Hot, Volatile, Magnetized Gas

The Million-Degree Solar Corona

The light we see coming from the Sun originates in the photosphere, a thin, tenuous layer of gas, only a few hundred kilometers thick, with a temperature of 5,780 K. We are looking at the round disk of the photosphere when we watch the Sun rise in the morning and continue on its daily journey across the sky. So it is natural to suppose that the photosphere is the surface of the Sun; however, being entirely gaseous, the Sun has no solid surface that divides the inside from the outside.

Moreover, the sharp outer rim of the Sun is illusory. A hot, transparent outer atmosphere envelops the photosphere and extends all the way to the Earth and beyond. Observing the Sun is like looking into the distance on a foggy day. At a certain distance, the total amount of fog we are looking through amasses to create an opaque barrier. The fog then becomes so thick and dense that radiation can penetrate no farther, and we can only see that far. When looking into the solar atmosphere, we similarly can see through only so much gas. For visible light, this opaque layer is the photosphere – the level of the Sun from which we receive our light and heat.

The unseen atmosphere just above the visible solar disk is far less substantial than a whisper and more rarefied than the best vacuum on the Earth. It is so tenuous that we see right through it, just as we see through the Earth’s clear air. The diaphanous atmosphere of the Sun includes – from its deepest part outward – the underlying photosphere, from the Greek word photos for “light”; the thin chromosphere, from the Greek word chromos for “color”; and the extended corona, from the Latin word for “crown.” We can observe the chromosphere and corona during a total solar eclipse, when the Moon blocks out the intense light of the underlying photosphere (Fig. 6.1).