Flare classes
Flares are classified by their peak X-ray flux measured by GOES satellites between 0.1 and 0.8 nm. The classes are A, B, C, M and X, each ten times stronger than the previous one. A letter followed by a number gives the detail: an M5.2 flare has a peak flux of 5.2 × 10-5 W/m², and X1.0 means 10-4 W/m². The flare class converter does the maths for you.
The NOAA radio blackout scale
| Scale | Flare class | Effect on HF |
|---|---|---|
| R1 Minor | M1 | Weak degradation on the sunlit side, brief loss of contact |
| R2 Moderate | M5 | Limited blackout on the sunlit side, tens of minutes |
| R3 Strong | X1 | Wide-area blackout of HF on the sunlit side, about an hour |
| R4 Severe | X10 | HF blackout on most of the sunlit side for one to two hours |
| R5 Extreme | X20 | Complete HF blackout on the whole sunlit side for hours |
Why it happens
X-rays arrive at the speed of light and suddenly increase ionisation in the D layer. The extra absorption swallows HF signals passing through it. This is a sudden ionospheric disturbance: it starts instantly, only on the daylight side, and fades over minutes to hours as the flare decays. Night-side paths are not affected by the flare itself.
After the flare
Some flares are followed by a coronal mass ejection (CME). If it points towards Earth it arrives one to three days later and can drive a geomagnetic storm. Energetic protons from large events can also cause polar cap absorption, which blocks polar paths for days. Follow the situation on the forecast and alerts page.
