Medieval Japanese poetry and buried trees help elucidate volatile space weather
On Earth, extreme solar activity often appears as beautiful, benign auroras. But venturing beyond the safety of the Earth's magnetic field, one faces the full brunt of a temperamental star that can suddenly erupt with flares ...
These outbursts occasionally trigger so-called solar proton events (SPEs), in which high-energy particles are flung towards Earth at up to 90% of the speed of light.
In 1972, a string of SPEs occurred between the Apollo 16 and 17 moon missions—had these coincided with either expedition, the astronauts would have been helplessly exposed to deadly particle radiation. As we return to the moon, understanding these sporadic events is becoming more urgent.
Now, researchers from the Okinawa Institute of Science and Technology (OIST) have shown a new approach for detecting historical SPEs, where they use medieval records to guide ultra-precise carbon-14 measurements of buried asunaro trees in Northern Japan.
Using this combined approach, the physicists have identified and dated an SPE to a period between winter 1200 to spring 1201 CE in the medieval period, when solar activity was extremely high. Their findings were published in the Proceedings of the Japan Academy, Series B.
Red aurora over Engaru, Hokkaido, Japan. Credit: Tomohiro M. Nakayama
Until last week, Apollo 17 was the last time humans left low-earth orbit to visit the moon. Several solar proton events occurred in the same year as Apollo 16 and 17; had these coincided, the astronauts would have been exposed to deadly radiation without protection. Credit: NASA (Public domain)
A hand-copied version of Fujiwara no Teika's diary, Meigetsuki, from the Edo period. The page shown includes references to "red lights in the northern sky" on the right-hand side. Credit: National Archives of Japan
The asunaro cypress tree samples, unearthed at Shimokita Peninsula in northern Aomori Prefecture. The sample is provided by Tohoku University. Credit: Hiroko Miyahara/OIST
Reconstructed solar cycles based on carbon-14 records, relative to baseline activity level at y=0, overlaid with historical records: orange circles denote the timing of aurora sightings, blue and red stars denote prolonged aurora events like those described in Meigetsuki, and black diamonds indicate the emergence of large sunspots. Credit: Miyahara et al., 2026