Too Much Light at Night, Right?
By Sanjana Narayanan
Imagine taking a nighttime stroll hundreds of years ago. Not only do you see many more stars than you could ever hope to see today, but the sky is so bright that you can actually distinguish pebbles on the ground and read a book.1 No, it’s not moonlight; in the words of Roman philosopher Pliny the Elder, it’s a “nocturnal sun.”
Reported observations of these “bright nights” date from 113 BCE to 1996 AD.1 However, 1909 marked the first real shift from qualitative descriptions of bright nights to a quantitative measurement of the phenomenon. In that year, scientists found convincing evidence that bright nights were not illuminated by starlight.1 About two decades later, Nobel prizewinning physicist Lord Rayleigh suggested that bright nights were not caused by aurora (commonly known as the Northern and Southern lights). Since then, researchers have been unable to pinpoint a definitive cause to the bright light phenomenon. However, recent research claims to have found a possible solution.
In June 2017, scientists Gordon Shepherd and Young-Min Cho from York University revealed the mechanism behind the oxymoronic nocturnal sun.
Their research, published in the journal Geophysical Research Letters, hinges on two scientific concepts: zonal waves and airglow.1 Zonal waves, also known as long waves or planetary waves, are large-scale, uniform movements of westerly winds along lines of longitude that minimize temperature imbalances in the Earth’s upper atmosphere.234 Airglow (or nightglow, in this case) is atmospheric light produced by a process called chemiluminescence.5 Oxygen molecules naturally exist in pairs of two oxygen atoms (written as O2), which have a low energy state.5 However, when the sun’s rays hit the oxygen molecules in the atmosphere, they divide into lone oxygen atoms, which individually have a higher energy state.5 At night, these oxygen atoms re-bond into their natural, low-energy O2 form.5 They release the excess energy given off from this process as a form of green light that is normally invisible to the human eye due to its low intensity.51
Using NASA’s Wind Imaging Interferometer (WINDII) and selecting data from the years 1992 and 1996, Shepherd and Cho analyzed atomic oxygen emission rates (as a way of measuring airglow) and the phases of zonal waves at locations where bright nights had been observed.1 First, they found that nocturnal suns are associated with extremely bright airglow.1 Second, these airglow anomalies are caused by the superposition of zonal waves 1 through 4.1 For this reason, the cumulative effect of all four waves aligning and peaking at the same place makes the airglow over ten times brighter at that location, creating what we perceive as a bright night
Not only does this study unravel an enigma that has existed for millennia, but it also adds another dimension to our understanding of the waves and other physical processes that take place in the earth’s atmosphere. Most importantly, in a world where skies are giving way to skylines, bright nights are coming perilously close to extinction thanks to light pollution.7 Even if a bright night does occur once every year, as Shepherd predicts, the chances that anyone actually observes it are contingent on the absence of artificial light coming from urban and suburban areas.8 The hope is that Shepherd and Cho’s data, as well as data from similar studies, can help scientists predict when and where we can catch a glimpse of the next spectacular nocturnal sun.8
- Shepherd, G. G., and Y. M. Cho. “WINDII Airglow Observations of Wave Superposition and the Possible Association with Historical “Bright Nights”.” American Geophysical Union, onlinelibrary.wiley.com/doi/10.1002/2017GL074014/epdf. Accessed 2 July 2017.
- “Long Wave.” American Meteorological Society Glossary of Meteorology, 20 Feb. 2012, glossary.ametsoc.org/wiki/Long_wave. Accessed 26 July 2017.
- “Long and Short Waves.” National Oceanic and Atmospheric Administration, National Weather Service, www.srh.noaa.gov/jetstream/constant/longshort.html. Accessed 26 July 2017.
- Scott, Anna. “Atmospheric Waves Awareness: An Explainer.” The Planetary Society, 20 Apr. 2016, www.planetary.org/blogs/guest-blogs/2016/atmospheric-waves-awareness.html. Accessed 26 July 2017.
- Case, Nathan. “What’s the Difference between an Airglow and an Aurora?” Real Clear Science, 4 Nov. 2016, www.realclearscience.com/articles/2016/11/05/whats_the_difference_between_an_airglow_and_an_aurora_110077.html. Accessed 12 July 2017.
- “Angular Wavenumber.” American Meteorological Society Glossary of Meteorology, 20 Feb. 2012, glossary.ametsoc.org/wiki/Hemispheric_wavenumber. Accessed 26 July 2017.
- Klein, Joanna. “Waves above the Earth May Have Once Caused a ‘Nocturnal Sun.'” New York Times, 23 June 2017, www.nytimes.com/2017/06/23/science/bright-nights-nocturnal-sun-zonal-waves.html. Accessed 12 July 2017.
- “Scientists Solve Mystery of Unexplained ‘Bright Nights.'” American Geophysical Union News, 21 June 2017, news.agu.org/press-release/scientists-solve-mystery-of-unexplained-bright-nights/. Accessed 26 July 2017.