Red and green colors predominate in this view of the Aurora Australis photographed from the Space Shuttle in May 1991 at the peak of the last geomagnetic maximum. The payload bay and tail of the Shuttle can be seen on the left hand side of the picture. Auroras are caused when high-energy electrons pour down from the Earths magnetosphere and collide with atoms. Red aurora occurs from 200 km to as high as 500 km altitude and is caused by the emission of 6300 Angstrom wavelength light from oxygen atoms. Green aurora occurs from about 100 km to 250 km altitude and is caused by the emission of 5577 Angstrom wavelength light from oxygen atoms. The light is emitted when the atoms return to their original unexcited state.
At times of peaks in solar activity, there are more geomagnetic storms and this increases the auroral activity viewed on Earth and by astronauts from orbit. Photographing them requires careful technique with long exposures and fast film (in this case ASA 1600). Such film can only be used on short-duration Shuttle flights and not from the Space Station because it is sensitive to radiation damage in orbit over time. The most recent astronaut photograph of aurora was taken before the April 2001 flurry of solar activity, and showed only a relatively low-energy green glow (see previous Earth Observatory posting).