Geodynamo

(geophysics) The self-sustaining process responsible for maintaining the earth's magnetic field in which the kinetic energy of convective motion of the earth's liquid core is converted into magnetic energy.

The mechanism thought to be responsible for the generation of the Earth's magnetic field through the convection of conducting fluids in the Earth's core.

Paleomagnetic measurements suggest that the Earth has possessed a magnetic field for at least 3.5 billion years. Geophysicists generally accept that the ambient magnetic field measured at the Earth's surface is due to electric currents flowing in its liquid iron core (see illustration). In the absence of electromotive forces, like those of chemical batteries, electric currents will decay as magnetic energy is converted to heat. Without some regenerative process to offset such natural ohmic dissipation in the Earth's core, any electric currents and the associated magnetic field would vanish in about 15,000 years. Regeneration of the field is necessary. In the Earth it is thought that the magnetic field is maintained by dynamo action, whereby the kinetic energy of convective motion in the Earth's liquid core is converted into magnetic energy. Since this process operates without an external energy source, the geodynamo is said to be self-sustaining. See also Geoelectricity; Geomagnetism; Paleomagnetism.

Anatomy of the Earth. The rocky mantle has a radius a = 6371 km (3959 mi), the liquid iron outer core has a radius c = 3485 km (2165 mi), and the solid inner core has a radius b = 1215 km (755 mi). The Earth's rotational vector is Ο.

It is not obvious how a simply connected conducting fluid body, like the Earth's core, functions as a dynamo without the induced currents simply short-circuiting and eliminating field generation. In fact, the electric current in a dynamo and the magnetic field that it sustains cannot be too simple; a theorem, due to T. G. Cowling, says that no axisymmetric, or even two-dimensional, dynamo magnetic field can exist. Although the magnetic north and south poles usually are nearly coincident with the geographic poles, indicating that the rotation arising from the Coriolis force plays an important role in the core's dynamics, it is no accident that the compass does not point toward true north everywhere on the Earth's surface—an inherent lack of symmetry. As a result, theoretical progress has been slow since scientists often take advantage of symmetry, should it be present, when solving mathematical equations. See also Coriolis acceleration; Magnetohydrodynamics; Magnetomotive force.

Geophysicists do, however, have a good qualitative understanding of how the geodynamo works. In the 1940s and 1950s, W. M. Elsasser and E. N. Parker first elucidated the so-called α-ο (alpha-omega) mechanism, by which core fluid motion can act as a dynamo if it consists of a combination of differential rotation and convective helical motion. Since then it has been shown mathematically that dynamo regeneration can arise from the turbulent motion of a rotating fluid. Although the α-ω mechanism probably describes how the field is amplified, it is the dynamics that ultimately governs the strength of the field.

There are two possible sources of the energy sustaining the fluid convection in the outer core—thermal and compositional. Thermal convection is perhaps most familiar, with heat sources, such as radioactive potassium, distributed over the volume of the outer core. With sufficient internal heating, the fluid is gravitationally unstable and, as a result, convection is sustained. Compositional convection is currently favored by most geophysicists as the energy source of the geodynamo. Although the core is primarily of iron, there are probably light impurities, such as sulfur. Due to the effects of pressure, as the Earth slowly cools, iron solidifies at the inner-core boundary. This causes the inner core to grow and leaves the lighter constituents behind in the fluid at the base of the outer core, supplying the buoyancy that drives the convection. See also Convection (heat).

The Sun is a familiar dynamo, and it reverses regularly almost every 11 years. So, why does the Earth's magnetic field not display such regularity? The difference is thought to be due to the presence in the Earth of a solid electrically conducting inner core, where the magnetic field can change only rather slowly by diffusion. Recent calculations suggest that because the inner core is electromagnetically coupled to the outer core, its presence acts to stabilize the magnetic field, so that only particularly large fluctuations of the field in the outer core are sufficient to overcome the damping effect of the inner core. See also Diffusion; Electromagnetism; Geophysics; Magnetic reversals.