In most photos from space, the atmosphere gradually fades away from pretty bright blue to essentially black, as you move away from the Earth surface, such as can be seen in this photo of the Earth from the ISS (also showing some noctilucent clouds):

NASA photo of Earth atmosphere
NASA photo of Earth atmosphere. Source: NASA

Or by night (again with noctilucent clouds):

NASA photo of Earth atmosphere by night
Earth atmosphere by night. Source: NASA

Yet in photos with an exposure long enough for stars and airglow to be clearly visible, the opposite appears to be the case:

ISS photo of Earth and milky way
NASA photo of Earth atmosphere and milky way. Source: NASA

In the third photo, the Earth is rather dark. Moving away from the Earth, the sky closest to the Earth is colourful, but faint enough for stars to be visible through it. Then the atmosphere appears to get brighter as you move further up, until it becomes abruptly black after a sharp edge. Higher up still we can see a faint but clearly visible band of red airglow. It's not aurora — aurora is higher up than airglow, and not so constant.

Another example, but with different colours (that could be a side-effect of exposure or editing):

ESA starry night
ESA starry night. Source: ESA via Wikimedia Commons


1 Answer 1


The glow observed in those pictures in the upper layers of the atmosphere is the airglow from OH molecules. OH emission peaks between 75 and 105 km of elevation (Blamont and Reed, 1967) with intensities peaking at different wavelengths, the most prominent peaks in the visible spectrum are 557nm (Green), and multiple other peaks between 620 and 750 nm (red)

In 1965 the OGO-II satellite was launched with the propose to measure airglow, the following image show a typical altitude vs intensity profile:

enter image description here

From Blamont and Reed, (1967). The peak around 100 km is mostly due to OH airglow and the broader peak around 250 km is mostly due to atomic oxigen.

Regarding the color observed in the pictures, while it is difficult to trust them due to photo editing, they are consisten with OH airglow, in the greens, yellows and reds. This is the airglow spectrum from Broadfoot and Kendall (1968), where I've highlighted in yellow the most important emission bands of OH in the visible range:

enter image description here

To get a sense of the color associated to each band here is a approximate scale (remember 1 nm = 10 Å)

enter image description here

From wikipedia

The fading yellow glow that goes below the brightest band can be explained as the OH glow at the same altitude band (75-105 km) but originating from the part of the atmosphere that is closer (or further) to the spacecraft from which the picture was taken. As that altitude band is a shell around the Earth and not a ring. For the same reason the peak intensity in the picture is likely a bit lower than altitude of the peak in emissions.

  • $\begingroup$ You know every earth science subject amazingly sir :-) $\endgroup$ Commented Aug 18, 2018 at 2:37

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