A Stellar Flare is a sudden flash of brightness observed near a Star’s surface. It involves a very broad spectrum of emissions, requiring an energy release of up to 6 × 1025 joules of energy for the Sun (roughly the equivalent of 160,000,000,000 megatons of TNT).
Flares are often, but not always, accompanied by a coronal mass ejection. The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space.
Stellar flares affect all layers of the star’s atmosphere (photosphere, chromosphere, and corona), when the plasma medium is heated to tens of millions of Kelvin, while the cosmic-ray-like electrons, protons, and heavier ions are accelerated to near the speed of light.
They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays, although most of the energy is spread over frequencies outside the visual range.
For this reason the majority of the flares are not visible to the naked eye and must be observed with special instruments. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior.
Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. X-rays and UV radiation emitted by solar flares can affect a planet’s ionosphere and disrupt long-range radio communications.
Direct radio emission at decimetric wavelengths may disturb the operation of radars and other devices that use those frequencies. The frequency of occurrence of solar flares varies, from several per day when the Star is particularly "active" to less than one every week when the Star is "quiet".
Large flares are less frequent than smaller ones.