The surface of the Sun is around 5,500 degrees, but its outer atmosphere is more than a million degrees hotter, and after decades of work physicists still cannot fully explain how the cooler layer heats the one above it.

The visible surface of the Sun, the photosphere, sits at around 5,500 degrees Celsius. Its outer atmosphere, the corona, runs to one or two million degrees, and in places hotter still. The cooler layer is below; the far hotter one is above it. At these scales the difference between Celsius and kelvin is rounding error, so the gap is real and enormous either way. That inversion has a name in solar physics. It is called the coronal heating problem, and it has been open since the early 1940s, when the temperature of the corona was first worked out from its spectrum. After more than eighty years it is still not fully resolved.

The surface of the Sun is around 5,500 degrees, but its outer atmosphere is more than a million degrees hotter, and after decades of work physicists still cannot fully explain how the cooler layer heats the one above it.. The visible surface of the Sun, the photosphere, sits at around 5,500 degrees Celsius. Its outer atmosphere, the corona, runs to one or two million degrees, and in places hotter still. The cooler layer is below; the far hotter one is above it. At these scales the difference between Celsius and kelvin is rounding error, so the gap is real and enormous either way. That inversion has a name in solar physics. It is called the coronal heating problem, and it has been open since the early 1940s, when the temperature of the corona was first worked out from its spectrum. After more than eighty years it is still not fully resolve...