A phenomenon rarely seen in centuries might have just appeared in the night sky over Colorado--a "Bright Night." Philosophers and scientists have reported Bright Nights for literally thousands of years. It's when an otherwise dark and moonless night fills with a soft glow, allowing observers to see distant mountains or read a newspaper. Pliny the Elder, an Army commander in ancient Rome, described the phenomenon as a "nocturnal sun" (~ 113 BCE).
Comment: Pliny may, or may not, have been describing airglow: The Cosmic Context of Greek Philosophy, Part One
In the modern world, Bright Nights are seldom seen. Most of our planet's human population lives in cities, and even rural landscapes are somewhat lit by distant urban bulbs. The "nocturnal sun" has been overwhelmed.
Or has it? Enter Aaron Watson, an experienced night-sky photographer who regularly visits the darkest of dark-sky sites in remote parts of Colorado. On multiple occasions this year he has photographed red auroras and green airglow invisible from any ordinary countryside. Last month, he was at one of his favorite inky-dark sites when he may have experienced a modern Bright Night.
"A 'Bright Night' sounds exactly like what I experienced!" says Watson.
As the landscape grew in brightness, Watson decided to photograph the sky. When he pointed his lens toward the stars, the viewscreen of his camera filled with green light. "It was a very strong green airglow--perhaps the brightest I have ever seen," he says. A 20 second exposure produced the photo shown above.
Bright Nights have been a mystery since at least the first century. Watson's photo could be a clue. It seems to confirm a hypothesis published in 2017 by Gordon Shepherd, then a professor at Canada's York University, who believed that Bright Nights were caused by intense displays of airglow.
Shepherd came to this conclusion using a satellite sensor he built himself: The Wind Imaging Interferometer (WINDII), which orbited Earth for 14 years onboard NASA's Upper Atmosphere Research Satellite. When he and co-author Youngmin Cho examined WINDII's archive, they found a number of apparent Bright Nights in the sensor's airglow data.
Airglow is just what it sounds like: A diffuse glow that fills the air. It is produced by photochemistry in Earth's upper atmosphere, with its green color coming from atomic oxygen. Frequently photographed by astronauts on the ISS, airglow can be detected by sensitive cameras on almost any dark night from all locations on Earth. Rarely, however, is it visible without a camera.
The amorphous glow in Watson's photo is just what one would expect of broad zonal wave activity. "The airglow was strong in all directions," recalls Watson. "It had a flat color density without ripples or other fine structures"--just as one would expect from Shepherd and Cho's hypotheses.
Bright Nights might not be a thing of the past, after all.
Would you like to experience a Bright Night? Good news: Solar activity is boosting airglow, making the job of zonal waves that much easier. On the next moonless night, find a dark-dark site far from city lights. Bring a newspaper to read--and let us know what happens.