Solved: The mystery of unexplained ‘bright nights’

WASHINGTON, DC — Dating back to the first century, scientists, philosophers and reporters have noted the occasional occurrence of “bright nights,” when an unexplained glow in the night sky lets observers see distant mountains, read a newspaper or check their watch.

The different layers of Earth’s airglow can be seen from the International Space Station as it orbits Earth. The very thin green layer above the bottom of the window occurs 95 kilometers (59 miles) above Earth’s surface; the red region above is a different type of airglow. The rectangle represents the portion of the airglow measured in a single WINDII image.

A new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union, uses satellite data to present a possible explanation for these puzzling historical phenomena.

The authors suggest that when waves in the upper atmosphere converge over specific locations on Earth, it amplifies naturally occurring airglow, a faint light in the night sky that often appears green due to the activities of atoms of oxygen in the high atmosphere. Normally, people don’t notice airglow, but on bright nights it can become visible to the naked eye, producing the unexplained glow detailed in historical observations.

Few, if any, people observe bright nights anymore due to widespread light pollution, but the new findings show that they can be detected by scientists and may still be noticeable in remote areas. Bright airglow can be a concern for astronomers, who must contend with the extra light while making observations with telescopes.

“Bright nights do exist, and they’re part of the variability of airglow that can be observed with satellite instruments,” said Gordon Shepherd, an aeronomer at York University in Toronto, Canada, and lead author of the new study.

A historical mystery

Historical accounts of bright nights go back centuries. Pliny the Elder described bright nights, saying, “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.”

European newspapers and the scientific literature also carried observations of these events in 1783, 1908 and 1916.

“The historical record is so coherent, going back over centuries, the descriptions are very similar,” Shepherd said.

Modern observations of bright nights from Earth are practically nonexistent. Even devoted airglow researchers like Shepherd and his colleagues have never seen a true bright night with their eyes. But even before the advent of artificial lighting, bright nights were rare and highly localized.

“Bright nights have disappeared,” Shepherd said. “Nobody sees them, nobody talks about them or records them any longer, but they’re still an interesting phenomenon.”

Airglow anomalies

Shepherd knew of the historical observations and could see bright night events reflected in airglow data from the Wind Imaging Interferometer (WINDII), an instrument once carried by NASA’s Upper Atmosphere Research Satellite (1991-2005), but he couldn’t explain why the phenomena occurred.

Earth’s airglow can be seen from the International Space Station as a greenish bubble 95 kilometers (59 miles) above the Earth’s surface. Credit: NASA

He and his co-author, Youngmin Cho, a research associate at York University, searched for mechanisms that would cause airglow to increase to visible levels at specific locations.

Airglow comes from emissions of different colors of light from chemical reactions in the upper reaches of the atmosphere. The green portion of airglow occurs when light from the sun splits apart molecular oxygen into individual oxygen atoms. When the atoms recombine, they give off the excess energy as photons in the green part of the visible light spectrum, giving the sky a greenish tinge.

To find factors that would cause peaks in airglow and create bright nights, the researchers searched two years of WINDII data for unusual airglow profiles, ruling out meteors and aurora, which have their own distinct signatures. They identified 11 events where WINDII detected a spike in airglow levels that would be visible to the human eye, two of which they describe in detail in the study.

Finally, the researchers matched up the events with the ups and downs of zonal waves, large waves in the upper atmosphere that circle the globe and are impacted by weather. When the peaks of certain waves aligned, they produced bright night events that could last for several nights at a specific location. These events were four to 10 times brighter than normal airglow and could be responsible for the bright nights observed throughout history.

“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,” said 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.”

Observing a bright night

From their data, the researchers estimate that at a specific location, visible bright nights occur only once per year, and their observation would rely on a sky watcher looking from a remote location on a clear, moonless night with dark-adjusted eyes. Shepherd estimates that a bright night occurs somewhere on Earth, at different longitudes, on about 7 percent of nights.

If an astronomer wanted to experience a bright night personally, Shepherd suspects that scientists could predict their occurrence if they monitored the waves continuously, so that they could calculate when their peaks would align.

The next challenge will be to reproduce the observed convergence of these waves through modeling and to consider the effects of other types of waves in the atmosphere, Scheer said.

