The Sun's outer atmosphere must be cooler than its visible surface.

The visible surface of the Sun, called the photosphere, is about 5,800 kelvin.

Ordinary experience suggests that moving away from a hot surface should mean getting cooler. For that reason, the faint atmosphere extending above the Sun was naturally expected to be cooler than the photosphere.

Its spectrum produced a puzzle.

When astronomers spread coronal light into a spectrum, they found bright lines that did not match familiar elements under ordinary laboratory conditions. For a time, some proposed that an unknown element—“coronium”—was responsible.

The answer was stranger.

Bengt Edlén showed that the mysterious lines came from familiar atoms, especially iron, that had lost many of their electrons. Removing that many electrons requires collisions at extraordinarily high energies, corresponding to temperatures around a million kelvin.

The corona was therefore not cooler than the visible surface. It was hundreds of times hotter.

That discovery created the coronal-heating problem: how does the Sun’s thin outer atmosphere become so hot above a much cooler photosphere? Magnetic activity, including wave heating and magnetic reconnection, is central to modern explanations, although the balance of mechanisms varies across the corona.

Temperature here measures the typical energy of particles. Because the corona is extremely sparse, its high temperature should not be confused with containing more total thermal energy than the much denser layers below.