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

Cosmogenic nuclides – 14 C from tree rings and 10 Be and 36 Cl 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 14 C concentrations in tree rings in the 19th century (1844–1876 CE) to search for any increases in 14 C 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 14 C data. Notably, no significant 14 C variations were observed between early and latewoods (0.0 ± 0.3‰), and the annual resolution 14 C data series displayed an error of ~ 0.8‰. Over the entire study period, no significant increase in 14 C concentrations characterized by other candidates of extreme SEP events such as the 774 CE event was detected in the annual 14 C 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).