The Sun. It’s right there in the middle of the Solar System. We’ve got spacecraft monitoring it 24 hours a day. You’d think we know just about everything there is to know about it, but the Sun harbors a great scientific mystery.
Before we go into that, let’s define some key terms.
- The photosphere: the surface layer of the Sun.
- The convective zone: the layer directly beneath the photosphere.
- The corona: the Sun’s “atmosphere,” located above the photosphere.
As heat radiates out from the Sun’s core, the temperature gradually decreases. In the convective zone, it’s several million degrees Celsius. In the photosphere, the temperature drops to merely a few thousand degrees Celsius.
That makes sense. As we get farther away from the source of all that heat, we’d expect the temperature to cool off a bit. So the corona should be cooler still, right?
Wrong. The temperature rapidly increases from a few thousand degrees to tens of thousands of degrees, and then peaks back in the range of several million degrees Celsius.
Why? I don’t know why. Nobody knows why, though it may have something to do with the Sun’s magnetic field. More recent research suggests “nanoflares” might have something to do with it.
But the truth is we do not know the reason for this temperature discrepancy. It is (at least for now) a scientific mystery.
Sources
The Sun’s Corona from NASA’s Imagine the Universe.
Explain the Coronal Heating Mystery from the Stanford Solar Center.
Layers of the Sun from The Sun Today.
Sun’s Magnetic Secret Revealed from Space.com.
Tiny “Nanoflares” May Solve Sun Mystery from Space.com.
Planet Pailly 
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