Solar activity. Transient pertubations of the solar atmosphere as measured by enhanced x-ray emission (see x-ray flare class), typically associated with flares. Five standard terms are used to describe the activity observed or expected within a 24-h period:
(1)  Very low - x-ray events less C-class
(2)  Low - C-class x-ray events
(3)  Moderate - isolated (1 to 4) M-class x-ray events
(4)  High - several (5 or more) M-class x-ray events, or (1 to 4) M5 or greater x-ray events
(5)  Very high - several (5 or more) M5 or greater x-ray events