Oceanographic effects on the geomagnetic field

Abstract

Three classes of ocean water movements are identified as significant sources of motionally induced electromagnetic (EM) fields in the frequency range 10-3?10-1 c h-1. These are: the barotropic steady-state and mesoscale eddies; tides; and internal waves, particularly at the inertial frequency. Other water motions may contribute significant EM lields such as tsunami and surface waves (Larsen 1971). The electric field components are more sensitive to the motional induction than the magnetic components since the magnetic field is a spatial average of electric currents. The vertical component of the electric field (VEF) is not contaminated by ionospherically induced signals and represents a purely occanographic signal. When the ratio of ocean length scale (D) to ocean depth (h) is large (i.e. D/h >> 1) the VEF can be approximated by [see full text for equation] where Ez is the VEF, Bn is the horizontal north component of the earth's steady magnetic field. and Ve is the east-west velocity of the local fluid (Harvey 1972; Malkus & Stern 1952). The interpretation of the horizontal components of the electric field is more complex. The horizontal components of the electric current are in general non-zero and depend on the velocity field and, in the time varying case, on the con- ductivity structure of the mantle. The degree of mutual induction depends on the ratio of the skin depth (dm) of the mantle and the length scale (D) of the occan current flow (Sanford 1971). Small dm/D ratios indicate a strong coupling between mantle and ocean movements.