Japanese researchers used historical reports of the unusual red aurora that was observed over Kyoto (35 ° N) in the 18th century to pinpoint the strength of the associated geomagnetic storm. The geomagnetic storm of September 17, 1770 could be 3-10% stronger than the largest storm in 200 years in 1859, known as the Carrington Event. This study provides insights that could help prepare for an unlikely, but possible future, severe geomagnetic storm.
Historical documents are becoming more accessible as they come out of private collections around the world. Researchers at the National Institute of Japanese Literature and the National Institute for Polar Research have studied a detailed picture from a Japanese manuscript of Seikai ("Understanding Comets") with related commentaries describing the red glow that occurs in the sky over Kyoto on September 17, 1770. Detailed descriptions of this event from the recently discovered diary of the Higashi-Hakura family from Kyoto were also investigated.
The Seikai painting of the red radiance shows the radial structure of the stripes, including fine rays within the stripes. The lower part and eastern edges are somewhat darkened. The heading on the right can be translated as follows: "Red vapor was active over the northern sky on September 17, 1770 at midnight."
The Higashi-Hakura family's diary details the view of the zenith glare over Kyoto, and its position relative to the Milky Way.
Using astrometric calculations of the heights of the Milky Way, as seen from Kyoto on September 17, 1770, the researchers were able to calculate the geometry of the red aurora. The success of describing the aurora in accordance with historical documents allowed researchers to estimate the strength of the geomagnetic storm responsible for the aurora in September 1770.
“The magnetic storm of September 17, 1770 was comparable to or slightly stronger than the magnetic storm of September 1859. The storm of 1859 was the largest geomagnetic storm during which technological effects were observed,”says Ryuho Kataoka, a scientist at the National Institute for Polar Research. "We are fortunate that the storm of 1770 preceded our dependence on electricity."
So, how likely are similar geomagnetic storms? “We are currently in a period of declining solar activity, which could mean the end of serious geomagnetic storms in the near future,” says Kataoka. “However, we actually witnessed an extremely fast coronal mass ejection on September 10, 2017, which could be powerful enough to cause a violent storm.” Fortunately, he passed the Earth.