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A region in the outer layers of the Sun where solar activity is taking place. Active regions develop where strong magnetic fields emerge from the subsurface layers. A variety of features may be observed in the photosphere, the chromosphere and the corona. Phenomena such as sunspots, plages and flares are evidence of an active region. Radiation is enhanced over the whole spectrum, from X-rays to radio waves, though in sunspots the localized decrease in temperature reduces the visual brightness. There is a large variation in the size and duration of active regions: they may last from several hours up to a few months. Electrically charged particles and the enhanced ultraviolet and X-radiation from active regions affects the interplanetary medium and the upper atmosphere of the Earth.
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