The corona is a highly rarefied and hot ionized gas, or
The corona is observable with the
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.
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).