A rare blue aurora over Japan stretched all the way to space. Now scientists think they know why

On the night of May 10 to 11, particles from a series of massive eruptions on the Sun reached Earth and caused a geomagnetic storm, resulting in rare auroras, including a distinctive blue glow over Japan, particularly over Honshu.

Physicists Sota Nanjo from the University of Electro-Communications in Tokyo and Kazuo Shiokawa from Nagoya University studied this phenomenon using images captured by citizen scientists in Japan. They published their findings in the journal Earth, Planets and Space.

The blue aurora was located at an altitude of 400 to 900 kilometers, surpassing even the orbit of the International Space Station, which operates between 370 and 460 kilometers above Earth. This contradicts traditional explanations for auroras in temperate latitudes. "This is the first time that a blue-dominant aurora has been observed in this form and with such longitudinal and magnetically aligned structures," the researchers noted, according to Focus Online.

Auroras occur when energetic charged particles from the Sun collide with Earth's magnetic field and upper atmosphere. When these solar particles hit Earth's magnetic field, they are deflected and accelerated toward the poles, where they collide with atmospheric atoms, generating a bright glow as these atoms release energy in the form of photons. Typically, auroras during solar storms are caused by excited oxygen atoms, which emit green and red light. Nitrogen atoms can emit blue and red photons, but nitrogen auroras are extremely rare because nitrogen atoms require significant energy to become excited.

During the intense geomagnetic storm on May 11, unusual auroras, including a pink glow and a distinctly blue glow, were observed. The researchers found that the aurora was arranged in longitudinal structures that ran along magnetic field lines for around 1,200 kilometers, which do not fit known models of auroral activity.

By analyzing images from two amateur researchers taken from different locations, Nanjo and Shiokawa were able to localize the blue aurora. The various angles of the photos allowed them to determine the structure, location, and height of the blue auroras more accurately. The blue polar light showed three stripe-like structures oriented along Earth's magnetic field lines, which is difficult to explain with the particles involved.

The researchers hypothesize that accelerated molecular nitrogen ions moving upward may be responsible for the blue aurora phenomenon, but how this occurs is still a mystery. "Because these ions remain only for a short time due to their high molecular weight and their rapid recombination to molecules," Kazuo Shiokawa explained, according to Focus Online. "How the nitrogen molecules get there is still 'surrounded by mysteries,'" he noted, according to Blick and Die Welt.

Currently, low-latitude auroras are thought to be related to the ring current, a torus-shaped current of charged particles trapped inside Earth's magnetosphere, circling the planet's equator. However, the researchers do not believe that the ring current explains what they observed. "In this study, a structure of several hundred kilometers was found in the blue-dominant aurora in the longitudinal direction, which is difficult to interpret by ENA activity only," Shiokawa said, according to Science Alert.

There are two main challenges in explaining the blue aurora. First, it is unclear how the solar storm excited the nitrogen molecules needed for the blue aurora. Second, nitrogen ions, due to their mass and rapid recombination times, should not survive at such high altitudes for significant periods. There is currently no explanation for the presence of nitrogen at such high altitudes.

The two scientists hope for many images from amateur researchers to understand the mysterious process.

This article was written in collaboration with generative AI company Alchemiq