Researchers have uncovered evidence of a powerful solar proton event that struck Earth around 1200 ce by analyzing carbon-14 spikes in ancient Japanese trees and cross-referencing medieval records of red auroras.
The study, led by scientists at the Okinawa Institute of Science and Technology, identified the sub-extreme event as occurring between the winter of 1200 ce and the spring of 1201 ce. It combined ultra-precise carbon-14 measurements from buried asunaro trees in Aomori Prefecture with historical accounts, including descriptions of red lights in the northern sky from a Japanese diary and similar reports from China.