WINDII Airglow Observations of Wave Superposition and the Possible Association with Historical “Bright Nights”

Gordon Shepherd and Young-Min Cho recently submitted to Geophysical Research Letters a paper entitled “WINDII Airglow Observations of Wave Superposition and the Possible Association with Historical ‘Bright Nights’” which appeared on July 4th. Bright nights is the name given to those nights which should be dark, with no moon, and yet they are bright enough to allow observers to see their surroundings, or distant objects, or even read a newspaper. These have been reported from Roman times, for example by Pliny the Elder, who wrote, “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 (~ 113 BCE), and many other times, giving an appearance of day during the night”. These reports continue in the European literature up until World War I, but afterwards fade away as the use of artificial light increases. Limited scientific observations began then, during which the name “bright nights” was applied, but there has never been an explanation of their origin.

The Wind Imaging Interferometer (WINDII) was launched in 1991 and it was seen immediately that the 557.7 nm atomic oxygen airglow was extremely variable, from place to place and one night to the next. This “airglow” emission is caused by the photo-dissociation of O2 in the daytime by solar ultraviolet radiation, followed by the recombination of the atomic oxygen during which the energy released emerges as green line photons. Some years later it was recognized that there might be a relation to “bright nights”, but without any obvious explanation of the mechanism. However, in the meantime there were WINDII studies of the longitudinal wind and airglow intensity variations, leading to a resolution of longitudinal waves 1, 2, 3 and 4, where the number indicates the number of waves around the Earth at a given latitude. Early this year Young-Min discovered that from time to time the peaks in these propagating waves could coincide, causing a large peak in intensity at a given longitude, and that this could explain the bright nights. WINDII observed such events 7% of the time for the whole Earth, but at a given limited location the probability would be only 10% of this, and allowing for full moon periods means that the bright nights would be observed only once per year. The fact that they were reported so frequently centuries ago can be accounted for by the fact that the whole population was living with dark nights and so when a bright night occurred anywhere over land, it would be reported.

Geophysical Research Letters identified this as an article of public interest and created a press release that has kept Gordon busy on the phone and in e-mails since then. The story was picked up by the New York Times, the CBC, and from them by The Guardian, as well as in many other places such as the India Times and Korean newspapers. He also heard from individuals, one in particular who told the story told to him by his grandfather, who was playing football with his friends after dinner in 1908. They played and played and when the grandfather got home he was chastised by his parents for staying up until midnight. He protested that he didn’t know it was midnight as he didn’t have a watch, and it was still light. This experience stayed with him for decades as he didn’t tell it to his grandson (the writer) until much later. Gordon confirmed with the grandson that it was almost certainly a bright night.

The Guardian article can be found here.

The article includes a beautiful photo of the sky (see featured image) at night taken from the Grasslands National Park in Saskatchewan, near where Gordon was born – but it is not a bright night. The whole experience has been a surprise to the authors and a bit of a puzzle as to why there is such public interest. Probably it is the historical part, which has a human content, and the fact that bright nights are no longer being seen (at least not reported) owing to the environment change of artificial lights. Like some animal species, perhaps they have gone forever, for the public.

By: Gordon Shepherd