Aurorae come in all sorts of colors.. green predominantly, but every once in a while, you will stumble across a blood red aurora. Like all aurorae, the colors you see will ultimately depend on the composition of the atmosphere. Green stems from atomic oxygen at lower altitudes, blue and purple come from gases like hydrogen and helium trapped within the ionosphere, and red aurorae, as rare as they are, come from oxygen as well, only the process is a little bit different. Notably, they make an appearance when atomic oxygen is present in the upper atmosphere, where collisions between atoms and molecules are less likely to occur.
This image features a red and yellow aurora, taken over Yellowknife in the Canadian Northwest Territories. Captured by Chul Kwon, it was one of around 20 images featured by the Royal Museums Greenwich in London. Come next month, one of the talented photographers will be named the 'astronomy photographer of the year.' See other contestants below. (See a larger image here)
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