Study uncovers cause of mysterious ‘bright nights’ phenomenon

TORONTO — The phenomenon in which a glow pervades across the sky on some nights has long had enigmatic origins, but science now has an answer, a new study finds.

Researchers at York University in Toronto used satellite data to determine that “bright nights”— the modern-day term used for the phenomenon in which evenings are so well-lit a person could check the time on their watch without issue — emanate from zonal waves in the upper reaches of the Earth’s atmosphere.

Previous hypotheses had posited that solar or meteoric factors contributed to bright nights, an occurrence that only a few have experienced in this age of extensive artificial light.

Pliny the Elder, an ancient Roman writer and philosopher, is believed to be one of the first to document the bright nights phenomenon, which was then termed the “nocturnal sun.” As recently as the early 20th century, the phenomenon was widely recorded in newspapers and scientific publications.

“The historical record is so coherent, going back over centuries, the descriptions are very similar,” says lead researcher Gordon Shepherd in a news release by the American Geophysical Union.

While bright nights were always rare and specific to certain regions, their presence ranges from extremely elusive to practically non-existent to today’s observer, regardless of where one resides. Scientists say the phenomenon is even rarer and practically non-existent these days because of air pollution.

Assuming one does happen to be in the right location at the right point in time, they will have an improved probability of seeing bright lights if the sky is clear and moonless, the researchers say.

About seven percent of all nights on Earth, a bright night occurs somewhere on the planet, they add.

“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,” says 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.”

The researchers hope to conduct additional research through modeling the convergence of zonal waves in the atmosphere, which could help predict where and when to expect a specific occurrence of bright nights.

The study’s findings were published in the journal Geophysical Research Letters.