The aurora borealis, or northern lights, illuminates the sky over Stockholm on April 6, 2000. The red and yellow-green colors of this auroral display are due to oxygen gas hovering at two different altitudes. Oxygen in the atmosphere more than 150 miles above the surface of the Earth glows red when exposed to fast-moving particles from the magnetosphere, while oxygen closer in appears yellow-green or green, the brightest and most common auroral hues.



This image is part of the PBS Nova documentary Magnetic Storm . Click here to watch.