The Geodynamo Theory

The first important contributions to dynamo theory were made by Edward Bullard (1907-1980) and Walter Elsasser (1904-1991).  They used magnetohydrodynamic dynamo theory in a liquid core to produce models for a self-sustaining dynamo.  (Merrill, McElhinny; p.13)

Bullard Elsasser

The Interior of the Earth

The innermost core of the Earth is a sphere of solid iron, surrounding the inner core is the liquid iron outer core.  Above the liquid iron outer core is the viscous molten rock mantle and above the mantle floats the hard crust upon which we live.  These layers and the materials they are made of have been determined by measuring the speed and route that large seismic waves travel through the Earth.  The liquid iron core can conduct electricity and since it is liquid it also is believed to swirl in a convection cell as liquid iron is heated near the inner core then rises and cools giving heat to the mantle above it.

The Coriolis Effect

The Coriolis Effect discovered by Gaspard-Gustave Coriolis (1792-1843) states that fluid flowing away from the equator invariably follows a curved path that swings it to the right in the northern hemisphere and to the left in the southern hemisphere because the Earth is rotating counterclockwise on its axis.  The Coriolis Effect explains why hurricanes spiral counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere.  (Luhr; p.446)

Convecting liquid iron core plus the coriolis effect equals Geodynamic Theory

As the liquid iron convects upward from the inner core its path is believed to be adjusted due to the Coriolis Effect causing the fluid flow to follow a helical pattern as it performs its cycle from the hot inner core to the cooler mantle.  The combination of an electrically conducting liquid iron core and the Coriolis Effect are believed to produce the self-perpetuating dynamo.  The Geodynamo cannot produce a magnetic field from nothing and it is believed that all that was required was a small initial magnetic field in place when the Earth was forming to cause electrical currents to flow through the Earth’s liquid iron core.  These electrical currents were propagated by the geodynamo and they induced the grand magnetic field which has remained constant for 3.5 billion years.  Core samples of Ancient rocks indicate the field direction and strength through geologic time.

The following excerpt from Earth's Magnetism in the Age of Sail explains the Geodynamo Theory succintly

"Understanding the processes that generate and shape the geomagnetic field requires a detailed analysis of the physics at work in the fluid outer core. To start with, one has to imagine the intense heat in the deep Earth, between three and six thousand degrees centigrade, trapped within a thick insulating layer of mantle rock. Our planet is still cooling down from the time of formation, while additional heat is released by the decay of radioactive elements, chemical processes, and liquid material solidifying onto the surface of the inner core. Secondly, in a fluid such as molten iron, thermal buoyancy drives convection, transporting the heat to overlying cooler regions. Thirdly, as the world turns around its axis, the whole of this white-hot cauldron is permanently spinning round, forming a rotating compound structure of a solid sphere (the inner core) inside a liquid sphere (the outer core) inside a second solid sphere (the mantle). This system of nested shells already creates quite complex patterns of fluid flow. The physical forces in such a rotating frame of reference then induce additional twists in the convective motions, deflecting rising fluid to rotate counterclockwise in the northern hemisphere and clockwise in the southern. This process creates slowly spinning rolls parallel to the Earth's axis, and a large ring-shaped region of flow aligned with the equator.

So far, we have a spherical pressure cooker roughly the size of the planet Mars, full of rather hot soup, constantly heated from the center. Now imagine this soup to be extremely well able to conduct electricity. A sufficiently large volume of a conducting fluid, rotating and convecting, is able to form and maintain a magnetic dynamo. All it needs to start is the presence of a small magnetic field, for instance, as a relict from the time of the Earth's formation. While heat and rotation propell the molten iron, a potentially small initial magnetic field will generate an electric current in the conducting liquid interior. An intricate pattern of current loops then forms a secondary magnetic field, which will interact with the fluid motion to amplify the original field. The induced electric currents would long ago have dissipated due to electrical resistance if the process did not constantly regenerate itself. The geodynamo converts fluid motion into moving electric charge into magnetic force, and will remain self perpetuating for as long as liquid iron continues to convect around the inner core." (Jonkers; p. 14-15)

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