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 (see facula). 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 10Ð100 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 (see electromagnetic radiation). 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 (see auroras). 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 Ü (386,000,000 æ) 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.