Friday, May 4, 2012

What is Night Glow/Airglow?



What are the different colors in the Atmosphere?
Even on a moonless night, away from any city lights, you can still see your hand.  Where is this light coming from?  Some of the light is from the stars in the Milky Way, but a majority of this light is created from the atmosphere itself.  Yes, the atmosphere creates light!  This is called night glow (AKA: airglow).  A star-filled night sky is 10 times brighter with airglow than without it. 

What causes Night glow?
Daytime UV radiation excites atoms of oxygen and nitrogen atoms/molecules in the thermosphere. These excited atoms and molecules can either return to their ground state (un-excited) and give off a photon in the process or bump into other atoms and molecules and create other molecules via a chemical reaction. The production of hydroxyl radicals (OH), nitric oxide (NO) and molecular oxygen give off light in process known as chemiluminescence.  This is the major component to night glow, and the reason why there is no such thing as a truly dark sky, even on a moonless night.

Yellow:
The "Sodium Layer" is a yellowish layer between 80-105km above the surface of the earth that has a average thickness of around 5km.  Meteors breaking up in the atmosphere (see below) act as the source of sodium for this layer.  Sodium atoms in this layer in an excited state radiate wavelengths ~589nm in the visible spectrum (yellow light).



Red:

Atomic Oxygen

The Red layer of night glow is seen at heights of 150-300km above the surface of the earth.  Red emissions are from excited –OH radicals.  OH radicals are created by the combination of ozone and hydrogen.

As seen in image below, there are non-uniformities, which can be caused by gravity waves.  Similar to how you can cause waves between two adjacent mediums of differing densities (on the surface of a lake at the interface between the denser water and the less dense air), gravity allows for ripples to form between atmospheric layers and thus supports these non-uniformities.


Green:

The green light given off is due to the combining of molecular oxygen and molecular nitrogen, at heights around 90-100 km from the surface of the earth, and excited oxygen. When oxygen and nitrogen combine, they form NO (nitric oxide), in a process that emits a photon typically ~558nm (Green). 

Blue:
The weak blue light is formed by excitation of molecular oxygen and can be seen at ~95km.




Airglow Light Wavelengths
Source



17 comments:

  1. Loved this post! Clear, concise explanation of a wonderful topic. Airglow is one of those phenomena people tend not to be aware of until it is pointed out to them. I knew of it, but thoroughly enjoyed learning the details of how it is generated. Thank you, Alex! (And your profile picture makes me smile every time I see it!)
    --Susan.
    (And congrats on publication in Science!!)

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  3. Hello Alex,
    As an astrophotographer, I've often noticed that airglow near the ocean tends to be green while airglow over land tends to be orange/red. Is there a reason for this or is it just coincidence that I have captured airglow in this pattern? I thought perhaps it had to do with evaporation over the ocean making the air more moist......but I'm not the scientist here :)

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