Images of the aurora borealis showing the structured continuum emission (Credit: Faculty of Science Research Team)
CALGARY — A puzzling atmospheric phenomenon that has long intrigued aurora watchers and scientists has finally been explained by researchers at the University of Calgary. These ghostly gray patches that materialize alongside the northern lights represent a new chapter in our understanding of Earth’s atmospheric light shows.
“You’d see this dynamic green aurora, you’d see some of the red aurora in the background and, all of a sudden, you’d see this structured – almost like a patch – gray-toned or white toned-emission connected to the aurora,” says lead author Dr. Emma Spanswick, associate professor with the Department of Physics and Astronomy, in a statement. “So, the first response of any scientist is, ‘Well, what is that?'”
In research published in Nature Communications, Spanswick’s team has documented and analyzed these mysterious gray-toned structures that materialize near active auroras. While scientists have known about the presence of weak background continuum emissions in the night sky for decades, this marks the first comprehensive study of their relationship with auroras and their distinctive characteristics.
The discovery was made possible by recent advances in camera technology that have revolutionized both scientific and amateur aurora photography. “Everyone has noticed the advancement in digital photography. Your cellphone can now take pictures of the aurora,” Spanswick notes. “That has flowed to the commercial sensor market now. Those types of sensors can now be found in more commercial, more robust sensors that we would use in science.”
Using an innovative network of high-resolution, broad-band color cameras called the Transition Region Explorer (TREx-RGB), researchers identified 30 separate events where these gray structures appeared in the night sky. The structures varied in size from about 10 to several hundred kilometers across and were always found either embedded within or immediately adjacent to active auroras.
To understand what makes these gray glows different from regular auroras, we need to look at how each type of light is produced. Traditional auroral lights occur when energetic particles from the sun collide with gases in our upper atmosphere, causing them to glow in characteristic colors – green from oxygen atoms, red from nitrogen molecules, and so forth. It’s similar to how neon signs work, with electricity exciting gas atoms to produce specific colors.
The newly discovered gray structures, however, appear to be produced through a different mechanism called chemiluminescence – the same process that makes fireflies glow or creates the ethereal blue light in ocean waves on some beaches. The research team concludes it’s “most certainly a heat source” and suggests that the aurora borealis are more complex than previously thought.
This research follows renewed interest in continuum emission sparked by the discovery of STEVE (Strong Thermal Emission Velocity Enhancement), a different atmospheric phenomenon that appears as a mauve-colored arc in the sky. “There are similarities between what we’re seeing now and STEVE,” explains Spanswick. “STEVE manifests itself as this mauve or gray-toned structure. To be honest, the elevation of the spectrum between the two is very similar but this, because of its association with dynamic aurora, it’s almost embedded in the aurora. It’s harder to pick out if you were to look at it, whereas STEVE is separate from the aurora – a big band crossing the sky.”
The research team used both imaging and spectroscopic techniques to study these formations. While the color cameras captured their shape and movement, a specialized instrument called a meridian imaging spectrograph analyzed the spectrum of light they emit. By analyzing the light from these gray patches using specialized equipment, researchers found that they glow with a consistent brightness across all colors of light, unlike regular auroras which tend to shine brightly in specific colors. The strength of this glow was much brighter than normal night sky background light, suggesting it’s definitely a real phenomenon rather than just a camera effect.
While regular auroras have captivated observers for centuries, this research shows there’s still much to discover about the light shows in our night sky. These newly documented gray patches represent an entirely different way that our atmosphere responds to solar activity, adding another piece to the complex puzzle of how space weather affects Earth. As camera technology continues to improve and more eyes turn to the sky, researchers expect to uncover even more secrets about these mysterious atmospheric phenomena.
Paper Summary
Methodology
Researchers used a network of specialized cameras called TREx-RGB, which capture true-color images of the night sky at high resolution and speed (3 frames per second). These cameras were paired with spectrographs that measure the precise wavelengths of light being emitted. The team analyzed data from two stations in Canada – Rabbit Lake and Lucky Lake – focusing on times when the moon was below the horizon to avoid interference. They identified 30 events where gray-toned structures appeared in the imagery and confirmed their unique spectral signature using the spectrograph data.
Results
The study found that these gray structures consistently appear near active auroras and produce a uniform increase in light intensity across all wavelengths, typically 20-60 Rayleighs per nanometer above background levels. The structures range from 10 to several hundred kilometers in size and can persist for several minutes. Spectral analysis showed they contain both a continuous spectrum component and discrete emission lines associated with aurora activity, suggesting a complex interaction between different atmospheric processes.
Study Limitations
The observations were limited to two geographic locations in Canada and only included events where both imaging and spectroscopic data were available. The study also relied on clear sky conditions and times when the moon was below the horizon. Additionally, the exact chemical mechanisms producing the gray glow could not be definitively determined with the available data, though the researchers propose nitric oxide and oxygen interactions as a likely explanation.
Discussion and Takeaways
This research reveals a previously unrecognized way that solar activity can affect Earth’s upper atmosphere. The discovery suggests that the interaction between solar storms and our atmosphere is more complex than previously thought, with multiple processes occurring simultaneously at different scales. The findings could have implications for understanding space weather effects on our atmosphere and potentially lead to improved forecasting of solar storm impacts.
Team and Training Impact
The research has also provided valuable opportunities for young scientists. Undergraduate student Josh Houghton, initially hired as an intern, played a crucial role in analyzing the data. “I was still learning things at the time,” he says. “I had just started my internship, and I very quickly got involved. It’s just very, very cool.” Houghton’s substantial contributions earned him authorship on the Nature paper as an undergraduate student, and he will continue this research through his honors thesis before pursuing a master’s degree at UCalgary.
Funding and Support
The research was made possible by the Transition Region Explorer (TREx), a University of Calgary project jointly funded by the Canadian Foundation for Innovation, the Government of Alberta, and the Canadian Space Agency. The TREx RGB and Spectrograph instruments are operated and maintained by Space Environment Canada with support from the Canadian Space Agency through its Geospace Observatory (GO) Canada initiative.
Publication Information
This study, titled “Association of structured continuum emission with dynamic aurora,” was published in Nature Communications (2024, Volume 15, Article number 10802) on December 30, 2024. The research was conducted by a team of scientists from the University of Calgary, NASA Goddard Space Flight Center, The Catholic University of America, Boston University, University of Alaska Fairbanks, and Mount Royal University.
