Medieval Japanese Poetry and Buried Trees Help Elucidate Volatile Space Weather
by Okinawa Institute of Science and Technology
edited by Sadie Harley, reviewed by Robert Egan
On Earth, extreme solar activity often appears as beautiful, benign auroras. However, once we move beyond the protective embrace of Earth's magnetic field, we are exposed to the full fury of a volatile star capable of unleashing devastating solar flares and coronal mass ejections.
During the year 1972, a series of solar proton events (SPEs) occurred between the Apollo 16 and 17 missions. Had an SPE coincided with any part of the missions, astronauts would have found themselves without protection against harmful particle radiation. As humanity prepares to return to the moon, gaining a deeper understanding of such sporadic solar events becomes increasingly important.
Researchers from the Okinawa Institute of Science and Technology (OIST) have unveiled an innovative technique for detecting historical SPEs, utilizing medieval records to guide ultra-precise carbon-14 dating of buried asunaro trees in Northern Japan. Through this interdisciplinary approach, physicists successfully identified an SPE occurring between winter 1200 CE and spring 1201 CE, coinciding with an period of heightened solar activity. Their findings were recently published in the Proceedings of the Japan Academy, Series B.
Professor Hiroko Miyahara from the OIST Solar-Terrestrial Environment and Climate Unit stated, "Previous research into historical SPEs has primarily concentrated on exceptionally rare and powerful solar events. Our analysis establishes a framework for identifying sub-extreme SPEs—events that, while less intense, occur more frequently and can be equally hazardous."
This discovery not only aids in pinpointing the potential for more frequent SPEs but also enhances our comprehension of the solar phenomena that give rise to these events.
Red Lights in the Northern Sky
The majority of high-energy protons from SPEs are diverted by Earth's magnetic field. However, at the poles, where geomagnetic field lines open into space, particles can penetrate the atmosphere and interact with its gases, forming compounds of carbon-14 that disperse globally. Consequently, measuring the carbon-14 content in preserved organic materials, such as buried trees, lets researchers trace solar activity fluctuations over the past 10,000 years.
Using ultra-precise measurement techniques developed over a decade-long investigation, scientists can now detect smaller fluctuations that conventional methods often overlook, thus identifying sub-extreme SPEs.
The initial clue for the study came from historical records—specifically from Meigetsuki, a diary penned by prominent Japanese courtier Fujiwara no Teika (1162–1241), who documented witnessing "red lights in the northern sky over Kyoto" in February 1204 CE. Though SPEs themselves do not directly cause auroras, they frequently accompany related space weather events, which provides researchers with a specific timeframe for investigation.
During the study, the researchers measured carbon-14 content in unearthed asunaro wood from the Aomori Prefecture in Northern Japan, identifying carbon-14 spikes that corresponded to a sub-extreme SPE.
Red aurora over Engaru, Hokkaido, Japan. Credit: Tomohiro M. Nakayama
Further corroborating the findings, the researchers utilized dendroclimatic studies—a dating method that analyzes tree ring growth patterns linked to regional climate—to narrow down the SPE to a time frame between winter 1200 CE and spring 1201 CE. This coincided with reports of a red, low-latitude aurora seen in China.
Conclusion and Implications
This novel research harnesses a combination of historical literature and advanced scientific techniques, illustrating how a multidisciplinary approach can yield new insights into the complex dynamics of solar activity and its historical implications. As humanity faces an era of renewed space exploration, understanding the unpredictability of solar events remains vital for astronaut safety and mission planning.
Key concepts: aurora, coronal mass ejection, solar activity, space weather, solar cycle
More information
Extremely Active Sun from 1190 to 1220 in the Medieval Period: Intercomparison of Historical Records and Tree-ring Carbon-14, Proceedings of the Japan Academy Series B (2026). DOI: 10.2183/pjab.102.011
Provided by Okinawa Institute of Science and Technology
Summary
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Combining medieval Japanese records with high-precision carbon-14 analysis of buried trees enabled identification and dating of a sub-extreme solar proton event (SPE) between winter 1200 and spring 1201 CE. This approach allows detection of more frequent, hazardous SPEs previously undetectable, and reveals that solar cycles during this period were shorter and more active than today's, improving understanding of historical space weather variability.
