CHROMOSPHERE

lowest layer of the Sun's atmosphere, several thousand kilometres thick, above the bright photosphere and below the extremely tenuous corona. Named by the English astronomer Joseph Norman Lockyer in 1868, the chromosphere (colour sphere) appears briefly as a bright crescent, red with hydrogen light, during solar eclipses when the body of the Sun is almost obscured by the Moon. Except during eclipses, it can be observed only with special instruments-i.e., the spectroheliograph or coronagraph. The lower chromosphere was formerly called the reversing layer because it was thought to be responsible for producing the dark lines of the solar spectrum that appear reversed against the bright continuous spectrum; actually the weak dark lines and bright continuum can be produced in essentially the same regions, stronger lines being produced at higher levels. The term reversing layer is now seldom used. The lower chromosphere is more or less homogeneous; the upper contains comparatively cool columns of ascending gas known as spicules, having between them hotter gas much like that of the corona, into which the upper chromosphere merges gradually. Temperatures in the chromosphere range from about 4,500 to 100,000 K, increasing with altitude. Solar flares and prominences are primarily chromospheric phenomena.