TWENTY-ONE FREQUENTLY ASKED QUESTIONS
AND THEIR ANSWERS

Following are 21 frequently asked questions, and their corresponding answers, about magnetism and, more specifically, the Earth’s magnetic field. The answers to each question are pitched at a technical level of expertise approximating that possessed by the curious individuals who have asked questions of us in the past. The answers should be comprehensible by anyone having taken a freshman level college physics class. Readers wishing to obtain a deeper understanding of the subject of geomagnetism are referred to our Introduction to Geomagnetism and the many articles already written on geomagnetism listed in the Further Reading page.

1. What is a magnetic field?

Fields fill the space between matter and they determine how it is that bits of matter can exert forces on other bits of matter at a distance. There are several different fields in nature, and their reality is demonstrated by our observation of the forces with which they are associated. So, for example, gravitational fields determine how it is that objects with mass are attracted together by a gravitational force. Electric fields determine how it is that objects with electric charge are attracted together by an electric force, if they have opposite electric charge, or repelled from each other, if they have the same electric charge. Interestingly, unlike an electric field, a magnetic field only comes into play when electric charges are moving. Magnetic fields determine how it is that electric currents, composed of moving electric charges, exert forces on other electric currents. Consider, then, two parallel wires, each with an electric current flowing in the same direction. By virtue of the magnetic field, they will be pulled toward each other, they experience an attractive force. If the currents are flowing in the opposite direction, then there will be a repulsive force between the wires. More generally, magnetic fields are generated by electric currents, the motion of electric charges, and, conversely, electric currents and the motion of electric charges can be induced by time-dependent magnetic fields. In fact, an electric generator works by the motion of magnetic fields.

2. What is a permanent magnet?

Most material is non-magnetic. It is composed of molecules made of atoms, each of which have electrons orbiting nuclear protons, but where the motion of one electron, essentially a tiny electric current, generates a magnetic field that is cancelled by the magnetic field generated by the motion of another electron. In magnetic materials this cancellation is incomplete, and so the atoms of the material have small net electric currents and they thus generate small magnetic fields. For various reasons having to do with the intricacies of atomic physics, this tends to happen for certain substances, like cobalt, nickel, and, of course, iron. Within these magnetic materials, the magnetic fields of the various atoms exert torques on the electric currents of their neighboring atoms, causing the atoms to align and their magnetic fields to add together constructively. As a result, the material exhibits a magnetic field. It is a ‘magnet’. Most magnets are like the bar magnet shown in the illustration, having a simple 'dipole' arrangement of a 'north' pole, where the field diverges, and a 'south' pole, where the field converges.

3. How does a compass work?

The needle of a compass is a small magnet, one that is allowed to pivot in the horizontal plane. The needle experiences a torque from the ambient magnetic field of the Earth. The reaction to this torque is the needle’s preferred alignment with the horizontal component of the geomagnetic field. The ‘north’ end of the compass needle is simply the north end of the magnet, and it is the end of the compass needle that points in the general direction of the geographic north pole; naturally, the ‘south’ end of the compass needle is the south end of the magnet and it points in the opposite direction, towards the general direction of the geographic south pole. Having said this, the preferred directionality of a compass can be affected by local perturbations in the magnetic field, like those set up by (say) a near-by electrical system; a compass can also be affected by local magnetization of the Earth's crust, particularly near large igneous or volcanic rock deposits.

4. What is declination?

At most places on the Earth's surface, the compass doesn’t point exactly toward geographic north. The deviation of the compass from true north is an angle called 'declination'. It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It might surprise you to know that at very high latitudes the compass can even point south! Declination is simply a manifestation of the complexity of the geomagnetic field. The field is not perfectly symmetrical, it has non-dipolar ‘ingredients’, and the dipole itself is not perfectly aligned with the rotational axis of the Earth. Interestingly, if you were to stand at the north geomagnetic pole, your compass, held horizontally as usual, would not have a preference to point in any particular direction, and the same would be true if you were standing at the south geomagnetic pole. Moreover, if you were to hold your compass on its side the north-pointing end of the compass would point down at the north geomagnetic pole, and it would point up at the south geomagnetic pole. Maps of declination, such as that shown below (contours of 10 degrees east), as well as other field components, and a program for determining the magnetic field at any geographic location, are given in the Models, Charts, and Movies pages of this website.

5. Is the Earth a magnet?

In a sense, yes. You probably know that the Earth is stratified; a section is pictured here. In radius it is composed of layers having different chemical composition and different physical properties. The crust of the Earth has some permanent magnetization, and the core of the Earth, the outer part of which is liquid iron and the inner of which is solid iron, generates its own magnetic field, sustaining the main part of the field we measure at the surface. So we could say that the Earth is, therefore, a ‘magnet’. But there is no giant bar magnet near the Earth’s center, despite the depictions you may have seen in elementary textbooks on geology and geophysics. Permanent magnetization, such as discussed in question 2, cannot occur at high temperatures, like temperatures above 650 degrees centigrade or so, when the thermal motion of atoms becomes sufficiently vigorous to destroy the ordered orientations needed to establish permanent magnetization. The core of the Earth has a temperature of several thousand degrees, and so, even though the core is the source of most of the geomagnetic field, it is not, itself, permanently magnetized.

