Earth's magnetic field: ocean current contributions to vertical profiles in deep oceans

The Earth's main magnetic field, arising in the core, should, in the ocean, have a well-defined uniform gradient with depth. Superimposed upon this un iform gradient may be magnetic signals due to a variety of sources. These i nclude crustal magnetization, the transient fluctuations arising external t o the Earth and causing secondary induced fields within it; and, the focus of the present paper, magnetic signals arising from the motional induction of seawater moving in the steady main magnetic field of Earth. There are ci rcumstances where theory predicts Such motionally-induced magnetic fields t o be of order 10(2) nT, and to vary with depth in a way which is directly r elated to the velocity profile. Exploratory soundings of the magnetic field with depth have been made in th e oceans around Australia, both to test these predictions, and to investiga te the practicability of measuring such profiles. The magnetic field parame ter observed has been that of the 'total field', which should sense the com ponent of the ocean velocity which lies in the magnetic meridian. The magne tometer has been lowered by cable from a ship and also operated free-fall t o the seafloor (and return). The observations appear both to confirm the th eoretical gradient of the main field where there is no ocean current and, w here ocean currents exist, to give evidence of their profiles resolved in t he direction of magnetic north. In particular, observations taken in an edd y of the East Australian Current show the correct contrast in sign for nort h and south flowing streams.