Airglow
Definition
A faint natural glow of the night sky produced by chemical reactions in the upper atmosphere — present even without auroras.
In plain language
Even on aurora-free nights the atmosphere glows faintly green and red as daytime solar energy is slowly released by chemical reactions. Long-exposure photos often capture it as bands of color that get mistaken for aurora.
The science
Airglow arises from chemiluminescent recombination (e.g., atomic oxygen recombination near 95 km producing 557.7 nm emission) rather than particle precipitation, occurring globally at all latitudes.
Why it matters for aurora viewing
Knowing airglow exists prevents false alarms: green bands in photos at low latitudes with quiet indices are usually airglow, not aurora.
See this in action on tonight's data.