APOD: Green Aurora over Sweden (2024 May 22)

[APOD Robot]

Green Aurora over Sweden

Explanation: It was bright and green and stretched across the sky. This striking aurora display was captured in 2016 just outside of Östersund, Sweden. Six photographic fields were merged to create the featured panorama spanning almost 180 degrees. Particularly striking aspects of this aurora include its sweeping arc-like shape and its stark definition. Lake Storsjön is seen in the foreground, while several familiar constellations and the star Polaris are visible through the aurora, far in the background. Coincidently, the aurora appears to avoid the Moon visible on the lower left. The aurora appeared a day after a large hole opened in the Sun's corona, allowing particularly energetic particles to flow out into the Solar System. The green color of the aurora is caused by oxygen atoms recombining with ambient electrons high in the Earth's atmosphere.

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[Ann]

Okay, Color Commentator speaks.

Green Aurora over Sweden. Image Credit & Copyright: Göran Strand

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And you, Göran Strand, my fellow Swede, that's another great picture from you. You have taken so many. Of all the pictures you have taken, this is my favorite:

Gibbous Moon beyond Swedish Mountain Image Credit & Copyright: Göran Strand

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The gibbous moon picture is from 2018, and it was the Astronomy Picture of the Day on November 19, 2018.

As for the shape of the aurora you have captured in today's APOD, it is so impressive, like a magnificent celestial portal or the most amazing set of draperies gracing the sky, like the curtains in an old theater house and the audience just waiting for the spectacle to come.

Of course, here the curtain is the spectacle. And... it's so green. About this color, ███.

So, yeah. This is a green-green aurora. The whole picture is like, █████████████████████. Except the green stuff is an arc and not a straight line.

So, yeah, this is why I rarely get excited by aurora images. They tend to be soooo gren, or soooo red, ███. Or soooo green and red, ██████. Yes, because when red and green auroras are seen together, the green aurora tends to be a yellower shade of green.

For this very reason, I have really, really liked some of the aurora pictures that have been posted recently, because they have been multicolored! Some of them have displayed hues of magenta, purple and even blue!!!

This aurora is partly magenta and purple! Credit: Mariusz Durlej

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This aurora shows a flare of blue at right, can you see it? Credit: Gabriela Levit.

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And just look at this amazing color bomb from Matthew Browne, can you believe it?

Aurora in Pembrokeshire, Wales, by Matthew Browne.

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So, yeah. Today's APOD is a beautiful picture, Göran Strand, because you are indeed a great photographer.

And now I'll go and look at something else.

Ann

[Christian G.]

Amazing aurora arch, amazingly not too far from overlapping the Milky Way arch!

[Gibo1973@msn.com]

Magnificent photograph. Why isn’t the aurora reflected in the water? Respectfully, John.

[Roy]

I’m having a hard time finding a coherent explanation of what causes the different colors. O2 molecules split because of ultraviolet light, then the O atom picks up an “ambient” electron, (not from the sun , only “particles” come from the sun, not protons and electrons) causing emission of green light or red light, depending on the altitude? And what about N2 and argon molecules?

[Chris Peterson]

I’m having a hard time finding a coherent explanation of what causes the different colors. O2 molecules split because of ultraviolet light, then the O atom picks up an “ambient” electron, (not from the sun , only “particles” come from the sun, not protons and electrons) causing emission of green light or red light, depending on the altitude? And what about N2 and argon molecules?

The primary charged particle responsible for auroras is the electron. When an electron collides with an atom or molecule in the atmosphere it ionizes it. When the atom relaxes back to its ground state it releases a photon, the color of which depends on the state of ionization. The most common color we see is green, which is produced by the ionization of single oxygen atoms, either by electrons from the Sun or by secondary electrons freed up when solar electrons strike diatomic nitrogen. Red is also produced by the ionization of single oxygen atoms, but at a higher altitude where the lower density changes the relaxation time dynamics. Other colors are much fainter and usually only seen photographically.

[Ann]

I’m having a hard time finding a coherent explanation of what causes the different colors. O2 molecules split because of ultraviolet light, then the O atom picks up an “ambient” electron, (not from the sun , only “particles” come from the sun, not protons and electrons) causing emission of green light or red light, depending on the altitude? And what about N2 and argon molecules?

