The telluric field in northern Germany induced by tidal motion in the North Sea

SUMMARY By Faraday's law of induction tidal currents of the North Sea induce electric currents which—to a certain extent—spread out far inland. Their influence on the telluric field in northern Germany was investigated. Time variations of the telluric field at six field sites in northern Germany were recorded continuously for at least 2 months during 1984 together with variations of the magnetic field at Göttingen. Transfer functions between the telluric and magnetic fields were calculated from the Dst‐ and D‐continuum from 0.5 to 8 cpd (cycles per day). They are presented as continuous functions of frequency by orthogonal polynomials. This serves (a) to estimate the local telluric DC‐distortion matrix by comparing the transfer function values of the Sq‐harmonics at 1, 2, 3, 4 cpd with undistorted impedances obtained by Schmucker's Sq‐analysis, (b) to eliminate that part of the induced telluric field which is correlated with magnetic field variations. In the uncorrelated part of the spectra a significant peak appears at the lunar frequency M2 (= 24/12.4206 cpd). The Chapman‐Miller method yields amplitude and phase of the M2‐variation in the uncorrelated telluric fields. Local influences are eliminated using the inverse of the distortion matrices mentioned above. The amplitudes then vary between 0.3 and 0.6 mV km ‐1 . The direction and phase of the uncorrelated telluric field at five field sites reflect well the pattern of the tidal current in the North Sea along the coast and coincide with the assumed motion‐induced electric currents.