HR: 1330h
AN: G22A-0301    [PDF]
TI: Electromagnetic Field Decadal Variations Generated by Zonal Oscillating Flows in the Earth's Core and Their Detectability by Using Submarine Cable Geoelectric Observations
AU: * Shimizu, H
EM: shimizu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Utada, H
EM: utada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Electric field observations by using 1,000 km scale submarine cables have been performed since early 1990's. One of the main purposes of the observations is to obtain observational constraints on the dynamics of Earth's core such as the strength and the distribution of the toroidal magnetic field and its variation at the core mantle boundary. Several constraints have been obtained until present, but the electromagnetic plausibility of them has not been examined. In this presentation, electromagnetic field variations generated by a simple spherical mean-field kinematic dynamo within an electrically conducting mantle are discussed. The field variations are assumed to be generated by perturbing a steady $\alpha^2$-dynamo with nearly cylindrically symmetric zonal oscillating flows (i.e. torsional oscillation type flows) having period of 30 years. It is confirmed that the kinematic dynamo can generate the observed amplitude of electric voltage variation ($\sim$100mV) naturally. The amplitude of voltage variation is controlled mainly by the energy state of the dynamo, i.e., the magnetic Reynolds numbers, and the strength of the toroidal field variation at the CMB is determined by the magnetic Reynolds numbers and the conductance of the D$^{\prime\prime}$ layer. Potential obstacles for the detection of the 100mV signal in 1,000km scale submarine cable voltage variations are the electric voltages induced by external magnetic field variations (magnetotelluric induction) and those induced by ocean flow (motional induction). Although the magnetotelluric current with decadal time scales seems negligibly small, the motional induced electric field variation can be as much as 100mV for 1,000km scale. It is necessary to know the time variation of ocean flux in order to discuss the electric voltages generated in the deep interior of Earth correctly.
DE: 1510 Dynamo theories
DE: 1560 Time variations--secular and long term
SC: Geodesy [G]
MN: 2003 Fall Meeting