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.
How Flares Are Actually Measured
Flare class is measured by NOAA's GOES satellites, which carry X-ray sensors pointed continuously at the sun. Each satellite records the peak X-ray flux in watts per square meter during the event; that raw measurement is what gets converted into the letter-and-number classification. Because the measurement is continuous and automated, flare classification happens in near real time — scientists (and space weather apps) typically know a flare's class within minutes of it happening, well before its potential CME even leaves the sun.
Where Flares Come From
Flares originate in active regions — areas of intense, tangled magnetic field on the sun's surface, usually associated with sunspot groups. As the sun's differential rotation and convection twist these magnetic field lines, they store enormous amounts of energy. A flare is what happens when that field snaps and reconnects, releasing the stored energy explosively as radiation across the spectrum, from radio waves through visible light to X-rays. Larger, more complex sunspot groups are generally capable of producing larger flares, which is why space weather forecasters closely track sunspot group classifications, not just current flare activity, to estimate near-term risk.
Frequently Asked Questions
Can a solar flare hurt me directly?
No. Earth's atmosphere and magnetic field absorb the radiation from flares before it reaches the ground. There's no direct health risk to people on the surface from any flare class.
Do all X-class flares cause geomagnetic storms?
No. A flare is a burst of radiation; a geomagnetic storm requires a separate CME of solar material aimed at Earth. Many X-class flares occur without an Earth-directed CME, and produce a radio blackout but no storm at all.
How much warning do we get before a flare's effects arrive?
The flare's radiation (X-rays, radio blackout risk) arrives in about 8 minutes, the time it takes light to reach Earth — essentially no warning. If a CME follows, that arrives 1–3 days later, giving much more lead time for any resulting geomagnetic storm.
Are solar flares getting more frequent?
Frequency rises and falls with the roughly 11-year solar cycle, not a long-term trend. Activity peaked around 2024–2025 during Solar Cycle 25's maximum and will gradually decline over the following years.
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.
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A note on this guide: Thresholds and criteria described here are general and can vary by NWS forecast office, region, and over time. This content is educational — always check your local National Weather Service office for the specific criteria that apply to your area.