Corona — The corona is the luminous “atmosphere” of the Sun extending millions of kilometers into space, most easily seen during a total solar eclipse.
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.
During periods of quiet Sun the corona is more or less confined to the equatorial regions, with “coronal holes” covering the polar regions. During the Sun’s active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity.
Generated by solar flares or large solar prominences, “coronal transients” are sometimes released. These are enormous loops of coronal material travelling outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that triggered them.