Neutron Monitor Records in Broader Historical Context

Man-made neutron monitors have provided a continuous detailed record of the cosmic-ray flux over only about the last 5 decades. Fortunately, nature operates its own detectors and offers the opportunity to extend the cosmic-ray records over much longer time scales. Two different types of `natural detectors' can be distinguished. The first is based on long lived radionuclides that are produced by cosmic-ray interactions in the atmosphere and subsequently become stored in archives such as ice sheets or tree rings. The second type are rocks that are exposed to cosmic-rays at a certain time and from then on integrate the production of cosmogenic nuclides over the whole exposure time. The analysis of ¹⁰Be in polar ice cores and ¹⁴C in tree rings clearly reveals solar and geomagnetic modulation of the cosmic-ray flux on different time scales ranging from decades (11-year Schwabe cycle) to millennia. This revised version was published online in August 2006 with corrections to the Cover Date.