The early 13th century was a notable time for solar activity, marked by intense solar storms. Scientists, led by Hiroko Miyahara of the Okinawa Institute of Science and Technology, uncovered evidence of a significant solar proton event (SPE) from this period. They analyzed tree rings from asunaro trees in northern Japan, which showed a carbon-14 spike linked to powerful solar reactions between 1200 and 1201 CE.
These solar events are caused by protons accelerated by solar flares and coronal mass ejections. While Earth’s magnetic field typically protects us from the majority of these protons, some manage to penetrate and create carbon-14, a stable isotope that can be measured in living organisms, including trees.
Miyahara’s research began with the diary of a Japanese poet, Fujiwara no Teika, who recorded seeing unusual red lights in the sky over Kyoto in February 1204. These lights likely resulted from auroras, which are often associated with solar activity. Interestingly, historical records from China also describe similar auroras, hinting at a widespread atmospheric phenomenon during that time.
The findings indicate that solar cycles back then were shorter—around seven to eight years—as opposed to the current average of 11 years. This suggests that the sun was more active during the early 13th century, leading to a greater frequency of SPEs.
Miyahara notes that sub-extreme SPEs, like the one they studied, are more common and can still be dangerous, despite being less intense than their extreme counterparts. He states, “Sub-extreme SPEs are more challenging to detect, but our method allows us to identify them and understand the conditions that lead to their occurrence.”
What makes this discovery intriguing is that the auroras were reported as the solar cycle was nearing its minimum, which defies the common understanding that more solar activity should correlate with solar storms. This unexpected observation opens up avenues for further research into solar dynamics.
Historical studies of isotopes such as carbon-14 and beryllium-10 indicate variations in solar cycle lengths over the last 11,000 years. While researchers believed we were currently experiencing the most intense solar activity in thousands of years, this new evidence suggests even stronger activity in medieval times.
The ongoing research sheds light on how past solar events can inform us about present and future solar behavior, reminding us of the sun’s complex and evolving nature. For more details on this research, you can view the published findings in the Proceedings of the Japan Academy, Series B here.
