asked questions
How much stronger is an X flare than an M flare?
Ten times. The flare scale is logarithmic — each letter (A, B, C, M, X) marks a tenfold jump in peak X-ray brightness. So an X1 is ten times an M1, which is ten times a C1, and so on.
What does the number after the letter mean?
It's a linear multiplier within that letter band. An M5 is five times an M1 and halfway to an X1; an X2 is twice an X1. The X class has no upper cap, so the strongest flares can be X10, X20 or higher.
Which flare classes affect Earth?
A and B flares are background noise. C flares are minor. M flares can cause brief high-frequency radio blackouts on the Sun-facing side of Earth. X flares can cause stronger, wider blackouts and — if a coronal mass ejection accompanies them — major geomagnetic storms and aurora.
Who measures flare class?
NOAA's GOES weather satellites carry X-ray sensors that watch the Sun continuously. The peak X-ray flux during a flare sets its class. NOAA's Space Weather Prediction Center publishes the results, which is the data behind our live flare tracker.
SEE IT LIVE
Everything in this guide is on the live Sun tracker.