Solar Flare Classes
Explained
Solar flares are ranked A, B, C, M, or X, with each letter roughly 10 times stronger than the one before it. Here's what each class means, and why X-class flares are the ones that make the news.
What Is a Solar Flare?
A solar flare is a sudden, intense burst of radiation from the sun's surface, usually released near sunspots where magnetic field lines twist and snap. Flares are measured by the peak intensity of X-ray output they produce, and classified on a letter scale.
Flares travel at the speed of light, so their effects — when they happen — arrive at Earth in about 8 minutes. That's different from a CME (coronal mass ejection), which is a separate burst of solar material that can take 1–3 days to arrive and is what actually drives geomagnetic storms and the aurora.
The Flare Classes: A Through X
Each letter class is 10 times more powerful than the one before it, and each class has a numeric sub-rating from 1–9 (except X, which can go higher).
A & B-Class — Background Level
The weakest flares, indistinguishable from normal background solar activity. No noticeable effect on Earth.
C-Class — Small
Minor flares with few to no noticeable effects on Earth. These occur frequently, even during quiet solar periods.
M-Class — Medium
Can cause brief radio blackouts at the poles and minor radiation storms. Strong M-class flares are sometimes accompanied by a CME.
X-Class — Major
The strongest category. Can cause wide-reaching radio blackouts and radiation storms, and are often paired with a CME capable of triggering a major geomagnetic storm days later. An X10 flare is 10 times stronger than an X1.
What a Flare Actually Does on Earth
Radio blackouts
X-rays from a large flare ionize the upper atmosphere on the sunlit side of Earth, which can degrade or black out high-frequency radio communication for minutes to hours — NOAA rates this as its own R1–R5 radio blackout scale.
No direct effect on people
Earth's atmosphere and magnetic field block the radiation from flares. There's no health risk to people on the ground — the effects are on radio signals, satellites, and (if a CME follows) the power grid.
A preview, not the main event
The flare itself passes in minutes. What matters for aurora-watchers is whether it launched a CME — that's the slower-moving material that actually triggers the geomagnetic storm days later.
The Strongest Flares on Record
The largest flare ever directly measured was an X28 in November 2003, which actually saturated the detector on NOAA's monitoring satellite — the true peak may have been even higher, with some estimates as high as X45. It triggered a strong radio blackout and was followed by a significant geomagnetic storm. More recently, an X9.3 flare in September 2017 — the strongest of that entire solar cycle — caused a wide-reaching HF radio blackout across the sunlit side of Earth, including much of the Americas.
The 11-Year Solar Cycle
Flare frequency isn't constant — it rises and falls with the sun's roughly 11-year activity cycle, tracked by sunspot number. Near solar maximum, X-class flares can occur several times a month; near solar minimum, the sun can go weeks or months without producing one at all. Solar Cycle 25 reached its maximum around 2024–2025, which is part of why aurora and strong geomagnetic storms became noticeably more frequent and widely visible during that period.
Track Solar Activity in StormCast
StormCast's Space Weather dashboard shows recent solar flare activity alongside the live Kp index, current geomagnetic storm level, and CME arrival forecasts — so you can see the full picture from flare to potential aurora.
