The outermost layer of the solar atmosphere, characterized by low densities (<1.0 10⁺⁹/cc) and high temperatures (>1,0 10⁺⁶deg.K).  It is not visible from the Earth except during a total eclipse of the Sun or by use of a special telescope called a coronagraph. ON-LINE GLOSSARY OF SOLAR-TERRESTRIAL TERMS of National Geophysics Data Center