Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004geoji.159..565l&link_type=abstract
Geophysical Journal International, Volume 159, Issue 2, pp. 565-572.
Astronomy and Astrophysics
Astronomy
1
Magnetic, Motional Induction, Ocean, Signals, Swell, Waves
Scientific paper
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 13s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, combined with motional induction theory.
Hitchman A. P.
Lilley E. M. F.
Milligan P. R.
Pedersen Ted
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