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The Record of Solar Forcing in Cosmogenic Isotope Data
Raimund Muscheler, Department of Earth and Ecosystem Sciences, Division of Geology, Lund
University, Sweden
Cosmogenic radionuclides are the most reliable proxies for reconstructing solar activity variations
thousands of years back into the past. Several characteristics of solar activity variations have been
identified in these records. These range from longer-term solar cycles (e.g., the 207-year cycle), the
bundling of solar minimum periods and possible longer-term changes in solar activity. Recently an
increasing number of studies have attempted quantitative reconstructions of solar activity changes based
on radionuclide records. 5,6,7,8
Disagreements between some of the results illustrate the difficulties in isolating the solar signal
from ice core 10 Be and tree-ring 14 C records. The challenges consist in identifying influences of weather
and climate on cosmogenic radionuclide records. Failure to correctly identify a climate impact in
cosmogenic radionuclide records could lead directly to an erroneous inference of a solar influence on
climate. Moreover, the geomagnetic influence has to be corrected for. For quantitative estimates of
absolute solar activity levels one has to correctly normalize the records and, in addition, there are different
estimates of the intergalactic cosmic ray spectrum. Depending on the applied spectrum one can obtain
different results.
Figure C.2 illustrates the potential and the problems of cosmogenic radionuclide-based
reconstructions of solar activity changes. It shows a reconstruction of the solar modulation function based
on 10 Be and 14 C together with the group sunspot number reconstruction. The calculations are based on the
production results from Masarik and Beer (1999), 9 and the data are corrected for the geomagnetic dipole
field intensity variations (Yang et al., 2000). 10 Figure C.2 shows that especially long-term changes in
solar activity or absolute levels of solar activity are uncertain. Depending on the geomagnetic field
correction and the interpretation of the radionuclide records one can get significant differences in the
results. The same applies to the past 100 years, which is a crucial period for connecting the radionuclide
records to observational data. For this period the 14 C record includes the difficulty of an anthropogenic
influence due to fossil fuel burning (Suess, 1953) 11 and nuclear weapon tests. 10 Be records for the last
century show differences between Greenland and Antarctica that add to the normalization uncertainty
(Raisbeck and Yiou, 2004). 12
Nevertheless, there is an agreement between different radionuclide records for the
majority of the supposed solar minimum periods during the past 10,000 years (see Figure C.2) and there
is good qualitative agreement with the sunspot record. Improvement in geomagnetic field
reconstructions, new knowledge about the intergalactic cosmic-ray spectrum, additional radionuclide
5 R. Muscheler, F. Joos, J. Beer, S.A. Mueller, M. Vonmoos, and I. Snowball, Solar activity during the last 1000
yr inferred from radionuclide records, Quaternary Science Reviews 26(1-2):82-97, 2007.
6 S.K. Solanki, I.G. Usoskin, B. Kromer, M. Schüssler, and J. Beer, Unusual activity of the Sun during recent
decades compared to the previous 11,000 years, Nature 431:1084-1087, 2004.
7 F. Steinhilber, J.A. Abreu, and J. Beer, Solar modulation during the Holocene, Astrophysics Space Science
Transactions , 4:1-6, 2008.
8 M. Vonmoos, J. Beer, and R. Muscheler, Large variations in Holocene solar activity: Constraints from 10 Be in
the Greenland Ice Core Project ice core, Journal of Geophysical Research 111:A10105, 2006.
9 J. Masarik, J. Beer, Simulation of particle fluxes and cosmogenic nuclide production in the Earth's
atmosphere, Journal of Geophysical Research 104(D10):12099-12111, 1999.
10 S. Yang, H. Odah, and J. Shaw, Variations in the geomagnetic dipole moment over the last 12000 years,
Geophysical Journal International 140:158-162, 2000.
11 H.E. Suess, Natural radiocarbon and the rate of exchange of carbon dioxide between the atmosphere and the
sea pp. 52-56 in Proceedings of the Conference on Nuclear Processes in Geological Settings , (National Research
Council Committee on Nuclear Science, ed.), University of Chicago Press, Chicago, Ill., 1953.
12 G.M. Raisbeck, and F. Yiou, Comment on “Millennium scale sunspot number reconstruction: Evidence for an
unusually active Sun since the 1940s”, Physical Review Letters 92(19):199001, 2004.
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