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Polar lights appear in different colours. Green light, for example, is caused by oxygene atoms which are stirred in a height of approx. 100km; red light is caused by oxygene atoms stirred in approx. 200km. Stirred nitrogene atoms emit a violet to pale blue light. To stir nitrogene atoms a very high level of energy is needed, so that these colours can only be seen when there a strong solar winds. As the solar wind outside the polar areas rarely reaches far into the atmosphere, the polar lights in the temperate zones are mostly red.
The earliest reports about polar lights reach back about 2,000 years. Several cultures in the north of America, Europe and Asia regarded them as activities of Gods and Spirits (as their dances or fights), but also as messages to humans. Especially in the medieval ages, polar lights in Europe were regarded as harbingers of doom (wars, plagues, famine); perhaps the dominant red colour of these aurorae, due to the latitude of Central Europe, contributed to this notion.
Vikings divined from polar lights that somewhere on Earth a big battle had been fought. In their view, the valkyries rode across the sky after each skirmish and chose those heroes who would from then on be feasting at Odin's table.
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