The outermost part of sun atmosphere is called the corona. Like the chromosphere, the corona was first observed in total eclipses. In contrast to the chromosphere, the corona has been known for many centuries: it was mentioned by the Roman historian Plutarch and discussed in detail by Kepler.
The corona extends millions of kilometers across the photosphere and emits about half as much light as the full moon. The reason we don’t see this light until there is a solar eclipse is the overwhelming brightness of the photosphere. Just as the bright lights of the city make it difficult to see the faint light of the stars, so the strong light of the photosphere hides the faint light of the corona.
While the best time to see the corona from Earth is during a total solar eclipse, it can easily be observed from an orbiting spacecraft. Its brightest areas can now be photographed with a special instrument, a coronagraph, which removes the glare of the sun from the image with a occulting disk (a circular piece of material held directly in front of the sun).
Studies of its spectrum show that the corona has a very low density. There are only about 109 atoms per cubic centimeter at the bottom of the corona, compared to about 1016 atoms per cubic centimeter in the upper photosphere and 1019 molecules per cubic centimeter at sea level in the Earth’s atmosphere. At higher altitudes, the corona becomes thinner very quickly, where it corresponds to a high vacuum according to laboratory standards of the earth. The corona extends so far into space, far past earth, that here on our planet we technically live in the atmosphere of the sun.