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 electromagnetic spectrum, from gamma rays to radio waves. They eject 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 twisted configurations. Their frequency of occurrence varies following the 11-year solar cycle.

Flares are classified in two ways: by their appearance in Ηα‎ light, and by their soft X-ray emission. In Ηα‎ 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. Very large X-class flares are much less frequent than smaller ones.

Flares go through different phases as they erupt. In the pre-flare phase, lasting a few minutes, the coronal material in the flare region slowly heats up and becomes more visible in soft X-rays and extreme ultraviolet. This is the trigger stage prior to the major release of energy. Next there is the impulsive phase, lasting up to 10 minutes, when most of the energy is released in the form of high-speed electrons and sometimes ions as well. The total energy released in very large events can be up to 10³² joules. Hard X-ray sources appear in the chromosphere during this phase, along with bright ribbons either side of the magnetic inversion line. The soft X-ray intensity also rises rapidly during the impulsive phase, but then starts to decrease slowly in the gradual phase; the hard X-ray intensity decreases more irregularly. In this decaying phase, the coronal plasma returns nearly to its original state.

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.