There are 3 categories: X-class flares are big; they are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized; they can cause brief radio blackouts that affect Earth’s polar regions. Minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small with few noticeable consequences here on Earth.
NOAA Space Weather After producing a pair of R3 (Strong) Radio Blackouts in quick succession yesterday morning (10/1142 and 10/1252 UTC), active Region 2087 produced yet another R3 event today at 11/0906 UTC. Impacts from this activity were short lived and affected HF communications for the daylit side of Earth at the time of the flare. Continuing chances for more events R3 or greater events exists. Unlike yesterday, a Coronal Mass Ejection (CME) is not believed to be associated with this latest impulsive event. A CME assoicated with the activity yesterday morning has been observed moving at a flank from Earth and a glancing blow to Earth from this event is expected on June 13. An outside chance of at most G1 (Minor) Geomagnetic storms remains in the forecast.