Extreme solar particle events of 775 CE, 994 CE, and 660 BCE are nearly two orders of magnitude stronger than those observed instrumentally. Because of the large observational gap between directly measured and historical events, it was unclear whether they can be produced by the Sun "normally" or from an unknown phenomenon. Recent works by Miyake et al. (2021, doi: https://doi.org/10.1029/2021GL093419) and Brehm et al. (2021, https://doi.org/10.1038/s41561-020-00674-0) start filling the gap with weaker yet extreme events approaching the detectability threshold. More such events are expected to be found in the future but the present result, if confirmed, would imply that the extreme solar events likely represent the high-energy/low-probability tail of the continuous distribution of solar eruptive events rather than a new unknown type of events. However, more statistic is needed for a solid conclusion. This would lead to better understanding of the processes producing such events that is important for their risk assessments for the modern technology. Plain Language Summary Hazards related to eruptive solar events such as flares, coronal mass ejections or particle storms are well-known during the recent decades and are studied by the Space Weather research discipline. However, as we know from historical proxy data, solar particle events (SPEs) can be a factor of 100 stronger than the directly observed ones and can potentially cause dramatic damages to modern technologies. With the huge observational gap between directly measured and historical events, it was not clear whether the latter can be produced by the Sun in a "normal" way or from an unknown phenomenon. A recent work by Miyake et al. (2021, https://doi.org/10.1029/2021gl093419) presents a new candidate for the extreme SPE dated to 5410 BCE discovered using high-precision measurements of radiocarbon in tree rings. Together with other recent results by Brehm et al. (2021, https://doi.org/10.1038/s41561-020-00674-0), it starts filling the gap. The result suggests that the extreme solar events likely represent the high-energy/low-probability tail of the continuous distribution of solar eruptive events. This would lead to a better understanding of the processes producing such events that is crucially important for assessments of the related risks for the modern technological society.
