Geophysics

Modeling Earth's Dynamo

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Science  26 May 2000:
Vol. 288, Issue 5470, pp. 1301
DOI: 10.1126/science.288.5470.1301b

Earth's magnetic field is thought to be generated by fluid motions in the iron-rich, liquid outer core. The geodynamo represents a closed system in which a homogeneous medium of rotating conductive fluid creates electric and magnetic fields without any external forcing or ferromagnetism, unlike mechanical dynamos. Computer models have simulated the geodynamo, but experimental simulation has been challenging because of the large amount of fluid needed and the requirement that the velocity of the fluid motion be high enough to excite a magnetic field (at lower velocities, the trivial zero-field solution of magnetic induction equation holds).

Gailitis et al. have completed a large-scale experiment in the joint Latvian and German Dynamo Facility in Riga which shows the development of a self-excited magnetic field in a rotating fluid. The experiment uses 2 cubic meters of liquid sodium contained in a multiple compartment cylinder. In the central section of the cylinder, sodium is propelled downward in a spiral flow at about 15 meters per second; the backflow travels upward in a coaxial cylinder. When the propeller was rotated at 2150 rpm at 250°C, a self-exciting magnetic field was observed. This observation confirms the theory of liquid metal dynamos and offers a new method for tackling the complexities of Earth's dynamo from a distance.—LR

Phys. Rev. Lett.84, 4365 (2000).

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