Time-dependent electromagnetic core-mantle coupling |
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| Authors: | M Stix PH Roberts |
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| Institution: | Kiepenheuer-Institut für Sonnenphysik, D-7800 Freiburg W. Germany;University of Newcastle, Newcastle upon Tyne, NE1 7RU Gt. Britain |
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| Abstract: | The magnetic field in the Earth's mantle is computed using a depth-dependent electrical conductivity, of form σ = σa(r/a)?α, and an approximation scheme in which the electromagnetic time constant of the mantle is assumed small compared with the time scales of the secular variation, and in which the induced currents and fields are obtained iteratively. We first associate the toroidal fields in the mantle with motions at the core surface (r = a) which create the observed geomagnetic field by flux rearrangement, and compute the resulting couple, Γ, parallel to the geographical axis. Using only zonal core motions, and values σa = 3 × 103ω?1m?1, α = 30 for the conductivity profile, we find that the toroidal induced fields create a couple, ΓT, that over most of this century has been roughly ten times greater than the poloidal part, ΓS, of Γ, and has the same sign. The total couple, Γ, has fluctuations of order 1018 Nm as required for the observed decade fluctuations in the length of the day. Its average is ～ ?1.5 × 1018 Nm, i.e., it is too large to remain unbalanced. We suppose that an equally important couple in the opposite sense is created by flux leakage from the core, and we estimate the necessary gradient of toroidal field in the core to be of order ?0.5 Gs km?1 at the core surface. During the course of the data analysis needed for the present work, we found some evidence for a torsional wave in the Earth's core with a period of ～ 60 y. |
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