Classifying Solar Flares
On May 13, 2013, an X2.8-class flare erupted from the sun -- the strongest flare of 2013 to date. This NASA image shows a close-up of the flare as seen by the Solar Dynamics Observatory in the 131 angstrom wavelength.
Solar flares occur when a buildup of magnetic energy on the sun is suddenly released. They usually erupt from sunspots, temporary dark and relatively cool patches on our star's surface where the local magnetic field is very strong.
Flares generate a burst of radiation across a wide range of the electromagnetic spectrum. They're often accompanied by coronal mass ejections (CMEs), which hurl enormous clouds of super-heated plasma into space.
Scientists classify strong solar flares into one of three categories: C, M or X (with A and B classes, too, for weaker eruptions). There's a tenfold increase in power from one class to the next, so an X flare is 10 times stronger than an M flare, and 100 times more powerful than a C.
There's also a finer gradation within each class, from 1 to 9 in the case of C and M flares. But the number goes higher for X flares, because they're at the top of the scale and the sun occasionally fires off eruptions more than 10 times stronger than an X1. For example, in 2003, the sun let loose a flare that registered as an X28 before overwhelming the sensors that measured it. It remains the most powerful solar flare ever recorded.