Since the first century, skywatchers have recorded instances of an unexplained green glow in the sky. Rather than distinct lights in the sky, these “bright nights” see the whole sky illuminate bright enough for distant landmasses to be seen or to allow nighttime outdoor reading. This little-understood phenomenon has been documented by writers like Roman author Pliny the Elder, who called it a “nocturnal sun.” Similar accounts of bright nights have been published in European newspapers throughout the 18th and 20th centuries. While plenty of examples of this phenomenon have been recorded, an explanation has never been found.
Until now, possibly. A new preprint study accepted by the American Geophysical Union claims to have finally solved the mystery of the “bright nights” phenomenon. A group of researchers from York University in Toronto poured through data gathered by the Wind Imaging Interferometer (WINDII) aboard NASA's Upper Atmosphere Research Satellite to find instances of bright nights when no other luminous objects were present. These instances were then compared with instances of atmospheric waves which form in the upper atmosphere. The data showed that when more than one of these waves sync up, they can create a higher concentration of oxygen atoms which emit green wavelengths of light as they recombine after being split by sunlight, causing a faint but visible green glow throughout the nighttime sky. Researchers estimate that at any given location on Earth, bright nights likely occur once a year.
Unfortunately, reports of this phenomenon have dwindled in recent decades as urban light pollution has made it nearly impossible to see in most areas. Gordon Shepherd, an atmospheric scientist at York University and the study’s lead author, says that these strangely bright nights are nevertheless worth studying. “Nobody sees them, nobody talks about them or records them any longer,” Shepherd says, “but they're still an interesting phenomenon.” Astronomers, for example, frequently have to contend with the occasional bright night which can obscure their views of celestial objects.