Sea-surface observations of the magnetic signals of ocean swells

F. E.M. Lilley*, Adrian P. Hitchman, Peter R. Milligan, Tina Pedersen

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    35 Citations (Scopus)


    Ocean swells have a magnetic signal, caused by the motional induction of sea water moving in the steady main magnetic field of Earth. To check the character of such signals at the sea surface, a magnetometer has been set free from a ship to float unrestricted on the surface of the ocean for periods of several days. The path of the floating magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship. Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetization, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically up to 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metres trough-to-peak in height. The power spectra of the magnetic signals show a consistent peak at period 13 s, appropriate for the known characteristics of ocean swell in the area. The power spectra then exhibit a strong (-7 power) fall-off as period decreases below 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory.

    Original languageEnglish
    Pages (from-to)565-572
    Number of pages8
    JournalGeophysical Journal International
    Issue number2
    Publication statusPublished - Nov 2004


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