Ciel au-dessus des Acores
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.