What is the Sun's outermost atmospheric layer?

The Sun’s outermost atmospheric layer is the corona. It forms the Sun’s extremely thin, extended outer atmosphere and is visible as a glowing halo during a total solar eclipse.

The corona begins above the chromosphere and stretches millions of kilometres into space, gradually merging with the solar wind. It is made of very diffuse plasma, so it is much less bright than the photosphere and normally cannot be seen through the glare of the uneclipsed Sun. Special solar telescopes and coronagraphs can block the photosphere’s light to study it.

The corona is also surprisingly hot, with temperatures often reaching around 1 to 3 million °C. That is far hotter than the photosphere beneath it, which is about 5,500 °C. Explaining this unusual temperature increase is known as the coronal-heating problem. The core is the Sun’s deepest region, while the photosphere and chromosphere are lower atmospheric layers; none is the outermost layer.