A powerful burst of solar radiation struck Earth in the early thirteenth century—yet it went largely unnoticed by medieval observers. Now, by combining historical texts with high-precision scientific analysis, researchers have uncovered new evidence of a solar proton event around the year 1200, offering fresh insight into how the Sun behaved during the Middle Ages.
The study, published in Proceedings of the Japan Academy Series B, draws on an unusual combination of sources: medieval chronicles describing strange lights in the sky, and microscopic traces of radiation preserved in ancient tree rings. Together, these reveal that the Sun was not only active during this period—it may have been far more volatile than it is today.
A Hidden Solar Event
Red aurora over Engaru, Hokkaido, Japan. Photo Credit: Tomohiro M. Nakayama
Solar proton events (SPEs) occur when the Sun releases bursts of high-energy particles that travel toward Earth at extraordinary speeds. While our planet’s magnetic field shields us from most of this radiation, traces of these events can still be detected. When such particles collide with Earth’s atmosphere, they create carbon-14, which is then absorbed into plants and preserved in tree rings.
By analysing these rings with exceptional precision, the research team identified a sudden spike in carbon-14 between the years 1200 and 1201. This spike points to a previously unknown solar proton event—one that was smaller than the most extreme events known from earlier periods, but still significant.
The researchers estimate that this event produced roughly 20 percent of the radiation associated with the famous 774–775 solar event, making it about 14 times stronger than the largest comparable event recorded in modern times. While not catastrophic, it would have posed a serious hazard to anyone exposed beyond Earth’s natural protection—highlighting the risks faced by future space missions.
Medieval Observers and the Night Sky
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. Image Credit: National Archives of Japan
One of the most intriguing aspects of the study is how it integrates medieval written sources. Around the same period, observers across Eurasia recorded unusual celestial phenomena, particularly red auroras seen far from the polar regions.
A key piece of evidence comes from the diary of the influential Japanese courtier and poet, Fujiwara no Teika, who witnessed “red lights in the northern sky over Kyoto” in February 1204. Another seven accounts appear in Chinese, Korean, and European sources in that year, suggesting that these were not isolated observations but part of a widespread atmospheric event.
However, the new research complicates the story. Despite the dramatic nature of the 1204 auroras, the carbon-14 data show no corresponding spike in radiation for that year. Instead, the solar proton event appears to have taken place earlier, between late 1200 and early 1201.
This finding highlights an important distinction: while auroras are often linked to solar activity, they do not necessarily coincide with the most intense bursts of radiation. In other words, the most visually spectacular events were not always the most dangerous.
A More Active Medieval Sun
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. Image credit: Miyahara et al., 2026
Beyond identifying a single event, the study sheds light on broader patterns of solar activity during the medieval period. The years around 1200 fall within what scientists call the Medieval Solar Activity Maximum, a time when the Sun appears to have been particularly energetic.
“The high-precision data not only allowed us to accurately date sub-extreme solar proton events, but it also lets us clearly reconstruct the solar cycles of the period,” explains Hiroko Miyahara, Professor at Okinawa Institute of Science and Technology Graduate University and lead author of the study. “Today, the Sun’s activity fluctuates over eleven-year-long cycles, but we’ve found that the cycle was just seven to eight years long back then, indicating a very active Sun. The SPE we have dated occurred at the peak of one of these cycles.”
Using their carbon-14 data, the researchers reconstructed solar cycles for this period and found that they lasted only seven to eight years, compared to the roughly eleven-year cycles observed today. Shorter cycles are associated with higher levels of solar activity, suggesting that the Sun was in an unusually vigorous state.
Unexpected Patterns in Space Weather
One of the study’s more surprising findings concerns the timing of solar events within these cycles. Today, the most intense space weather events tend to occur near the peak of the solar cycle. The reconstructed data suggest that the 1200–1201 proton event followed this pattern, occurring at or near a solar maximum.
Yet other phenomena tell a different story. Some of the prolonged auroral displays recorded in medieval sources—particularly those in 1203 and 1204—appear to have taken place during the declining phase or even near the minimum of the cycle. This runs counter to modern observations and suggests that solar behaviour during periods of heightened activity may not follow the same rules seen today.