“Maybe it’s an almost dead question,” Shepherd said. “I’m having the last word before it dies.”

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The paper: http://onlinelibrary.wiley.com/doi/10.1002/2017GL074014/abstract;jsessionid=5056FE6A8D4CB497417BB3809C28591B.f02t03

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51 thoughts on “Solved: The mystery of unexplained ‘bright nights’

    • “Shepherd knew of the historical observations and could see bright night events reflected in airglow data from the Wind Imaging Interferometer (WINDII), an instrument once carried by NASA’s Upper Atmosphere Research Satellite (1991-2005), but he couldn’t explain why the phenomena occurred.”

      • “1783, 1908 and 1916.”
        “Bright nights have disappeared,” Shepherd said.

        Well if you only get one or two in a century and half he’s a bit quick to say they have “disappeared”. Lamentable what constitutes observational science these days.

      • Greg,
        Did you actually READ the entire article? Those dates are of literature mentioning the events, not the only dates they occurred.

        “Shepherd estimates that a bright night occurs somewhere on Earth, at different longitudes, on about 7 percent of nights.”

        That’s 25-26 times per year.

  1. If an astronomer wanted to experience a bright night personally, Shepherd suspects that scientists could predict their occurrence if they monitored the waves continuously, so that they could calculate when their peaks would align.

    A testable hypothesis, the hallmark of good science. It also seems to me that it is an excellent opportunity to use citizen scientists in remote locations testing the predictions of the researchers. It would save the considerable expense of sending in researchers.

    • An observer is not a scientist. The science comes from interpreting the observational evidence and formulating a working hypothesis.

      • You don’t have to be a scientist to observe something. Interpreting the evidence, formulating a hypothesis, and predicting when and where an event will occur=science. Having someone at that location on that date verify that “Yep, there’s a bright sky tonight” is a good idea, and scientists use non-scientists to make observations all the time.

      • “The observer is a lab tech.”

        First-most lab techs have degrees in science.
        Second-I’m observing you make repeated and completely illogical and unintelligent comments here and I’m not a lab tech.

      • “When the peaks of certain waves aligned, they produced bright night events that could last for several nights at a specific location.”

        Based on their ability to predict when the peaks will align, and where the brightness should be visible, it should be possible to set up light meters prior to and watch the readings surge and then return to normal.

      • Ceilometers are fairly common at airports, even remote ones. I wonder if there’s a way to mine ceilometer data to catch the bright night phenomena.

  2. Interesting. Maybe this explains the biblical narrative where the promised people were in a battle and needed more daylight:

    From Joshua 10
    On the day the Lord gave the Amorites over to Israel, Joshua said to the Lord in the presence of Israel:

    “Sun, stand still over Gibeon,
    and you, moon, over the Valley of Aijalon.”
    13 So the sun stood still,
    and the moon stopped,
    till the nation avenged itself on[b] its enemies,
    as it is written in the Book of Jashar.

    The sun stopped in the middle of the sky and delayed going down about a full day. 14 There has never been a day like it before or since, a day when the Lord listened to a human being. Surely the Lord was fighting for Israel!

    • The battle was during the daytime, so no, a bright night doesn’t explain that impossible passage.

  3. I didn’t spot the place where they blamed the absence of bright-nights on cagw, it must’ve been in the request for further funding.

  4. Nobody was reading their newspaper or checking their watch by night in the first century. But, still interesting.

    • “Nobody was reading their newspaper or checking their watch by night in the first century. ”

      No, …But they *were* trying to get home in the middle of ancient cities that had no street lamps or other sources of light, and could be pitch black at night. Waiting for torch bearers could be a pain, and people would *notice* when the night was bright enough they did not need Moonlight, or torchlight to get home after leaving some friend’s party!

  5. This is a description of constructive interference patterns and harmonics. This would confirm something I have been cogitating for a while.
    Q: How do you explain the fluctuations of climate and weather if the energy input from the sun is ‘constant’?
    A: The system dynamics migrate back and forth from dissonance and chaos into harmonic resonance and interference patterns. There are areas/periods of constructive interference that amplify the energy levels on a local level, and areas/periods of destructive interference that cancel energy levels. The net energy in the system remains constant yet the effect is peaks and valleys of harmonic resonance in the system. This is expressed as high and low pressure systems, jet stream meanderings, ENSO and all the features of this crazy chaotic system.

