The sun emitted a mid-level solar flare, an M6.6-class, peaking at 11:32 pm EDT on Oct. 28, 2014 – the latest in a series of substantial flares from a giant active region on the sun that first erupted with a significant solar flare on Oct. 19. NASA's Solar Dynamics Observatory, which constantly observes the sun, captured images of the event.

A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end.  The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time.

Source: NASA