Transient perturbations 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:
Very low x-ray events less than C-class.
Low - C-class x-ray events.
Moderate - isolated (one to 4) M-class x-ray events.
High - several (5 or more) M5 or greater x-ray events.
The general term for all forms of short-lived phenomena on the Sun, including solar flares, sunspots, prominences, etc., indicating that the Sun is an active star.