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Jun 18, 2019
Looking up into the night sky with my own eyes, pretty much all I see is dark. There's bright spots here and there, like our nearest planetary neighbors, and of course the moon. But besides that, I can see only the brightest of stars. My night vision is poor, it's one of the reasons I enjoy going out with my camera and photographing the night sky so much. Even though I couldn't see most of what is up there in the sky, I still was generally aware of what was going on up there. It wasn't until I started doing night photography that I became aware of one thing though, a phenomenon that is known as airglow. I started seeing color in some of my photos, sometimes wildly streaking color, and in some of the night photography groups on Facebook I saw it mentioned. It wasn't until recently though, when some of the people joining me on my night photography adventures starting asking questions about it, that I decided to really look up what airglow is all about and what causes it.
The first 2 questions that typically get asked when people see the colors in night sky photos are "Are those colors real?" and "Can you see that with your own eyes?" The answer to the first question is yes, those colors are out there, but the answer to the second question is a little more complicated. Airglow is the dimmest source of light in the night sky, in fact if you're not in a really dark location far from city lights you may not even be able to pick it up in your photographs. Even if you have good night vision and your eyes have adjusted to the darkness, you have to look for it purposefully, as it will appear like faint clouds or haze in the sky. Even if you start to notice it, it is too dim to activate the cone cells in our retinas that give us color vision, so you won't see the colors. Only our highly sensitive cameras can make out the different colors being given off. It is most visible 10-15 degrees above the horizon. Lower than that and the light is absorbed by denser air, and higher in the sky it diffuses more and becomes dimmer.
So what causes airglow? I first saw it described as being similar to the process that causes aurora, and the colors that are given off are similar. That's only partially true however. They due form at similar altitudes in the atmosphere (60-65 miles), and they both are caused by excitation of atoms and molecules (typically oxygen). The reactions that cause them are different though. If you're familiar with aurora, you know that it is caused by the solar wind impacting the earth's atmosphere and magnetic field. Nitrogen & oxygen are excited, and as they return to rest they give off the green & red light that causes the aurora. The closer you are to the earths poles, the more prominent it will be. Airglow is caused by the ultraviolet light from the sun stripping electrons off of oxygen & nitrogen atoms & molecules. As the electrons reunite, they give off the colors of airglow. The brightest is the green light coming from the oxygen atoms, and it is the most prominent color of airglow you will see in photographs. Red airglow is second most common, caused by reactions with oxygen & nitrogen atoms much higher in the atmosphere, at an altitude of 90-185 miles. Yellow light will occasionally be given off by excited sodium atoms shed by meteors, and occasionally there will be some weaker blue light given off by oxygen molecules as well. Unlike the aurora, airglow is always there and exists anywhere on earth. The intensity of the airglow does vary from night to night, and even throughout the night. It's intensity is affected by how much energy is being given off by the sun, and its appearance is affected by winds in the atmosphere as well. This can cause the streaking that you see in some photos like the one below. If you want to learn even more about this phenomenon that brings color to our night sky, some internet searching will give even more details. My main source that I used for this post is a great article on Universe Today written by Bob King. The night sky is a fascinating thing to study, and this is just another way that my photography has added to my knowledge.
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