The alteration of the magnetic field of transient variations by an axisymmetric island in a large ocean

It is shown how the density of electric currents flowing in a horizontal conducting slab (the sea) may be calculated by B.G. Galerkin's method of moments, when a uniform electric field acts in the slab. The slab is uniform except for a hole, symmetrical about a vertical axis (an island). The cross-section of the hole may be arbitrary. Electric current streamlines are drawn for an island whose shape is a frustum of a cone. The magnetic field of the disturbing currents is numerically computed using successive over-relaxation. These results are compared with a two dimensional model which is a modification of the model due to Ashour and Chapman (1965). It is shown that introducing the vertical current component in the sea affects the magnetic field at the surface of the island by only a few per cent of the total magnetic field. The computer model is applied to the study of the island effect on Oshima and Jarvis Islands. The computed results for Oshima Island explain the observed enhancement of Zat some points and the reversal, in fact, of Z at other points on the island.