Astronauts Capture Aurora Australis South of Australia
This video of the Aurora Australis was created from a sequence of still shots taken by astronauts on board the International Space Station. The images were acquired on September 11, 2011 as the ISS orbit pass descended over eastern Australia, reached the "bottom" of the orbit, then changed direction to an ascending pass just east of New Zealand during the night hours -- in essence describing a wide "U" over the region. Dense cloud cover obscures the land and sea surface during much of the video.
The aurora displays a sinuous green ribbon shape -- with occasional hints of red near the upper extent -- throughout the video. The unique viewing perspective from the ISS cupola allows for a sense of the 3-dimensional nature of the phenomena as it varies in apparent length, width, and thickness as the ISS orbits above it.
Like its northern hemisphere counterpart the Aurora Borealis, the Aurora Australis occurs when ions in the solar wind collide with atoms of oxygen and nitrogen in the upper atmosphere. Atoms excited by these collisions emit light as they return to their original energy level, creating the visible aurora. The most commonly observed color of aurora is green, caused by light emitted by excited oxygen atoms at wavelengths centered at 0.558 micrometers, or millionths of a meter. Red aurora are generated by light emitted at a longer wavelength (0.630 micrometers), and other colors such as blue and purple are also occasionally observed.
Throughout the video the curvature of Earth's surface, or limb, is visible. The thin yellow-gold line above the limb is airglow, caused by the release of energy from atoms and molecules high in the atmosphere that are excited by ultraviolet radiation during the day. The sense of motion of the ISS is accentuated by the changing star field in the background, and the rotation of a station solar panel array across the view at image right.