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On certain nights in Japan, people may notice a faint red glow spreading low across the horizon. The dim crimson haze can be easy to overlook, but it is created by powerful activity taking place far above Earth. Streams of charged particles from the Sun travel toward our planet and interact with Earth's magnetic field. When these particles collide with oxygen atoms high in the atmosphere, the atoms release energy as soft red light, producing rare auroras visible from the ground.
A new study published in the Journal of Space Weather and Space Climate reports that researchers from Hokkaido University and the Okinawa Institute of Science and Technology observed unusual red auroras above Japan at far greater heights than expected. The glowing displays extended roughly 500 to 800 kilometers above Earth's surface.
Rare Red Auroras Reach Extreme Heights
Auroras are most commonly seen near the polar regions, where charged solar particles interact with Earth's atmosphere to create shimmering lights in the sky. In Japan and other areas farther south, auroras are much less common and are usually linked to powerful geomagnetic storms. These events typically occur at altitudes of around 200 to 400 kilometers.
The new findings challenge that understanding.
"We found that red auroras can extend to extremely high altitudes even during those storms that are measured as moderately intense. I was really surprised because I didn't expect such tall auroras to appear even during moderately intense storms," says Tomohiro M. Nakayama, lead author of the study. "This suggests that these storms may actually be stronger than conventional indices indicate."
The research team studied five auroral events recorded in Hokkaido between June 2024 and March 2025. During those periods, bursts of charged particles from the Sun compressed Earth's magnetosphere, the invisible magnetic shield surrounding the planet. Although standard measurements classified the storms as moderate, the compression itself appeared unusually intense.
Solar Wind May Be Hiding Stronger Storms
Researchers believe dense streams of solar wind squeezed Earth's magnetic field so strongly that the upper atmosphere became heated and expanded upward. This may have pushed the region where red auroras form to much higher altitudes than scientists normally expect.
At the same time, the movement of charged particles may have masked the true intensity of the storms, making them appear weaker according to traditional space weather measurements.
To investigate the phenomenon, scientists combined satellite observations with photographs captured by citizen scientists across Japan. By studying the angles of the auroras in those images and mapping them along Earth's magnetic field lines, the researchers were able to estimate how high the glowing structures reached into the atmosphere.
The participation of skywatchers across the country proved especially valuable. Observations from multiple locations allowed the team to study rare auroral events in greater detail than would have been possible using conventional monitoring networks alone.
Why These Auroras Matter for Satellites
The findings may have important implications beyond the visual spectacle of auroras. When Earth's upper atmosphere heats up and expands, satellites orbiting the planet experience greater atmospheric drag. This extra resistance can gradually alter satellite trajectories and cause some spacecraft to lose altitude faster than expected.
"As the number of satellites in low Earth orbit continues to grow, understanding these effects is increasingly important," says Nakayama. "Our results could help improve space weather forecasting and support safer satellite operations."
English (US)