Humans are often attracted by the splendor of aurora when looking up at the night sky, but outside the protective umbrella of the Earth's magnetic field, solar activity can be extremely lethal. Strong flares and high-energy charged particle eruptions are enough to pose fatal threats to astronauts and spacecraft. Recently, a research team from the Okinawa Institute of Science and Technology (OIST) in Japan successfully identified a solar proton event that occurred between 1200 and 1201 AD by combining precise measurements of carbon isotopes in tree rings with medieval document records, providing key clues for reconstructing past solar activity and improving future space weather forecasts.
The so-called Solar Proton Events (SPE) refer to the sun ejecting high-energy protons and other particles toward the earth during a violent explosion. These particles can fly towards the earth at speeds up to nearly 90% of the speed of light. In 1972, the sun experienced multiple such events between the Apollo 16 and Apollo 17 missions. If astronauts were performing missions in deep space at that time, they would likely have encountered fatal doses of radiation in the absence of effective shielding. As manned moon landings and more far-reaching manned spaceflight programs are put back on the agenda, how to identify and assess the risks of such "sudden solar storms" has become increasingly important.
The OIST research team adopted a new "interdisciplinary evidence collection" method this time: on the one hand, they selected buried Hinoki cypress (asunaro) wood samples unearthed from the Shimokita Peninsula in Aomori Prefecture, northern Honshu, Japan, and carried out high-precision measurements of the carbon 14 content in them year by year; on the other hand, they relied on the medieval Japanese aristocrat Fujiwara Sadaie's diary "Meigetsuki" and the aurora records in Chinese historical materials from the same period to lock in periods of suspected strong solar activity, and then "match points" to look for anomalies in the tree ring data.
Carbon-14 is a radioactive carbon isotope generated when high-energy particles bombard the earth's atmosphere. It enters plants with atmospheric circulation and is fixed in the tree rings of that year, which is equivalent to "leaving a negative" for that year's cosmic rays and solar activity. In the past, scientists have used this method to reconstruct the history of solar activity on a time scale of about 10,000 years. However, to capture "sub-extreme" solar proton events with slightly lower intensity and higher frequency requires both extremely high measurement accuracy and as narrow a time search window as possible.
This is where historical documents come into play. Fujiwara Sadake (1162–1241) recorded in his diary that in February 1204 AD, he saw "a red light appearing in the northern sky" in Kyoto. Although solar proton events themselves do not directly produce aurora, they are often accompanied by strong solar activity that can stimulate auroras. This record provides key clues to the research team. Based on this, they selected tree-ring samples from adjacent years for focused testing, and finally discovered a significant abnormal increase in carbon-14 between the winter of 1200 and the spring of 1201, indicating that a solar proton event of "sub-extreme" intensity occurred at that time.
In order to further pinpoint the year and verify the correlation, the team also used dendrochronology and dendroclimatology techniques to refine the time boundaries of the event through comparative analysis of tree ring width and climate patterns. In addition to Japanese documents, there are also rare records of low-latitude red auroras in Chinese history books from the same period. This is highly consistent with the time period indicated by the carbon-14 anomalies in the tree rings, further supporting the existence of this solar event.
Research leader Professor Yuko Miyahara from the Solar-Earth Environment and Climate Research Unit of OIST pointed out that previous research on historical solar proton events focused on a very small number of "super events", and the significance of this work is to provide a methodological basis for identifying more common but still obviously harmful "sub-extreme" events. The energy of such events is about 10% to 30% of the most extreme events known. Although it will not cause a global disaster, it is enough to pose severe challenges to the safety of orbiting satellites, deep space missions and future lunar bases.
Through high-precision carbon-14 measurements, the research team not only locked in a specific solar proton event, but also reconstructed the details of the solar activity cycle from around 1190 to 1220 AD. Analysis shows that, unlike today's solar activity cycle of about 11 years, the solar cycle at that time was only about 7 to 8 years, and it was in an extremely active stage. The solar proton event identified this time occurred at the peak stage of one of the cycles. This result provides important evidence for understanding the sun's activity patterns in different periods.
Professor Miyahara emphasized that carbon-14 data alone is not enough to fully restore the behavior of the sun, and must be mutually corroborated with sunspot and auroral observation records in historical documents. She pointed out that through comprehensive comparison of tree-ring data and literature records, researchers can more accurately reconstruct the timing of solar activity and thus better understand the conditions and characteristics that produce extreme space weather events. For example, the reconstruction results show that although this solar proton event occurred at the peak of solar activity, some long-lasting low-latitude auroras in history seem to have fallen near the bottom of their reconstruction cycles. This "abnormal" phenomenon implies that the sun may also trigger special space weather events through different mechanisms during the minimum period.