A study of the solar activity variability in the past millennium by precise radiocarbon measurement

本项目旨在利用加速器质谱法，利用太阳活动极小期树木年轮的放射性碳，研究近千年来太阳活动的变化和行星际磁场的结构。为此，我们精心制备样品，高精度地测量放射性碳含量，并试图找到sperer极小期和Maunder极小期放射性碳含量的微小变化。采用名古屋大学的加速器质谱计（AMS）。我们注意到在样品制备和AMS操作中如何实现高精度测量。本项目构建的系统制备的样品质量高、均匀性好，通过对标准样品的评价表明，该系统可达到0.2%的高精度。在实际测量中，总体误差取决于AMS的测量时间和稳定性，由统计误差决定。虽然由于机器的维护使得长时间的测量和大的目标电流难以达到高精度，但对相同的树木年轮样品进行重复测量可以实现高精度。我们高精度地测量了蒙德极小期和斯波尔极小期树木年轮样品的放射性碳含量。对时间序列数据的分析使我们了解太阳活动极小期宇宙射线通量的变化。结果表明，即使在大极小期，太阳活动也存在11年的变化，但在深极小期，11年的变化可能较弱，极小期的周期长度可能较长，太阳磁场有反转的迹象。我们还测量了公元1000年左右的放射性碳含量。我们发现了一个可能是由于11年太阳活动变化引起的周期结构，并讨论了超新星爆炸对AD1006的影响的可能性。

This project aimed to investigate variability of solar activity and structure of interplanetary magnetic field in the last millennium by radiocarbon in tree rings during grand minima of solar activity with accelerator mass spectrometry. For this purpose we prepared samples and measured radiocarbon content with great care on high precision and tried to find small change of radiocarbon content in the Spoerer and Maunder Minima. An accelerator mass spectrometer(AMS) of Nagoya University was used.We took care how high precision measurement could be achieved in preparation of samples and also in operation of AMS. We could prepare samples with high quality and uniformity by using the system constructed in this project and showed that high precision of 0.2% is achievable by evaluation of standard samples. In actual measurement overall error is determined by statistical error depending on measurement time and stability of AMS. Although the machine maintenance made it difficult to try long measurements and large target current for high precision, repeat of measurement for the same tree ring samples could realize the high precision.We measured radiocarbon content in tree ring samples from the Maunder Minimum and the Spoerer Minimum with high precision. Analysis of time sequential data gives knowledge on variation of cosmic ray flux during the solar activity minima. It was revealed that the 11-year variation of solar activity exists even in the grand minima, however, the 11-year variation could be weak in deep minima, the cycle length, which is normally 11 years, might be possibly longer in minima, and signs of reversal of solar magnetic field are seen. We also measure radiocarbon contentin the period of around AD1000. We found a periodic structure probably due to 11-year solar activity change and discussed a possibility of the effect of supernova explosion in AD1006.