A ground-based facility dedicated to observing the northern lights has captured intriguing images of a pale grey glow among the vibrant green and red streaks of the aurora. This unusual light has left scientists curious, as no one has been able to explain it fully—until now.
Researchers at the University of Calgary are using new spectral data to shed light on this mystery. They compare the grey and white patches to a recently identified phenomenon called ‘STEVE,’ which stands for Strong Thermal Emission Velocity Enhancement.
Emma Spanswick, a physicist at the university, describes how this mysterious grey tone appears alongside the typical green and red auroras. “You see a dynamic green aurora, then suddenly a structured patch of grey or white connected to it,” she explains. This unusual light raises the immediate question: “What is that?”
Most of the time, the Sun’s clouds of charged particles stay contained by its gravity and magnetic fields. Occasionally, some particles escape and travel toward Earth. While our planet’s magnetic field usually redirects these particles away, a small portion can collide with our atmosphere. This collision causes atmospheric molecules to glow, creating a stunning display of colors—mostly green and pink due to oxygen and nitrogen at different altitudes. Yet, the sporadic appearances of white and grey have puzzled scientists.
Interestingly, the Earth’s atmosphere can also produce light through other processes. For example, sunlight can cause molecules to break apart and reform, resulting in a faint glow known as nightglow.
Years ago, aurora enthusiasts named a peculiar mauve-gray light ‘STEVE.’ This glow’s distinct spectrum blends to form white and grey colors and is different from traditional auroras. Scientists are investigating whether STEVE’s mysterious light could be related to the pale patches seen now, suggesting that both may result from chemical reactions in the atmosphere.
“There are similarities between what we’re seeing now and STEVE,” says Spanswick. “This grey-toned structure is closely tied to the auroras.” Unlike STEVE, which stands apart in the sky, the new grey patches are linked to the dynamic auroras, making them harder to spot.
To better study these patches, researchers are now using a high-resolution sky-imaging observatory called the Transition Region Explorer. This tool captures detailed color profiles of auroras, allowing scientists to analyze the specific wavelengths contributing to the white glow.
The team discovered that these grey patches can range from tens to hundreds of kilometers in size and typically appear within active auroras. They suspect that the heat released in certain auroral displays may trigger chemical reactions, causing the release of the light we see.
While the exact processes behind this glow remain theoretical, they may offer new insights into the beautiful and complex nature of auroras. Further research, including laboratory simulations and more field observations, could unveil more about this captivating spectacle in our skies.
This enlightening study was published in Nature Communications.
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