The field guide · Sun
Solar flares: what the letters mean.
A solar flare is a sudden release of electromagnetic radiation from the Sun. The A, B, C, M and X classes describe peak soft X-ray flux measured near Earth. Each class begins ten times above the previous one; flare class alone does not predict a geomagnetic storm.
The classification measures a specific signal
The class is based on peak flux in the 0.1–0.8 nm soft X-ray band, not the apparent size of an orange feature in a solar photograph. NOAA’s flare reference explains the measurement and its connection to radio blackouts.
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| Class begins at | Peak soft X-ray flux |
|---|---|
| A1 | 10⁻⁸ W/m² |
| B1 | 10⁻⁷ W/m² |
| C1 | 10⁻⁶ W/m² |
| M1 | 10⁻⁵ W/m² |
| X1 | 10⁻⁴ W/m² |
Open chart at full size ↗The figure shows class thresholds, not observed flares. Its logarithmic axis makes each tenfold step equally spaced. X is not capped at X9: a sufficiently large event can have a larger multiplier.
How to read M5 or X2
The number multiplies the class threshold. M5 is 5 × 10⁻⁵ W/m². X2 is 2 × 10⁻⁴ W/m², four times M5 in this particular peak-flux measure. This arithmetic does not mean four times every possible consequence on Earth.
When reading a headline, retain both the class and the event time. A large flare from yesterday is not a statement about the Sun’s current X-ray flux.
Why a flare can affect radio quickly
Flare radiation travels at the speed of light. Increased X-ray and extreme-ultraviolet input can alter ionization on Earth’s sunlit side, affecting high-frequency radio propagation. NOAA reports radio-blackout severity with its R scale.
That effect is different from the later arrival of a cloud of magnetized plasma. Mixing the two creates misleading claims such as “the flare will arrive in three days.” The CME guide explains what can take much longer to travel.
A bright solar image is not a storm forecast
The archival SDO film below shows a prominence in false-color extreme ultraviolet imagery. It helps make solar structure visible, but it is not a visible-light photograph, a current observation or a labelled example of a particular X-ray flare class.
To judge a specific event, use its instrument report and original date. A dramatic visual and a high X-ray class still do not establish that an Earth-directed CME occurred or that a particular G-level storm followed.
Sources
- NOAA SWPC: flare classification and radio effects.
- NASA SVS: July 1, 2013 solar prominence, SDO/AIA 304.
- NOAA SWPC: CMEs are eruptions of plasma and magnetic field.
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