The solar corona is the outermost layer of the solar atmosphere and is a plasma, a completely ionised gas. Above the chromosphere, the gas in the solar atmosphere become less and less dense, but its temperature rises again, going from 10,000 K at the base of the transition zone to 2,000,000 K in the corona. This surprising corona heating phenomenon is currently still one of the big research topics in solar physics research.
The hot gas in the transition region, and then the corona, emit ultraviolet and X-radiation observable from satellites. The transition region is being observed by the SOHO EIT experiment in different UV lines produced by atoms heated to different temperatures. The corona itself is regularly observed in the X-ray region by the Japanese YOHKOH satellite.