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"Aurora borealis, the Northern Lights, photographed over spruce trees near Fairbanks, Alaska. Auroras are luminous displays that appear in the night sky at high latitudes; they form rays, bands, arcs, streamers & curtains, often coloured red or green. They are caused by charged subatomic particles from the Sun interacting with atoms & molecules in Earth's upper atmosphere (above about 100 km). The reason that they appear at high latitudes is that the particles from the Sun are concentrated over the poles by Earth's magnetic field." (KEYSTONE/SCIENCE PHOTO LIBRARY/JACK FINCH)