  6. Reading newspapers by airglow so dim that light pollution overwhelms it everywhere in the world except the most remote places? I doubt it. I can believe reading large newspaper headlines by that though. Human night vision does not have the resolution needed to read the print in newspapers under the headlines.

    • Apparently historical events were a lot brighter, if accounts are accurate. However, they are centennial events. That explains the recent lack.

      • the researchers searched two years of WINDII data

        QBO has a period of about 2.4y and they looked at 2y of data. Historical records relate strong events every century or so… and they look at 2y of data.

        Fine. I suppose it’s a start.

      • Seriously, do you have a reading comprehension problem Greg? if you do, I apologize for being blunt.

        The article said that the WINDII flew in space between the years 1991-2005. A total of 14 years. Examining just TWO years worth of that 14 years of data, “They identified 11 events where WINDII detected a spike in airglow levels that would be visible to the human eye, two of which they describe in detail in the study.” So….there’s normal-average-occurring all the time airglow, and then there’s SPIKES in airglow levels that are actually visible to the human eye. In just two years worth of data (somewhere between 1991 and 2005), they found 11 of these events…

        SO….if these events were “centennial events”, as you mistakenly state they are, then 11 events within a two year period does not indicate a “recent lack” but an enormous and sudden ABUNDANCE of events.

        BUT…you fail to realize that you are not reading the article correctly, because it never states that these events ONLY happen every 100 years. It says they happen on at least 7% of earth nights! That we can no longer SEE them easily due to light pollution, does not mean they are not happening, or that there is a recent lack OF them. Because there was not “light pollution” in 113 BCE or 1783 or the “many other times” (stated in the article) they were easier to see AND got way more attention than they do now. You are making a personal ASSUMPTION that the “historical records relate strong events every century or so” that is not backed up by the article in any way.

  7. So….oxygen atoms…split by the SUN…emit ENERGY when they recombine….and sometimes there is so much of this ENERGY in one place at the same time that it can actually become visible to the naked eye.
    Can we logically assume then, that it happens constantly, but 93% of nights it’s just not concentrated enough to be visible?

    I ask because oxygen currently makes up 20% (ish) of our atmosphere. Has the amount of “oxygen combination energy” ever been measured? Calculated? How much energy is produced? Does it affect climate? Do atmospheric models address it?

  8. large waves in the upper atmosphere that circle the globe and are impacted by weather.

    AlGorian cause and effect myopia.

    More like the waves affecting weather. But don’t want to give the impression that natural phenomena could affect weather and ( God forbid ) climate, do we?

  9. Great post. I love this site, as I love science in general (being a real scientist).

  10. In fact, about 98% of Earth is NOT light-polluted. Lit urban areas are that little compared to the world the rest of us live in.

    • I think he’s wrong to assume/say that “few, if any, people observe bright nights anymore due to widespread light pollution” . Simply because people don’t report it, doesn’t mean they don’t observe it. I grew up in a rural area and I’ve experienced bright nights often. Never occurred to me that I should call someone to report it! :)

  11. Few, if any, people observe bright nights anymore due to widespread light pollution…

    Now even light is pollution? What next? Silver, garlic, holy water and crucifixes? Where I come from the long, dark and cold season is celebrated with Christmas and June solstice with bonfires. The local greens hate both traditions, but latter to a degree greens can be objectively characterised as rock conservatives. A bit off the topic, but worth translating and reading for fun https://www.verkkouutiset.fi/kotimaa/kokko-51393

  12. I have noticed bright nights lately, here in the mountains at 2000′. It gets dark up here at night when the Moon is down. I have noticed that when looking out the window at times in the middle of the night. Every once and awhile there is enough light to where I can readily make out the surrounding view from my window, and to see animals such as deer, bear, etc.

  13. Modelers? Atmospheric scientists? Climate scientists? Anyone? Bueller?
    How much energy is discharged when oxygen atoms recombine in the atmosphere?
    And has that energy been measured AND addressed in climate science??

