The mystery of the ‘bright nights’ has been solved

The previously unexplained phenomena are supposedly caused by waves converging in the upper atmosphere


Airglow can occur in a variety of colours, as shown in this image, and they can occur in different circumstances. Image credit: NASA

Bright nights are a virtually nonexistent phenomenon these days, due to worldwide light pollution, but this occurrence has been documented as far back as the first century. Researchers have analysed satellite data to suggest a likely explanation, which is this illumination comes from the when zonal waves in the upper atmosphere merge and create a peak in the airglow.

This historic enigma has occurred for centuries, and the reason we know this is because we not only have European newspapers and scientific literature from 1783, 1908 and 1916 describing the events, Pliny the Elder from the first century documented these bright nights.

The ancient Roman author described it as, “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 (around 113 BC) and many other times, giving an appearance of day during the night.”

Gordon Shepherd, the lead researcher and aeronomer at York University in Toronto, Canada says: “The historical record is so coherent, going back over centuries, the descriptions are very similar.”

In these modern days, the bright nights are practically a myth now because of the increased demand for artificial lighting. Shepherd explains: “Bright nights have disappeared. Nobody sees them, nobody talks about them or records them any longer, but they’re still an interesting phenomenon.”

Earth’s airglow is located 95 kilometres (59 miles) above the surface, and in this image is seen as a greenish bubble from the International Space Station. Image credit: NASA

As this event is not recorder any longer, these researchers had to examine data from the Wind Imaging Interferometer (WINDII), an instrument that was previously functioning on NASA’s Upper Atmosphere Research Satellite (1991 to 2005).

Shepherd and his team pursued a natural mechanism that would explain the airglow. What is known is that airglow comes from emissions of different colours, which in this case, the oxygen molecules in the upper atmosphere absorb energy and then reemit photons in the green part of the visible light spectrum.

After ruling out meteors and aurorae among several other things, the team of researchers correlated the events to the oscillations of zonal waves, which are large waves in the upper atmosphere that circle the globe and are impacted by weather. When these waves superimpose, they produce the bright nights that are four to ten times brighter than normal airglow.

The data suggests that visible bright nights occur only once a year, but to observe such an event; you must be in a remote location with clear skies, including no moon and dark-adjusted eyes. All these specifications are very rare these days, but as Shepherd states, “Maybe it’s an almost dead question, I’m just having the last word before it dies,” says Shepherd.

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