Solar Flare Class Table
| Class | Peak soft X-ray flux (W/m²) | NOAA radio blackout scale | Practical read |
|---|---|---|---|
| A | below 10-7 | — | Background hum near the instrument floor |
| B | 10-7 – 10-6 | — | Quiet-Sun variation, nothing detectable downstream |
| C | 10-6 – 10-5 | — | Routine small flares, dozens per day near solar maximum |
| M | 10-5 – 10-4 | R1 (M1) to R2 (M5) | Radio operators notice; brief HF degradation on the sunlit side |
| X | above 10-4 | R3 (X1) up to R5 (X20) | Wide-area HF blackout and navigation errors possible |
Solar flares are graded by the soft X-ray brightness they deliver at Earth, measured continuously by the GOES satellites, and the grading is a logarithmic ladder: A, B, C, M, X - each class ten times stronger than the one before. A C flare is ten Bs; an X flare is ten Ms and a hundred Cs. That geometry is the chart's first lesson: the alphabet is not a list of five kinds of flares but five decades of one dial, and the Sun spends most of its time wiggling in A, B and C while the news only notices M and X.
The second lesson is that the flare letters plug directly into technology through the NOAA radio blackout scale: an M1 flare triggers an R1 minor blackout (about 2,000 events per solar cycle), an M5 an R2, an X1 the strong R3 wide-area HF blackout (175 per cycle), and an X10 or X20 the severe-to-extreme R4-R5 events that black out high-frequency radio across the entire sunlit face of the Earth. The most powerful flare ever measured, on 4 November 2003, saturated the GOES detectors at an estimated X28 - later revised to X45 - and the 1859 Carrington Event reconstructs to about X45 from magnetometer records.
How to use
- Read a flare report by its letter and number as one value: the letter is the power-of-ten class and the number is the position inside it, so an M5.7 flare sits at 5.7 times 10 to the minus 5 watts per square meter - stronger than every M1-M4, weaker than every M6+, and one full class below the smallest X flare.
- Convert a flare to its consequences with the R column: radio users care whether the class crosses an R trigger (M1, M5, X1, X10, X20) more than the exact flux - R1-R2 mean brief degradation and navigation errors, R3 is loss of HF contact for about an hour on the sunlit side, and R4-R5 are the events that push airlines off polar routes.
- Weight the class against the solar cycle: near solar maximum the daily flare count is dominated by C-class events with M flares several times a week, while X flares arrive a handful of times per cycle - so a single X-class headline says less about a rising trend than the week's full A-to-X distribution does.
Frequently asked questions
What is the strongest solar flare class?
X is the top of the published ladder, and it has no ceiling: the class simply means a soft X-ray flux above 10 to the minus 4 watts per square meter, so an X flare can be an X1 or, in principle, an X100. The largest ever measured by instruments was the 4 November 2003 flare, which saturated the GOES detectors - initially estimated at X28, later revised to about X45 - and the 1859 Carrington Event, reconstructed from its magnetic recordings, is estimated at roughly X45 (plus or minus 5). When reports speak of an X-class flare without a number, that usually means X1, the floor of the class and a strong R3 radio blackout.
What is the difference between an X-class flare and a coronal mass ejection?
A flare is a burst of electromagnetic radiation - light and X-rays - that reaches Earth at the speed of light in about 8 minutes; a coronal mass ejection is an eruption of actual solar plasma that takes one to three days to arrive. They often erupt from the same region and are graded on separate scales: the flare by its X-ray class (this table) and the CME by its speed and Earth-directed mass. The radiation of a flare is what causes radio blackouts in real time, while the CME's magnetic cloud is what later drives the geomagnetic storms measured by the Kp index - which is why a big X flare followed by an Earth-directed CME is the two-punch sequence space weather forecasters watch for.
How often do M and X class flares occur?
It swings with the 11-year cycle. NOAA's R-scale statistics make the rhythm concrete: R1-level events (M1 class) run about 2,000 per solar cycle - roughly 950 days with at least one - while R2 (M5) events occur on about 300 days, R3 (X1) on about 140, R4 (X10) on just 8, and R5 (X20) is rarer than once per cycle. Near solar maximum, C-class flares fire dozens of times a day and M flares several times a week; at minimum, weeks can pass with nothing above B.
Can a solar flare affect GPS or airline flights?
Yes, through two separate doors. The flare's X-ray radiation arrives in minutes and changes the ionosphere, degrading GPS accuracy and absorbing HF radio - the R-scale events in the table, which is why polar-route airlines reroute during major flare activity. The associated coronal mass ejection, if Earth-directed, arrives days later and drives the geomagnetic storm (the Kp/G scale) with its own satellite-drag, navigation and power-grid effects. The X45-class 2003 Halloween storms demonstrated both doors at once: real-time radio blackouts from flares and, days later, G5 geomagnetic storms from the CMEs.