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).