Airglow is a faint, continual emission of light originating from the Earth’s upper atmosphere, typically between 80 and 400 kilometres in altitude. While often mistaken for distant starlight, it forms a distinct phenomenon with unique characteristics and scientific significance.
- Emission Process: Airglow primarily results from chemiluminescence. Solar radiation ionizes atmospheric molecules like oxygen, nitrogen, and sodium during the day. These excited molecules later recombine with other particles at night, releasing energy as light in specific wavelengths.
- Spectral Colours: Different molecules emit light at characteristic colours:
- Green: Primary emission from excited oxygen atoms.
- Red: Mainly from sodium atoms, contributing to the reddish band above the horizon.
- Blue and violet: Emissions from hydroxyl (OH) and nitric oxide (NO) molecules.
- Visibility and Variations: Airglow intensity varies due to altitude, wavelength, and location. Magnetic storms can enhance brightness, creating spectacular displays. Astronauts observe airglow as a luminous band encircling Earth.