{kuh-roh'-nuh }

The corona is a highly rarefied and hot ionized gas, or plasma, observed to extend from just above the photosphere of the Sun to a distance of several solar radii. Its outer extension supplies mass to the outflowing solar wind.

The corona is observable with the coronagraph, or with the unaided eye during a total solar eclipse, as a faint glow about 1 million times less bright than the solar disk. During periods of magnetic activity, intricate loop and radial plume structures can be seen. Visible coronal light is photospheric radiation scattered from electrons in the ionized coronal medium, whose density ranges between 109 particles per cm3 near the limb, to about 106 at a distance of 2 solar radii above it. The coronal gas also emits X rays and ultraviolet light, which are radiated by ionized elements such as magnesium, silicon, and iron when they collide with fast-moving electrons in the coronal plasma. Strong X-ray and ultraviolet emissions from certain other stars suggest that stellar coronae are common.

Above the active regions associated with areas of strong and extended photospheric magnetic field, the solar coronal emission is enhanced by about a factor of 10, mainly because of the locally higher plasma density. Elsewhere, X-ray and ultraviolet-light pictures have revealed regions of low density, called coronal holes, that occur particularly frequently at the north and south solar poles. These holes have been associated with strong magnetic storms on Earth.

The corona has been under paricular study by scientific satellites. In 1991 a Japanese satellite named Yohkoh was launched for this purpose, and in 1998 NASA launched a scientific satellite named Trace (Transitional Region and Coronal Explorer). The latter carries a telescope that remains focused on the Sun, studying the corona and the transition zone between the corona and the solar surface in different light frequencies. Images obtained by Trace in 2000 suggest that the extreme temperatures observed in the corona (several million degrees K) come from pillars of fiery gas at the bottoms of looping arcs of magnetic fields that stretch hundreds of kilometers above the solar surface.

Peter Foukal

Bibliography: Bray, R. J., et al., Plasma Loops in the Corona (1990); Foukal, Peter, Solar Astrophysics (1990); Linsky, Jeffrey L., and Serio, Salvatore, eds., Physics of Solar and Stellar Coronae (1993); Phillips, Kenneth J. H., Guide to the Sun (1992); Watanabe, T., et al., Observational Plasma Astrophysics (1998).