Research Article

No signature of extreme solar energetic particle events in high-precision ¹⁴C data from the Alaskan tree for 1844–1876 CE

¹ Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan
² National Museum of Japanese History, Sakura 285-8502, Japan
³ Institute for Advanced Research, Nagoya University, Nagoya 464-8601, Japan
⁴ Space Physics and Operations Division, RAL Space, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxfordshire OX11 0QX, UK
⁵ Nishina Centre, Riken, Wako 351-0198, Japan
⁶ Laboratory for Ion Beam Physics, ETH Zürich, Zürich 8093, Switzerland

^* Corresponding author: fmiyake@isee.nagoya-u.ac.jp

Received: 31 August 2023
Accepted: 19 November 2023

Abstract

Cosmogenic nuclides – ¹⁴C from tree rings and ¹⁰Be and ³⁶Cl from ice cores serve as an effective proxy for past extreme solar energetic particle (SEP) events. After identifying the first signature of an extreme SEP event in 774 CE, several candidates have been found in these proxy archives, such as 993 CE, 660 BCE, and 7176 BCE. Their magnitudes have been estimated to be tens of times larger than that of the largest SEP event ever observed since the 1950s. Although a detailed survey of such extreme SEP events is ongoing, the detection of intermediate-sized SEP events that bridge the gap between modern observations and extreme events detected in cosmogenic nuclides has not progressed sufficiently, primarily because of the uncertainties in cosmogenic nuclide data. In this study, we measured ¹⁴C concentrations in tree rings in the 19th century (1844–1876 CE) to search for any increases in ¹⁴C concentrations corresponding to intermediate-size extreme SEP events. We utilized Alaskan tree-ring samples cut into early and latewoods to suppress the potential seasonal variations in intra-annual ¹⁴C data. Notably, no significant ¹⁴C variations were observed between early and latewoods (0.0 ± 0.3‰), and the annual resolution ¹⁴C data series displayed an error of ~ 0.8‰. Over the entire study period, no significant increase in ¹⁴C concentrations characterized by other candidates of extreme SEP events such as the 774 CE event was detected in the annual ¹⁴C data. The present result imposes a constraint on the SEP fluence when the largest class of recorded solar storms occurred (especially those in 1859 CE and 1872 CE).