Solar activity reconstructed over the last 7000 years: The influence of geomagnetic field changes

The long-term solar activity, as manifested by sunspot number, has been recently reconstructed on multi-millennium time scales by S.K. Solanki et al. (2004) from the measured concentration of C-14 in tree rings. The exact level of the reconstructed solar activity depends, however, on independently evaluated data of the geomagnetic dipole strength variations. Recently, a new series of the palaeomagnetic dipole moment reconstruction for the last 7000 years has been presented by M. Korte and C.G. Constable (2005a) on the basis of a thorough analysis of global samples. The new palaeomagnetic series yields a systematically lower dipole moment in the past, compared to the earlier geomagnetic reconstructions. We have revised the earlier sunspot activity reconstruction since 5000 BC, using the new geomagnetic data series, and found that it is roughly consistent with the previous results during most of the period, although the revised sunspot number values are in general higher. Nonetheless, it is confirmed with the new palaeomagnetic series that the Sun spends only 2-3% of the time in a state of high activity, similar to the modern episode. This strengthens the conclusion that the modern high activity level is very unusual during the last 7000 years.