A brilliant eruption on the Sun above a sunspot, thought to be caused by release of magnetic energy. Flares reach maximum brightness within a few minutes, then fade away over about an hour. They eject a burst of atomic particles into space at up to 1 000 kps. When these particles reach Earth they can cause radio blackouts, disruptions of the Earth's magnetic field, and aurora.

In 2003 astronomers using the Solar and Heliospheric Observatory (SOHO) observed the biggest solar flare ever recorded. Solar flare strength is given an ‘X’ designation ranging from a minimum X1 up to X20 (the latter being the magnitude of the previous largest recorded solar flare, in 2001). The 2003 flare was so powerful that it overloaded the measuring devices, and estimates of its magnitude placed it at around X28.