Irregular coastline and channeling effects in three-dimensional geomagnetic perturbation models

Two models, one with two small islands in a relatively narrow channel between two larger land masses and the other with an island near an irregular coast, are investigated with a view to assessing the importance of small-scale features. The perturbation of a uniform alternating geomagnetic field of various frequencies is studied by a numerical technique in which the inhomogeneous models are represented by cells of different conductivity within a three-dimensional mesh. The results are presented as contour plots of the amplitudes of the various field components over the surface of the conducting region, as well as by selected profiles of the field component amplitudes and phases. The channeling of currents around the higher resistivity island structures is apparent in the results. The effects of the small-scale structure of the coastline are pronounced in the model, and indicate that considerable care needs to be taken in selecting sites for experiments and in interpreting the results.