Propagation Parameters of Sea Surface Waves Inferred from Observations from Two Closely Spaced Vector Magnetometers.

Abstract

The alternating magnetic dynamo field of sea surface waves, a consequence of their Lorentz electric field, has been observed with a pair of simultaneously operated, closely spaced tri-axial magnetometers. Measurements from a magnetometer located in the centre of a tiny, uninhabited island served to compensate measurements from a near-shore magnetometer for magnetic pulsations of ionospheric origin, leaving the water wave dynamo field, effective close to shore only, as the dominant residual magnetic field. Amplitude and frequency of waves and swell were recorded with a vertical accelerometer (wave rider buoy), floating on the sea surface. The wave rider data are in good agreement with those obtained from the magnetometers. Amplitude and phase relations between the three vector components of the magnetic oscillations yield a sea surface wave vector which is consistent with the swell propagation direction usually found in that area. The magnetic field data further demonstrate that the water mass motion close to shore was not confined to a vertical plane (as would be the case for freely propagating gravity waves in the open ocean). The motion rather took place in a plane inclined at about 40° from the horizontal, which is roughly twice the inclination of the island flanks. We conclude that the magnetometer measurements yield a reasonably accurate description of the surface wave water motion within about one wavelength from shore.

Sea-surface observations of the magnetic signals of ocean swells

“…The magnetic fields of ocean waves have been the subject of theoretical investigation by Longuet-Higgins et al (1954), Weaver (1965), Beal & Weaver (1970), Podney (1975), Chave (1984) and Weaver (1997). Maclure et al (1964) and Ochadlick (1989) report measurements in agreement with the theory of Weaver (1965) and observations of the magnetic signals of ocean waves are also reported by Fraser (1966) and Watermann & Magunia (1997). The topic is naturally closely associated with the electric signals of such waves, which are studied in papers such as Cox et al (1978) and .…”

Apparent Aeromagnetic Wavelengths of the Magnetic Signals of Ocean Swell

“…Maclure et al (1964) and Ochadlick (1989) report measurements in agreement with the theory of Weaver (1965), and observations of the magnetic signals of ocean waves are also reported by Fraser (1966) and Watermann and Magunia (1997). In particular, Ochadlick (1989) reports aeromagnetic measurements over deep water.…”

Analysis of the Magnetic Signature of Surface Waves Measured in a Laboratory Experiment

“…In the 1950s, the electromagnetic field fluctuations resulting from the movement of seawater across Earth's magnetic field caught the attention of physical oceanographers. This sparked research that led to the development of the theory of electromagnetic fields caused by ocean currents and surface and internal ocean waves based on Maxwell's equations (Von Arx 1950;Longuet-Higgins et al 1954;Crews and Futterman 1962;Maclure et al 1964;Warburton and Caminiti 1964;Weaver 1965;Fraser 1966;Larsen 1973;Beal and Weaver 1970;Sanford 1971;Podney 1975;Ochadlick 1989;Watermann and Magunia 1997;Lilley et al 2004). Podney (1975) did an extensive theoretical summary of electromagnetic fields generated by surface and internal waves.…”