Vertical electric field fluctuations at the floor of the Tasman Abyssal Plain

Abstract

A record of vertical electric field fluctuations occuring naturally in the ocean has been obtained at a site (36°14′S, 152°15′E) off the coast of easter Australia, on the floor of the Tasman Abyssal Plain in water of depth 4836 m. The free-fall and self-contained sea-floor instrument measured ambient voltage fluctuations between two low-noise silver-silver chloride electrodes connected by a vertical insulated wire 160 m long. The record returned is 107 days in duration, with readings taken every 28 1 8 s. The data obtained are interpreted in terms of east-west fluid current motion past the wire, causing electromagnetic induction with the horizontal north component of the earth's steady magnetic field. These motional fields are observed on a wide range of time scales. Spectral analysis of the data shows inertial oscillations and shorter period internal waves, with good agreement between observation and the theory of Chave (1984, Journal of Geophysical Research, 89, 10519–10528). There is also a peak in the power spectrum at the frequency of the semi-diurnal tides. The shortest period signals occurring are interpreted as due to some form of turbulence. Aperiodic motions on time scales of days are attributed to mesoscale activity in the East Australian Current system.