Scientists have solved the centuries-old unexplained mystery of "bright nights", the unusual glow that appears in the sky after dark and lets observers see distant mountains, read a newspaper or check their watch.
Researchers suggest that when waves in the upper atmosphere converge over specific locations on the Earth, it amplifies naturally occurring airglow, a faint light in the night sky that often appears green due to the activities of atoms of oxygen in the high atmosphere.
Generally, it is not easy to spot the airglow but on bright nights it produces an unexplained glow that can be seen with the naked eye. There is a lot of written record about the unusual glow in historical observations.
Historical accounts of bright nights go back centuries.
European newspapers and the scientific literature carried observations of these events in 1783, 1908 and 1916.
According to Gordon Shepherd, an aeronomer from the York University in Toronto in Canada, Bright Nights do exist and they are a part of the variability of airglow that can be observed with satellite instruments.
Shepherd, who is also the lead author of the study published in the journal Geophysical Research Letters, further stated that the historical record is so coherent, it goes back over centuries and the descriptions are very similar.
• Modern observations of bright nights from Earth are practically nonexistent due to pollution. Even dedicated airglow researchers have never seen a true bright night.
• However, even before the advent of artificial lighting, bright nights were rare and highly localised.
• Researchers could see bright night events reflected in airglow data from the Wind Imaging Interferometer (WINDII), an instrument once carried by NASA’s Upper Atmosphere Research Satellite (1991-2005).
• They searched for mechanisms that would cause airglow to increase to visible levels at specific locations.
• Airglow comes from emissions of different colours of light from chemical reactions in the upper reaches of the atmosphere. The green portion of airglow occurs when light from the sun splits apart molecular oxygen into individual oxygen atoms.
• When the atoms recombine, they give off the excess energy as photons in the green part of the visible light spectrum, giving the sky a greenish tinge.
• To find factors that would cause an increase in airglow and create bright nights, researchers searched two years of WINDII data.
• They identified 11 events where WINDII detected a spike in airglow levels that would be visible to the human eye, two of which have been described in detail in the study.
• These events were four to 10 times brighter than normal airglow and could be responsible for the bright nights observed throughout history.
The Researchers estimated that visible bright nights occur only once per year at a specific location and their observation would rely on a sky watcher looking from a remote location on a clear, moonless night with dark-focussed eyes.
Further, Shepherd estimates that a bright night occurs somewhere on Earth, at different longitudes, on about seven percent of nights.