The solar cycle was several years shorter and the sun was experiencing an unusually active phase at the beginning of the 13th century — at least, that's the story told by evidence left behind in tree rings and historical records that suggest a burst of protons and enhanced coronal mass ejections battered Earth between the years 1200 and 1204 CE.
More specifically, scientists led by Hiroko Miyahara of the Okinawa Institute of Science and Technology (OIST) Solar–Terrestrial Environment and Climate Unit in Japan have found remnants of a dramatic solar proton event in tree rings dating back 825 years.
A solar proton event, or SPE, is a barrage of protons that are accelerated to nine-tenths the speed of light by solar flares and coronal mass ejections. SPEs can be extremely dangerous, threatening astronauts and spacecraft. While Earth's magnetic field can keep out most of the protons, occasionally some burst through Earth's magnetic shield and descend into the atmosphere where they collide with atmospheric gases, creating atoms of a kind of carbon (carbon-14) that drift around the planet and become incorporated into living organisms — including trees.
Miyahara's team used "Meigetsuki," which is the diary of a Japanese courtier and poet named Fujiwara no Teika who lived between 1162 and 1241, as a starting point in their search for historic SPEs. The diary said that, in February of 1204, the poet saw "red lights in the northern sky over Kyoto." Kyoto is located at 35 degrees north, which is a rather low latitude to be witnessing an aurora since auroral lights are usually confined to the poles — the stronger the solar storm, the closer to the mid-latitudes the lights are seen. (It must have been especially strange to Fujiwara no Teika, who would not have known what he was looking at.)
Chinese astronomers also witnessed a red aurora around the same time period. Although SPEs are not directly responsible for the creation of an aurora, they are connected to the coronal mass ejections that do cause auroral lights. Therefore, an unusually strong aurora is a good starting point in the search for historical SPE.
In the remains of buried asunaro trees in northern Japan from the turn of the 13th Century, Miyahara's team found carbon-14 contained within tree rings dating back to the period between the winter of 1200 and the spring of 1201. The tree rings are the smoking gun of a "sub-extreme" SPE.
"Previous studies on historical SPEs have focused on rare, extremely powerful events," Miyahara said in a statement. "Our paper provides a basis for detecting sub-extreme SPEs."
Sub-extreme SPEs are more common and have energies about 10% to 30% of the most extreme SPEs, but are still dangerous.
"Sub-extreme SPEs are more challenging to detect, but our method now allows us to efficiently identify them and better understand the conditions under which they are more likely to occur," said Miyahara.
By applying what's known as "dendroclimatology," which is the study of how the spacing between tree rings records climatic changes, Miyahara's team were able to deduce that the sun must have been supercharged during this period.
"The high-precision data not only allowed us to accurately date sub-extreme solar proton events, but also lets us clearly reconstruct the solar cycles of the period," said Miyahara. "Today, the sun's activity fluctuates over 11-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."
Intriguingly, the aurora registered by Japanese and Chinese observers came when the solar cycle was winding down to its periodic minimum. This is contrary to what is normally expected, which is that enhanced space weather occurs when the Sun is more active, flinging off coronal mass ejections.
"This is unexpected, and we're excited to look further into what solar conditions could cause this," said Miyahara.
Studies of the isotopes carbon-14 and beryllium-10 in ice cores, tree fossils and sediments show that the solar cycle has varied in length and activity over the past 11,000 years, so the discovery that the cycle was shorter in the early part of the 13th Century is not necessarily a surprise. However, beginning around 1940, we have been in what had been thought to be the strongest period of solar activity for 9,000 years — that is, until this new discovery that suggests there was an even stronger spell in medieval times.
For all that we know about our nearest star, it is clear we still have much to learn, but history can be our guide to filling in the sun's secret past.
The findings are reported in 2026 Volume 102 Issue 4 of the journal Proceedings of the Japan Academy, Series B.
