The corona is the outer atmosphere of the Sun. It extends many thousands of kilometers (miles) above the visible "surface" of the Sun, gradually transforming into the solar wind that flows outward through our solar system.
The material in the corona is an extremely hot but very tenuous plasma. The temperature in the corona is more than a million degrees, surprisingly much hotter than the temperature at the Sun's surface which is around 5,500° C (9,940° F or 5,780 kelvins). The pressure and density in the corona is much, much lower than in Earth's atmosphere.
The corona is above the Sun's lower atmosphere, which is called the chromosphere. A relatively narrow area called the transition region separates the corona from the chromosphere. Temperatures rise sharply in the transition region, from thousands of degrees in the chromosphere to more than a million degrees in the corona. The density of plasma falls rapidly through the transition region moving upward from the chromosphere to the corona.
We normally cannot see the solar atmosphere, including the corona. The surface of the Sun is far too bright to allow a glimpse of the much fainter corona. During a total solar eclipse the wispy corona briefly comes into view as the Moon blocks out the solar surface. A special instrument called a coronagraph allows astronomers to view the corona at other times. Some coronagraphs are used with ground-based telescopes; others are carried on satellites.