Colorful auroras lighted up Japan’s Honshu and Hokkaido islands on May 11, 2024, due to a fierce magnetic storm. Typically, auroras at these lower latitudes exhibit red hues from oxygen atom emissions. However, on this particular night, a striking salmon pink aurora was visible, alongside an unusually tall blue-dominant aurora that made its appearance just before midnight.
On May 11, 2024, Japan witnessed a unique blue-dominant aurora. This spectacular event was documented through videos and photos taken by citizens, which were later examined by Sota Nanjo, a postdoctoral researcher at the Swedish Institute of Space Physics, and Professor Kazuo Shiokawa from Nagoya University. These experts propose an alternative theory. They suggest that nitrogen molecular ions may be propelled into the magnetosphere via an unidentified mechanism. This new finding invites further exploration into how nitrogen ions, known to influence Earth during geomagnetic storms and auroras, could remain at high altitudes.
A brilliant display of colorful auroras illuminated Japan’s Honshu and Hokkaido islands on May 11, 2024, as a result of a powerful magnetic storm. While auroras seen at lower latitudes usually appear red due to the emission from oxygen atoms, on this occasion, observers noted a salmon pink glow throughout the evening and an unusually tall blue-dominant aurora just before midnight.
Many recorded the celestial phenomenon with smartphones and casual cameras, allowing researchers to merge public observations with their scientific analysis of the event.
In a new study, experts scrutinized the captured videos and images of the blue-dominant aurora to estimate its extent, corroborating their findings with spectrophotometer data. This research, spearheaded by Sota Nanjo and Professor Kazuo Shiokawa, was published in the journal Earth, Planets and Space.
The efforts of Nanjo and Shiokawa resulted in the first depiction of the blue-dominant auroras’ spatial structure during a magnetic storm. They discovered that these auroras formed longitudinal structures aligned with magnetic field lines, a first for low-latitude blue-dominant auroras. The aurora was determined to spread approximately 1200 km in longitude, comprising three distinct structures, and ranging in height from 400 to 900 km above the Earth.
These revelations could reshape our comprehension of blue auroras. The ring current, a toroidal region of charged particles orbiting the Earth, is believed to generate energetic neutral atoms (ENAs) that contribute to low-latitude auroras, including the red ones. According to this model, the magnetic storm likely energized the ENAs, producing a vibrant light display.
However, Shiokawa notes that their findings don’t align neatly with this explanation. “This study identified a structure several hundred kilometers long within the blue-dominant aurora in the longitudinal direction, which is hard to explain solely by ENA activity. Furthermore, ENAs should not create auroral patterns that are aligned with the magnetic field lines, as seen here.”
Another theory considered was that resonant scattering of nitrogen molecular ions, influenced by sunlight, accounted for the aurora. Yet, the research indicates a different event occurred since sunlight typically penetrates only to about 700 km—not the 400 km in which the team made their observations.
Their results hint at the exciting possibility of an unknown mechanism. “Our observations imply that some process may have caused nitrogen molecular ions to accelerate upwards, leading to the formation of the blue-dominant aurora,” said Shiokawa.
“Currently, we do not fully understand how large nitrogen molecular ions can survive at such altitudes,” he elaborated. “These ions, due to their heavier weight and brief dissociative-recombination intervals, shouldn’t normally endure for extended periods, yet they are detected at significant heights. The underlying mechanism remains unclear.”
Regular sightings of blue-dominant auroras, like the one in Japan, may provide valuable insights into the principles governing nitrogen presence at these elevations. Since understanding the outflow of nitrogen molecular ions into the magnetosphere is crucial for comprehending geomagnetic storms and space radiation, these findings could shed light on the processes occurring hundreds of kilometers above our heads.
