Aurora borealis or northern lights are visible in the sky above a lighthouse to the village of Strand near Tromso in northern Norway, 22 February 2018. Aurorae are caused by the interaction between energetic charged particles from the sun and gas molecules in the upper atmosphere of the earth, about 100 kilometres up. Streams of charged particles, called solar winds, flow out into space continuously from the sun at speeds of 400 to 500 kilometers 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. (Photo by Martial Trezzini/EPA/EFE)
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