On August 9, 2011 at 3:48 a.m. EDT, the sun emitted an Earth-directed X6.9 flare, as measured by the NOAA GOES satellite. These gigantic bursts of radiation cannot pass through Earth's atmosphere to harm humans on the ground, however they can disrupt the atmosphere and disrupt GPS and communications signals. In this case, it appears the flare is strong enough to potentially cause some radio communication blackouts.
The image here was captured by NASA's Solar Dynamics Observatory (SDO) in extreme ultraviolet light at 131 Angstroms.
What does it take to be an X-class flare? X-class are the largest solar flares. More
here. (Plus, an awesome image of a solar flare in fluorescent green.)