Cosmogenic Radionuclides as an Extension of the Neutron Monitor Era into the Past: Potential and Limitations

The cosmogenic radionuclides, Be-10, C-14 and others, provide a record of the paleo-cosmic radiation that extends > 10,000 years into the past. They are the only quantitative means at our disposal to study the heliosphere prior to the commencement of routine sunspot observations in the 17th century. The cosmogenic radionuclides are primarily produced by secondary neutrons generated by the galactic cosmic radiation, and can be regarded, in a sense, as providing an extrapolation of the neutron monitor era into the past. However, their characteristics are quite different from the man-made neutron monitor in several important respects: (1) they are sensitive to somewhat lower cosmic ray energies; (2) their temporal resolution is similar to 1 to 2 years, being determined by the rapidity with which they are sequestered in ice, biological, or other archives; (3) the statistical precision for annual data is very poor (similar to 19%); however it is quite adequate (similar to 5% for 22-year averages) to study the large variations (+/- 40%) that have occurred in the paleo-cosmic ray record in the past between grand solar minima and maxima. The data contains noise caused by local meteorological effects, and longer-term climate effects, and the use of principal component analysis to separate these system effects from production effects is outlined. The concentrations of Be-10 decreased by a factor of two at the commencement of Holocene, the present-day interglacial, due to a 100% increase in the ice accumulation rates in polar regions. The use of the Be-10 flux to study heliospheric properties during the last glacial is discussed briefly.