Nav: Home

Scientists solve mystery of unexplained 'bright nights'

June 21, 2017

WASHINGTON, DC -- Dating back to the first century, scientists, philosophers and reporters have noted the occasional occurrence of "bright nights," when an unexplained glow in the night sky lets observers see distant mountains, read a newspaper or check their watch.

A new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union, uses satellite data to present a possible explanation for these puzzling historical phenomena.

The authors 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.

Few, if any, people observe bright nights anymore due to widespread light pollution, but the new 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," said Gordon Shepherd, an aeronomer at York University in Toronto, Canada, and lead author of the new study.

A historical mystery

Historical accounts of bright nights go back centuries. Pliny the Elder described bright nights, saying, "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 BCE), and many other times, giving an appearance of day during the night."

European newspapers and the scientific literature also carried observations of these events in 1783, 1908 and 1916.

"The historical record is so coherent, going back over centuries, the descriptions are very similar," Shepherd said.

Modern observations of bright nights from Earth are practically nonexistent. Even devoted airglow researchers like Shepherd and his colleagues 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," Shepherd said. "Nobody sees them, nobody talks about them or records them any longer, but they're still an interesting phenomenon."

Airglow anomalies

Shepherd knew of the historical observations and 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), but he couldn't explain why the phenomena occurred.

He and his co-author, Youngmin Cho, a research associate at York University, 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 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.

"This [study] is a very clear, new approach to the old enigma of what makes some night skies so remarkably bright, and the answer is atmospheric dynamics," said Jürgen Scheer, an aeronomer at Instituto de Astronomía y Física del Espacio in Buenos Aires, who was not connected to the study. "We now have a good idea which dynamical phenomena are behind [airglow] events of extreme brightness."

Observing a bright night

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. Shepherd estimates that a bright night occurs somewhere on Earth, at different longitudes, on about 7 percent of nights.

If an astronomer wanted to experience a bright night personally, Shepherd suspects that scientists could predict their occurrence if they monitored the waves continuously, so that they could calculate when their peaks would align.

The next challenge will be to reproduce the observed convergence of these waves through modeling and to consider the effects of other types of waves in the atmosphere, Scheer said.

"Maybe it's an almost dead question," Shepherd said. "I'm having the last word before it dies."
The American Geophysical Union is dedicated to advancing the Earth and space sciences for the benefit of humanity through its scholarly publications, conferences, and outreach programs. AGU is a not-for-profit, professional, scientific organization representing 60,000 members in 137 countries. Join the conversation on Facebook, Twitter, YouTube, and our other social media channels.

American Geophysical Union

Related Atmosphere Articles:

Physics: An ultrafast glimpse of the photochemistry of the atmosphere
Researchers at Ludwig-Maximilians-Universitaet (LMU) in Munich have explored the initial consequences of the interaction of light with molecules on the surface of nanoscopic aerosols.
Using lasers to visualize molecular mysteries in our atmosphere
Molecular interactions between gases and liquids underpin much of our lives, but difficulties in measuring gas-liquid collisions have so far prevented the fundamental exploration of these processes.
The atmosphere of a new ultra hot Jupiter is analyzed
The combination of observations made with the CARMENES spectrograph on the 3.5m telescope at Calar Alto Observatory (Almería), and the HARPS-N spectrograph on the National Galileo Telescope (TNG) at the Roque de los Muchachos Observatory (Garafía, La Palma) has enabled a team from the Instituto de Astrofísica de Canarias (IAC) and from the University of La Laguna (ULL) to reveal new details about this extrasolar planet, which has a surface temperature of around 2000 K.
An exoplanet loses its atmosphere in the form of a tail
A new study, led by scientists from the Instituto de Astrofísica de Canarias (IAC), reveals that the giant exoplanet WASP-69b carries a comet-like tail made up of helium particles escaping from its gravitational field propelled by the ultraviolet radiation of its star.
Iron and titanium in the atmosphere of an exoplanet
Exoplanets can orbit close to their host star. When the host star is much hotter than our sun, then the exoplanet becomes as hot as a star.
Astronomers find exoplanet atmosphere free of clouds
Scientists have detected an exoplanet atmosphere that is free of clouds, marking a pivotal breakthrough in the quest for greater understanding of the planets beyond our solar system.
Helium detected in exoplanet atmosphere for the first time
Astronomers have detected helium in the atmosphere of a planet that orbits a star far beyond our solar system for the very first time.
Mountain erosion may add CO2 to the atmosphere
Scientists have long known that steep mountain ranges can draw carbon dioxide (CO2) out of the atmosphere -- as erosion exposes new rock, it also starts a chemical reaction between minerals on hill slopes and CO2 in the air, 'weathering' the rock and using CO2 to produce carbonate minerals like calcite.
The changing chemistry of the Amazonian atmosphere
Researchers have been debating whether nitrogen oxides (NOx) can affect levels of OH radicals in a pristine atmosphere but quantifying that relationship has been difficult.
Hubble observes exoplanet atmosphere in more detail than ever before
An international team of scientists has used the NASA/ESA Hubble Space Telescope to study the atmosphere of the hot exoplanet WASP-39b.
More Atmosphere News and Atmosphere Current Events

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Rethinking Anger
Anger is universal and complex: it can be quiet, festering, justified, vengeful, and destructive. This hour, TED speakers explore the many sides of anger, why we need it, and who's allowed to feel it. Guests include psychologists Ryan Martin and Russell Kolts, writer Soraya Chemaly, former talk radio host Lisa Fritsch, and business professor Dan Moshavi.
Now Playing: Science for the People

#538 Nobels and Astrophysics
This week we start with this year's physics Nobel Prize awarded to Jim Peebles, Michel Mayor, and Didier Queloz and finish with a discussion of the Nobel Prizes as a way to award and highlight important science. Are they still relevant? When science breakthroughs are built on the backs of hundreds -- and sometimes thousands -- of people's hard work, how do you pick just three to highlight? Join host Rachelle Saunders and astrophysicist, author, and science communicator Ethan Siegel for their chat about astrophysics and Nobel Prizes.