I did a May 25 post about What a Meteor Shower Looks Like From the ISS. When I saw the above photo on Space.com I knew I had to do a followup. This photo was taken by Sophie Adenot, an astronaut for ESA who’s currently deployed on the ISS.
This is the aurora australis — the southern hemisphere equivalent of our northern aurora borealis. Adenot said the display was so bright that she needed to adjust her camera settings from their usual “aurora mode.”
The prominent green curtain is produced by charged particles from the Sun (the solar wind) colliding with oxygen atoms at altitudes of 60 to 150 miles. This is the easiest color for the human eye to perceive. You can actually see this “curtain” undulating before your eyes responding to variations in the solar wind.
There’s two other colors of aurora that get overwhelmed by the bright green. The thin reddish ring just above Earth’s edge is caused by solar particles colliding with high-altitude oxygen at heights above 150 miles. These faint, long-lasting displays typically occur during periods of intense solar activity, which we are currently experiencing. A red aurora is also visible on outside structures of the space station itself.
Finally, you’ll also notice a faint blue-violet color over the entire field of view. That’s solar particles colliding with nitrogen molecules at altitudes below 60 miles.
When a molecule of any gas is struck by a charged particle, it gets kicked up to a temporary high energy state. When it decays back to normal in a process called photoemission, it releases a photon of light, the color of which depends on specific energy levels within the molecule. Given the constituent gases of Earth’s atmosphere (mainly nitrogen and oxygen) This means aurorae have a limited palette of colors.
Adenot noted aurora viewing is quite popular on the ISS, and when there’s a good display the crew fight for their chance at the window. She said that although auroras are expected during intense solar activity, this was one of the most spectacular displays she’s ever seen.
Next Week in Sky Lights ⇒ Why We Don’t See Green Sunsets