    • Yes it is called bond energy.
      There are tables available it has been calculated and measured.
      This is basic school chemistry

      for oxygen it is 498 kJ/mol

      • Thanks Owen. Because it can actually produce ‘visible light’ in high enough amounts, is it short wave energy or long wave?
        Do climate models account for it?

      • As Owen indicates, the process is well understood & quantified. The energy needed to split O2 certainly does “warm” the atoms themselves, however recombination emits that same energy as visible light in a very narrow green band (typically), and the amount emitted is so faint that while our eyes typically perceive the darkness between the stars (on nights when they’re visible) to be slightly less dark than the shadows of mountains or trees, it is nevertheless perceived as a dark shade of gray or indigo (not green) since there’s just not enough energy to activate the green cones in our eyes.

        The green light recombination emits isn’t really categorized as “short/long-wave” either; it’s just visible light. Also, that particular emission band (~558 nm, which isn’t the ONLY band that produces airglow, but it is by far the main one) tends to either reflect or pass through just about everything between the ground & the TOA, so I can’t imagine that airglow contributes any significant energy to the climate system from the jump, let alone that the difference between airglow emission on “bright nights” vs. regular ones should be significant.

        Now, I can’t prove it isn’t, just can’t imagine it is, either. For instance, airglow is so faint that even though it’s about 3 orders of magnitude brighter during the day than it is at night, we can’t see it at ALL from the ground during daylight hours because of the Rayleigh scattering that turns our skies blue. . . which makes sense, since we can’t see most stars during the day either & airglow is much fainter than starlight. IOW, if we’re having to account for airglow, then we’ll need to start accounting for starlight/moonlight in our climate understanding as well, maybe. ^_^

        For a more complete discussion of airglow emissions, try this link: http://www.atoptics.co.uk/highsky/airglow2.htm

    • When I was young and dumb there was a saying, “He’s out in the Ozone!”, an alternative to just saying. “He’s out there!”. (Nuts!)
      Hmmm…Al Gore used to be all hyper about the “Ozone Hole”.
      Now he’s all hyper about CO2 from fossil fuels and “Global Warming”.
      (Well, at least until the price is right.)
      Al Gore is a failed (but a profiteering) politician.
      Might such as he be the connection to “climate science”?

  14. There are still plenty of people who work and live in remote areas who would still see this phenomenon if it were present regularly…. Strange that you don’t hear much about it?
    As a trawlerman I never observed “bright nights” as described here…. But on really cold nights in Tropical Queensland… about 15 degrees lattitude, I did observe an atmospheric magnification effect of stars in the night sky.
    It was a very clear and cold night (for the tropics) about 12 degrees, and the wind was ESE, blowing at about 25 to 30 knots. The atmosphere was clear. We decided to have the night off, because it was too rough and we didn’t feel like bashing around for the low catch rate we would be expecting at that time of year.
    Anyway at about nine o’clock at night, the anchor started dragging and wouldn’t dig back in so we started the main engine went on deck and winched it up and moved back to the anchorage… While up on the bow doing this I noticed that the stars above were unusually “bright” and some groups were quite large and I didn’t really recognize them, not that I’m much for identifying many star constellations. But I took note of a group of stars in a cockeyed square/trapezoid shape as we steamed back into the anchorage, after we’d finished I look back up….. and the Stars were “Gone”. Well three were sort of there, but smaller and duller…. So I went into the wheelhouse and retrieved a set of binoculars and scanned the area of the sky where I’d seen the four stars….. Under magnification, all four were back again.
    All I could put it down to, was that the Cold air layer was shaped in such a way for a short while, that it magnified that part of the sky from my position of observation through it…..
    I’ve only seen, or noticed this effect twice in all my years fishing and being in remote areas. I’ve had plenty of cold nights were the stars seemed brighter and larger, but only twice have they been so outlandishly and obviously so.

      • My comment was not in regard to gases. The sky is, in fact, considered to have a blackbody temperature. A search on ‘”blackbody temperature of the sky”‘ yields 33,700 hits. It’s commonly taken as approx. 3 or 4K. Under the conditions discussed here, it may (or may not) deviate from that significantly, with a consequent effect on radiation transmission to space. What do you want to bet that it appears in climate models as a constant?

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