Transient perturbations of the solar atmosphere as measured by enhanced x-ray emission, typically associated with flares.

 Solar activity is described (also applicable on WWV and WWVH) according to the number of flares which occur during the day. Activity is basically classified as follows:

Very Low : X-ray events less than class C.
Low : C-class x-ray events.
Moderate : Isolated (one to 4) M-class x-ray events. High : Several (5 or more) M-class x-ray events or isolated (1 to 4) M5 or greater x-ray events.
Very High : Several M5 or greater x-ray events.

Mr. Vaino K. Lehtoranta