Airglow - The Green sheen of the night
Night sky pictures such as this sometimes have some green sheen in them which often is mistaken for at best Auroras (northern lights in the northern hemisphere), camera noise or worse, bad post processing by the photographer. Unless you are the northern latitudes, the chances of capturing Aurora Borealis is like finding a Polar Bear in the Texas Hill Country. It ain’t happenin!
More than likely it is a phenomenon called “airglow”. Airglow’s subtle radiance arises from excitation of a different kind. Ultraviolet light from the daytime sun ionizes or knocks electrons off of oxygen and nitrogen atoms and molecules; at night the electrons recombine with their host atoms 80km or higher, releasing energy as light of different colors including green, red, yellow and blue.
Aurorae are at similar heights and are also the light of excited atoms. There is a difference, however, auroral excitation is by collisions with energetic particles whereas daytime short wavelength solar radiation produces the airglow via chemical excitation of which electronically excited oxygen atoms are the main component.
A couple of months ago on a night when I tried out a few combinations of my camera gear and lenses at Chinmaya Mangalam, a spiritual retreat in Barry, TX, this one stood out for its tell tale green band above the tree line on the left of the dome which is about 10-15 degrees above the horizon where you would normally find airglow. But this image was shot 60 miles from the heart of Downtown Dallas. I have not seen airglow in pictures shot so close to cities that too one of the largest metropolitan areas in the United States.
I wanted to be doubly sure what I was seeing here was indeed airglow. So, I started reading about it and found that one of the most renowned researchers and experts in the Airglow Imaging Studies is right here in Dallas. Dr. Brian Tinsley, a native New Zealander is Professor Emeritus of Physics at University of Texas at Dallas and for 50 years has been actively researching Aeronomy. After a couple of email exchanges, I sent him this picture to which he replied:
That is a fine photograph of the milky way and night sky that you have taken. Yes, it is possible that the green coloration in the lower left of the sky is the 557.7 nm airglow. However there could be contributions of scattered light from mercury and sodium lights in towns below the horizon. The rest of the light in the sky as a whole, in the spaces between the stars, would be scattered starlight and zodiacal light.