Unprecedented Blue Auroras Over Japan Challenge Existing Theories on Atmospheric and Geomagnetic Processes

In May 2024, a mysterious blue aurora appeared over Japan, sparking significant scientific interest and challenging existing theories about Earth’s atmospheric processes. The auroras, that were observed above the islands of Honshu and Hokkaido, were different from the typical green or red auroras seen at higher latitudes.

This unusual event, occurring at altitudes far higher than typical auroras, was linked to a powerful geomagnetic storm and included both blue and salmon-pink glows. These rare hues and the auroras’ location left the scientific community puzzled and eager to explore this phenomenon further.

Auroras are generally caused by charged particles from the sun interacting with Earth’s magnetosphere, exciting atmospheric gases like oxygen and nitrogen. This interaction typically occurs between altitudes of 100 and 200 kilometers, where green and red colors are emitted.

However, the blue auroras seen over Japan were observed at altitudes between 400 and 900 kilometers, much higher than expected. This discrepancy raised questions about the mechanisms that led to such a high-altitude aurora and prompted researchers to investigate the cause of the blue hues.

A team of scientists from Japan and Sweden, led by Sota Nanjo and Professor Kazuo Shiokawa, conducted research using both public data and their own findings. They proposed that the blue auroras were likely caused by nitrogen molecular ions, which might have been accelerated to high altitudes by an unknown atmospheric process.

Nitrogen ions, typically unstable at such heights, may have persisted in the upper atmosphere longer than expected, providing a potential explanation for the blue color. However, the exact mechanism behind this remains unclear, and further research is needed to understand how these ions can survive at such extreme altitudes.

The appearance of these blue auroras also challenges the most widely accepted model for aurora formation, which involves the ring current—a region of charged particles trapped in Earth’s magnetic field. According to this model, the energetic neutral atoms (ENAs) produced by these particles should interact with the atmosphere to create auroras.

However, the blue auroras observed over Japan did not align with this theory. Notably, the auroras exhibited longitudinal structures, aligning with Earth’s magnetic field lines, a feature not previously seen in low-latitude auroras. This inconsistency suggests that the current understanding of auroral formation may need to be revised.

One of the key mysteries surrounding the blue auroras is the presence of nitrogen molecular ions at such high altitudes. These ions are generally unstable and have a short lifespan at the heights where the blue auroras were observed.

The discovery of nitrogen ions at altitudes above 400 kilometers contradicts existing scientific knowledge, as such ions should not be able to survive under normal atmospheric conditions. This raises the possibility that a new, unidentified atmospheric process may be at work, accelerating nitrogen ions to these extreme altitudes and contributing to the formation of these unusual auroras.