Japanese researchers used historical accounts of the unusual red aurora that was observed in the sky over Kyoto (35 ° N) in the 18th century, to clarify the strength of the geomagnetic storm associated with it. The geomagnetic storm on September 17, 1770, may be 3-10% stronger than the most powerful 200-year-old storm of 1859, known as the “Carrington Event.” This study gives an idea that can help prepare for an unlikely, but possible future strong geomagnetic storm.
Historical documents are becoming more accessible, as they come out of private collections around the world. Researchers from the National Institute of Japanese Literature and the National Institute of Polar Studies, studied a detailed picture from the Japanese manuscript “Seikai” (“comprehension of comets”) with the corresponding comments describing the red radiance that occurs in the sky over Kyoto on September 17, 1770. Detailed descriptions of this event were also investigated from the recently discovered Diary of the Higashi-Khakur family from Kyoto.
In the picture of the red radiance from the Seikai manuscript, a radial stripe structure is shown, which includes small rays inside the bands. The lower part and the eastern edges are somewhat darkened. The title on the right side can be translated as follows: “On September 17, 1770, at midnight, the red sky was active above the northern sky.”
In the diary of the Higashi-Hakur family, the appearance of the anti-aircraft glow over Kyoto and its position relative to the Milky Way are described in detail.
Using astrometric calculations of the heights of the Milky Way, as it was seen from Kyoto on September 17, 1770, the researchers were able to calculate the geometry of the red aurora. The success of the description of the aurora in accordance with historical documents allowed the researchers to estimate the power of the geomagnetic storm responsible for the radiance in September 1770.
“The magnetic storm of September 17, 1770 was comparable to the magnetic storm of September 1859, or was slightly stronger than it. The storm of 1859 became the strongest geomagnetic storm during which technological effects were observed, “says Ryuho Kataoka, a scientist from the National Institute of Polar Research. “We were fortunate that the storm of 1770 preceded our dependence on electricity.”