Stellar corona
A corona is produced by the diffraction of light from either the Sun or the Moon through a cloud of water droplets. It consists of a series of concentric colored rings which appear around the celestial object but which are actually at cloud height. Coronae differ from haloes in that the latter are formed by refraction from ice crystals. Reddish colors always occupy the outer part of a corona's ring.
The corona is the luminous "atmosphere" of the Sun, most easily seen during a total solar ecplipse. An interesting feature of the corona is the fact that it is much hotter than the visible "surface" of the Sun; the photosphere is approximately 6000°C compared to the corona at over one million °C. The corona is much less dense than the photosphere, however, and so produces less light. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and sonic pressure waves from below. The outer edges of the sun's corona are constantly being lost as solar wind.
A corona discharge is an electrical discharge brought on by the ionization of gas surrounding a conductor, which occurs when the potential gradient exceeds a certain vale but is not sufficient to cause sparking.