“Historical literature provides a candidate time window, and dendroclimatology enables direct intercomparison between detected SPE and reports of sunspots and auroras recorded in literature. Integrated approaches like these are necessary to accurately reconstruct past solar activity, helping us better understand the characteristics of extreme space weather,” concludes Miyahara. “For example, while the SPE we found occurred near the peak of the solar cycle, some of the prolonged low-latitude aurora recorded in the literature seems to fall near the minimum of our reconstructed solar cycle. This is unexpected, and we’re excited to look further into what solar conditions could cause this.”
Combining Science and History
The asunaro cypress tree samples, unearthed at Shimokita Peninsula in northern Aomori Prefecture. The sample is provided by Tohoku University. Photo Credit Hiroko Miyahara/OIST
The study underscores the value of combining scientific and historical evidence. Medieval chronicles provide crucial clues about when unusual events occurred, while tree-ring data offer the precision needed to confirm and date them.
As the researchers note, neither approach is sufficient on its own. Historical records can be incomplete or ambiguous, while scientific methods require guidance on where to look. Together, however, they allow for a far more detailed reconstruction of past solar activity.
Looking Back to Prepare for the Future
Understanding past solar behaviour is not just of historical interest. Solar proton events pose real risks to modern technology and human activity, particularly as space agencies plan new missions to the Moon and beyond. By identifying and analysing events like the one in 1200–1201, scientists can better assess how often such events occur and under what conditions.
The medieval period, it seems, has more to teach us than previously thought. Far from being a quiet era in terms of solar activity, it may have witnessed some of the most dynamic and complex behaviour of our star—recorded not only in scientific data, but in the observations of those who watched the skies centuries ago.
The article, “Extremely active Sun from 1190 to 1220 in the Medieval Period: Intercomparison of historical records and tree-ring carbon-14,” by Hiroko Miyahara, Ryuho Kataoka, Kazuaki Yamamoto, Fuyuki Tokanai, Toru Moriya, Mirei Takeyama, Hirohisa Sakurai, Motonari Ohyama, Kazuho Horiuchi, and Hideyuki Hotta, is published in Proceedings of the Japan Academy, Series B. Click here to read it.
Top Image: Staatsbibliothek zu Berlin Ms. germ. fol. 1416 fol. 94r
A powerful burst of solar radiation struck Earth in the early thirteenth century—yet it went largely unnoticed by medieval observers. Now, by combining historical texts with high-precision scientific analysis, researchers have uncovered new evidence of a solar proton event around the year 1200, offering fresh insight into how the Sun behaved during the Middle Ages.
The study, published in Proceedings of the Japan Academy Series B, draws on an unusual combination of sources: medieval chronicles describing strange lights in the sky, and microscopic traces of radiation preserved in ancient tree rings. Together, these reveal that the Sun was not only active during this period—it may have been far more volatile than it is today.
A Hidden Solar Event
Solar proton events (SPEs) occur when the Sun releases bursts of high-energy particles that travel toward Earth at extraordinary speeds. While our planet’s magnetic field shields us from most of this radiation, traces of these events can still be detected. When such particles collide with Earth’s atmosphere, they create carbon-14, which is then absorbed into plants and preserved in tree rings.
By analysing these rings with exceptional precision, the research team identified a sudden spike in carbon-14 between the years 1200 and 1201. This spike points to a previously unknown solar proton event—one that was smaller than the most extreme events known from earlier periods, but still significant.
The researchers estimate that this event produced roughly 20 percent of the radiation associated with the famous 774–775 solar event, making it about 14 times stronger than the largest comparable event recorded in modern times. While not catastrophic, it would have posed a serious hazard to anyone exposed beyond Earth’s natural protection—highlighting the risks faced by future space missions.
Medieval Observers and the Night Sky
One of the most intriguing aspects of the study is how it integrates medieval written sources. Around the same period, observers across Eurasia recorded unusual celestial phenomena, particularly red auroras seen far from the polar regions.
A key piece of evidence comes from the diary of the influential Japanese courtier and poet, Fujiwara no Teika, who witnessed “red lights in the northern sky over Kyoto” in February 1204. Another seven accounts appear in Chinese, Korean, and European sources in that year, suggesting that these were not isolated observations but part of a widespread atmospheric event.
