Why do they appear in different colours?
An aurora's colours are set by the composition of the atmospheric gases, the altitude at which it forms, the density of the air and the level of energy released.
gas and altitude.
The most commonly seen colour. It forms when charged particles collide with oxygen at low altitude (≈100–300 km).
Four colours, three gases.
- The most common colour. It forms when charged particles collide with oxygen at low altitude.Green
- Forms at the aurora's lowest layer, when electrons excite nitrogen in the atmosphere. Far rarer than the other colours.Pink
- In the higher layers of the atmosphere, collisions of oxygen atoms produce red instead of green. In the thin air this colour takes about two minutes.Red
- Hydrogen and helium gases produce these colours. Because of how the human eye is built, it struggles to tell them apart.Blue & Violet
Time is a colour too.
At high altitude the atmosphere is less dense, so red light needs more time and energy to form — about two minutes. Green takes around a second. Every curtain you see in the sky is, in fact, a chemistry burning at different speeds.
How does it look to the naked eye?
Unlike the frozen image in a photograph, to the naked eye auroras are in constant motion — a form dancing across the sky. Most of the time they stir as though a thin veil were being drawn. Though some patterns of movement seem to repeat, they never compose the same view from the same point in the same order, so each time they offer a unique sight.
+Added · 2020·A new form: the DunesThe book describes four colours; in 2020 a new form joined the list. Finnish sky-hunters spotted green, parallel waves resembling sand dunes — photographed simultaneously from Finland and Sweden on 7 October 2018 and published in AGU Advances in January 2020. Nicknamed “the Dunes,” it is a single-colour wave field at about 100 km altitude with a ~45 km wavelength, caused by atmospheric “mesospheric bore” waves that make oxygen glow within a channel between layers. A discovery night photography handed to science.
STEVE — the sky's violet ribbon.
Discovered in 2016 by aurora watchers in Alberta, Canada, this atmospheric optical phenomenon glows in green and pink tones, stretching across the sky as a ribbon. It forms as extremely hot atomic particles move rapidly within the sub-auroral ion drift. Strong Thermal Emission Velocity Enhancement — STEVE for short — closely resembles an aurora yet is an entirely different formation. It is seen far more rarely and lasts a shorter time. The green “picket fence” beneath it hides tiny, point-like “streaks,” found in 2020 with the help of sky-hunters, that still aren't fully explained.
- +The Dunes · a new auroral form
- +STEVE · the sky's violet ribbon
- ○Original · book edition