Induction in the oceans

The induction of electric currents in the oceans is considered to be of major interest and may also throw light upon the conductivity structure of the mantle. Two aspects are considered: (1) induction by the solar quiet variations which have principal periods of 24, 12, 8 and 6 hr, and (2) induction by the ocean tides which principally have a semi-diurnal lunar period of 12.45 hr. Two classes of model may be devised: those in which the oceans are considered to be insulated from the mantle, and those in which there is a true electrical connection between the two. It appears that methods of calculating the electric currents are outstripping conductivity models. State of the art recommendations are made on what appears to be the best methods to use and these are demonstrated by using simple analogies. Particular attention is called to the Hewson-Browne technique for solving the integral equations which often emerge. It is noted that the formulations of the integral equations are variations of Weaver's method for plane configurations. Problems in which the ocean is isolated can be solved adequately by the zeroth order of the method of matched asymptotic expansions. Curiously this latter technique is simple in principle, involves only the numerical solution of a partial differential equation on the surface of the globe, and has been fecund in producing solutions for a variety of models. Induction by the ocean tides is also considered. Recommendations are made on the most promising lines appearing in present-day theories.