IGY Bulletin, Number 5, November 1957 - Airglow measurements during the IGY

The second article in this issue of the IGY Bulletin returns to the topic of airglow.  As I read the article and looked a some more recent web pages and videos on airglow, I had to remind myself that the whole reason for the IGY was to add to our scientific knowledge about many geophysical phenomena. So whatever you might read or view about airglow (or pretty much anything else!) today may be somewhat different than what the IGY Bulletin said almost 65 years ago because, well, scientific knowledge is cumulative and is always subject to revision and improvement. Of course that is also true in any other area, such as understanding viruses and climate change. Duh. 

This article's topic (as will many others) involves the electromagnetic spectrum, so let's start with a little pictorial refresher of that. The diagram below shows the electromagnetic spectrum with commonly named types of radiation, and the corresponding wavelengths in nanometers (nm, one billionth of a meter) and frequencies in hertz (Hz, cycles per second). The visible portion of the electromagnetic spectrum is the narrow region with wavelengths between about 400 and 700 nm. Wavelength and frequency are related by the equation c = λ x f, where c is the speed of light (3 x 10⁸ meters per second).

The electromagnetic spectrum (Chemistry Library)

The Bulletin article starts off with a description of airglow:

A faint, usually invisible glow -- the night airglow -- occurs in the earth's atmosphere and is a subject of considerable scientific interest and study during the IGY. Although not generally identifiable by the naked eye, airglow supplies much of the light of the night sky. One investigator has roughly estimated its contribution to the visible light -- in the absence of moonlight and aurora -- at 40%, which is somewhat greater than that contributed by starlight.

This video gives a contemporary summary of airglow:

Prominent colors found in airglow include green from an atomic oxygen transition; yellow from sodium; red from oxygen; infrared from hydoxyl (OH). I asked myself, sodium in the atmosphere? Turns out there is a sodium layer in the atmosphere which originates from the ablation of meteorites.

Airglow is due to emission from excited states of atoms and molecules formed by processes resulting (directly or indirectly) from solar radiation. Airglow is an example of luminescence, the spontaneous emission of light by a substance (not resulting from heat, which would be incandescence). It includes chemiluminescence (the emission of light as a result of a chemical reactions) and fluorescence (emission of photons/light after the excitation of atoms and molecules to higher but unstable energy states).

Airglow originates in the mesosphere part of the atmosphere, at altitudes of about 80-100 km, as shown in an earlier post.

The article states that nine countries were to participate in IGY airglow studies. For the USNC-IGY program, photometers (instruments that measures the strength of electromagnetic radiation) were to periodically scan the skies to measure airglow. Pen-and-ink records  would be transferred to punch cards which were fed into computers. Various corrections would then be made to compute the absolute intensity of the airglow in different areas of the sky.

The heights of the airglow were to be determined using rocket soundings, triangulation from different ground stations, and analyzing the increase of intensity towards the horizon.

Airglow has dynamic characteristics in space and time that were also to be studied during the IGY.

One of the books I have in my IGY library -- Geophysics and the IGY: Proceedings of the Symposium at the Opening of the International Geophysical Year -- does have an article on airglow, which bears similarity to this one in the IGY Bulletin. Perhaps the author is the same, although Bulletin articles are not attributed. The title page and table of contents for the book are shown below. One article on The Night Airglow, the scan of which is here, is by Franklin E. Roach. Darn, this book is another one of those good IGY resources I own that I haven't really much looked at.