A rare crimson aurora glowing over northern Japan may be a sign that some solar storms are more powerful than scientists once believed.
Researchers studying faint red auroras observed above Japan in June 2024 found that the displays stretched far higher into Earth's atmosphere than expected, reaching altitudes between roughly 310 and 500 miles (500 to 800 kilometers) — which is unusually high for a storm that was not considered especially severe by conventional geomagnetic indices, according to a statement from Hokkaido University.
The discovery suggests that even "moderately intense" geomagnetic storms — which are a disturbance in Earth's magnetic field caused by charged particles and magnetic energy from the sun — may carry far more energy than previously thought, challenging scientists' understanding of how these space weather events develop and how their strength is measured.
"I was really surprised because I didn't expect such tall auroras to appear even during moderately intense storms," Tomohiro M. Nakayama, lead author of the study, said in the statement. "This suggests that these storms may actually be stronger than conventional indices indicate."
Auroras are usually seen near the poles as bright, shimmering lights produced when charged particles from the sun collide with gases in Earth's upper atmosphere. When they appear farther south, in areas like Japan, they are generally linked to strong geomagnetic storms and occur at lower altitudes of around 124 to 249 miles (200 to 400 kilometers).
The unusual auroras were photographed over Hokkaido, Japan, where observers captured diffuse red glows hanging low over the horizon. Unlike the vivid green curtains commonly associated with the northern lights, these auroras appeared as soft crimson veils spread across the night sky.
This contrast in auroras is because different gases — and different altitudes — produce different colors. Green auroras, the most common type, form when energetic particles excite oxygen atoms about 60 to 150 miles (100 to 250 km) above Earth. Red auroras also come from oxygen, but they occur much higher in the atmosphere, where the air is extremely thin and oxygen atoms can release a dim red glow before colliding with other particles. Blue and purple auroras are typically linked to nitrogen molecules.
Red auroras are generally faint and usually occur at very high altitudes, making them less commonly seen compared to green displays. They are also more often associated with especially strong geomagnetic storms capable of pushing auroral activity farther from the poles.
What's more is that Japan sits much farther south than the regions where auroras are usually visible. While powerful solar storms can occasionally push the auroral oval toward lower latitudes, scientists did not expect such extensive high-altitude red emissions during a storm categorized as only moderate.
The team analyzed five auroral events recorded between June 2024 and March 2025, combining observations from Hokkaido with satellite data and photographs captured by citizen scientists across Japan. By measuring the elevation angles of the auroras in the images and tracing them along Earth's magnetic field lines, the team reconstructed how high the glowing structures extended into the atmosphere. Their analysis suggests that an unusually dense solar wind — streams of charged particles flowing from the sun and interacting with Earth's magnetic field — may have fueled the rare crimson auroras even without an officially "extreme" geomagnetic storm, according to the statement.
The study comes as the sun remains near the peak of Solar Cycle 25, a period of heightened solar activity that has already produced spectacular aurora displays across the globe. In May 2024, one of the strongest geomagnetic storms in decades pushed auroras deep into mid-latitude regions around the world. Understanding these unexpected auroras could improve forecasts of dangerous space weather, which can disrupt satellites, GPS systems, communications and even power grids during extreme solar storms.
"As the number of satellites in low Earth orbit continues to grow, understanding these effects is increasingly important," Nakayama said in the statement.
Their findings were published May 19 in the Journal of Space Weather and Space Climate.
