Seeking a seafloor magnetic signal from the Antarctic circumpolar current

Abstract

Motional electromagnetic induction by ocean currents is a basic phenomenon of geophysics, with application to the monitoring of ocean transport. One of Earth's strongest ocean currents is the Antarctic circumpolar current (ACC). This paper explores the magnetic signals that should be generated by the ACC, and reports an experiment in which a magnetometer recorded natural variations of Earth's magnetic field on the floor of the Southern ocean for some five months in 1996. Magnetometer records from Kingston, Tasmania, and from Macquarie Island give reference information concerning magnetic storms and substorms. The instrument was sited in the region of the major oceanographic subantarctic flux and dynamics experiment (SAFDE), where the ACC passes south of Tasmania, between the major topographic features of the South Tasman rise and the Australia–Antarctica spreading ridge. The SAFDE records give comprehensive control on the actual ocean current flow at the time of the magnetic recording, and allow a magnetic signal to be predicted, in terms of the seafloor conductance. The seafloor conductance for the area is however low, and the amplitude of the predicted signal is low. The seafloor observations confirm that the signal is weak against the effects of ionospheric signals. In future experiments, the choice of sites with thicker seafloor sedimentation would increase the ACC magnetic signal to be observed. The magnetometer measurements have a result relevant for the SAFDE, in confirming that the correction of electric data for seafloor conductance is small. There is also the result for seafloor magnetic observatories for which motional induction effects are unwanted, that such an observatory can operate even under the ACC, and be substantially protected from motional induction effects by low seafloor conductance.