Compasses point to the magnetic north pole. Birds follow the magnetic field lines during their seasonal migrations. Earth’s magnetic field probably has subtle effects on human physiology as well. These are important to know but the reason we should learn more about this subject is that the Earth’s magnetic field protects us from dangerous cosmic rays. Life on Earth would not be possible without the shield created by Earth’s magnetic field. It is quite possible that life on Mars was destroyed after it lost it’s magnetic field.
Dipole Component of Earth’s Magnetic Field
The magnetic field lines are directed into the Earth on the Northern Hemisphere and directed out of the Earth in the Southern Hemisphere. The figure on the left was obtained from the geodynamo simulations of G. A. Glatzmaier and P. H. Roberts . Measurements and simulations both confirm that there is a clear dipole component. In other words the Earth’s magnetic field to first approximation acts like a bar magnet.
Geologic record shows that the strength of the dipole component has been decreasing for the last 2000 years. Since 1830’s the dipole strength decreased by 10%. This is a little alarming because we rely on the dipole field strength for protection from cosmic rays.
I should point out that the the energy contained in Earth’s magnetic field is not decreasing. Only the dipole component is decreasing.
Magnetic Polarity Reversals
According to geologic record in the minerals, Earth’s magnetic dipole polarity reversed many times during the planet’s 4.5 billion year history. The average time between reversals is 250 thousand years. The recent decrease in the dipole moment naturally worries us because no reversal occured in the last 780 thousand years. It seems that we are overdue for a reversal.
Formation of the Inner Core
During the formation of Earth the molten metallic iron separated from the surrounding mixture of silicates and oxides and sank to the center. This is how the liquid iron core was formed.
How do we know that the core is liquid and the core contains iron? The evidence comes from Seismological measurements which indicate that the density of the core is just 10% less than the density of pure iron. This means the core is mostly iron. Light elements rise into the fluid outer core, while denser elements form the solid inner core.
The thickness of Earth’s crust (litosphere) is between 5 to 30 km. The mantle is 2900 km thick. The fluid outer core is about 2200 km thick. There are turbulent convection currents inside the fluid outer core. The temperature in the core is about 5000 degrees Celcius similar to the temperature at the surface of the Sun (the temperature in Sun’s corona is millions of degrees Celcius, 3 orders of magnitude higher than the temperature at the surface of the Sun).
What is Driving the Geodynamo?
The energy driving the geodynamo is part thermal and part chemical.
- hot bouyant liquid iron in the lower part of the outer core rises upward
- liquid iron loses some of its heat at the mantle boundary and cools and sinks.
- this process (thermal convection) transfers energy from bottom to top.
- heat escaping from the upper core causes the solid inner core to grow larger, the chemical process involving iron crystals and the growing inner core enhances the convection mechanism.
- Earth’s rotation through the Coriolis effect deflects the rising iron fluid into spiral or helical paths.
- Three-dimensional numerical simulations of the geodynamo suggest that a super-rotation of Earth’s solid inner core relative to the mantle is maintained by magnetic coupling between the inner core and an eastward thermal wind in the fluid outer core. This mechanism, which is analogous to a synchronous motor, also plays a fundamental role in the generation of Earth’s magnetic field.
- The liquid iron carries electric charge and when electric charges move in curved paths they produce magnetic field. This is how the Earth’s magnetic field is created.
- The convection mechanism and the super-rotation of the inner core keeps the geodynamo going.
Earth’s Inner Core Is Rotating Slightly Faster
Earth’s solid inner core rotates slightly faster than the rest of Earth. Glatzmaier & Roberts  found that the inner core rotates relative to the mantle in the eastward direction at an angular velocity of 2° to 3° per year. This phenomenon is now known as the super-rotation of the inner core. Subsequent seismological measurements confirmed the existence of the super-rotation but the magnitude was found to be 0.3° to 0.5° per year, or about one extra revolution each 900 years .
 Glatzmaier, G., and Roberts, P., 1996, Rotation and magnetism of Earth’s inner core: Science, v. 274, p. 1887–1891.
 Zhang, J., Song, X., Li, Y., Richards, P., Sun, X., and Waldhauser, F., 2005, Inner core differential motion confirmed by earthquake waveform doublets: Science, v. 309, p. 1357–1359.