The geomagnetic coast effect

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.