Abstract Carbon-14 ( 14 C) analysis of tree rings has revealed multiple rapid increases in 14 C ( 14 C spikes) during the Holocene, e.g., in ca. 774 CE, 993 CE, 660 BCE, and 7176 BCE. These 14 C spikes have been replicated by several tree-ring 14 C records and other cosmogenic nuclide records, such as 10 Be and 36 Cl from ice cores. These events are considered to have been caused by extreme solar energetic particle (SEP) events, with the scale of the SEP events estimated to be several dozen times larger than the largest SEP event detected in modern observations. As such, studies on past SEP events using cosmogenic nuclides have advanced in recent years; however, several issues regarding such events remain unknown. This paper reviewed recent studies on the following topics: 1) further exploration of extreme SEP events over the long term, and 2) exploration of intermediate-sized SEP events between those detected by modern observations and the extreme SEP events detected by cosmogenic nuclides. Future research directions for these topics were also discussed.
Fusa Miyake (Sun,) studied this question.