Last night I had an Astrophotography Workshop at Three Cliffs Bay. After almost cancelling because it was cloudy and raining when we met, we spent a good few hours out on the cliffs taking photos before going back to the cars. When my client had left, I walked all the way back out to the cliffs by myself. Half way, it started raining again and I almost turned around and headed back to the car, but I decided to persist and walk to the viewpoint I love. I’m glad I did because the rain soon cleared when I arrived at the viewpoint, the conditions were fantastic with the night sky clear and no moon. I took a photo and saw something unusual – green! My first thought was the Northern Lights as there was an Aurora alert out, but I quickly realised I was looking south and you have to look north to see the Aurora. That meant it must be airglow (see an explanation below). I spent a few minutes perfecting the composition and then took this photo:
As soon as I saw the photo, I knew I would be happy with the end result! I left the viewpoint and eventually headed home, getting to bed just gone 2am! I was up again at 6:15am to make sandwiches for the kids and to do the school run. Coffee featured prominently the next day…!
I spent some time later identifying the constellations:
What is “Airglow”?
Airglow is a process called “chemiluminescence” which is responsible for the glowing atmosphere.
Sunlight deposits energy into the atmosphere during the day, some of which is transferred to oxygen molecules (e.g. O₂). This extra energy causes the oxygen molecules to rip apart into individual oxygen atoms. This happens particularly around 100km in altitude. However, atomic oxygen isn’t able to get rid of this excess energy easily and so acts as a “store” of energy for several hours.
Eventually the atomic oxygen does manage to “recombine”, once again forming molecular oxygen. The molecular oxygen then releases energy, again in the form of light. Several different colours are produced, including a “bright” green emission.
Reference: RealClear Science