BITES

 

Holocene solar activity inferred from global and hemispherical cosmic-ray proxy records

DOI: https://doi.org/10.1038/s41561-024-01467-5 (Nilsson et al. 2024)

Cosmogenic radionuclides, produced in the atmosphere by high energy cosmic rays and measured in well-dated and high-resolution natural archives, can be used to reconstruct variations in solar activity on Holocene timescales and longer. Extracting variations in solar activity from radionuclide data is challenging due to uncertainties in the shielding effect of the geomagnetic field, which is the other main factor controlling the radionuclide production rates. In this study, we show that accounting for differences in hemispherical production rates, related to geomagnetic field asymmetries, helps resolve so far unexplained differences in Holocene solar activity reconstructions. Based on our joint analysis of radionuclide and geomagnetic data, we find no compelling evidence for long-term variations in solar activity and show that variations in cosmogenic radionuclide production rates on millennial timescales and longer, including the 2,400-year Hallstatt cycle, are explained by variations in the geomagnetic field. Our results also suggest an on-average stronger dipole moment during the Holocene, associated with higher field intensities in the Southern Hemisphere.

 

Figure 1: Model predictions of variations in (a) geomagnetic dipole moment, (b) large-scale field asymmetry and (c) solar activity over the past 9000 years. Shaded areas represent 95% uncertainty ranges.