The geomagnetic secular‐variation timescale in observations and numerical dynamo models

The knowledge of the spatial power spectra of the main geomagnetic field and of its secular variation makes it possible to define typical timescales τ n for each spherical harmonic degree n . Investigating both observations and numerical dynamos, we show that a one‐parameter law of the form τ n = τ SV / n is satisfied for the non‐dipole field, given the statistical way the observed τ n are expected to fluctuate. Consequently, we determine the corresponding secular‐variation timescale τ SV from either instantaneous or time‐averaged spectra, leading to a value of 415 ± 45 55 yr for recent satellite field models. In the broader context of geomagnetic data assimilation, τ SV could provide a sensible and convenient means to rescale the time axis of dynamo simulations.