Near most coastlines the vertical component of the geomagnetic variation field is abnormally large and correlates positively with the inland horizontal component. This phenomenon is known as the geomagnetic coast effect. The few coastal locations at which it is absent are tectonically anomalous. The ratio of vertical to horizontal components decreases inland at a rate which depends on the geological nature of the continent. The effect varies only slightly with period, reaching a broad maximum between periods of 30 and 90 min. A number of models have been investigated by either calculation or analogue modeling, but the complete problem dealing with finitely conducting irregular oceans overlying a less conducting lithosphere has not yet been solved. Eddy currents flowing entirely in the seawater would be induced only by the vertical component of the magnetic field. Observations, combined with analogue model experiments, suggest that induction by horizontal components is also important. This requires flow of current in the lithosphere beneath the oceans.

https://www.leif.org/EOS/RG017i008p01999.pdf

The geomagnetic coast effect

In addition to a review of global induction studies of the Earth, the results of recent investigations of the electrical conductivity of the lunar interior are presented. Information on the magnetic environment of Venus, Mercury, Mars and Jupiter is tabulated, there being insufficient data to infer their electrical conductivity. Regional induction studies of the Earth's crust and upper mantle using geomagnetic deep-sounding and magnetotelluric techniques are summarised with a fuller discussion of studies in the UK and Kenya. The instrumentation used in observatory, field and laboratory studies is described.

http://iopscience.iop.org/article/10.1088/0034-4885/39/6/001/pdf

The electrical conductivity of the Earth and planets

The electric field in the beach along the First Narrows of the Strait of Magellan was recorded for 1 month. The observed signals correlate with direct tidal current measurements from the Narrows. As an aid to control of the heavy ship traffic through the strait the onshore electric field is shown to be a practical real time monitor for tidal currents.

Electric field recording of tidal currents in the Strait of Magellan

Summary.

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.

/pdf/induction-in-the-oceans-pf976gt1qc.pdf

N.J.W. Webber

Papers

Electric and magnetic fields associated with sea tides in the English Channel