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Sudden short-lived brightenings of small areas of the Sun's upper chromosphere/inner corona that are optically visible usually only in the monochromatic light of certain strong Fraunhofer lines. They represent an explosive release of energy – in the form of particles and radiation – that causes a temporary heating of the surrounding medium and may accelerate electrons, protons, and heavier ions to high velocities.

A typical flare attains its maximum brilliance in a few minutes and then slowly fades over a period of up to an hour. Although attempts have been made to classify them according to their structure, the most objective system considers the maximum area covered by the flare and to a lesser extent its intensity. The overwhelming majority of flares are small, covering areas of less than several hundred million square kilometers, but those that are larger are sometimes associated with a number of interesting phenomena.

During the ‘flash phase’ of a large flare, when it suddenly increases in brightness and rapidly expands to its maximum extent, material may be ejected in the form of a surge, spray, or (for a particularly energetic event) fast ejection – in which a compact portion of the flare is expelled without fragmentation, with a velocity of about 1000 km s–1. Large flares may initiate a Moreton wave, which is a magnetohydrodynamic shock wave that spreads out from the center of disturbance, in a sector of about 90°, as it travels with a velocity of the order of 1000 km s–1 across the vertical magnetic fields of the inner corona. This causes a depression and subsequent relaxation of the underlying chromospheric network and may induce distant filaments, perhaps several hundred thousand kilometers from the flare, to undergo several damped vertical oscillations. Filaments in the vicinity of the flare may also be activated, before and/or during the flare, by changes in the configuration of the local magnetic field. This activation usually takes the form of increased internal motion or flow along the filament (small surges), sometimes accompanied by a gradual ascent of the filament itself. The aftermath of large flares may include coronal condensations of relatively dense material at temperatures of up to 4 000 000 K.

The effects of energetic flares are by no means confined to the Sun. Fade-out of short-wavelength radio signals is often experienced on the daylight hemisphere of the Earth and is caused by a temporary strengthening (by increased ionization) of the reflecting property of the D layer of the ionosphere (60–90 km altitude), which suppresses the passage of the signals to the higher layers from where they are normally reflected. This is accompanied by a sudden increase in the electrical conductivity of the E layer (90–120 km altitude) and by disturbances of the Earth's magnetic field. Ultraviolet radiation from the flare is responsible for these effects. X-rays may also be emitted and solar radio emission (from the inner corona) frequently exhibits an intense burst or series of bursts at centimeter or meter wavelengths. Occasionally, within about half an hour of the flare, energetic charged particles (energies up to 1010 eV) reach the Earth, and within about 26 hours, on average, less energetic charged particles may also arrive. These latter particles spiral around the Earth's magnetic field lines, causing geomagnetic storms and their luminous counterpart, auroral displays.

The nature of flares and the physical mechanism responsible for them are not completely understood. They invariably occur in active regions, close to the line of inversion (see sunspots), where the gradient of the horizontal component of the magnetic field is steepest and therefore stresses are greatest. It is thought that energy is released when the stressed field reconnects to a lower potential energy configuration, and that this produces the flare; but much remains to be done before a quantitative picture will emerge.

A phenomenon similar to solar flares, but far more energetic, is thought to be responsible for the rapid brightening of flare stars.

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