APOD: Airglow Over Germany (2012 Sep 05)

[APOD Robot]

Airglow Over Germany

Explanation: Does air glow? It does, but it is usually hard to see. When conditions are right, however, a faint glow about 90 kilometers up can be observed, most easily with a wide-angle long-duration camera exposure. The same airglow can also frequently be seen looking down -- in pictures taken from Earth orbit -- as a faint arc hovering above the surface. Pictured above between the beige clouds, above the curving Earth, behind the streaking airplane, and in front of the sparkling stars are some green bands of airglow. The glow is predominantly created by the excitation of atoms by ultraviolet light from the Sun, with the bands resulting from density fluctuations caused by upward moving atmospheric gravity waves. The above image was taken in mid-July above Weikersheim, Germany. Lightning and aurorae can also cause air to glow, but result from particle collisions and are more fleeting.

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[Beyond]

Why, yes it did. Very nicely, it would seem.

[Chris Peterson]

From a really dark site- one with no light pollution at all- it's remarkable how light the sky is once you are dark adapted. We like to think of those sites having inky black skies, but such skies don't exist. You can actually read large print under the glow of the darkest sky.

People have asked here before about how many more stars you might see from space, or from the surface of the Moon. The answer is not much more than you see from the ground, since our atmosphere doesn't attenuate the stars very much. But without airglow, the space between the stars, seen from space, would be utterly black. Such a sky, filled with stars, must be surreal and awe inspiring.

[revloren]

Allright, I always thought of myself as an astronomy nerd, but...airglow? Gravity waves? I always thought that gravity was constant. This is over my head.

[Chris Peterson]

Allright, I always thought of myself as an astronomy nerd, but...airglow? Gravity waves? I always thought that gravity was constant. This is over my head. :?

"Gravity waves" is an unfortunate term. There are gravitational waves, which are ripples in spacetime created by moving mass, and there are gravity waves, which occur in a fluid, and create all sorts of common wave effects, like pond ripples and repetitive cloud structures. The terms can be confusing in their similarity, even though they describe radically different concepts.

[Moonlady]

I love this APOD! So much to discover!

And I see a part of a cropfield circle

[ritwik]

dang !! im 23 and hadn't known anything about gravity waves up until now

Click to play embedded YouTube video.

[NGC3314]

People have asked here before about how many more stars you might see from space, or from the surface of the Moon. The answer is not much more than you see from the ground, since our atmosphere doesn't attenuate the stars very much. But without airglow, the space between the stars, seen from space, would be utterly black. Such a sky, filled with stars, must be surreal and awe inspiring.

Nitpick here - from a good dark site on Earth, almost half the visible-wavelength skyglow comes from a combination of zodiacal emission (sunlight scattered from dust grains in the inner Solar System) and its interstellar counterpart, the general starlight scattered from dust in the Milky Way. To get really really dark skies you'd need to go past the asteroid belt (although there is a local hint looking at areas of the sky which are simultaneously far from the ecliptic plane and the galactic plane). Pioneers 10 and 11 got a lot of data on this during their cruises to Jupiter, and one student drew the unenviable task of correcting their wide-field measures for the light of all catalogues foreground stars to get better estimates of the dust component. There was one plan to use simultaneous space- and ground-based measures to really separate airglow, zodiacal light, and distant background. The situation is rather different if we go outside visible light - the sky background from space is thousands of times darker in the infrared (despite dust emission), and it's really dark in the UV as well (dust scatters better but the general starlight intensity is much lower).

[Chris Peterson]

Nitpick here - from a good dark site on Earth, almost half the visible-wavelength skyglow comes from a combination of zodiacal emission (sunlight scattered from dust grains in the inner Solar System) and its interstellar counterpart, the general starlight scattered from dust in the Milky Way.

Yes, that's a good point. You'd need to be in the outer system or beyond to get the really dark skies. I think the ratio of light scattered from dust to light from airglow is quite variable, however. At solar max, my zenithal sky brightness is 20.8 mag/arcsec^2; at solar min, it is 21.3, so there is quite a range (very obvious visually) in background brightness. The contribution from dust is constant (I can see the gegenschein at both solar min and solar max).

