What is the name for the Sun's outer-most atmosphere?

The Sun's outermost atmosphere is called the corona. It is a vast, extremely thin envelope of plasma that extends millions of kilometres into space and gradually merges with the solar wind.

The corona lies above the photosphere, the Sun's visible surface, and the chromosphere. Although the Sun's surface is roughly 5,500 kelvins, parts of the corona reach temperatures of more than 1 million kelvins. This surprising temperature increase is known as the coronal-heating problem and is linked to the Sun's magnetic fields and energetic waves.

The corona is usually hidden by the Sun's intense glare, but it becomes visible during a total solar eclipse as a pearly white halo. Special instruments called coronagraphs can also block the Sun's disk so scientists can study it. The corona is not the same thing as the chromosphere, which is a lower atmospheric layer seen as a reddish rim during some eclipses.