Earth's Magnetic Field: Origins

The Earth has a dipole magnetic field that is roughly aligned with its rotational axis. A dynamo effect in the Earth’s molten outer core is the most likely source of most of the magnetic field.

Overview

The Earth’s magnetic field is primarily a dipole field, meaning it has two well-defined magnetic poles, called north and south (the prefix “di” comes from a Greek term for “two”). This is the type of field produced by a bar magnet or an electric current flowing in a wire loop. The Earth’s iron core once was thought to act like a giant bar magnet, possibly due to a remanent field frozen in place from some primordial magnetic field that existed when our solar system was forming. Now, however, electrical currents produced by fluid motions in the molten outer part of the Earth’s iron core are theorized to be the source of the magnetic field. This conclusion is based on models of the Earth’s interior structure and variations in the magnetic field over both historic and geologic timescales.

The ultimate source of any magnetic field is the movement of electric charges. Wires carrying electric currents, for example, have magnetic fields around them because of the electric charges (electrons) moving through the wires. The electrons surrounding the nucleus of an atom are moving, and this produces a minute magnetic field. In a magnet, the atoms are aligned in such a way that these small fields add together to produce the larger field of the magnet. The conclusion, therefore, is that electric currents within the Earth produce its magnetic field through a process referred to as the geodynamo.

To determine the Earth’s interior structure, sismic waves from earthquakes act as probes as they pass through the Earth. They reveal that the Earth’s interior consists of three major zones or layers: the crust, mantle, and core. The crust and underlying mantle are composed mainly of rocky material, which is a good electrical insulator. The innermost region, the core, is composed of metals, most probably iron with a small percentage of nickel and an even smaller percentage of other elements, and thus it is a good electrical conductor. The inner core (out to a radius of about 1,300 kilometers) is solid, but the outer core (from 1,300 to about 3,500 kilometers radius) is molten.

The temperature of the solid inner core is estimated to be about 5,800 kelvins. Heat flowing outward through the outer molten core sets up convection currents, in which hotter, less dense fluid rises. When it transfers its heat to the overlying mantle, the fluid cools, becomes denser, and sinks. The cnvection may also be partly driven chemically. If iron crystallizes at the bottom of the molten outer core to add to the solid inner core, the remaining fluid contains less iron and so is less dense, augmenting the thermally driven upward motion. Simple convection currents are deflected by the Earth’s rotation in a process called the Coriolis effect. Computer models of the outer molten core that incorporate both convection and rotation show that the fluid moves in a number of spiraling columns aligned roughly parallel with the Earth’s rotational axis.

All that is needed to “jump start” the geodynamo is a weak background magnetic field, perhaps provided by the solar wind or a remanent primordial field. As the metallic fluid moves through the background field, electrical currents are induced that in turn generate their own magnetic fields. This produces a positive feedback that reinforces the electrical currents and the overall magnetic field. The approximate alignment of the fluid’s spiral motion with the rotational axis produces a dipole field with the magnetic poles near the rotational poles. The energy to produce the stronger magnetic field comes from the motion of convection and rotation. This process does not continue generating an ever-increasing field; it levels off since it becomes harder to generate an even stronger field as the field strength increases.

The geodynamo process explains many features of the Earth’s magnetic field. Although the magnetic poles apparently remain close to the rotational poles, there is a shift in the position of the magnetic poles and changes in the field strength over periods of years to centuries; this could be caused by changes in the convection currents within the molten outer core.

Furthermore, paleomagnetic studies indicate that the Earth’s magnetic field has reversed polarity at irregular intervals many times in the geologic past, the last reversal occurring about 700,000 years ago. The geodynamo process is unstable over long periods of time and can decay and regrow with changed polarity. Geologists have constructed models of dynamos that are simple versions of the geodynamo, and when set in operation, these models display changes in the field’s intensity and polarity. The geodynamo explains the origin of about 90 to 95 percent of the Earth’s magnetic field; the rest probably comes from fields associated with magnetic minerals in the Earth’s crust, more complicated irregularities in the convective motions of the molten outer core, and external sources such as the solar wind interacting with the Earth’s ionosphere.

Methods of Study

The Earth’s magnetic field is generated in its interior. A clue to the composition of the interior is provided by the Earth’s average density. Dividing the Earth’s mass by its volume shows the average density to be about 5.5 times the density of water. Common rocks from the surface are about three times denser than water. Therefore a portion of the Earth’s interior must be much denser than surface rocks in order to yield the average value. Only metals have the required density, but some metals, such as aluminum, are too low in density, and others, such as uranium, are too high. Still others, such as gold and silver, are close to the required density but are too rare. Iron is a good candidate, since it has the right density and is fairly abundant.

