The Coronal Heating Problem – why is the Sun’s corona so hot?

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Written explanation of the coronal heating problem

When we move away from hot things we get cooler. At least that’s how it usually works. Except the Sun’s outer layer – its corona – is hotter than the solar surface. This is called the solar coronal heating problem. It seems there’s something at the surface injecting energy into the corona somehow.


It could be nanoflares. A solar flare is the rapid release of stored magnetic energy. A nanoflare is a tiny solar flare with a billionth of the energy of a normal flare. Each one would barely add any energy to the corona, but a lot of them could add up to a lot of energy.


Equally the culprit could be Alfven waves. An Alfven wave is a wave along a magnetic field line. Just as a wave on the ocean can dump a surfer onto the beach, so Alfven waves along magnetic fields line could deposit energy from the Sun’s surface into the corona. Astronomers don’t yet know if either of these is the answer or whether it is a mixture of both. The next generation of solar observatories could help us to understand why the corona is so hot and solve the coronal heating problem once and for all.

Image: NASA/Aubrey Gemignani

the solar coronal heating problem - why is the Sun's corona so hot