Poet's 1204 Aurora Was Real, but Not the Solar Radiation Blast Found in Tree Rings

Fujiwara no Teika wrote it down: on February 21, 1204, and for two nights after, red and white stripes stretched across the northern sky over Kyoto. The Japan Times The poet was not wrong. He was just writing about a different event than the one that actually matters.

Eight hundred years later, a team of researchers found the solar proton event — the actual radiation blast from the sun — that left a radioactive fingerprint in ancient Japanese tree rings. It was not in 1204. It was between winter 1200 and spring 1201 CE, three to four years before Teika picked up his brush. And when the team measured its scale, it came back 14 times larger than the February 1956 solar storm, which is the most powerful event we have ever recorded with instruments. OIST News Release Scientific American

The paper, published April 10, 2026 in the Proceedings of the Japan Academy, Series B, reconstructs the episode using two independent records that do not normally talk to each other: the diaries of a 13th-century Japanese poet, and the carbon-14 signature locked inside centuries-old trees. Neither record is complete on its own. Together, they tell a story that our space-age instruments cannot.

Solar proton events are bursts of high-energy particles thrown outward by the sun, the radiation hazard that makes deep space travel so difficult to survive. The particles strip nitrogen from the atmosphere, leaving a spike in carbon-14 that trees absorb and remember. The same particles would kill an unshielded astronaut. Smithsonian Magazine OIST News Release

But the 1200-1201 event was not the only notable finding. The researchers also found that solar cycles around 1200 ran just seven to eight years, compared with 11 today. That compressed rhythm would have made the sun more volatile overall. Proceedings of the Japan Academy, Series B Nine such extreme solar storms — called Miyake events after the scientist who first identified them — have been identified in the last 15,000 years, occurring roughly every 200 to 500 years. University of Leeds

The last confirmed Miyake event before the space age was in 993 AD. We have been watching the sun with instruments for roughly 70 years. That is not long enough to see the full range of what it can do. Scientific American

The 1972 episode illustrates the stakes. A string of solar proton events fired off between the Apollo 16 and Apollo 17 missions. NASA later calculated the radiation dose at 1,000 to 4,000 millisieverts, enough to be lethal. Neither crew was in space at the wrong time by luck alone. OIST News Release If a Miyake-class event struck today, power grid transformers could be permanently damaged and communications satellites knocked offline for months. University of Leeds

The combination is the contribution. Tree rings give physicists a geochemical record of the sun's behavior stretching back millennia. Historical documents give historians a record of what people actually saw in the sky. The researchers matched them by dating the carbon-14 spike to within a season, then cross-referencing against Japanese astronomical diaries from the same period. The result is a precise reconstruction of an event that left no obvious trace in any single source — only in the pairing of two. OIST News Release

Teika saw something real in the winter sky over Kyoto. But auroras and solar proton events are not the same phenomenon: the particles that cause auroras do not carry enough energy to produce the carbon-14 spikes. Smithsonian Magazine An aurora can appear without an SPE. An SPE can occur without producing a conspicuous aurora at mid-latitudes. Teika's diary is a record of the sky. The tree rings are a record of invisible radiation. They are reading two different books.

Some scientists caution that both events fall within the same extended active period of the sun — roughly 1190 to 1220 CE — and that a single super-active sun phase may be responsible for both the aurora and the carbon-14 spike, just at different times. Scientific American The paper's authors do not claim the two are unrelated. They identify them as separate in time but note they are consistent with a period of exceptional solar activity.

The practical question is what this means for models used to design radiation shielding for the Artemis program and for estimating the survivability of crewed deep space missions. Those models are calibrated on the space-age record: roughly seven decades of direct measurement. That record contains the 1956 event, the 1972 near-miss, and the Carrington Event of 1859, which remains the benchmark for extreme space weather. The 1200-1201 data point, if it holds, suggests the Carrington-class events may not be the ceiling. They may be the floor.

Sub-extreme solar proton events — smaller than Miyake-class storms but still large enough to be hazardous — occur more frequently than the space-age record implies and are more challenging to detect. ScienceDaily The authors' method, combining annual-resolution tree-ring dating with historical records, offers a way to find them, and a way to find the really large ones: the ones our instruments have simply not been watching long enough to catch.

The sun is not done.