For context
Astronomers rank solar flares by strength using five categories: A, B, C, M and X. The A-class flares are the weakest sun storms, while the X-class events are the most powerful solar flares.
This ranking system was designed by the National Oceanic and Atmospheric Administration and resembles the Richter scale used for earthquakes in that each category is 10 times stronger than the one before it, NASA officials have said. So a B-class solar flare is 10 times stronger than an A-class event, while a C-class solar storm releases 10 times more energy than B-class flare (or 100 times more energy than an A-class event).
The categories are also broken down into subsets, from 1 to 9, to pinpoint a solar flare's strength. Only X-class solar flares have subcategories that go higher than 9. The most powerful solar flare on record occurred in 2003 and was estimated to be an X28 on the solar flare scale, NASA officials said.