MadSci Network: Chemistry

Re: What causes the green light in the sky.

Date: Wed Jan 28 16:44:28 2004
Posted By: John Christie, Faculty, Dept. of Chemistry,
Area of science: Chemistry
ID: 1074771751.Ch

I believe the answer you are referring to is my answer in the Astronomy archive for October 1996.

The question/answer reference number is 844624659.As

The green colour comes from chemical reactions in the upper atmosphere that emit light. These reactions are triggered by radiation from the sun, but many of them are slow reactions, so the light from them is still emitted at nighttime. There are a number of different sources of light, each providing its own characteristic colour. They are known collectively as "the airglow". The light is very faint, and is about 10 times stronger at daytime than at nighttime. But it almost impossible to observe directly from the Earth's surface in daylight. Atmospheric scientists also talk about "the dayglow" and "the nightglow".

The strongest emissions of light are in the infrared, just beyond the wavelength where they might be visible. If human eyes were tuned just a little differently, the sky might look quite bright at night! The green colour comes mostly from a reaction where an excited state of atomic oxygen, singlet-S, changes to another lower excited state, singlet-D, and gives out the excess energy as light. The normal ground state of atomic oxygen is triplet-P. This gives most of the visible colour to the nightglow, but there are many other processes involving different types of atoms, molecules, radicals, and ions. Small amounts of visible red light (also atomic oxygen) and yellow light (sodium) are also present in the nightglow. The main infrared components are due to molecular oxygen and hydroxyl radicals.

There is a thorough discussion of the airglow in Richard P Wayne "Chemistry of Atmospheres" chapter 7. Unfortunately it is also rather technical and a bit difficult for a non-expert to follow. You might also try web searches on airglow or nightglow. As always with the web, be careful to look at just who is posting, and what credentials they have. Some of the sites don't quite "get it right".

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