Airglow

Airglow is light naturally emitted by an atmosphere. On Earth it arises in the upper atmosphere through several processes: during the day, solar radiation excites atmospheric atoms and molecules; at night, chemical reactions and recombination among particles altered by sunlight can emit photons as they return to lower-energy states. The particular mechanism depends on the species and atmospheric conditions. Airglow is distinct from sunlight scattered by the atmosphere and from the more episodic aurora, even though the phenomena can occur in similar regions and may appear together in images.

Different atmospheric gases, altitudes and excitation processes produce different emission bands, so airglow can appear in colors including green, red and orange. The glow is usually faint compared with stars and city lights. From the ground, observers generally need dark skies and sensitive imaging; from orbit, cameras can view the luminous atmospheric layers against the dark background of space. NASA imagery from the International Space Station shows red and green airglow along Earth's curved horizon.

Airglow is more than a visual effect. Its emissions can be used as tracers of conditions and movement in the upper atmosphere. NASA notes that the glow carries information about temperature, density and composition, and that patterns in the emissions can reveal large-scale winds that move material through the ionosphere. Satellite missions that image airglow use it as a diagnostic to investigate how solar energy, Earth's magnetic environment and processes from lower atmospheric levels influence the region where the atmosphere transitions toward space.

Both airglow and auroras involve atmospheric emissions, but their energy sources differ. Airglow is associated with day-to-day solar radiation and atmospheric chemical processes; auroras are driven by energetic charged particles and disturbances in Earth's magnetosphere. Airglow is generally a persistent, subtle glow spread over broad areas, whereas auroral displays are more variable and often concentrated around polar regions. These distinctions help scientists use each phenomenon to study different aspects of the upper atmosphere and space weather.