I am, unfortunately, not a great fan of auroras, which means that I don't know a lot about them. But you may want to look at this page: https://www.webexhibits.org/causesofcolor/4D.html

Here you can find the following tidbits of information:

The color of the aurora depends on the wavelength of the light emitted. This is determined by the specific atmospheric gas and its electrical state, and the energy of the particle that hits the atmospheric gas. The atmosphere consists mainly of nitrogen and oxygen, which emit the characteristic colors of their respective line spectra. Atomic oxygen is responsible for the two main colors of green (wavelength of 557.7 nm) and red (630.0 nm). Nitrogen causes blue and deep red hues.

So the two main colors of the aurora are green at 557.7 nm, which looks like this, ███ and red at 630 nm, which looks like this: ███. In both cases, the color comes from oxygen.

The red aurora is formed higher up in the atmosphere than the green aurora:

The strong, green light originates at altitudes of 120 to 180 km. Red Northern Lights occur at even higher altitudes, while blue and violet occur mostly below 120 km. When the sun is "stormy," red colors occur at altitudes of 90 to 100 km.

It seems obvious to me that it is harder to create a blue nitrogen aurora than a green or a red oxygen one. Since the blue aurora is so rare, it has to require a lot of energy to create it. But in the many pictures of auroras that have been submitted to Starship Asterisk* and other sites from the latest solar storm, there have indeed been auroras that have also contained the color blue. That makes sense, if we consider that the latest solar storm was a particularly violent one.

Ionized nitrogen causes both blue and red auroras. Don't ask me why.

Click to view full size image

Ann

[Chris Peterson]

It seems obvious to me that it is harder to create a blue nitrogen aurora than a green or a red oxygen one. Since the blue aurora is so rare, it has to require a lot of energy to create it. But in the many pictures of auroras that have been submitted to Starship Asterisk* and other sites from the latest solar storm, there have indeed been auroras that have also contained the color blue. That makes sense, if we consider that the latest solar storm was a particularly violent one.

Keep in mind that very few of the aurora pictures we see look like what we see with our eyes. As with most astronomical images, a camera captures much more detail and color than our eyes are capable of. The blue color of auroras is not very bright, and is in a part of the spectrum where our visual sensitivity is very low. So it will show up in images where nothing is visually evident at all. (This is also true of red and green in auroras... at my low latitude, I've captured auroras on camera that were completely invisible to the eye except for a slight colorless sky glow that could come from a variety of sources.)

[Roy]

Thanks for that reference, Ann. It makes it very clear what is thought to happen. Do we know of any experiments to replicate this? Maybe when neon signs were being invented?i
Speaking of seeing auroras at low latitude, I did see the 1989 event, living then in Massachusetts near Boston, about 41 degrees N latitude ( I’m now in Maine, 45 degrees N lat.).

[Chris Peterson]

Thanks for that reference, Ann. It makes it very clear what is thought to happen. Do we know of any experiments to replicate this? Maybe when neon signs were being invented?i
Speaking of seeing auroras at low latitude, I did see the 1989 event, living then in Massachusetts near Boston, about 41 degrees N latitude ( I’m now in Maine, 45 degrees N lat.).

It is quite tricky to create either the 557.7 nm emission or the 630 nm emission in the lab, because both involve forbidden transitions of monoatomic oxygen which must be at a very, very low density.

[Ann]

It seems obvious to me that it is harder to create a blue nitrogen aurora than a green or a red oxygen one. Since the blue aurora is so rare, it has to require a lot of energy to create it. But in the many pictures of auroras that have been submitted to Starship Asterisk* and other sites from the latest solar storm, there have indeed been auroras that have also contained the color blue. That makes sense, if we consider that the latest solar storm was a particularly violent one.

Keep in mind that very few of the aurora pictures we see look like what we see with our eyes. As with most astronomical images, a camera captures much more detail and color than our eyes are capable of. The blue color of auroras is not very bright, and is in a part of the spectrum where our visual sensitivity is very low. So it will show up in images where nothing is visually evident at all. (This is also true of red and green in auroras... at my low latitude, I've captured auroras on camera that were completely invisible to the eye except for a slight colorless sky glow that could come from a variety of sources.)

Point taken, Chris, but my point was that blue auroras are very rare even in photographs. But during the recent solar storms, several "photopgraphic" blue auroras were seen.

Ann