Amateur photographs and smartphone videos captured the striking aurora, providing critical data for scientists to combine with their observations. The collaborative effort led to a study published in *Earth, Planets and Space*, headed by Sota Nanjo from the Swedish Institute of Space Physics and Professor Kazuo Shiokawa of Nagoya University's Institute for Space-Earth Environmental Research (ISEE).
The study marked the first successful visualization of the spatial structure of a blue-dominant aurora during a geomagnetic storm. Researchers discovered that the auroras featured longitudinal structures aligned with magnetic field lines, a pattern never before documented in low-latitude blue auroras. Spanning approximately 1,200 kilometers in longitude, the aurora consisted of three distinct structures and ranged in altitude from 400 to 900 kilometers.
"This study revealed a structure several hundred kilometers wide in the blue-dominant aurora along the longitudinal direction, which is difficult to explain using existing ENA (energetic neutral atom) activity models," Professor Shiokawa said. "Moreover, ENAs are unlikely to create auroral structures aligned with magnetic field lines, as seen here."
Auroras are often linked to the Earth's ring current - a torus of charged particles surrounding the planet. Energized ENAs are thought to generate low-latitude auroras, including red ones. However, the blue aurora's characteristics suggest an alternative process. Researchers proposed nitrogen molecular ions, potentially accelerated upward by an unknown mechanism, as a plausible explanation.
The study explored another theory involving resonant scattering of nitrogen molecular ions due to sunlight. Yet, this explanation fell short since sunlight only penetrates to altitudes of 700 kilometers, whereas the aurora was observed at 400 kilometers.
"Our findings suggest that nitrogen molecular ions may have been accelerated upward by a mechanism yet to be identified, leading to the blue-dominant aurora," Shiokawa explained.
The discovery raises questions about how heavy nitrogen molecular ions exist at high altitudes despite their short lifespans. Shiokawa added, "Understanding this mystery is crucial as nitrogen molecular ion outflow plays a significant role in geomagnetic storm dynamics and the radiation environment in space."
Repeated observations of blue-dominant auroras could unlock further insights into these enigmatic processes and improve our understanding of geomagnetic storms and upper-atmosphere phenomena.