Aurora borealis or northern lights display over silhouetted conifer trees. Aurorae are caused by the interaction between energetic charged particles from the Sun and gas molecules in the upper atmosphere about 100 kilometres up. A stream of charged particles, called the solar wind, flows out into space continuously from the Sun at speeds of 400-500 kilometres per second. On reaching Earth, the charged particles are drawn by Earth's magnetic field to the poles, where they collide with gas molecules in the upper atmosphere, causing them to emit light. Photographed from Finland in early October 2002.

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