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SYFY WIRE Bad Astronomy

The softly glowing night sky

By Phil Plait

There's a lot more going on over your head than you know.

For example, the atmosphere of the Earth thins out gradually the higher you go, and when you get to about 100 kilometers (60 miles) up, different physical processes become important. One of them is called chemiluminescence -- light produced by chemical processes. This can make the upper atmosphere glow in different colors. It's faint, and best seen from space... where we conveniently keep several astronauts. Neuroscientist and amateur video maker Alex Rivest has collected pictures of this airglow taken by astronauts and made this eerie and beautiful time lapse video:

Alex took the original astronaut pictures and enhanced them somewhat to bring out the faint airglow. You can see it in lots of pictures taken from the space station, and I've commented on it many times. One thing I've been meaning to do, though, is find out what the physical process is that's causing the air to glow, and why it creates different colors -- you can clearly see green, yellow, and red glow in many of the pictures!

Alex comes to the rescue on that as well. On his blog, he discusses how he made the video and why the air glows (based on a somewhat terser explanation at the Atmospheric Optics website).

The way this works is simple in general, though complicated in detail -- much like everything else in the Universe! Basically, during the day, in the upper atmosphere ultraviolet light from the Sun pumps energy into oxygen molecules (called O2; two oxygen atoms bound together -- this is the stuff we breathe). This energy splits the molecules apart into individual atoms, and these atoms have a little bit of extra energy -- we say these atoms are in an excited state. Like a jittery person who's had too much coffee, they want to give off this energy. They can do this in a couple of ways: they can emit light, or they can bump into other atoms and molecules and react chemically with them.

If you have an excited oxygen atom sitting in space all by its lonesome, it can either dump that energy by emitting green light or red light. Usually, it'll emit green light in less than a second after becoming excited, and it'll emit red light on much longer timescales, like minutes. This is important, so bear with me.

At a height below about 95 km, the atmosphere is thick enough that collisions between atoms happens all the time. In fact, an excited oxygen atom doesn't have to wait very long (usually microseconds) before another atom or molecule bumps it. If collisions happen faster, on average, than about once every 0.1 seconds, then an oxygen atom doesn't have enough time to emit green light before getting smacked by another atom or molecule. When that happens, the other atom can steal its energy, and no green light is emitted. So below that height we don't see any green emission.

At heights of 95 - 100 km or so, collisions happen less frequently, giving the oxygen atom time to blow out a green photon (a particle of light). So at that height we do see the green glow. This layer is thin, like the shell of a bubble, and we see it as an arc due to limb brightening (which you can read about here if you want details). In the picture above, you can see it as a very thin green arc above the diffuse yellow glow (which I'll get to; hang tight). Normally it wouldn't be very bright, but looking along the edge of the shell is like looking through a very long slab that stretches for hundreds of kilometers. The light builds up, making it bright enough to see.

Higher up, above 100 km, the oxygen atoms are much farther apart because the density is lower. The odds of two of them colliding are a lot lower, so the time between collisions can be pretty long, long enough to give the oxygen atoms time to emit red photons. That's why we see that red glow higher up, where the air is ethereally thin.

As I said above, the oxygen atoms can also smack into other molecules and react chemically. When there's hydrogen and nitrogen around, one of those chemical end products is what's called a hydroxyl radical -- an oxygen and hydrogen atom bound together (designated OH-). These radicals can vibrate, like weights attached to either end of spring, and emit red light in the process as well. That also contributes to the red sky glow at great heights.

I've pointed out this red smear in various astronaut photos before (like here and here), and wondered what it was. Now I know!

There's more going on, too. Below that green line (at roughly 50 - 65 km high) is a somewhat fuzzier yellow glow. It turns out that's from sodium, which emits yellow light when it's excited. It was thought for a long time that this sodium might be coming from sea salt blown into the air, but it turns out to have a more heavenly source: meteors! As these tiny rocks from space burn up in our upper atmosphere, they leave sodium behind. It's not much, but sodium is a very enthusiastic atom, and glows brilliantly. So even though there's much less of it than oxygen, it's still pretty bright.

There are other processes, too, which contribute different colors at fainter amounts. For example, when two oxygen atoms combine to form an O2 molecule, it has a bit of residual energy left over. It can get rid of that by emitting a blue photon. This is usually pretty faint, and occurs at 95 km, right at the bottom of the green layer. That's not a coincidence! Remember, that's the height where collisions become frequent, so that same process that quenches the green glow -- oxygen atoms smacking into each other -- is what causes the blue glow.

And if all this sounds familiar, it may because these same processes are what causes aurorae to glow at different colors, too! In that case, though, the source of energy isn't light from the Sun, but fast subatomic particles from the solar wind or solar storms. These come zipping in like little bullets, slam into the air, and blast apart oxygen molecules like shrapnel. After that, the process of the atoms giving off energy is pretty much the same as what I've outlined above.

The levels of complexity of all this get serious pretty rapidly past what I've described, with electrons jumping from one energy level to another, Einstein coefficients, forbidden transitions, and collision probability cross-sections. You can find out all about those online if you wish, and more power to you if you do.

But in fact, I'd say this whole topic seems to go from relatively simple to fiendishly complex in an almost -- ahem -- quantum leap.

Still, it's fascinating, and I had a lot of fun poking around websites and quantum mechanics descriptions trying to figure this all out. The pictures taken of the Earth from space are always lovely and engaging and awe-inspiring, but they become even more so when there is understanding -- when there's science -- behind them.

Knowing is always better. Always.

Image credit: NASA

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