The Earth’s interior structure can be probed using seismic waves produced by earthquakes. Body waves from the earthquake travel through the Earth’s interior. Their speed and direction of travel are determined by the density and elastic properties of the material through which they are traveling. The waves also are reflected off boundaries between different layers. When the transmitted and reflected waves reach the surface, they are recorded on seismographs. Analysis of the seismograms obtained at seismic stations all around the globe reveals that the Earth has three main zones or layers: the surface crust, the mantle, and the central core. One type of body waves, called S waves, are transverse waves that can travel only through solids, not liquids. Their presence or absence recorded on seismograms reveals which parts of the interior are solid or liquid. The crust and mantle are solid (except for isolated pockets of molten material called magma). The inner part of the core also is solid, but the outer core is molten.

Models of Earth’s interior that combine chemical composition estimates with seismic-wave data indicate that the crust and mantle are composed of rocky material, while the core (both molten outer and solid inner parts) is mostly iron, with some nickel and other elements. The composition and pressure from these models can be used to calculate the melting-point temperature as a function of depth. The mechanical properties of the layers (whether solid or liquid, as indicated by seismic waves) can then be used to determine whether the actual temperature is below or above the calculated melting-point temperature. Anchored by measurements of heat flow at the surface and the increase of temperature with depth recorded in mines and wells, the geotherm—a graph of actual temperature versus depth—can be drawn. This is how the temperature of the Earth’s core, about 5,800 kelvins, is determined, showing that there is enough heat energy to drive the convection necessary for the geodynamo process.

Remanent magnetism, the evidence of past magnetic fields preserved in some igneous and sedimentary rocks, can be measured with various types of magnetometers. These data, together with records of changes in the Earth’s magnetic field during historic times, show how the field varies in strength and orientation and has even reversed polarity many times in the past.

Context

The geodynamo mechanism that generates the Earth’s magnetic field also can explain the presence or absence of magnetic fields for other planets, their moons, and the sun. Mars and Earth’s moon have extremely weak magnetic fields, probably because their iron cores are so small, and they may have cooled to the point that their iron cores are no longer molten so convection cannot occur. Venus also has an extremely weak field; although it is nearly the same size and mass as Earth and probably has a similar internal structure with a molten outer core, it rotates very slowly.

Mercury has a magnetic field about an order of magnitude stronger than the fields of Venus, Mars, and the moon, but about two orders of magnitude weaker than the field of the Earth; although Mercury rotates slowly, its relatively large iron core might still have a molten convective zone. Jupiter’s magnetic field is more than ten times stronger than Earth’s, and Saturn’s is about two-thirds as strong as Earth’s; their fields are probably due to convection occurring in the liquid metallic hydrogen in their interiors (Jupiter has much more than Saturn) and their rapid rotation. Jupiter’s moons Europa and Ganymede also have small magnetic fields, probably due to convection in electrically conductive salty oceans beneath their icy crusts. Images from the Hubble Space Telescope gathered in 2015 revealed that Ganymede's oceans are larger than all of the oceans on Earth combined. The Hubble images showed auroras in the moon's skies (like those on Earth) which are caused by the interaction of Ganymede's magnetic field with that of Jupiter. It was the shimmering quality of the auroras suggested the presence of water. The sun has a strong magnetic field that reverses polarity every eleven years; it is produced by the movement of ionized gas in the convection zone of the sun’s interior.

Bibliography

Aron, Jacob. "Aurora Reveals Jupiter Moon's Secret Subsurface Sea." New Scientist, 12 Mar. 2015, www.newscientist.com/article/dn27151-aurora-reveals-jupiter-moons-secret-subsurface-sea/. Accessed 23 Jan. 2023.

Dobrijevic, Daisy. "Earth's Magnetic Field: Explained." Space.com, 6 July 2022, www.space.com/earths-magnetic-field-explained. Accessed 23 Jan. 2023.

Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge UP, 2004. Print.

Garland, G. D. Introduction to Geophysics. 2d ed. Philadelphia: Saunders, 1979. P.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Brooks, 2005. Print.

Howell, Elizabeth. "Astrobiology Top 10: Ganymede's Auroras Point to Ocean Below Its Icy Surface." Astrobiology. Astrobio.net, 26 Dec. 2015. Web. 29 Dec. 2015.

Lapedes, D. N., ed. McGraw-Hill Encyclopedia of Geological Sciences. New York: McGraw, 1988. Print.

Merrill, R. T., and M. W. McElhinney. The Earth’s Magnetic Field. New York: Academic, 1983. Print.

Motz, L., ed. Rediscovery of the Earth. New York: Reinhold, 1979. Print.

Smith, D. G., ed. The Cambridge Encyclopedia of Earth Sciences. New York: Crown, 1981. Print.

Stacey, F. D. Physics of the Earth. New York: Wiley, 1977. Print.

Tarbuck, Edward J., Frederick K. Lutgens, Dennis Tasa, and Scott Linneman. Earth: An Introduction to Physical Geology. 13th ed. Pearson, 2019.

Vogel, Shawna. Naked Earth: The New Geophysics. New York: Plume, 1996. Print.