Why Is the Aurora Green, Red or Purple? Colours Reveal Altitude
Auroral colours are not random photographic filters. They appear when energetic particles transfer energy to atoms and molecules in the upper atmosphere. The gas, particle energy and altitude determine the emitted light.
Green: the most common colour
Green is produced mainly by atomic oxygen at roughly 100–150 kilometres altitude. Human vision is particularly sensitive to this wavelength, so green dominates many observations and photographs.
Red, blue and purple
Oxygen at greater heights can emit red, often visible at the tops of curtains or during major storms at lower latitudes. Molecular and ionised nitrogen contributes blue, purple and pink tones, especially in lower regions and energetic edges.
Why cameras see more
Camera sensors collect light throughout an exposure and record colours too faint for human night vision. An aurora that appears greyish to the eye may look green in a photograph without the camera inventing it.
Every colour is a signature revealing which gas was excited and at what height.
Reading an auroral curtain
A purple lower edge may indicate energetic precipitation at relatively low altitude; a green band often corresponds to common oxygen emission; a red top may come from much higher oxygen. These heights are approximate and emissions can overlap.