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What is Airglow?

Recent photo from the ISS showing airglow

In many of the photos that we have featured recently from astronauts aboard the International Space Station, a glowing greenish-yellow band can be seen just above Earth’s limb. I’ve been asked before what this is, so I thought I’d explain it here. This is a phenomenon known as “airglow”.

A photochemical reaction that occurs high in the atmosphere, airglow is the result of various atoms, molecules and ions that get excited (chemistry-excited, that is… not “whee!”-excited) by ultraviolet radiation from the Sun and then release that energy as visible – as well as infrared – light when they return to their “normal” state. It’s not entirely unlike glow-in-the-dark toys or paint!

This light is most visible to the crew of the ISS when it is orbiting over the night side of the planet, and thus is seen in images like the one above. It appears like a thin band because viewing the atmosphere at a shallow angle – rather than directly down through it – increases the airglow layer’s relative visibility.

Most of visible airglow comes from oxygen atoms and molecules, which glow green… as commonly seen in the aurora. Other contributing elements include sodium and nitrogen. While present in the atmosphere at all layers, the region that glows visibly is typically constrained to a narrow band 85 – 95km (53-60 miles) high. The band itself is usually about 6 – 10km (4-6 miles) wide. The reason for this is that below those heights the atoms and molecules are more concentrated and collide more readily, releasing their energy sooner, and above it the density of the atoms is too low to do much colliding at all (to put it very simply.)

There are a lot of other factors involved with airglow as well, such as temperature and altitude, as well as different kinds of airglow depending on when in the day they occur. Nightglow is not exactly the same as dayglow, and then there’s even twilightglow… one could say there’s a lot glowing on in the upper atmosphere!

I’m here all week, folks.

You can read more about airglow in this informative article by the Institute of Astronomy and Astrophysics (Instituto de Astronomía y Física del Espacio) in Buenos Aires. Image credit: NASA.

 

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

Comments on this entry are closed.

  • John B October 8, 2011, 6:52 PM

    It’s not green – It’s been Photoshopped to hide it’s real color – the color of smog – That’s because it IS smog. Got any more oil company PR propaganda for us?

    • Baris Bicer October 8, 2011, 6:56 PM

      I have some propaganda for you. Get out.

    • Lights in the Dark October 9, 2011, 12:06 AM

      Nope, no Photoshop involved. I use Photoshop every day of my career and I can tell you straight out there was none used on this image. I didn’t even crop it. ;)

  • Brian York October 8, 2011, 7:04 PM

    There’s also a fair bit of airglow (also occasionally referred to as geocoronal emission) in the ultraviolet, particularly the Oxygen doublet around 1300 angstroms, and Lyman Alpha at 1215. They’re the bane of the cosmic origins spectrograph on Hubble.

  • John B October 9, 2011, 6:41 AM

    Nobody – paper – media writes or publishes articles about why brown stuff at the top of the atmosphere IS air pollution. The articles don’t exist – nobody gets paid to do such a piece – no paper – internet website has any incentive to publish such an article. Oil companies and fossil fuel corporations have incentive to pop up articles and columns out of no where propagandizing that brown stuff at the top of the atmosphere ISN”T air pollution but rather (insert incredibly overly non understandable scientific jargon here). I rest my case. They’re all over the internet everywhere regularly. Explanations why such phenomena are in no way related to cars and diesel trucks and smog.All the time – day in day out. They become easy to spot.

    • Anonymous October 9, 2011, 12:56 PM

      Why would smog glow? Have over 500 people from 38 countries who have been into space all been paid by oil companies to keep quiet? Is data from environmental monitoring satellites systematically altered before transmission to Earth? Are blog articles only written by people who get paid for their opinion?

      Energy from the sun hits atoms which vibrate and re-emit the energy as visible light. Stop me if I’m getting too technical for you.

    • Torbjörn Larsson October 9, 2011, 3:10 PM

      Oh, where you serious with that conspiracy theory bullshit, on a science website to boot?

      Conspiracy theories are always the least likely just so story for anything by construction (untestable, generally purposefully so; complicated, when simple explanations suffice). The odd thing is that only in the presence of an actual rare conspiracy they can become observable. (o.O) They don’t fly high on science sites.

      Airglow is a commonly observed geophysical phenomena. That said, it is still researched. (I remember an article on a faint new airglow mechanism deeper in the atmosphere, previously unobserved.)

      But it is most definitely not caused by pollution, or even having pollution participate. Even this green oxygen airglow starts at 90 km. The bulk (~ 99 %) of aerosols are found in the troposphere, i.e. up to ~ typically 10 km. The dust content at the airglow region is _many_ order of magnitudes lower than the oxygen content those emissions we see.

    • Lights in the Dark October 9, 2011, 7:20 PM

      Three facts (real facts, not make-believe facts): 1. smog doesn’t glow, 2. the information on airglow and its mechanics came from an Argentinian science institution, and 3. I have never gotten a check from a fossil fuel company for my articles. And if they offered to pay me to write a false science bit for them, I’d tell them to find someone else.

  • Anonymous October 9, 2011, 12:47 PM

    Airglow is similar to Airbrushing. It’s what Gaia uses when she’s feeling a little jaded or when she has forgotten to use her “Worth It” cosmetics. Gaia does prefers the term Airglow to “geocoronal emission” dear Brian Y.

  • Anonymous October 9, 2011, 1:48 PM

    Presuming–hoping–this IS natural atmospheric airglow, excited by the Sun’s UV radiation, these orbital images leave me with a profound sense wonder over our Home Planet, framed in the bigger celestial picture.

    Circle the Solar System, survey our star’s family of battered worlds: some stripped naked of air envelopes ( if ever they were so enclosed ), while others are smothered in toxic brews, from what one could describe as catastrophe smog: Venus ( one hell of a place! ). While yet others, are embroiled with howling storms of endless fury, were no terra-firma exists to rest ( before being torn to shreds ).

    And among them, some which give tantalizing hints, they once were more life friendly!

    Then return back to a serene Earth orbit ( alert to flying debris ). And stand in humbled awe at the life-teeming ocean world slowly turning below, with its emerald-hued continental masses, virtually alive with animated activity. And, there, in the distant horizon of this “oasis in space”, the luminous green arc of its life-supporting atmosphere, colored in its dominant floral hue.

    A wonder, reflecting a wonder!

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