Solar Sources of Cosmogenic Isotope Variations

Raimund Muscheler [], National Center for Atmospheric Research, Climate and Global Dynamics Division, Boulder, Colorado; Juerg Beer and Maura Vonmoos, Swiss Federal Institute of Environmental Science and Technology (EAWAG); Fortunat Joos and Simon A. Mueller, Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland; and Ian Snowball, Geobiosphere Science Center, Quaternary Sciences, Lund University, Sweden.

    The production rates of cosmogenic radionuclides are influenced by the geo- and heliomagnetic shielding of the galactic cosmic rays. This relationship is well known and makes cosmogenic radionuclide records the most reliable recorders of solar activity variations extending the period of direct solar observations.

    There are two basic uncertainties in using cosmogenic radionuclide records to reconstruct past changes in heliomagnetic shielding (parameterized as solar modulation parameter):

I.The geomagnetic influence has to be known very well. For example, reconstructions of the intensity of the geomagnetic dipole field exhibit large errors during the Holocene that lead to correspondingly large uncertainties in the long-term changes in solar activity.

II.  Cosmogenic radionuclide records can be influenced by changes in climate. The problem is illustrated by two reconstructions of past changes in solar activity that are based on two different 10Be records. While Bard et al. (2000) conclude that recent solar activity is high but not exceptional, Usoskin et al. (2003) claim that the past 50 years are characterized by a phase of unusually high solar activity.

            The comparison of different ice core 10Be records with the tree-ring 14C data allows us to identify periods where the radionuclide records are influenced by changes in climate. This comparison illustrates the uncertainties that are involved using radionuclides to reconstruct past changes in solar activity. Nevertheless, the generally good agreement between different records indicates that we can infer robust estimates of past changes in solar modulation.