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. - Space Daily

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…