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.
|Number of pages||9|
|Journal||Geophysical Journal International|
|Publication status||Published - 1996|
- Core-mantle boundary
- flow imaging