coronal heating problem

The coronal heating problem is a long-standing puzzle in solar physics: the Sun's outer atmosphere, the corona, is hundreds of times hotter than the surface beneath it, and no one is completely sure why. It is as if the air a few metres above a campfire were far hotter than the flames themselves.

Here is the puzzle in numbers. The visible surface, the photosphere, is about 5,500 degrees Celsius. Just above it the gas cools a little, as you would expect when you move away from a heat source. But then, moving outward through the chromosphere into the corona, the temperature shoots up to one to three million degrees. Heat is not supposed to flow from a cooler place to a much hotter one on its own, so something must be actively pumping energy into the thin coronal gas. The energy almost certainly comes from the Sun's magnetic field, but exactly how is debated: leading ideas include countless tiny magnetic explosions (nanoflares) and waves travelling along magnetic field lines that dump their energy high up.

The coronal heating problem matters because it is a genuine, current frontier of physics — not a settled textbook fact. Missions like the Parker Solar Probe and Solar Orbiter were built largely to fly close enough to measure what is going on. Understanding it also bears on the heating of other stars' atmospheres and on the physics of hot, magnetised gas throughout the universe.