Cosmogenic Radiocarbon and the Solar Activity

Galactic cosmic-ray intensity is modulated by the change of the solar activity. When cosmic ray particles enter the earth's atmosphere, radiocarbons (C-14) are produced through nuclear interactions. Therefore, variation of the atmospheric C-14 reflects the change of solar activity. We have measured the C-14 concentration in tree rings with a time resolution of 1 year and obtained the information of solar activity variation in the past. The length of the Schwabe cycle in the sun has been examined using the C-14 content for the Maunder Minimum, which is one of grand minima of the solar activity. The result shows that the length of the Schwabe cycle in the Maunder Minimum was 14 years, but not 11 years as the average length of the present sun. Solar activity in other grand minima and some normal periods has also been investigated and the results are compiled.