What is a simple definition of a corona?

The chromosphere merges into the outermost region of the Sun's atmosphere is called the corona. The corona extends for millions of miles into space above the photosphere. Usually, we cannot see the corona because of the brightness of the photosphere. However, during a total solar eclipse, the corona shines beautifully against the dark sky. The corona has a density about 0.0000000001 times that of the Earth's sea-level atmosphere. It is very hot of about a millions of Kelvin. Because of this high temperature, the bulk of the radiation from the corona is emitted at ultraviolet and X-ray wavelengths. Magnetic fields on the Sun seem to play an important part in heating the gas to such a high temperature. However, the exact way that this happens is not well understood.