Scientists Used Medieval Poems and Trees to Uncover a 13th-Century Solar Surge

For obvious reasons, it is extremely difficult to track space weather events from very early on – unless we are talking about extremely powerful, extraordinary events. But for understanding broader weather patterns, these rarities aren’t the most ideal markers. So, scientists tried to turn to an unexpected source for help: poetry.

In the winter of 1204, Fujiwara no Takea, a famous Japanese poet, wrote about “red lights in the northern sky of Kyoto” over three nights. Researchers at the Okinawa Institute of Science and Technology (OIST) in Japan decided to conduct ultra-precise carbon dating of buried trees from that period. Their analysis, published today in Proceedings of the Japan Academy, Series B, revealed a peak in solar activity between 1200 and 1205 CE, demonstrating the potential of the historical literature in studying space weather patterns from much earlier.

“The high-precision data not only allowed us to accurately date sub-extreme solar proton events, but it also allowed us to unambiguously reconstruct the solar cycles of the period,” study lead author and OIST physicist Hiroko Miyahara said in a statement.

easier said than done

Technically speaking, existing instruments are quite capable of characterizing carbon-14 spikes in tree rings, which represent solid physical evidence of powerful solar events. The team behind the new findings went one step further, spending 10 years perfecting the method of collecting ultra-precise measurements of the carbon-14 content in organic matter.

But an (arguably) bigger issue was deciding which period of historical time – more than 10,000 years – would give them the best bang for their buck. This did not help because this method, as efficient as it was, took a lot of time and effort to implement. In addition, the team wanted to study the faint occurrences of solar proton events (SPEs), during which high-energy particles fall to Earth at up to 90% the speed of light. Although these “sub-extreme” SPEs are not as harmful, they still occur frequently and potentially pose a risk to space missions, the researchers explained.

History saves the present

And so, researchers turned to historical records. In addition to Teka, the team found Chinese and French documents from around the same time that mentioned similar observations. Ancient astronomical records from Korea and China also describe sunspot and red auroral activity with consistent frequency between 1193 and 1258 CE. According to the paper, this indicates that East Asian scientists observed an increase in solar activity during this time.

Based on this preliminary analysis, the team collected carbon-14 data from buried Asunaro trees in northern Japan. SPEs trigger a wave of high-energy particles spreading through Earth’s atmosphere, and some of the particles collide with atmospheric gases, creating carbon-14 compounds, which are incorporated into CO2 and absorbed by trees during photosynthesis. As expected, the team confirmed a “sudden surge” in carbon-14 content, which they dated to between 1200 and 1201 AD.

Although that time frame doesn’t exactly match Teka’s observations of the aurora, a Chinese record clearly mentions a red, low-latitude aurora in that year, the team reports in the paper. But Teka’s writing is still consistent with an overall increase in solar activity around this time.

medieval solar cycle

A wholesale review and comparison of historical records allowed researchers to reconstruct solar cycles between 1190 and 1220. Interestingly, the analysis revealed that the solar cycle in the 13th century lasted between seven and eight years, as opposed to the 11 years we know today.

Charlotte Pearson, a dendrochronologist at the University of Arizona who was not involved in the study, told Scientific American, “This work helps build a picture of past solar activity far beyond the measured and observational record.” “What’s particularly nice about it is that you get two records for the price of one – you get solar events and solar cycles in year-by-year detail.”

“Such integrated approaches are essential for accurately reconstructing past solar activity, which will help us better understand the characteristics of extreme space weather,” Miyahara said. “For example, while the SPEs we found occurred near the peak of the solar cycle, some of the longer-lived low-latitude auroras recorded in the literature appear to occur near the minimum of our reconstructed solar cycle. This is unexpected, and we are excited to see further what solar conditions might be causing it.”