[emc]

There’s a ton of science in today’s APOD.
How does one find the time to pursue all the details!?
I’m looking forward to retirement but I keep getting further behind on my APOD exploration…

[neufer]

I’m looking forward to retirement but I keep getting further behind on my APOD exploration…

<<A stud farm or stud in animal husbandry, is an establishment for selective breeding of livestock. The word "stud" comes from the Old English stod meaning "herd of horses, place where horses are kept for breeding" Historically, documentation of the breedings that occur on a stud farm leads to the development of a stud book. Male animals made available for breeding to outside female animals are said to be "standing at stud," or at "stud service," referencing the relatively high probability that they are kept at a stud farm. During the Middle Ages, stud farms were often managed as part of a monastery. At the time, few people apart from monks could read and write, and so they were charged with the responsibility of recording pedigrees. The Carthusian monks are famous for their role in breeding the Andalusian horse in Spain, while monasteries in Bavaria were responsible for the original Rottaler horse.>>

[emc]

Hmmm, maybe I’ll retire early! But I still may not be able to catch up on my APOD.

[bystander]

Hmmm, maybe I’ll retire early! But I still may not be able to catch up on my APOD.

I'm not sure Mrs. Ed would approve, however appealing a stud farm may be to you.

[Ann]

I'm thinking of the color of this airglow. It resembles the color of green aurorae, but it isn't the same. Green aurorae are often yellow-green, about 560 nm or so, but the color of this airglow appears purely green and not at all yellow. If I were to guess at the wavelength, I'd say perhaps 530 nm or so.

I thought that ultraviolet light was responsible for green aurorae, but apparently it is more responsible for airglow than for aurorae?

But I guess I shouldn't ask too much, because the answer may well be found in one of the links of today's APOD. I must admit that I haven't read them.

Ann

[Chris Peterson]

I thought that ultraviolet light was responsible for green aurorae, but apparently it is more responsible for airglow than for aurorae?

The only role of ultraviolet light in auroras is to dissociate molecular oxygen to atomic oxygen, which is ionized by high energy particles, emitting light at 557.7 nm. This is the green light of auroras (to me it looks very green, with no yellow... but color is quite subjective). Although the mechanism creating airglow is quite different from auroras, the green component of airglow is the same 557.7 emission line. Of course, in practice, this line is often mixed with other emission lines as well, which results in a wide range of possible perceived colors.

Images of airglow and auroras are usually made with digital color cameras, which are poor at accurately reproducing emission line spectra. So in images of these phenomena, you'll encounter a wide range of apparent color, as well.

[Ann]

Thanks, Chris. Very interesting. In this APOD, as well as in today's, the airglow looks very purely green to me. But in the picture taken from ISS(?) of Comet Lovejoy seen from space over the Earth, the airglow looks definitely more yellow-green and more like the typical color of aurorae.

So I find it very interesting that it is the same 557.7 nm wavelength that causes the green color.

Ann

[NGC3314]

At solar max, my zenithal sky brightness is 20.8 mag/arcsec^2; at solar min, it is 21.3, so there is quite a range (very obvious visually) in background brightness. The contribution from dust is constant (I can see the gegenschein at both solar min and solar max).

Oh, sure, rub it in about having dark skies!

As far as I know, one of the first astronomers to really take advantage of the general dependence of airglow intensity on solar cycle was Chip Arp - most of the photographs in his atlas of peculiar galaxies were taken during the minimum in the early 1960s, using then-new plates with improved green sensitivity and going deeper than most previous photos. (Today, I continue to be struck by the number of amateur imagers who routinely image deeper still - not only are digital detectors powerful, they and the needed processing expertise have proliferated widely).

[emc]

Hmmm, maybe I’ll retire early! But I still may not be able to catch up on my APOD.

I'm not sure Mrs. Ed would approve, however appealing a stud farm may be to you.

She would laugh… hysterically… it would be embarrassing!