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Abstract

Earth’s electromagnetic response has been restricted to periods shorter than one year, except for an early estimate by Harwood & Malin for the 11-year sunspot cycle. This restriction arose because the separation of the internal and external fields remains challenging for periods between 100 days and 11 years. We have extracted external dipole field variations and their inductive response from 117 years of hourly geomagnetic data from 192 observatories. Multi-taper spectral analysis and band averaging in frequency was used to estimate power spectral density and transfer functions between the two time series. A clear peak at the 11-year solar cycle appears in the external field spectrum, along with a smaller but discernible peak at the first harmonic of 5.5 years and associated signatures in the coherency spectrum. Error bars on the transfer function were estimated from the statistics of the cross-spectrum and a parametric bootstrap, and the data can be fit via a smooth 1D Occam inversion to recover electrical conductivity with depth. A Bayesian inversion approach allows an assessment of conductivity jumps in the models, detecting a highly conductive core required by the data. The well-documented jump in conductivity between the upper and lower mantle is clearly seen, and a second jump in conductivity at depth of 1,800 km suggests another change in mineralogy or conduction mechanism. Our estimate of lower mantle conductivity can be used to quantify core-mantle coupling and the filtering effect on rapid variations in the magnetic field originating in the core.