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.
- 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.
- A region of very hot, tenuous gas that stretches out of the galactic
plane in spiral galaxies such
as the Milky Way; also known as the galactic corona.