Canadian researchers explain rare 'bright nights' phenomenon
Tuesday, June 27, 2017, 6:59 PM - Thanks to two Canadian scientists, a rare and mysterious phenomenon, known in ancient times as 'nocturnal sun', may finally have an explanation.
Imagine going outside in the middle of the night, expecting pitch blackness, only to find yourself under a mysterious glow from the sky that is bright enough to to read by.
For those of us living in or near large urban centres, that is not so unusual. The phenomenon of light pollution is a simple fact of life around cities these days. Light from signs, streetlamps, vehicles and windows (and light from these sources reflected upwards by the ground) shines up into the night, is reflected or refracted by water vapour or clouds, and it artificially illuminates the sky. This washes out the stars and can even act as a supplemental light source for those trapped underneath it.
People living in the far north or far south are certainly familiar with the auroras, as well, which can, at times, shine quite brightly in the night sky.
The phenomenon of "bright nights", though, is quite different from both modern urban light pollution and auroras, however, and according to a new study by Gordon Shepherd and Young-Min Cho, from York University in Toronto, it may be a concentration of a different light from the sky, known as "airglow", that is amplified by conditions in the atmosphere.
"Bright nights do exist, and they’re part of the variability of airglow that can be observed with satellite instruments," Shepherd, who studies the upper atmosphere, told the American Geophysical Union.
Airglow captured in central France on the 13th of August, 2015. Credit: Clame Reporter/Wikimedia
From Shepherd and Cho's study, which is to be published in the AGU journal Geophysical Research Letters, people have been seeing 'bright nights' going back at least 2,000 years, long before urban sprawl became a problem. Not only that, but when they were observed and characterized on November 8, 1929 by physicist Robert Strutt, the 4th Baron Rayleigh and son of the man who developed the theory of Rayleigh scattering (why we see a blue sky during the day), he was able to show that they were not related to the auroras, and were a phenomenon all to themselves.
Auroras happen due to high-energy protons and electrons from the Sun, which get trapped by Earth's magnetic field and stream in to the atmosphere near the poles, to interact with air molecules in the region of the upper atmosphere known as the thermosphere, between 100-200 km up. The highly energetic particles transfer energy to the air molecules the encounter, and the air molecules dump that excess energy as light of different wavelengths and colours.
Airglow, on the other hand, is seen near the boundary of the thermosphere and the mesosphere, about 8-100 km up, and is produced by a few different effects. Oxygen molecules split into their component atoms by direct sunlight can emit this light when they recombine into molecules, it can happen due to cosmic rays hitting the upper atmosphere, or due to chemical reactions between atoms and ions in the upper atmosphere, that directly emit light (chemiluminescence).
Auroras and airglow, seen from the International Space Station. Credit: Alexander Gerst/ESA/Scott Sutherland
Airglow is quite faint, however, and is best viewed near the horizon, especially when viewing the limb of the Earth from orbit, as the effect becomes concentrated at that angle. So, how can faint airglow amplify to the point where you can read by it?
Using two years worth of data from the Wind Imaging Interferometer (WINDII), which Shepherd managed as primary investigator when the instrument flew on NASA's Upper Atmosphere Research Satellite (UARS) from 1991 to 2005, Shepherd and Cho found that instances of bright night occurred when the waves at different latitudes of the upper atmosphere had their peaks line up along lines of longitude - a condition known as superposition. This superposition puts a lot of molecular and atomic oxygen together in one spot in the upper atmosphere, thus concentrating the light from all of their airglow emissions, and producing the bright night effect over the ground below.
"When the peaks of the different waves superimpose, then the intensities add up ... by about a factor of 10, that's enough to make it visible to the human eye," Shepherd said in an interview with CBC News.
In some cases, when these superimposed waves form a standing wave - one where the wave peak remains stationary in the flow for some time - these bright night events have been found to last for several nights over a single location.
More and less, in the future
Two aspects of human activity are having opposite impacts on this phenomenon.
If anyone is looking forward to seeing one of these bright night events, the impacts of climate change may cause them to happen more often, as stronger storms would tend to produce stronger upper atmospheric waves and thus stronger and more persistent superpositions.
"So if climate change increases the amount of severe weather, it could increase the frequency of bright nights as well," Shepherd told CBC News.
As our cities expand and urban sprawl closes the gaps between them, the increase in the amount and expanse of light pollution is going to make these bright nights far more difficult to see.
Anyone who still can manage to get far enough away from city lights may still catch one, from time to time.