Determination of a steady velocity field in a rotating frame of reference at the surface of the Earth's core

Abstract

SUMMARY We relax the steady-motions theorem by solving for a steady velocity field at the surface of the core in a frame of reference drifting at a linear rate with respect to an observer fixed in the mantle frame of reference. We make the frozen-flux approximation, and compare the misfit of the secular variation (SV) predicted by the drifting velocity field with that from a steady velocity field fixed to the mantle frame of reference. The decrease of the misfit to the geomagnetic SV across the period 1960-80 is substantial, but is marginal across the interval 1930-60. The drift rate changes sign at the 1970 geomagnetic ‘jerk’ epoch, indicating a change in phase speed between the mantle and core flow. The marginal decrease in misfit prior to 1960 is inadequate to fit the SV data, necessitating a more complex drift function or perhaps a fully time-dependent flow. The results suggest that the SV is driven by deep-seated convection rather than from the core-mantle boundary.