A searching of past large Solar Proton Event by measuring carbon-14 content in tree-rings

Abstract:

Radiocarbon (¹⁴C) is produced by incoming cosmic rays to the Earth. The produced ¹⁴C becomes ¹⁴CO₂ and is absorbed by trees by photosynthesis. If a large Solar Proton Event (SPE) had occurred in the past, tree-rings would record such an event as a sudden ¹⁴C increase within 1-year.

Recently we found two signatures of large SPEs in AD 774-775 and AD 993-994 by the measurement of the ¹⁴C content in tree-rings. Some studies have estimated a scale of the AD 775 event as ten to dozens of times larger than the largest SPE on record. There is the possibility that a lot of such events are hidden in the periods when the ¹⁴C content has not been measured with a 1-year resolution. If we detect such events, we are able to discuss a detailed occurrence rate of the large SPE which is very important factor to prepare for future large SPEs.

We are planning to search for the past large SPEs by the measurements of ¹⁴C content in Japanese trees for this 5000 years. In this thesis, we are going to explain the plan and problems.