In a recent paper (Usoskin et al., 2002a), we have reconstructed the
concentration of the cosmogenic 10Be isotope in ice cores from the
measured sunspot numbers by using physical models for 10Be production in
the Earth's atmosphere, cosmic ray transport in the heliosphere, and evolution
of the Sun's open magnetic flux. Here we take the opposite route: starting from
the 10Be concentration measured in ice cores from Antarctica and
Greenland, we invert the models in order to reconstruct the 11-year averaged
sunspot numbers since 850 AD. The inversion method is validated by comparing
the reconstructed sunspot numbers with the directly observed sunspot record
since 1610. The reconstructed sunspot record exhibits a prominent period of
about 600 years, in agreement with earlier observations based on cosmogenic
isotopes. Also, there is evidence for the century scale Gleissberg cycle and a
number of shorter quasi-periodicities whose periods seem to fluctuate in the
millennium time scale. This invalidates the earlier extrapolation of
multi-harmonic representation of sunspot activity over extended time intervals.Comment: Submitted to A&