A research group led by Associate Professor Hiroko Miyahara and Senior Researcher Ryuho Kataoka of the Okinawa Institute of Science and Technology Graduate University, and Professor Kazuaki Yamamoto of the National Institute of Japanese Literature, in collaboration with Yamagata University, Hirosaki University, and Nagoya University, has established a technical foundation for exploring past solar proton events ("SPEs") by combining historical documents with high-precision carbon-14 analysis.
They successfully identified that a SPE occurred between the winter of 1200 and the spring of the following year, a period when solar activity was extremely intense. This achievement is expected to contribute to improving the accuracy of space weather forecasting and the results were published in the Proceedings of the Japan Academy, Series B on April 10.
Intense solar activity triggers SPEs, where high-energy particles are ejected toward Earth at high speeds. While Earth's magnetic field blocks most high-energy protons from SPEs, the impact outside the magnetosphere and atmosphere is significant, raising concerns about radiation exposure for astronauts and failures of satellites. Similarly, solar flares caused by active solar phases lead to various problems such as power outages, as well as the observation of red "low-latitude auroras" on the ground.
While it has become possible to predict such magnetic storms to some extent, many points remain unexplained, and accurate forecasting has not yet been realized. Identifying and analyzing a large number of past SPEs is considered crucial for improving prediction accuracy, and searches for SPEs have been conducted primarily across the past 10,000 years. However, detection precision has been low, with only massive events occurring once every few thousand years being detectable. It is estimated that events roughly 10% of that scale occur more than ten times as frequently.
Previously, the research group established a foundation for efficiently capturing relatively small-scale SPEs, which are difficult to detect, in buried trees using an accelerator mass spectrometer at Yamagata University. However, this method is expensive, making comprehensive analysis over long historical periods difficult.
To address this, the group first focused on classical books (books from before the Meiji era) to narrow down the years when the sun might have caused an SPE. They searched for records of auroras and sunspots.
Their search of the diary Meigetsuki by Fujiwara no Teika and the Omuro Soshoki preserved at Ninna-ji Temple in Kyoto revealed descriptions of what appear to be low-latitude auroras occurring for three consecutive days from February 21 to 23, 1204. Records of sunspots and low-latitude auroras were also concentrated in the years 1200 and 1205 outside of Japan. While SPEs themselves are not directly linked to aurora occurrences, they often happen alongside the space weather phenomena that cause auroras.
Next, using samples of asunaro (thujopsis) excavated in northern Aomori Prefecture, the research group analyzed the carbon-14 content in the tree rings using their established method. This takes advantage of the fact that radiation generated by solar flares interacts with atomic nuclei in the atmosphere to create carbon-14, which is then incorporated into tree rings through photosynthesis.
A sudden spike in carbon-14 indicates a high probability that a solar flare occurred, and the scale and timing can be narrowed down to a period of about six months by accounting for the timing of photosynthesis. As a result, they confirmed for the first time that a large-scale SPE occurred between the winter of 1200 and the spring of 1201.
This event was approximately 20% of the scale of the largest known SPE (774-775 AD) and would have been difficult to detect with conventional precision. It is more than ten times larger than the largest SPE directly observed by humanity, which occurred in February 1956. Conversely, no increase was confirmed for the year 1204 mentioned in Meigetsuki. This suggests that sudden increases in carbon-14 are not always associated with SPEs.
By strictly estimating the impact of carbon-14 increases caused by SPEs, the researchers were also able to determine accurate solar activity cycles. They found that the solar cycle, which currently lasts 11 years, had shortened to 7-8 years at that time. Furthermore, while records of red low-latitude auroras are typically confirmed during the declining phase of solar activity, it was discovered that the low-latitude auroras in Meigetsuki occurred near a "valley" (minimum) in the solar cycle.
Miyahara said, "By performing concentrated analysis in combination with classical books, we have become able to detect large SPEs efficiently, or at least we have constructed the foundation for doing so. By continuing such analyses, we hope to acquire a vast amount of data on SPEs that are rare, occurring once every hundred or several hundred years, but relatively frequent. We expect this will provide clues to understanding the trends of when these events are likely to occur."
Journal Information
Publication: Proceedings of the Japan Academy, Series B
Title: Extremely active Sun from 1190 to 1220 in the Medieval Period: Intercomparison of historical records and tree-ring carbon-14
DOI: 10.2183/pjab.102.011
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.