Aurora borealis corona, 19th century artwork. This so-called 'corona' occurs, when the stream of charged particles flow exactly in the direction of the observer. Auroral displays are caused by interactions between energetic charged particles from the Sun, and the Earth's atmosphere. Moving at 400-500 kilometres a second, the charged particles of the solar wind are drawn by Earth's magnetic field to the poles, where they collide with gas atoms and molecules, causing them to emit light. Artwork from 'A Treatise on Meteorology: With a Collection of Meteorological Tables' (1868) by the American mathematician Elias Loomis (1811-1889).
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