SOLAR FLARE TYPES. “Scientists classify strong solar flares into three categories: C, M and X, with C being the weakest, M being mid-level and X the strongest. X flares are 10 times more powerful than M flares. X2 and X3 flares are twice and three times as potent, respectively, as X1 flares.”
http://www.feelguide.com/2015/03/17/severe-geomagnetic-storm-flares-causing-widespread-sleep-disturbances-othe-human-health-effects/