ELECTRIC AND MAGNETIC FIELDS INDUCED BY DEEP SEA TIDES

A lunar semidiurnal variation (12·4206 h period) induced by tides in the deep ocean is demonstrated in (i) a month's observations of the horizontal electric field and magnetic declination at a sea floor site located 600 km off the California coast in 4·4 km of water, in (ii) a month's observations of the vertical magnetic field at a coastal site near Cambria, California, and in (iii) two years' observations of the vertical magnetic field at the island magnetic observatory on San Miguel, Azores. Comparison of the sea floor, coastal, and island observations with simultaneous continental magnetic observations permits an estimation of that part of the variation due to a lunar ionospheric oscillation. The observed oceanic induced variation at the sea floor and coastal sites are found to agree well with tidal induced fields computed for a model of the Earth's electrical conductivity and lunar semidiurnal tide. The model assumes (i) a flat semi‐infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (ii) non‐conducting atmosphere, continent and upper mantle, and (iii) superconducting mantle at a uniform depth beneath the ocean. The observed oceanic induced vertical magnetic field at San Miguel is found to agree qualitatively with tidal induced fields computed for an island model. This model assumes (i) a uniformly conducting small circular island, (ii) a flat infinite ocean of uniform depth and conductivity, rotating at a uniform rate appropriate to the latitude of interest, (iii) a non‐conducting atmosphere and upper mantle, and (iv) a superconducting mantle at a depth much greater than the size of the island. 1968 Royal Astronomical Society