A sudden release of energy in the Sun's corona, lasting up to several hours or, exceptionally, more than a day. Flares usually occur within about 175 000 km of the Sun's surface (one-quarter of a solar radius). They emit radiation over the whole spectrum, from gamma rays to radio waves. They also throw out high-speed particles (electrons, protons, and atomic nuclei), at speeds up to about 70 % of the speed of light, which reach the Earth in 15 min or so, depending on their trajectory. Only the most energetic flares are visible in white light. Flares occur in active regions with complex magnetic fields, the largest flares being in the most complex regions. Most of the energy may be released in the first few minutes, with an impulsive stage that may last only a few seconds. The total energy released can be up to 1027 joules; there is no well-defined lower energy limit.
Flares are classified in two ways: by their appearance in Hα light, and by their soft X-ray emission. In Hα the term subflare is given to the smallest events, and the scale then runs from 1 to 4 with increasing area; a brightness code is added from faint (f ), via normal (n), to bright (b). In soft X-rays (0.1–0.8 nm) flares are classified as C, M, or X according to increasing strength, with subdivisions from 1 to 9.
In Hα, the flare may start with the disappearance of a filament (i.e. a prominence seen from above), bright areas developing into ribbons either side of the magnetic inversion line. There is generally a rapid expansion stage called the flash phase. There are often hard X-ray and microwave radio bursts forming an early impulsive stage, with soft X-rays rising more gradually to maximum a few minutes later, followed by a decline (the decay or cooling phase). Flares derive their energy from the energy stored in magnetic fields, although the exact mechanism is not known. According to one theory, flares occur when oppositely directed magnetic field lines reconnect. Particles are accelerated at the impulsive stage to give the hard X-ray emission, while the soft X-rays are emitted by a very hot (20 million K) plasma contained within coronal loops.
Subjects: Astronomy and Astrophysics.