A solar flare is a sudden flash of brightness observed over the Sun's surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy (about a sixth of the total energy output of the Sun each second or 160,000,000,000 megatons of TNT equivalent, over 25,000 times more energy than released from the impact of Comet Shoemaker–Levy 9 with Jupiter). They are often, but not always, followed by a colossal coronal mass ejection. The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space. These clouds typically reach Earth a day or two after the event. The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies.
Flares occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Scientific research has shown that the phenomenon of magnetic reconnection is responsible for the acceleration of the charged particles. On the Sun, magnetic reconnection may happen on solar arcades – a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low arcade of loops leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection is the origin of the particle acceleration. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a coronal mass ejection. This also explains why solar flares typically erupt from what are known as the active regions on the Sun where magnetic fields are much stronger on average.
Although there is a general agreement on the flares' causes, the details are still not well known. It is not clear how the magnetic energy is transformed into the particle kinetic energy, nor is it known how the particles are accelerated to energies as high as 10 MeV (mega electron volt) and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop. Scientists are unable to forecast flares, even to this day.
Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square metre, W/m2) of 100 to 800 picometre X-rays near Earth, as measured on the GOES spacecraft.
Within a class there is a linear scale from 1 to 9.n (apart from X), so an X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. X class flares up to at least X28 have been recorded.
However, the extreme event in 1859 is theorised to have been well over X40 so a Z class designation is possible.