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When the charged particles flowing outward from the Sun (the solar wind) hit the Earths magnetic field, they are channeled down the magnetic field lines to the ionosphere at the North and South Poles. The impact of these particles on atmospheric molecules causes the molecures to emit light, which forms the visible aurora. This visualization shows the development of the aurora over the North Pole for about three hours on April 17, 1999, as seen by the ultraviolet VIS Earth Camera on the POLAR spacecraft. The two main features of these ultraviolet images are the very bright ultaviolet emission from the reflected solar radiation on the dayside of the Earth and the bright ring of the auroral oval circling the North Pole. The aurora seen in this visualization is the diffuse aurora, a very large scale bright band that is actually too dim to be seen well from the ground by the human eye. What we normally think of as the aurora are the even brighter curtains of light within the diffuse auroral caused by very energetic electrons. These curtains are too small scale to be seen in this image. The diffuse aurora appears as a ring around the pole rather than as a bright spot over the entire pole because the solar particles actually spend extended time wandering about within the Earths magnetic field before traveling down a very select set of magnetic field lines to the Earth. Near the end of this three hour period, the spacecraft was getting so close to the Earth that the edges of the globe were outside the cameras image, which accounts for the growing circular data gaps over Asia and the Pacific Ocean.
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