Citizen scientist data helps unravel the mystery of low-latitude blue auroras

On May 11, 2024, colorful aurora borealis appeared over the islands of Honshu and Hokkaido in Japan, triggered by an intense geomagnetic storm. Auroras observed at low latitudes usually appear red due to the emission of oxygen atoms. However, on this day, a salmon-pink aurora was observed all night, and just before midnight, an unusually high aurora with a blue tone appeared.

Smartphone videos and amateur photos captured the event, allowing scientists to combine public data with their own research to study the phenomenon.

In the new study, the researchers analyzed videos and images of the blue-dominated aurora to estimate the area of ​​the phenomenon, and confirmed their estimates with a spectrophotometer.

The study, published in the journal Earth, Planets and Space, was led by Sota Nanjo, a postdoctoral researcher at the Swedish Institute for Astrophysics in Sweden, and Professor Kazuo Shiokawa from the Institute for Space-Earth Environment (ISEE) at Nagoya University. In Japan.

Nanjo and Shiokawa’s research has for the first time visualized the spatial structure of the blue-dominated aurora during storms. Researchers discovered that the aurora has a longitudinal structure that follows magnetic field lines, making it the first confirmed blue-dominated aurora at low latitudes.

It was also determined that the aurora spans about 1,200 km in longitude and consists of three separate structures, with altitudes ranging from 400 to 900 km.

Nanjo and Shiokawa’s discovery may change our understanding of the blue aurora. The ring current, a donut-shaped region of charged particles surrounding the Earth, is thought to be the source of energetic neutral atoms (ENA) that produce low-latitude aurorae, including the red aurora. According to this model, the storm could have activated the ENA and produced the colorful light display.

However, this group of findings cannot be easily explained by this mechanism. Professor Shiokawa said, “In this study, we found a structure of several hundred kilometers in the vertical direction of the aurora, which is dominated by blue, but this is difficult to interpret based solely on the activity of ENA.Also, ENA creates the structure of the aurora. It is unlikely that the magnetic field lines observed in this study.

Another possibility was that the aurora was due to resonant scattering of nitrogen molecular ions caused by sunlight irradiation. However, the group’s study suggests that a different process was occurring, as sunlight only reached 700 kilometers instead of the 400 kilometers the researchers observed.

Rather, the results may point to an interesting possibility of an unidentified process. “Our findings suggest that nitrogen molecular ions were accelerated upward by some mechanism and may have been involved in the formation of the blue-dominated aurora,” Shiokawa said.

“So far, it is not well understood how large nitrogen molecular ions can exist at such high altitudes,” he continued. “Such ions cannot easily exist for long periods of time due to their heavy mass and short dissociation and recombination time intervals. However, they are observed at high altitudes, and the process is shrouded in mystery. ”

Overall, repeated observations of blue-dominated auroras, such as those observed in Japan, may provide clues to understanding the principles behind how nitrogen is found at these altitudes. There is.

The process by which nitrogen molecular ions escape into the magnetosphere is important for everything from understanding magnetic storms to the cosmic radiation environment, so these discoveries will help us understand the processes occurring hundreds of kilometers above us. It may be helpful.