The largest known geomagnetic storm ever was found to be the cause of an enormous red aurora seen across Japan in the Edo period, a research group from the National Institute of Polar Research announced in an online publication in the academic journal Space Weather on Sept. 18.
Accounts of this aurora can be found all over the country. For instance, a painting of the red aurora fanning out radially with white bands appears in the old Kyoto text "Seikai" (star analysis). However, details about the scope of the event seen on Sept. 17, 1770, were previously unknown. The polar research team led by associate professor Ryuho Kataoka discovered a diary belonging to a member of the Higashihakura family in the collection of Azumamaro Shrine in Kyoto's Fushimi Ward that recorded a detailed description of the aurora.
"The red clouds covered half of the northern sky, encroaching on the Milky Way," it was written in the diary. "Half of the sky was enveloped in red clouds. A sliver of white clouds penetrated the Milky Way." Along with these and other descriptions, the positioning and the scale of the aurora were also described in detail.
The group used the diary as well as geomagnetic data from the time to create a computer simulation of how the aurora would have appeared to Kyoto residents, and the resulting shape was a perfect match to what was reported.
Auroras occur due to geomagnetic storms caused by solar flares and other extraterrestrial sources disturbing the Earth's geomagnetic field. The storm simulated by Kataoka's team is estimated to be larger than the current record-holding storm that occurred in 1859.
"I'm surprised that we were able to simulate (the storm) with this much accuracy using old paintings and writings alone," Kataoka said.