Solar flares are powerful and rapid eruptions that occur in the atmosphere of the Sun in magnetically active regions. These eruptions are closely associated with sunspots and faculae. Astronomical observations have indicated that similar eruptions take place on other stars.

Solar flares typically exhibit a rapid increase of X-ray and ultraviolet emissions to 10–100 times their normal level. The increase takes place in tens of seconds. A slower but closely related increase is also seen in radio waves of centimeter wavelengths, and some flares also produce powerful bursts in meter wavelengths. The rapid initial rise is followed by a period of decreasing emissions that may last several hours.

Solar flares were for a long time associated with magnetic storms on Earth, involving disturbances in radio communications, fluctuations in electric power transmission, and increased auroral activity. This association is now discounted, and such storms are instead closely associated with so-called coronal "holes." These are cooler regions of the solar corona from which the Sun's magnetic field lines point out radially and the solar wind flows out at about twice its normal velocity.

A large flare may cover a billion sq km (386,000,000 sq mi) of the Sun's surface. The total energy involved in radiations, relativistic particles, and thermal plasma from a large flare may reach 10 to the 32d power ergs.

Peter Foukal

Bibliography: Culhane, J. L., and Jordan, C., eds., The Physics of Solar Flares (1992); De Cicco, Dennis, "Solar Activity on the Upswing," Sky & Telescope, February 1998; "Don't Blame Solar Flares," Sky & Telescope, June 1994; Hill, R. E., "Flares!," Astronomy, February 1992; Phillips, K. J. H., Guide to the Sun (1995); Somov, Boris V., Physical Processes in Solar Flares (1992).