Photo credit: Jeff Miller, Oregon State University.
In solar physics, the corona is the tenuous uppermost level of the Sun's (or another star's) atmosphere; lying immediately above the chromosphere, it consists of hot (one to four million K), low-density (about 10-16 g/cm3) gas that extends for millions of kilometers from the Sun's surface. The so-called white-light corona, visible during a total eclipse or with a coronograph, has three components. The faint E corona (emission corona) results from emission lines, including forbidden lines of calcium, iron, and some other elements. The K corona (or continuum corona), which is the innermost part of the corona closest to the photosphere, extending to about two solar radii, is caused by sunlight scattering off electrons. The F corona (Fraunhofer corona), lying outermost, is caused by sunlight scattering or reflecting off dust in interplanetary space. X-rays, too, come from the corona (as well as from solar flares) but not from all parts equally. Movies made from X-ray pictures show that the corona is extraordinarily dynamic with an appearance that changes, not only daily but over the course of a solar cycle. At solar maximum the dominant features are coronal loops and streamers associated with active regions, but at minimum these give way to coronal holes at each pole and a sheet-like structure near the equator.
In planetary astronomy, a corona is a circular to elongate feature (pl. coronae) on the surface of a planet or moon surrounded by multiple concentric ridges. Coronae are thought to be formed by hot spots.