3 The Proxy Method of Past Solar-Activity Reconstruction

In addition to direct solar observations, described in Section 2.2.1, there are also indirect solar proxies, which are used to study solar activity in the pre-telescopic era. Unfortunately, we do not have any reliable data that could give a direct index of solar variability before the beginning of the sunspot-number series. Therefore, one must use indirect proxies, i.e., quantitative parameters, which can be measured nowadays but represent different effects of solar magnetic activity in the past. It is common to use, for this purpose, signatures of terrestrial indirect effects induced by variable solar-magnetic activity, that is stored in natural archives. Such traceable signatures can be related to nuclear (used in the cosmogenic-isotope method) or chemical (used in the nitrate method – Section 5.2) effects caused by cosmic rays (CRs) in the Earth’s atmosphere, lunar rocks or meteorites.

The most common proxy of solar activity is formed by the data on cosmogenic radionuclides (e.g., 10Be and 14C), which are produced by cosmic rays in the Earth’s atmosphere (e.g, Stuiver and Quay, 1980Jump To The Next Citation PointBeer et al., 1990Jump To The Next Citation PointBard et al., 1997Jump To The Next Citation PointBeer, 2000Jump To The Next Citation Point). Other cosmogenic nuclides, which are used in geological and paleomagnetic dating, are less suitable for studies of solar activity (see e.g., Beer, 2000Jump To The Next Citation Point). Cosmic rays are the main source of cosmogenic nuclides in the atmosphere (excluding anthropogenic factors during the last decades) with the maximum production being in the upper troposphere/stratosphere. After a complicated transport in the atmosphere, the cosmogenic isotopes are stored in natural archives such as polar ice, trees, marine sediments, etc. This process is also affected by changes in the geomagnetic field and climate. Cosmic rays experience heliospheric modulation due to solar wind and the frozen-in solar magnetic field. The intensity of modulation depends on solar activity and, therefore, cosmic-ray flux and the ensuing cosmogenic isotope intensity depends inversely on solar activity. An important advantage of the cosmogenic data is that primary archiving is done naturally in a similar manner throughout the ages, and these archives are measured nowadays in laboratories using modern techniques. If necessary, all measurements can be repeated and improved, as has been done for some radiocarbon samples. In contrast to fixed historical archival data (such as sunspot or auroral observations) this approach makes it possible to obtain homogeneous data sets of stable quality and to improve the quality of data with the invention of new methods (such as accelerator mass spectrometry). Cosmogenic isotope data is the only regular indicator of solar activity on the very long-term scale but it cannot always resolve the details of individual solar cycles. The redistribution of nuclides in terrestrial reservoirs and archiving may be affected by local and global climate/circulation processes, which are, to a large extent, unknown for the past. However, a combined study of different nuclides data, whose responses to terrestrial effects are very different, may allow for disentangling external and terrestrial signals.

 3.1 The physical basis of the method
  3.1.1 Heliospheric modulation of cosmic rays
  3.1.2 Geomagnetic shielding
  3.1.3 Cosmic-ray–induced atmospheric cascade
  3.1.4 Transport and deposition
 3.2 Radioisotope 14C
  3.2.1 Measurements
  3.2.2 Production
  3.2.3 Transport and deposition
  3.2.4 The Suess effect and nuclear bomb tests
  3.2.5 The effect of the geomagnetic field
 3.3 Cosmogenic isotope 10Be
  3.3.1 Measurements
  3.3.2 Production
  3.3.3 Transport
  3.3.4 Effect of the geomagnetic field
 3.4 Towards a quantitative physical model
  3.4.1 Regression models
  3.4.2 Reconstruction of heliospheric parameters
  3.4.3 A link to sunspot numbers
 3.5 Solar activity reconstructions
 3.6 Verification of reconstructions
  3.6.1 Comparison with direct data
  3.6.2 Meteorites and lunar rocks: A direct probe of the galactic cosmic-ray flux
  3.6.3 Comparison between isotopes
 3.7 Summary

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