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Propagation of electromagnetic fields in the coastal ocean with applications to underwater navigation and communication

We examine the propagation of low‐frequency electromagnetic (EM) waves in the coastal ocean produced by controlled or motional impressed sources. Four important modes are the direct, up‐over‐down, down‐over‐up, and “beach” modes. The analyses of these modes are complicated by the varying bathymetry in the coastal region. We derive criteria to determine (1) which modes are important for given parameters; (2) a “matched phase” condition describing both when the up‐over‐down and down‐over‐up modes interfere constructively in the shallow zone and when the beach mode becomes important; and (3) a low‐frequency cutoff, below which the EM fields are not sensitive to the details of the coastal geometry. We verify the theoretically derived criteria with numerical examples and finally discuss the importance of our results in designing navigation and communications applications for subsurface vehicles and instruments.