The field guide · Sun
What is a solar radiation storm?
A solar radiation storm is an elevated energetic-particle event, classified by NOAA from S1 to S5 using proton measurements. It is different from a geomagnetic storm on the G scale and a flare-related radio blackout on the R scale.
What is being measured?
Solar eruptions can accelerate charged particles to high energies. NOAA’s S scale uses the flux of protons with energies of at least 10 MeV. The S1 threshold is 10 proton flux units, or pfu. A pfu expresses particles per square centimeter, per second, per steradian.
NOAA’s radiation-storm explanation describes these measurements and their relationship to GOES observations. This is a particle-flux quantity, not an X-ray flare class or a magnetic-field strength.
The five S levels
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| Level | Name | Proton flux threshold, ≥10 MeV |
|---|---|---|
| S1 | Minor | 10 pfu |
| S2 | Moderate | 100 pfu |
| S3 | Strong | 1,000 pfu |
| S4 | Severe | 10,000 pfu |
| S5 | Extreme | 100,000 pfu |
These thresholds classify the measured environment. They do not directly specify a person’s absorbed dose. The full NOAA scale describes possible effects in the relevant settings.
Why the distinction matters
Radiation-storm products are particularly relevant to space operations, spacecraft electronics and some polar or high-altitude aviation contexts. They should not be translated into a universal claim that everyone at ground level is experiencing the same exposure.
For operational decisions, use the appropriate official product and qualified procedures. A general educational page cannot replace spacecraft, airline or radiation-protection guidance.
Particle arrival is not CME arrival
Energetic particles and a CME’s bulk plasma are different parts of a solar event. They do not have to arrive together. A radiation-storm alert is therefore not proof that the main CME cloud has already reached Earth.
Likewise, a high X-ray flare class is not a conversion formula to an S level. Read the actual proton product for the particle event and the relevant magnetic index for Earth’s geomagnetic response.
How to read a particle plot
Check the energy threshold, units, time range and whether the plot uses a logarithmic axis. Multiple traces may represent different particle-energy channels. Comparing their heights without the legend can compare unlike quantities.
If a data stream has a gap, preserve it as missing. An absent measurement does not establish that particle flux fell to zero. The same principle appears in our chart-reading guide, even though the instruments and physical quantities are different.
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