Scientists have finally managed to find the answer to the centuries old puzzle surrounding bright nights – a phenomenon when an unexplained glow in the night sky lets observers see distant mountains or check out time in their watch and even read a newspaper.
The puzzle is one of the oldest one known to humans and according to a study to be published in Geophysical Research Letters, satellite data has helped scientists offer a possible explanation. In the study scientists explain that when waves in the upper atmosphere converge over specific locations on Earth, it amplifies naturally occurring airglow.
This natural airglow is a faint light in the night sky that often appears green due to the activities of atoms of oxygen in the high atmosphere. Normally, people don’t notice airglow, but on bright nights it can become visible to the naked eye, producing the unexplained glow detailed in historical observations.
Airglow comes from emissions of different colors 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 peaks in airglow and create bright nights, the researchers searched two years of WINDII data for unusual airglow profiles, ruling out meteors and aurora, which have their own distinct signatures. They identified 11 events where WINDII detected a spike in airglow levels that would be visible to the human eye, two of which they describe in detail in the study.
Finally, the researchers matched up the events with the ups and downs of zonal waves, large waves in the upper atmosphere that circle the globe and are impacted by weather. When the peaks of certain waves aligned, they produced bright night events that could last for several nights at a specific location. These events were four to 10 times brighter than normal airglow and could be responsible for the bright nights observed throughout history.
From their data, the researchers estimate that at a specific location, visible bright nights occur only once per year, and their observation would rely on a sky watcher looking from a remote location on a clear, moonless night with dark-adjusted eyes. If an astronomer wanted to experience a bright night personally, authors suspects that scientists could predict their occurrence if they monitored the waves continuously, so that they could calculate when their peaks would align.