Green colors of the aurora are dominant in this photograph taken from the International Space Station. These are Aurora Borealis (northern hemisphere auroras) occuring over Canada.
Auroras are caused when high-energy electrons pour down from the Earth’s magnetosphere and collide with atoms. Green aurora occurs when excited oxygen atoms return to their original state.
At times of peaks in solar activity, there are more geomagnetic storms, increasing the amount of auroras observed and allowing people to see them at lower latitudes.
Why is this image unique?: Astronauts exploit their unique vantage point to document auroral activity from above, profiling the vertical structure of auroras. By using a digital camera with a long exposure time, astronauts can capture a part of the light from the multicolored displays they observe, and downlink those images to Earth.
ISS003-E-6152, 4 October 2001
The Gateway to Astronaut Photography of Earth, http://eol.jsc.nasa.gov