APOD: Rainbow Airglow over the Azores (2021 Apr 18)

[APOD Robot]

Rainbow Airglow over the Azores

Explanation: Why would the sky glow like a giant repeating rainbow? Airglow. Now air glows all of the time, but it is usually hard to see. A disturbance however -- like an approaching storm -- may cause noticeable rippling in the Earth's atmosphere. These gravity waves are oscillations in air analogous to those created when a rock is thrown in calm water. The long-duration exposure nearly along the vertical walls of airglow likely made the undulating structure particularly visible. OK, but where do the colors originate? The deep red glow likely originates from OH molecules about 87-kilometers high, excited by ultraviolet light from the Sun. The orange and green airglow is likely caused by sodium and oxygen atoms slightly higher up. The featured image was captured during a climb up Mount Pico in the Azores of Portugal. Ground lights originate from the island of Faial in the Atlantic Ocean. A spectacular sky is visible through this banded airglow, with the central band of our Milky Way Galaxy running up the image center, and M31, the Andromeda Galaxy, visible near the top left.

<< Previous APOD

This Day in APOD

Next APOD >>

[RocketRon]

Starry starry night ....

[orin stepanek]

I don't ever recall seeing Rainbow Airglow in my lifetime! Maybe I have and never paid any attention!

[Chris Peterson]

I don't ever recall seeing Rainbow Airglow in my lifetime! Maybe I have and never paid any attention! 🌈

That's because you can't see it. It's too dim to stimulate human color vision. To the eye, this scene would have appeared as the faintest of gray structure in the sky- so faint that most people probably wouldn't notice it at all. It's really a photographic phenomenon.

[johnnydeep]

Rainbow Airglow over the Azores

Explanation: Why would the sky glow like a giant repeating rainbow? Airglow. Now air glows all of the time, but it is usually hard to see. A disturbance however -- like an approaching storm -- may cause noticeable rippling in the Earth's atmosphere. These gravity waves are oscillations in air analogous to those created when a rock is thrown in calm water. The long-duration exposure nearly along the vertical walls of airglow likely made the undulating structure particularly visible. OK, but where do the colors originate? The deep red glow likely originates from OH molecules about 87-kilometers high, excited by ultraviolet light from the Sun. The orange and green airglow is likely caused by sodium and oxygen atoms slightly higher up. The featured image was captured during a climb up Mount Pico in the Azores of Portugal. Ground lights originate from the island of Faial in the Atlantic Ocean. A spectacular sky is visible through this banded airglow, with the central band of our Milky Way Galaxy running up the image center, and M31, the Andromeda Galaxy, visible near the top left.

What does "The long-duration exposure nearly along the vertical walls of airglow ..." mean?

Also, note these are "gravity waves" not "gravitational waves"! That is, waves in materials caused by gravity as opposed to ripples in space-time caused by gravity.

Finally, what are the two almost straight dark horizontal bands (along with two fainter less defined ones) - cloud layers? EDIT: ok, yeah, they must be clouds.

[orin stepanek]

I don't ever recall seeing Rainbow Airglow in my lifetime! Maybe I have and never paid any attention!

That's because you can't see it. It's too dim to stimulate human color vision. To the eye, this scene would have appeared as the faintest of gray structure in the sky- so faint that most people probably wouldn't notice it at all. It's really a photographic phenomenon.

Thanks Chris; I didn't miss anything than!

[VictorBorun]

what my eyes see in the pic:
cyan (only in some places, must be an accessory bottom or belly part of an ionosphere wave) to green to yellow to orange to red to magenta

what I read:

95 km high, faint blue range glow
emitted by O₂ molecules;

90 to 100 km high, 558 nm, green
emitted by neutral O atoms. Below an excited O atom that takes about 0.7 second to decay to another lower energy excited state is too often kicked by an N₂ molecule instead; moreover the extreme UV sunlight is less intense and there are fewer oxygen atoms

some 5 km within 80 to 105 km range, 589.2 nm, yellow-orange
emitted by neutral Na atoms in a 5 km thick layer (rather thin). Higher up it's Na⁺ ions, lower down it's Na₂O and other molecules. Nowdays this sodium layer is used for laser guide stars at leading-edge observatories;

150 to 300 km high, 630 nm, red
emitted by neutral O atoms in another excited state. Below an excited O atom that takes about 107 seconds to decay to another lower energy excited state is too often kicked instead.

There is a mismatch: the red thick top is missing and a magenta top can be seen instead. Is it just visual merge with a blue bottom of another wave beside?