The cause of this light show was an especially strong blast of solar wind - electrically charged particles shot out from the sun at incredible speeds.
This is one example of what scientists call 'space weather,' dealing with the interaction between the sun and the Earth. Not all consequences are pretty, some are outright dangerous.
The sun is a giant nuclear fusion reactor. Protons are smashed together in the core to create a helium nucleus, releasing energy according to Einstein's equation E=mc^2.
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