Astronomy Magazine
Glossary · The Sun & Space Weather
Solar flare classes
A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays.
Flares are graded by peak X-ray brightness on a scale where each letter is ten times the last: A, B, C, M, X. Within a letter the number is linear, so an X2 is twice an X1, and the X class has no ceiling — the strongest measured, in 2003, saturated the detectors and was reconstructed as about X45. C-class flares are routine and harmless. M-class can black out high-frequency radio on the sunlit side of Earth for minutes. X-class can do that and more, and if the flare comes with a well-aimed coronal mass ejection, the interesting part arrives a day or two afterwards.
Where it has come up lately
Related terms
Coronal mass ejection
A billion tonnes of solar atmosphere thrown into space, and the cause of most serious aurorae.
Kp index
A 0–9 scale of how disturbed Earth's magnetic field is right now.
The solar cycle
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Sunspot
A patch of the Sun where the magnetic field is strong enough to hold the heat back.
Questions
What is Solar flare classes?
A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays.
Why does it matter?
M-class can black out high-frequency radio on the sunlit side of Earth for minutes. X-class can do that and more, and if the flare comes with a well-aimed coronal mass ejection, the interesting part arrives a day or two afterwards.