Palaeomagnetic intensity data: an Achilles heel of solar activity reconstructions

Reconstructions of solar activity prior to AD 1600 rely on estimates of the past production rates of cosmogenic radionuclides in the Earth's atmosphere. These rates are modulated by the strength of the solar magnetic field and the intensity of the Earth's geomagnetic dipole. One of the largest uncertainties associated with these reconstructions originates from palaeointensity data and to show how errors in palaeomagnetic data influence reconstructions of solar activity we present different radiocarbon based reconstructions of the solar magnetic modulation parameter (Φ) during the Holocene. We considered geomagnetic field changes according to (i) a global archaeomagnetic compilation, (ii) a geomagnetic field model covering the last 7000 years and (iii) a regional sediment relative palaeointensity record. Comparison between alternative solar modulation reconstructions shows that the uncertainties in palaeomagnetic data prevent us from making robust conclusions about solar variability at timescales longer than a few hundred years. Our analyses indicate that the geomagnetic dipole field dominates radiocarbon production at timescales longer than 500 years although a combination of solar and geomagnetic modulation could apply to all timescales. Long-term trends depend on which palaeomagnetic data set is used for correction and there are additional unexplained differences between ¹⁰Be and ¹⁴C records that question results about long-term changes in solar activity. The higher-resolution geomagnetic field records, which are usually very much smoothed in stacked records, also indicate that part of the shorter-term changes (~500 yr) in the radiocarbon production rate could be attributed to the geomagnetic dipole field. To test this hypothesis and improve our knowledge of past solar activity a higher resolution palaeomagnetic reconstruction of geomagnetic dipole moment is required.