However, the new research complicates the story. Despite the dramatic nature of the 1204 auroras, the carbon-14 data show no corresponding spike in radiation for that year. Instead, the solar proton event appears to have taken place earlier, between late 1200 and early 1201.
This finding highlights an important distinction: while auroras are often linked to solar activity, they do not necessarily coincide with the most intense bursts of radiation. In other words, the most visually spectacular events were not always the most dangerous.
A More Active Medieval Sun
Beyond identifying a single event, the study sheds light on broader patterns of solar activity during the medieval period. The years around 1200 fall within what scientists call the Medieval Solar Activity Maximum, a time when the Sun appears to have been particularly energetic.
“The high-precision data not only allowed us to accurately date sub-extreme solar proton events, but it also lets us clearly reconstruct the solar cycles of the period,” explains Hiroko Miyahara, Professor at Okinawa Institute of Science and Technology Graduate University and lead author of the study. “Today, the Sun’s activity fluctuates over eleven-year-long cycles, but we’ve found that the cycle was just seven to eight years long back then, indicating a very active Sun. The SPE we have dated occurred at the peak of one of these cycles.”
Using their carbon-14 data, the researchers reconstructed solar cycles for this period and found that they lasted only seven to eight years, compared to the roughly eleven-year cycles observed today. Shorter cycles are associated with higher levels of solar activity, suggesting that the Sun was in an unusually vigorous state.
Unexpected Patterns in Space Weather
One of the study’s more surprising findings concerns the timing of solar events within these cycles. Today, the most intense space weather events tend to occur near the peak of the solar cycle. The reconstructed data suggest that the 1200–1201 proton event followed this pattern, occurring at or near a solar maximum.
Yet other phenomena tell a different story. Some of the prolonged auroral displays recorded in medieval sources—particularly those in 1203 and 1204—appear to have taken place during the declining phase or even near the minimum of the cycle. This runs counter to modern observations and suggests that solar behaviour during periods of heightened activity may not follow the same rules seen today.
“Historical literature provides a candidate time window, and dendroclimatology enables direct intercomparison between detected SPE and reports of sunspots and auroras recorded in literature. Integrated approaches like these are necessary to accurately reconstruct past solar activity, helping us better understand the characteristics of extreme space weather,” concludes Miyahara. “For example, while the SPE we found occurred near the peak of the solar cycle, some of the prolonged low-latitude aurora recorded in the literature seems to fall near the minimum of our reconstructed solar cycle. This is unexpected, and we’re excited to look further into what solar conditions could cause this.”
Combining Science and History
The study underscores the value of combining scientific and historical evidence. Medieval chronicles provide crucial clues about when unusual events occurred, while tree-ring data offer the precision needed to confirm and date them.
As the researchers note, neither approach is sufficient on its own. Historical records can be incomplete or ambiguous, while scientific methods require guidance on where to look. Together, however, they allow for a far more detailed reconstruction of past solar activity.
Looking Back to Prepare for the Future
Understanding past solar behaviour is not just of historical interest. Solar proton events pose real risks to modern technology and human activity, particularly as space agencies plan new missions to the Moon and beyond. By identifying and analysing events like the one in 1200–1201, scientists can better assess how often such events occur and under what conditions.
The medieval period, it seems, has more to teach us than previously thought. Far from being a quiet era in terms of solar activity, it may have witnessed some of the most dynamic and complex behaviour of our star—recorded not only in scientific data, but in the observations of those who watched the skies centuries ago.
The article, “Extremely active Sun from 1190 to 1220 in the Medieval Period: Intercomparison of historical records and tree-ring carbon-14,” by Hiroko Miyahara, Ryuho Kataoka, Kazuaki Yamamoto, Fuyuki Tokanai, Toru Moriya, Mirei Takeyama, Hirohisa Sakurai, Motonari Ohyama, Kazuho Horiuchi, and Hideyuki Hotta, is published in Proceedings of the Japan Academy, Series B. Click here to read it.
Top Image: Staatsbibliothek zu Berlin Ms. germ. fol. 1416 fol. 94r
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