Solar superstorm of AD 774 recorded subannually by Arctic tree rings

• Published in: Nature communications 2018-08, Vol.9 (1), p.3495-8, Article 3495
• Main Authors: Uusitalo, J., Arppe, L., Hackman, T., Helama, S., Kovaltsov, G., Mielikäinen, K., Mäkinen, H., Nöjd, P., Palonen, V., Usoskin, I., Oinonen, M.
• Format: Article
• Language: English
• Subjects: 639/33/34; 639/33/525/870; 704/445/823; Atmosphere; Carbon; Carbon 14; Carbon cycle; Carbon fixation; Comets; Cosmic rays; Dependence; Geographical variations; Humanities and Social Sciences; Hypotheses; multidisciplinary; Photosynthesis; Science; Science (multidisciplinary); Solar storms; Tree rings
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-05883-1

Recently, a rapid increase in radiocarbon ( 14 C) was observed in Japanese tree rings at AD 774/775. Various explanations for the anomaly have been offered, such as a supernova, a γ-ray burst, a cometary impact, or an exceptionally large Solar Particle Event (SPE). However, evidence of the origin and exact timing of the event remains incomplete. In particular, a key issue of latitudinal dependence of the 14 C intensity has not been addressed yet. Here, we show that the event was most likely caused by the Sun and occurred during the spring of AD 774. Particularly, the event intensities from various locations show a strong correlation with the latitude, demonstrating a particle-induced 14 C poleward increase, in accord with the solar origin of the event. Furthermore, both annual 14 C data and carbon cycle modelling, and separate earlywood and latewood 14 C measurements, confine the photosynthetic carbon fixation to around the midsummer. Tree rings retain information of sudden variations of ancient radiocarbon ( 14 C) content, however the origin and exact timing of these events often remain uncertain. Here, the authors analyze a set of Arctic tree rings and link a rapid increase in 14 C to a solar event that occurred during the spring of AD 774.