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

Reconstruction of solar activity for the last millennium using $^\mathsf{10}$Be data