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.