Medieval Writings and Tree Rings Helped Researchers Track a Solar Storm From 800 Years Ago and Reconstruct Past Solar Cycles

Diary entries by the Japanese poet Fujiwara no Teika, along with other historical sources from across Asia and Europe, played pivotal roles in a new study

Margherita Bassi | Daily Correspondent

April 16, 2026

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In February of 1204, Japanese poet Fujiwara no Teika recorded a strange sight. He penned diary entries about red lights illuminating the night sky for three consecutive nights.

Now, more than 800 years later, those observations, along with other historical sources and tree rings, have helped researchers track down a historical solar event a few years before the auroras Teika observed. The findings, described in a study published April 10 in the journal Proceedings of the Japan Academy, Series B, not only present a new approach for finding similar events long ago, but also suggest that the sun’s activity cycle was a few years shorter during the early 13th century than it is today.

When our stellar companion becomes very active, it can spew plasma into space and shoot high-energy particles toward Earth at up to 90 percent the speed of light. These sun-induced storms of radiation are called solar proton events, or SPEs.

Our planet’s magnetic bubble protects humans from most of the sun’s harmful particles, but astronauts in space remain vulnerable. Two of NASA’s moon missions had close calls in 1972, when a series of SPEs flung radiation in our direction between the Apollo 16 and 17 missions, per a statement.

“Radiation exposure caused by solar proton events is the greatest barrier to human space exploration,” study co-author Hiroko Miyahara, a physicist at Okinawa Institute of Science and Technology in Japan, tells Discover’s Rosie McCall. “So, it is essential to improve our prediction capabilities, particularly for large events.”

While identifying past SPEs might seem unrelated to predicting future solar activity, the two are closely intertwined. Sometimes, the high-energy particles can sneak through Earth’s magnetic shield and interact with gases in the atmosphere, resulting in rare forms of atoms like carbon-14. Trees take in the radioactive version of carbon, recording the SPE within their yearly growth rings.

Searching trees for carbon-14 works well when looking for the most powerful solar storms. But milder SPEs require a more time-consuming method. Knowing when and where to look for tree records of these “sub-extreme” events can speed up the process.

Enter Teika, who wrote of “red lights in the northern sky over Kyoto”—presumably auroras—in February 1204 in his diary, titled Meigetsuki. Usually, “each aurora will disappear within 24 hours, so this three-day event suggests recurrent large solar flares had occurred,” which can trigger SPEs, Miyahara tells Discover.

Additional historical documents from Japan, China, Korea, Italy, France and Germany hinted that the sun was very active from the late 12th century through the early 13th century.  So, the team focused on reconstructing the past solar cycles from that period using the carbon-14 method, which has high precision.

They examined the radioactive carbon in the rings of buried asunaro trees in northern Japan and surprisingly found no evidence of an SPE associated with the auroras Teika witnessed. Instead, they identified a carbon-14 spike pinned to a solar storm sometime between the winter of 1200 and the spring of 1201. That coincided with a red aurora recorded by people in China.

“These phenomena were always documented because they were believed to affect people’s lives and were linked to fortune-telling,” Kazuaki Yamamoto, a study co-author and a scholar at the National Institute of Japanese Literature, tells the Japan Times’ Tomoko Otake. “By examining them from a scientific perspective, researchers … can offer new insights.”

Reconstructing the sun’s activity from that time using recorded sunspots and the trees’ carbon-14 patterns revealed that solar cycles lasted only seven to eight years, hinting that the star was super active. Currently, the sun follows 11-year-long cycles.

What’s more, the newfound 13th-century SPE took place at the peak of one of the solar cycles, the team found. Based on its associated carbon-14 spike, the event likely packed 14 times the punch of an event in February 1956, the most powerful solar storm measurable from Earth’s surface documented in the modern era.

The study “helps to build the picture of past solar activity far back beyond the measured and observational record, and what is especially cool about it is that you get two records for the price of one—you get solar events and solar cycles in year-to-year detail,” Charlotte Pearson, a tree ring researcher at the University of Arizona who did not participate in the study, tells Scientific American’s Jacek Krywko.

“Historical literature provides a candidate time window, and dendroclimatology”—studying links between tree rings and past climates—“enables direct intercomparison between detected SPE and reports of sunspots and auroras recorded in literature,” Miyahara says in the statement. “Integrated approaches like these are necessary to accurately reconstruct past solar activity, helping us better understand the characteristics of extreme space weather.” 

Margherita Bassi | Read More

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.