The corona is the outermost layer of the Sun's atmosphere, visible during total
solar eclipses.
SOHO coronagraph observations.
For many years it was known that the light from the corona showed bright
emission lines in the green and red, but no one could identify the wavelengths with
any known elements. There were suggestions that it might be ``coronium'', an element
that existed only in the corona of the Sun. In 1939 they were identified with iron atoms
which had been ionized between 9 and 14 times. To do this requires temperatures in
the range 1 - 2 million degrees Kelvin. This is the temperature of the solar corona.
How it becomes so hot remains an open question in astrophysics.