Earth is a truly magical place.
We know it’s not the kind of thing you would expect to read on a science website, but just a look at the photo above – a pre-dawn photo taken by an Expedition 62 crew member on the International Space Station (ISS) back in March.
In this image, taken when the ISS flew just south of the Alaskan Peninsula, the fantastic colors you see are particles interacting in the Earth’s upper atmosphere in different ways, resulting in two completely different atmospheric phenomena in one photograph.
Truly magical to look at, but also easily explained with some science.
The first phenomenon is the aurora – the glowing green, red-tipped phenomenon on the left side of the image. Auroras emerge when charged particles from the solar wind hit the Earth’s magnetosphere – a kind of protective mantle where such particles are at the grating of our magnetic field.
Mixed with atmospheric gases such as oxygen and nitrogen, the particles create the colors we know as the aurora.
Excited by solar wind, oxygen atoms at the highest altitude release this excess energy as the red glows, while the green is caused by exciting oxygen and nitrogen molecules that release energy at lower altitudes.
But the aurora is only one part of this particular shot. Straight to the image above, take a look at the yellow-red light band right above the curve of our planet. It’s called an ‘air glow’, and it’s subtler than the aurora, but just as cool.
To understand airglow – more specifically night glass – you need to remember that the night sky is never completely dark, not even once you have extracted light pollution, starlight and diffused sunlight.
Instead, atoms produce ’emissions’ from being in their excited state. For example, oxygen that is released during the day recombines and releases its extra energy as photons at night. Nitrogen molecules and reactions between nitrogen and oxygen also contribute to this luster.
Earth glass. (NASA)
The photons released in this case appear green, as in this image above, but yellow sometimes occurs at a lower layer (about 80 to 100 kilometers above the earth’s surface).
Meteors break up in this layer of the atmosphere, releasing sodium atoms into the air, which is why it is called the natural layer; exciting sodium atoms will create a distinct yellow luster.
As a bonus, the rising sun behind Earth causes the edge of the planet to appear dark blue. This happens for the same reason that the sky is blue during the day – when sunlight hits the molecules in our atmosphere, blue light (one of the shortest wavelengths) is scattered, while other colored light usually transmits.
We told you, absolutely magical.
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