Induction in the Oceans Explained and Analysed in Terms of Magnetic Merging

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Summary Geomagnetic induction in the oceans at high frequencies, corresponding to periods of less than 12 hr, say, for a typical ocean, is explained and analysed in terms of the transport and merging of magnetic field lines. The explanation helps our physical understanding of problems of induction in the oceans by micropulsations. The complete time-varying part of the electromagnetic field, excluding the Earth’s main field, is able to penetrate the conductor at high frequencies by transport of magnetic field lines through the agency of a large electric field near the edge. This electric field also tends to cause a large electric current there when the ocean has no edge slope. Magnetic neutral points in the time-varying part of the field form within the small thickness of the conductor. This enables the magnetic topology to pass through its various changes. The explanation leads to an analysis which applies to oceans of non-uniform depths, and in particular to oceans with an edge slope. A description is given of the way in which the electric current concentrates near the edge of the ocean and the width for this ‘ boundary layer’ is derived. In a previous paper (Hutson, Kendall & Malin 1972, henceforth referred to as HKM) an iterative method, believed to be new in the context of geomagnetism, has been suggested for dealing with the theoretical solution of model induction problems involving wide ranges of frequencies. Here we concentrate upon a physical analysis of the effects to be expected, particularly in the neighbourhood of a coastline. In keeping with HKM we shall try to retain a general approach, in the hope that the results will be found applicable to a wide range of problems with common features. Throughout this paper we exclude the Earth’s main magnetic field from the configuration. As the Earth’s main field varies very slowly with time, its effect on geomagnetic induction problems of the type considered here is negligible. When we refer to neutral points we therefore mean neutral points in the time-varying part of the magnetic field. In the next section we analyse induction phenomena for a particular example in terms of the merging and disappearance of magnetic field lines. In the later sections we shall draw general conclusions and make predictions. Throughout Sections 2-5 we restrict ourselves to the case of an isolated thin conductor with an inducing magnetic field imposed upon it whilst in Section 6, we examine the effects of an underlying superconducting layer. To discuss magnetic merging we shall have to take some account of the finite thickness of the conductor, but we shall otherwise assume that its thickness is negligible.