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A new power spectrum and stochastic representation for the geomagnetic axial dipole


 Earth’s internal magnetic field is dominated by the contribution of the axial dipole whose temporal variations are wide ranging and reflect characteristic timescales associated with geomagnetic reversals and large scale paleosecular variation, ranging down to decadal and sub-annual field changes inferred from direct observations. We present a new empirical power spectrum for the axial dipole moment (ADM) based on composite magnetic records of temporal variations in the axial dipole field that span the frequency range 0.1/My to 5× 105/My (periods from 10 million to 2 years). The new spectrum is used to build a stochastic representation for these time variations, based on an order 3 autoregressive (AR) process and placed in the context of earlier stochastic modelling studies. The AR parameter estimates depend on the frequency of transitions in the spectral regime and may be influenced by Ohmic diffusion, advection, and torsional oscillations in Earth’s core. In several frequency ranges across the interval 200 to 5000 /My (5000 to 200 year periods) the empirical power spectrum lies above the AR3 model and may be influenced by Magneto-Coriolis (MC) waves in Earth’s core. The spectral shape and parameter estimates provide a potentially useful guide for developing assessments of whether numerical dynamo simulations meet criteria for being considered Earth-like.