IonoFlux field guide
GOES X-ray class
What A, B, C, M and X-class solar X-rays mean for HF radio.
In simple terms
The X-ray class describes the strength of a solar flare as seen by NOAA satellites. A, B and C are smaller; M and X are stronger.
How to read it
A strong flare on the Sun-facing side can increase ionospheric absorption on the daylight side. Effects can arrive quickly and often affect the lower HF bands first.
A practical example
During an M-class flare, a daylight 20 m path can suddenly become noisy or disappear. Check the alert time and try again after conditions settle.