A solar flare is a sudden burst of energy erupting from the sun's corona. When the sun's magnetic field twists around sunspots, it can create a barrier that prevents energy from flowing outward. After enough energy builds up below the surface of the sun, it can burst out suddenly, sending a wave of energy out into the solar system.
Solar flares expel energy across the electromagnetic spectrum. A single flare can contain a burst of radio waves, gamma rays and other charged particles. In addition flares sometimes accompany larger events called coronal-mass ejections, in which the sun expels magnetically charged clouds of gas. Flares accompanied by coronal-mass ejections tend to have a greater impact and can cause more widespread disruption than solitary flares.
There are three classes of flares. C-class flares are the smallest and cause little disruption. M-class flares can trigger radio blackouts and radiation storms. X-class flares are the largest and can ionize large sections of the Earth's atmosphere and create vivid geomagnetic storms.
The sun undergoes an approximately 11-year activity cycle that influences how many sunspots, flares and coronal-mass ejections occur on a daily basis. During the quiet periods, the sun may send out one flare a week, whereas in a more active period, it may expel multiple flares in a single day.