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 τn for each spherical harmonic degree n. Investigating both observations and numerical dynamos, we show that a one‐parameter law of the form τn = τSV/n is satisfied for the non‐dipole field, given the statistical way the observed τn are expected to fluctuate. Consequently, we determine the corresponding secular‐variation timescale τSV from either instantaneous or time‐averaged spectra, leading to a value of 415 ±4555 yr for recent satellite field models. In the broader context of geomagnetic data assimilation, τSV could provide a sensible and convenient means to rescale the time axis of dynamo simulations. Key Points The geomagnetic correlation times follow a universal behaviour This behaviour can be described by a simple one‐parameter law This parameter value allows for a consistent time‐rescaling of numerical models