Cosmogenic Evidences for Past SEP Events
2019 July 22
Terrestrial cosmogenic nuclides (such as 14C, 10Be, and 36Cl) are mainly produced by galactic cosmic rays. In contrast, a certain amount of these nuclides is also produced by solar energetic particles (SEPs) derived from sporadic solar events such as solar flares and CMEs. Cosmic ray increase events in AD 774/775, AD 993/994 (or 992/993), and ~BC 660 have been discovered so far using 14C data in tree rings (Miyake et al. 2012, 2013, Park et al. 2017). It is considered that the most plausible cause of these events is an extreme SEP event whose energy spectrum is very hard based on 14C analyses of tree rings and 10Be and 36Cl analyses of ice cores (e.g. Mekhaldi et al. 2015, Miyake et al. 2015, 2019, Büntgen et al. 2018, O'Hare et al. 2019). These SEP events are estimated to be several dozen times larger than the largest events found in direct observation, which might have a serious impact on modern society. Therefore, it is important to investigate an occurrence rate of past extreme events. In recent years, surveys of past SEP events are actively conducted by cosmogenic nuclides measurements with high time resolution (~1-year resolution). Here, the detected past SEP candidates and a further survey of similar events are reviewed. This manuscript will be updated by 31 August.