STRENGTH OF ELECTRIC CURRENTS AND ZONAL MAGNETIC-FIELDS AT THE TOP OF THE EARTHS CORE - METHODOLOGY AND PRELIMINARY ESTIMATES

A new method is described for estimating: (a) the meridional electric current density, jθ, (b) the vertical growth rate of the zonal magnetic field, ∂Bφ/∂r, or its scale-height, Bφ/∂Bφ/∂r) and (c) the vertical growth rate of the vertical current density, ∂jr/∂r, at a few isolated points on the top surface of the Earth's core from observations of the internal geomagnetic field at the Earth's surface. The theoretical technique rests on combining unaccelerated, gravitationally-driven Boussinesq fluid dynamics of the core with frozen-flux electromagnetism, the mantle being treated as a spherically symmetric insulator. Insertion into this theory of main field models for epochs 1965, 1975 leads to preliminary values for these quantities of magnitude: (a) jθ ∼ 1 A/m2, (b) ∂Bφ/∂r ∼ 10-6 T/m or Bφ/(∂Bφ/∂r) ∼ 10 m, (c) ∂jr/∂r ∼ 10-6 A/m3. Some geophysical implications of these estimates are discussed. © 1979.