Ancient Diaries and Preserved Timber Reveal Unpredictable Hazards of Medieval Solar Proton Events

OIST researchers use 13th-century Japanese diaries and tree-ring data to identify hazardous solar proton events and a shortened medieval solar cycle.

By: AXL Media

Published: Apr 10, 2026, 8:54 AM EDT

Source: Information for this report was sourced from OIST Graduate University

Bridging Historical Literature and High Precision Radiocarbon Analysis

A multidisciplinary team in Japan has developed a novel method for tracking hazardous space weather by synchronizing medieval court diaries with the chemical signatures found in ancient wood. While extreme solar proton events (SPEs) have been studied previously, sub-extreme events remain difficult to detect due to their smaller atmospheric footprint. By using historical records to narrow their search window, researchers from the Okinawa Institute of Science and Technology (OIST) have established a new framework for identifying these sporadic outbursts, which present significant radiation risks to modern space exploration and satellite infrastructure.

The Role of Medieval Poetry in Identifying Space Weather Windows

The investigation began with a literary clue found in the Meigetsuki, the personal diary of the prominent 13th-century Japanese poet and courtier Fujiwara no Teika. In February 1204 CE, Teika recorded observations of "red lights in the northern sky over Kyoto," a description consistent with low-latitude auroras. While these auroras are not caused directly by solar protons, they frequently occur alongside the same type of intense solar activity that triggers SPEs. This historical eyewitness account provided researchers with a specific chronological target for their laboratory analysis.

Analyzing Carbon-14 Spikes in Preserved Asunaro Timber

To confirm the solar event, the team examined the carbon-14 content of buried asunaro trees unearthed in Aomori Prefecture. When high-energy solar protons collide with Earth’s atmosphere, they create carbon-14 isotopes that are eventually absorbed into the rings of growing trees. Using an ultra-precise measurement technique developed over a decade, the physicists identified a significant carbon-14 spike. By correlating these findings with dendroclimatic dating, they were able to place a sub-extreme SPE between the winter of 1200 CE and the spring of 1201 CE.