Dating back to the first century CE, philosophers, scientists, and reporters have noted the occasional occurrence of the phenomena known as ‘bright nights,’ when an unexplained glow in the night-time skies lets observers see distant mountains and read newspapers. A new study from York University, Canada, uses thermospheric data from the Wind Imaging Interferometer (WINDII), an instrument once carried by NASA’s Upper Atmosphere Research Satellite, to present an explanation for these historical phenomena.
Noctilucent clouds over Orlando. Image credit: Space Exploration Technologies Corporation.
York University Professor Gordon Shepherd and his colleague, Youngmin Cho, suggest that “when waves in the upper atmosphere converge over specific locations on 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.”
“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,” they said.
“Few, if any, people observe bright nights anymore due to widespread light pollution, but the findings show that they can be detected by scientists and may still be noticeable in remote areas.”
Bright airglow can be a concern for astronomers, who must contend with the extra light while making observations with telescopes.
“Bright nights do exist, and they’re part of the variability of airglow that can be observed with satellite instruments,” Prof. Shepherd explained.
Historical accounts of bright nights go back millennia.
Pliny the Elder, a 1st-century CE Roman naturalist, natural philosopher and author of the 37-volume encyclopedia Naturalis Historia, wrote: “The phenomenon commonly called ‘Nocturnal Sun,’ i.e. a light emanating from the sky during the night, has been seen during the consulate of C. Caecilius and Cn. Papirius (113 BC), and many other times, giving an appearance of day during the night.”
One well-documented event occurred in early June, 1783, and was repeated on the nights of June 26-30.
Another was described in a quote, from M. Toucher at Viroflay on June 30, 1908: “At 20 h, it was very easy to distinguish the objects in the non-artificially lighted streets. It is this overall brightness which caught my attention, because of the shortage of coal, the darkness was complete in the streets deprived of electricity.”
“At 22 h 30 I could make more complete observations. Very clear sky, full of stars which shine to the horizon. No moonlight. All the details of the landscape are visible. In the garden, we could recognize all the objects without any difficulty. Pebbles (diameter 10 mm to 15 mm) shine on the dark ground.”
Another event observed in France on December 23, 1916 was also seen by a member of the Société Astronomique de France from Copenhagen.
“The historical record is so coherent, going back over centuries, the descriptions are very similar,” Prof. Shepherd said.
Modern observations of bright nights from Earth are practically nonexistent. Even devoted airglow researchers have never seen a true bright night with their eyes. But even before the advent of artificial lighting, bright nights were rare and highly localized.
“Bright nights have disappeared. Nobody sees them, nobody talks about them or records them any longer, but they’re still an interesting phenomenon,” Prof. Shepherd said.
Prof. Shepherd knew of the historical observations and could see bright night events reflected in the WINDII data, but he couldn’t explain why the phenomena occurred.
He and Dr. Cho searched for mechanisms that would cause airglow to increase to visible levels at specific locations.
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 team searched two years of WINDII data for unusual airglow profiles, ruling out meteors and aurora, which have their own distinct signatures.
The authors 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, they 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.
They estimate that a bright night occurs somewhere on Earth, at different longitudes, on about 7% of nights.
If an astronomer wanted to experience a bright night personally, the authors suspect that scientists could predict their occurrence if they monitored the waves continuously, so that they could calculate when their peaks would align.
The research is published online in the journal Geophysical Research Letters.
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G.G. Shepherd & Y.-M. Cho. WINDII Airglow Observations of Wave Superposition and the Possible Association with Historical ‘Bright Nights.’ Geophysical Research Letters, published online June 6, 2017; doi: 10.1002/2017GL074014
