A solar flare is a sudden intense brightening of a small part of the Sun's chromosphere in the vicinity of a plage or facula and often near a sunspot group. The flare develops in a few minutes and may last several hours. Most of the energy consists of ultraviolet radiation; intense X rays are also emitted, along with cosmic rays and less energetic particles. The particles travel much slower than the light of the flare and reach the vicinity of Earth a day or two later; they pose a potential radiation hazard to human beings in space. Flare-emitted radiation and particles, by their interaction with Earth's magnetic field and ionosphere, disrupt radio communications and cause auroras.
Excerpt from the Encyclopedia Britannica without permission.