Medieval poems and tree rings reveal a 13th-century solar surge.

Researchers combined medieval observations and ultra-precise carbon-14 dating of buried trees to identify a peak in solar activity around 1200–1205 CE.

Researchers at the Okinawa Institute of Science and Technology used medieval writings together with high-precision carbon-14 measurements of buried trees to study past space weather. The team reviewed historical records, including a 1204 poem by Fujiwara no Teika that describes red lights over Kyoto, and additional Chinese, Korean and French documents. They focused on sub-extreme solar proton events, which are weaker than the largest known events but occur more often. The analysis and tree-ring dating produced a dated signal indicating elevated solar activity in the early 1200s. What researchers report: - The team spent about a decade refining ultra-precise carbon-14 measurements in organic material to detect smaller solar proton events in tree rings. - Carbon-14 data from buried asunaro trees showed an abrupt increase dated to sometime between 1200 and 1201 CE. - Historical records include Fujiwara no Teika’s 1204 note of red northern lights over Kyoto and other contemporary entries from China, Korea and France; East Asian astronomical records also report sunspots and auroral activity between 1193 and 1258 CE. - From combined tree-ring and textual evidence, the researchers reconstructed solar cycles from 1190 to 1220 and found cycles lasting about seven to eight years, shorter than the modern 11-year average. Summary: The study illustrates how historical literature and precise tree-ring carbon dating can extend the record of solar activity beyond modern observations and provide year-to-year detail on events and cycle length. Researchers report that the combined evidence helps characterize past solar behavior and that further investigation is planned to better understand unexpected timing of low-latitude auroral reports relative to their reconstructed cycle minima.