Earth's Magnetic Field at Present

The study of the Earth’s magnetic field is important from both academic and practical perspectives. The Earth is the only planet of the inner solar system with a strong magnetic field, and this provides clues about the formation of the Earth and the other inner planets. The Earth’s magnetic field deflects and traps high-energy charged particles, providing a shield to protect life. It can disrupt modern communication and electrical systems, but it also can point to the location of ore deposits.

Overview

The study of the Earth’s magnetic field is a branch of geophysics, which combines geology and physics to investigate various physical characteristics of the Earth. The ultimate source of any magnetic field is moving electrical charge, such as an electric current flowing in a wire. Approximately 90 to 95 percent of Earth’s magnetic field is thought to be produced by electrical currents in the Earth’s molten metallic outer core, a mechanism referred to as the geodynamo.

The Earth’s field is predominantly a dipole field, meaning it has two magnetic poles; the prefix “di” is derived from the Greek word meaning “two.” This is the type of field produced by a bar magnet or an electric current flowing in a wire loop. By definition, the north pole of a bar magnet is the end that points northward on Earth at the present time. Since like magnetic poles repel and unlike magnetic poles attract, that means the Earth’s magnetic south pole is located in the Northern Hemisphere, and the Earth’s magnetic north pole is located in the Southern Hemisphere. Magnetic field lines are a way of visualizing the direction of a magnetic field, and by convention they point in the direction the north pole of a bar magnet would point. Magnetic field lines leave the Earth’s surface in the Southern Hemisphere, arc over the Earth, and reenter the Earth in the Northern Hemisphere. The magnetic poles are the two places where the field lines leave and enter the Earth’s surface precisely vertically.

The pole in the Northern Hemisphere is called the north magnetic pole (but remember it is a magnetic south pole), and the pole in the Southern Hemisphere is called the south magnetic pole (although it is a magnetic north pole). At first this may seem confusing, but note the distinction in terminology: the geographic hemisphere that the pole is in comes before the words “magnetic pole,” while the type of pole comes between words “magnetic” and “pole.” The field’s strength is about 0.6 gauss (a unit of magnetic induction) at the magnetic poles and about 0.3 gauss at the magnetic equator, where the field lines are horizontal. (For comparison, a small bar magnet has a field strength of about 1 gauss.) The difference in strength is due to the field lines bunching together at the magnetic poles and spreading apart at the magnetic equator.

The magnetic poles are not located at the geographic or rotational poles of the Earth, which are the two points where the rotational axis of the Earth intersects the surface. At the beginning of the twenty-first century, the north magnetic pole is located in the Arctic Ocean north of Canada and west of Greenland, approximately 900 kilometers from the geographic North Pole. The south magnetic pole is located in the ocean between Antarctica and Australia, approximately 2,900 kilometers from the geographic South Pole. Notice that the magnetic poles are not symmetrically located in relation to the rotational poles. By 2020, the north magnetic pole was located at N 86.50°, W 164.04° and the south magnetic pole was found at S 64.07°, E 135.88°.

The magnetic poles also are not stationary; rather, they wander around the polar regions at varying speeds of up to tens of kilometers per year. Since the early 1900s, the north magnetic pole has moved roughly north-northwest about 1,300 kilometers, while the south magnetic pole has moved from Antarctica northward out into the ocean toward Australia. Because the geodynamo, the theoretical source of the Earth’s magnetic field, is driven partly by the Earth’s rotation, it is presumed that over long periods of time, the positions of the two magnetic poles average out to roughly the locations of the geographic poles. In addition, measurements of the field’s strength since the mid-nineteenth century indicate that it is decreasing at a rate of about 6 percent per century. Archaeomagnetic evidence indicates the field was approximately twice as strong two millennia ago, and before that, around 3,500 BCE, it was only about one-half the present strength.

These changes in direction and strength over timescales of years to millennia are called secular variations. They are thought to be due to changes in the geodynamo operating in the Earth’s molten outer core. Considering its past behavior, scientists cannot predict what the magnetic field will do in the future. It may continue to decrease, or it may increase. If it were to continue decreasing at the present rate, the field would drop to zero in about 1,600 years. This might lead to a magnetic reversal, in which the field reforms but with its polarity reversed. Paleomagnetic measurements of past magnetic fields preserved in some rocks indicate this has occurred many times in the geologic past, the last time about 700,000 years ago.

The Earth’s magnetic field also exhibits small, rapid changes in direction and strength over periods of hours to days, due to a variety of external effects. For example, the gravitational fields of the sun and moon distort the atmosphere of the Earth, in the same manner as ocean tides. Movement of electrically charged particles in the atmosphere produces a weak contribution to the magnetic field that changes with the relative positions of the sun and moon.

The sun continually blows electrons, protons, and other electrically charged particles outward from its surface at speeds of hundreds of kilometers per second, a phenomenon known as the solar wind. When these charged particles encounter the Earth’s magnetic field, they interact with it, producing a boundary called the magnetopause. Inside the magnetopause is the magnetosphere, the region in which the Earth’s magnetic field is dominant. The solar wind changes the shape of the Earth’s field. The side facing the sun is pushed in toward the Earth by the solar wind so that the magnetopause is about 60,000 kilometers, or 10 Earth radii, from the Earth, while the field pointing away from the sun is elongated into a magnetic tail that can extend farther than the orbit of the moon.

