Excited gases emit specific light
Incoming energetic particles transfer energy to atoms and molecules in the upper atmosphere. The excited species later release that energy as photons. Different transitions produce characteristic wavelengths, so auroral colour is linked to atmospheric composition and physical conditions.
The process is often compared with a gas-discharge sign, but the upper atmosphere is far less dense and the timing of emissions matters. Some excited states are more likely to be quenched by collisions at lower altitude before they can radiate.
Why green is common
A familiar green emission comes from atomic oxygen, commonly at roughly 100–200 kilometres altitude. The atmosphere and particle precipitation often create favourable conditions for this line, and the human eye is relatively sensitive to green under brighter low-light conditions.
A fixed altitude should not be treated as a hard border. Auroral forms span ranges of height and energy. NASA's public science material uses approximate bands precisely because real atmospheric conditions vary.
Red can come from higher oxygen
Atomic oxygen can also emit deep red light, often from higher and thinner parts of the upper atmosphere. The long-lived excited state is more able to radiate where collisions are less frequent. Red can form above green in tall rays or dominate some diffuse, low-intensity events.
Red aurora seen far from the usual oval may sit low on the horizon and can be confused with airglow, cloud illumination or camera artefacts. Direction, motion and corroborating observations help interpretation.
Nitrogen contributes blue, purple and pink
Excited molecular and ionised nitrogen contribute blue and violet emissions, while mixtures with red and green can appear purple or pink. Energetic precipitation reaching lower altitudes can create bright lower borders in rapidly changing displays.
Colour names describe perception, not a one-to-one altitude meter. Emissions overlap along the line of sight, and a camera combines them through its sensor, filter array, white balance and processing.
Why photographs look different from eyesight
At low brightness, human vision loses colour sensitivity and a faint aurora may appear grey or pale even when a long exposure records green. Modern sensors accumulate more photons over time and often reveal colour or structure that was subtle to the eye.
GreenHalos shader colours communicate relative modelled activity; they are not measured emission wavelengths or a prediction of the colour an observer will see. The legend therefore says auroral activity visualisation, from low to high.
Evidence register
Primary sources
Sources were checked on . Each record identifies which sections it supports.
- Auroras NASA Science Supports: Excited gases emit specific light; Why green is common; Red can come from higher oxygen; Nitrogen contributes blue, purple and pink
- Aurora Infographic NASA Scientific Visualization Studio Supports: Excited gases emit specific light; Why photographs look different from eyesight