During a solar eclipse, the SunΓÇÖs corona or upper atmosphere emits the ghostly glow seen here. An eclipse allows us to observe this part of the solar atmosphere that is ordinarily obscured by the bright light from the SunΓÇÖs surface. Physical conditions in the corona are extreme: temperatures, in excess of 1 million degrees Kelvin, are so intense that the corona is the only region of the solar atmosphere that emits significant x-ray emission. The corona is also sparse; it has less than one millionth the density of the solar surface.
The corona presents the solar astronomer with many puzzles. The high temperatures, for one. Moving away from the SunΓÇÖs interior, temperatures steadily decrease, which is exactly what one
