Constraining electromagnetic core-mantle coupling

Electromagnetic core-mantle coupling is one candidate for explaining length of day (LOD) variations on decadal time scales. This coupling has traditio nally been calculated from models of core surface now (U) over right arrow, but core flow inversions are nor unique. Unfortunately, the main torque con tribution depends on the toroidal part del x (r) over right arrow Phi, of t he vector field ((U) over right arrow B-r), which is left undetermined by t he radial induction equation that links core flows to magnetic field data u nder the frozen-flux hypothesis. (B-r is the radial magnetic field at the c ore surface.) We have developed a new method that bypasses flow inversions. Using the vector field ((U) over right arrow B-r) directly, we estimate it s toroidal part by imposing that ((U) over right arrow B-r) vanishes where B-r = 0. The calculation of the electromagnetic torque is also hindered by the uncertainty in the conductivity structure of the lower mantle. However, we conclude that the conductivity of the lower mantle has to be much large r than current estimates derived from diamond anvil cell experiments to mak e the electromagnetic torque significant. The region beneath Africa and the mid-Atlantic (where there is the most significant (B-r = 0) curve apart fr om the magnetic equator) is particularly important. Here, the field Phi is relatively well constrained, and this is the single region that contributes most to the torque. Following Holme [Holme, R., 1998a. Electromagnetic cor e-mantle coupling 1: explaining decadal changes in the length of day. Geoph ys. J. Int. (132) 167-180.] we show nevertheless that small changes to Phi suffice to recover the torque Gamma(LOD) required to explain LOD variations provided that the lower mantle conductivity is high enough. We find a valu e similar to Holme [Holme, R., 1998b. Electromagnetic core-mantle coupling 2: probing deep mantle conductance. In: Gunis, M., Wysession, M.E., Knittel , E., Buffett, B.A. (Eds.), The Core-Mantle Boundary Region. AGU, pp. 139-1 51.] for the minimum conductance (10(8) S). However, we were not able to si multaneously recover Gamma(LOD) and constrain (U) over right arrow to be ta ngentially geostrophic. (C) 1999 Elsevier Science B.V. All rights reserved.