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Abstract:
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-Hz field changes inferred from direct observations. We present a new empirical power spectrum for the geomagnetic field that spans frequencies from 10-15 Hz to 20 kHz (periods ranging from 10 million years to 5x10-5 seconds). This new spectrum is a composite based on axial dipole variations in time varying paleomagnetic field models, models based on direct measurements, and shorter period observations that encompass both internal and external field variations. Variations in the axial dipole field are used to build a stochastic representation 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 200–5000 /Myr (5000 to 200 yr 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.
