| Summary: | The main goal of this chapter is to demonstrate that 10 Be is a useful tool to reconstruct the history of solar activity. A comparison of neutron flux at the Earth's surface with the number of sunspots exhibits a clear negative correlation which is due to solar wind interaction with the galactic cosmic-ray flux. Since cosmogenic 10 Be production is proportional to the n-flux in the atmosphere, it also shows the inverse correlation with solar activity. Other sources of 10 Be variations in geologic reservoirs are changes of the geomagnetic dipole field and transport, and deposition processes within the atmosphere. Polar ice cores record the atmospheric fallout over the last ca 100,000 yr. Detailed 10 Be studies in these cores reveal the expected negative correlation and phase lag with sunspots. Periods of low solar activity, such as the Maunder minimum, are clearly reflected by higher 10 Be concentrations. In order to reduce the climatic signal introduced by atmospheric transport, 10 Be records from Greenland and Antarctica were combined and compared with 14 C tree-ring data. The generally good agreement of these comparisons strongly indicates that the main source of the short-term 10 Be variations is solar modulation of the cosmic rays and that several Maunder-minimum-type periods occurred during the last few millenia.
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