Some of the solar wind particles, particularly electrons and protons, are trapped by the Earth’s magnetic field. These form the Van Allen belts, which were discovered in 1958 by Dr. James Van Allen while analyzing data from a charged particle detector he had placed aboard Explorer 1, the first successful US satellite. The inner belt is a torus about 3,000 kilometers above the magnetic equator; the outer belt is a larger torus about 14,000 kilometers above the magnetic equator.

The number of sunspots increases and decreases over a cycle of eleven years. Sunspots are just one of the more obvious manifestations of solar magnetic activity, and the sun reverses magnetic polarity with each eleven-year cycle. During times of maximum solar activity, solar flares are most likely to erupt from its surface. These flares eject large numbers of highly energetic, electrically charged particles out into the solar system. If they encounter the Earth’s magnetic field, they can produce magnetic storms that cause wild variations in the Earth’s field. This in turn can disrupt modern communication and electrical distribution networks. It is at these times, when the sun is most active, that auroras (the northern and southern lights) are most common. Increased numbers of charged particles from the sun are deflected by the Earth’s magnetic field and enter the Earth’s upper atmosphere near the magnetic poles, where they excite air molecules, causing them to glow.

Lightning is a very rapid electrical discharge in the atmosphere; electrical charges can flow from the ground to clouds, from clouds to the ground, or from cloud to cloud. Locally, this strong but brief electrical current produces a very large increase and then decrease in the background field strength.

Magnetic anomalies distort the dipole shape of the main field. Some of these anomalies probably result from more complicated flow patterns in the molten outer core, while others probably are associated with rock units that are rich in iron. Two of the strongest known are located near Kursk, Russia, and in northern Manitoba, Canada. Running parallel to the ocean ridge-rift system are bands or strips of seafloor with alternate normal and reversed magnetic polarity that enhance or weaken the present field over them. The strips preserve a record of the Earth’s past magnetic field, frozen into the igneous rocks (mainly basalt and gabbro) that cooled from lava that oozed out along the ridge-rift, and then was pushed away from the ridge-rift as new lava oozed out. This provides evidence of magnetic field reversals in the geologic past and support for the concept of seafloor spreading (one of the key parts of plate tectonics). Small anomalies can even result from man-made iron objects.

Methods of Study

The orientation of the magnetic field at any point on Earth is specified by two angles called declination and inclination. Declination is the angle between true north (the direction of the geographic or rotational north pole) and the horizontal component of the magnetic field line at that point. Thus declination is the angle between true north and the direction an ordinary compass needle points. Inclination is the angle between a horizontal line and the downward tilt of the magnetic field line at that point. Inclinations are downward (positive) in the northern hemisphere and upward (negative) in the Southern Hemisphere. The magnetic poles are located where the inclination is 90° , specifically 90° down (positive) for the pole in the Northern Hemisphere and 90° up (negative) for the pole in the Southern Hemisphere. The magnetic equator is located where the inclination is 0°.

Around the world, 130 permanent magnetic observatories have been established to record any changes in the magnetic field. It was at observatories in London and Paris that secular variations of the field were first recognized in the 1600s. Early observatories could measure only the declination and inclination of the field. Declination was measured with a compass-like device and inclination with a magnetized rod balanced so that it could pivot freely in a vertical plane.

Magnetometersfor the measurement of magnetic field intensity were first developed in the mid-1800s, and a number of different types are in use today. In conjunction with the magnetic observatories on the ground, some satellites carry magnetometers for the measurement of the field from orbit, and they provide readings for virtually the entire globe.

Portable magnetometers can detect local field anomalies due to things under the surface. Geologists use them to prospect for magnetic iron ore deposits, and archaeologists use them to search for buried iron artifacts.

Context

When magnetic storms occur, modern communication and electrical distribution networks can be disrupted. Also on these occasions, auroras are more likely to occur and be seen over larger areas. The magnetic field interacts with electrically charged particles and prevents many of them from reaching the Earth’s surface. It is possible that a decrease in the field would lead to more particles reaching the surface, perhaps producing greater numbers of genetic mutations or cancers. Changes in the field strength have been suggested as a cause of some of the mass extinctions that have occurred in the geologic past. There was evidence that suggested that a weakening in the magnetic field could result in a reversal of the magnetic poles in the near future. A 2015 study by the Massachusetts Institute of Technology and Rutgers University, however, suggested that Earth's magnetic field is returning to a historical average, instead of weakening to a point that the poles would reverse.

Bibliography

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. London: Saunders, 1979. Print.

Jones, Andrew. "Earth's Magnetic Poles Probably Won't Flip Soon, After All." Space.com, 6 July 2022, www.space.com/earth-magnetic-field-unlikely-to-flip. Accessed 23 Jan. 2023.

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.

"Wandering of the Geomagnetic Poles." National Oceanic and Atmospheric Administration, www.ngdc.noaa.gov/geomag/GeomagneticPoles.shtml. Accessed 23 Jan. 2023.