Aurora colours
Green and red come from oxygen at different heights, blue and purple from nitrogen, and cameras see more colour than eyes.
Each gas gives off its own colours, and the height at which particles stop decides which gases they hit.
| Colour | Wavelength | Comes from | Height |
|---|---|---|---|
| Green | 557.7 nm | Atomic oxygen | Mostly around 100 to 150 km |
| Red | 630.0 and 636.4 nm | Atomic oxygen | Above about 200 km |
| Blue and violet | 427.8 nm and others | Ionised nitrogen molecules | Mostly below about 120 km |
| Pink and purple | Mixtures | Nitrogen (lower edge) with red and blue | Lower edges of bright curtains |
Why green is the usual colour
Oxygen's green light comes from an excited state that lasts under a second, so the atom usually gets to shine before it bumps into a neighbour and loses the energy. A 2023 study of about 58,000 camera measurements over northern Finland and Sweden found green aurora peaking at an average of about 115 km.
Why red is high and rare
Oxygen's red light comes from a state that lasts about 110 seconds. Lower down, the air is dense enough that the atom is knocked out of that state before it can shine, so red light only escapes from the thin air above about 200 km. Red takes more energy to show strongly, which is why all-red skies and red tops on curtains belong to big storms, and why red is what people far from the pole often see low on the horizon.
Pink fringes
During bright displays, faster particles reach below 100 km and light up nitrogen, giving a pink or purple lower edge to the curtains.
Eye or camera
In dim light your eyes rely on rod cells, which can't tell colours apart, so faint aurora looks grey or greyish-green. Only brighter aurora wakes up the colour-sensitive cones. A camera adds light up over several seconds and has no such limit, so photos show more colour, and more aurora, than you saw. People also differ: some see more colour than others standing next to them.