The dynamo effect is a geophysical theory that explains the origin of the Earth's main magnetic field in terms of a self-exciting (or self-sustaining) dynamo. In this dynamo mechanism, fluid motion in the Earth's outer core moves conducting material (liquid iron) across an already existing, weak magnetic field and generates an electric current. (Heat from radioactive decay in the core is thought to induce the convective motion.) The electric current, in turn, produces a magnetic field that also interacts with the fluid motion to create a secondary magnetic field. Together, the two fields are stronger than the original and lie essentially along the axis of the Earth's rotation.
The example presented in above figure illustrates how these factors might generate a self-sustaining magnetic field. Assume first (A) that there is present an initial poloidal magnetic field (one lying in meridian planes). Suppose next that the innermost part of the field line is embedded in a fluid rotating more rapidly than the outer parts of the fluid. In good conductors, magnetic field lines are nearly frozen into the fluid and have to move as the fluid moves. After many rotations a field line will "wrap up" around the rotation axis, creating a large toroidal field (one lying in planes perpendicular to the rotation axis). Since the conductivity is not perfect, the toroidal loop may diffuse through the fluid, disconnecting itself from the original poloidal field (B).
Next, consider the effect of radial fluid motion on the toroidal field. At various points in the liquid core, fluid is rising in cells driven by thermal convection. The rising fluid carries with it the toroidal magnetic field. As it rises, the Coriolis force deflects the fluid and causes it to spin around the central axis of the cell, thereby twisting the magnetic field. After a rotation of about 270 the magnetic field lines begin to twist about themselves and can diffuse through the conductor, disconnecting from the toroidal loop (C). At this stage, the rising loop is oriented in a meridian plane with the field pointing in the same direction as the original field--i.e., poloidal. Finally, small loops may merge into a single large loop, recreating the initial poloidal field (D). In cells of sinking fluid, the toroidal field wraps in the opposite direction and the poloidal loops have the opposite polarity. If the sinking process were exactly symmetrical, field loops produced in this manner would cancel loops created by rising fluid. Thus, for the process to create a net field of the correct sign, loops produced by sinking fluid must be weaker than loops resulting from rising fluid.
The dynamo theory was proposed by the German-born American physicist Walter M. Elsasser and the British geophysicist Edward Bullard during the mid-1900s. Although various other mechanisms for generating the geomagnetic field have been proposed, only the dynamo concept is seriously considered today.