6. How does the core generate a magnetic field?

This is explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, as the result of radioactive heating and chemical differentiation, the outer core is in a state of turbulent convection. This sets up a process that is a bit like a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy. Basically, the motion of the electrically conducting iron in the presence of the Earth's magnetic field induces electric currents. Those electric currents generate their own magnetic field, and, as the result of this internal feedback, the process is self-sustaining, so long as there is an energy source sufficient to maintain convection. The depiction of the geodynamo shown here is only schematic; in fact, the fluid motion and the form of the magnetic field inside the core are still the subject of intensive research.

7. Why do models and charts of the geomagnetic field need to be periodically updated?

Models and charts of the magnetic field at the Earth’s surface need to be periodically updated because the field is constantly changing in time. The same fluid motion in the Earth’s core that sustains the main part of the magnetic field also causes the field to slowly change in spatial form, a time-dependence known as ‘secular variation’. This variation can be seen in all vectorial parts of the magnetic field, but it was first noticed in declination several hundred years ago, since it is that quantity that is so important for navigation. In fact, the demands of navigators helped to motivate, centuries ago, some of the original studies of the Earth's magnetic field. On average the declination at the Earth’s surface changes by about a fifth of a degree per year.

8. Is it true that the magnetic field occasionally reverses its polarity?

Yes. We know this from an examination of the geological record. When lavas are deposited on the Earth’s surface, and subsequently freeze, and when sediments are deposited on ocean and lake bottoms, and subsequently solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. This type of magnetization is known as 'paleomagnetism'. Careful measurements of oriented samples of faintly magnetized rocks taken from many geographical sites allow scientists to work out the geological history of the magnetic field. We can tell, for example, that the Earth has had a magnetic field for at least 3.5 billion years, and that the field has always exhibited a certain amount of time-dependence, part of which is normal secular variation, like that which we observe today, and part of which is an occasional reversal of polarity. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, because the rotation of the Earth is an important dynamical force in the core, where the main part of the field is generated. Occasionally, however, the secular variation becomes sufficiently large such that the magnetic poles end up being located rather distantly from the geographic poles; we say that the poles have undergone an ‘excursion’ from their preferred state. Now, we know from physics that the Earth’s dynamo is just as capable of generating a magnetic field with a polarity like that which we have today as it is capable of generating a field with the opposite polarity. The dynamo has no preference for a particular polarity. Therefore, after an excursional period of enhanced secular variation, the magnetic field, upon returning to its usual state of rough alignment with the Earth’s rotational axis, could just as easily have one polarity as another. The consequences of polarity reversals for the compass are dramatic. Nowadays, the compass points roughly north, or, more precisely, the north end of the compass points roughly north at most geographical locations. However, before the last reversal, which was about 780,000 years ago, the polarity was reversed compared to today's, and the compass would have pointed roughly south, and before that reversed state the polarity was like that which we have today, and the compass would have pointed roughly north, and so on. The timings of reversals forms the so-called 'geomagnetic polarity timescale', shown here at the right. During a reversal, between polarities, the geometry of the magnetic field is much more complicated than it is now, and a compass could point in almost any direction depending on one’s location on the Earth and the exact form of the mid-transitional magnetic field. One of the things that is interesting about reversals is that there is no apparent periodicity to their occurrence. Reversals are random events. They can happen as often as every 10 thousand years or so, and as infrequently as every 50 million years or more. Questions about reversals are very popular with the general public, and further information can be found in the references given in the Further Reading page of this website.

9. What causes the magnetic field to reverse its polarity?

Nothing. That answer might surprise you, but the fact that the field occasionally reverses is simply a property of the continuous, on-going behavior of the Earth's dynamo. There is no ‘cause’ per se. In answering the previous question we discussed the phenomenology of polarity reversals, what they are and how they might affect a (hypothetical) compass, but with respect to the physics of the process itself, some lessons can be learned from the laboratory. It is possible, for example, to design a machine, an electrical-magnetic-mechanical dynamo consisting of spinning metal disks and coils of wire which, when supplied with mechanical energy, sustains its own magnetic field. Depending on the details of the apparatus, the magnetic field can be steady, with no time dependence at all, or it can reverse periodically, like the Sun’s magnetic field does every eleven years, or it can reverse randomly, bouncing back and forth in an orbit around two preferred states (opposite polarities) like the Earth’s magnetic field does. It is also possible to write down the mathematical equations that describe the behavior of this laboratory system – the equations describe what is popularly known as ‘chaos’, and, even though the laboratory system is relatively simple, its equations have some similarity to those describing the dynamics of the Earth’s core. In summary, then, nature allows for different kinds of dynamos, some of which just simply have the property that they undergo occasional random reversals. The Earth' core happens to be one of those dynamo types.

10. Could magnetic reversals be caused by meteorite or cometary impacts? Could reversals be caused by melting of the polar ice caps or some sort of planetary alignment?

One of the most important jobs that a scientist has is to determine, from among all the possible causes and effects in nature, which are the most important and strictly and necessarily causally related, and which are simply insignificant and essentially unrelated. Although extremely unlikely, we will admit that it might be possible for a reversal of the Earth’s magnetic field to be triggered by a meteorite or cometary impact, or even for it to be caused by something more ‘gentle’, such as the melting of the polar ice caps, as you suggest. But remember, from our discussion following the previous question, self-contained dynamical systems, some of which can be built in the laboratory, can exhibit randomly reversing behavior. They can do this without any outside influence. The Earth's dynamo is a natural example of such a self-contained, randomly-reversing dynamical system. Therefore, invoking an external mechanism for causing the Earth’s polarity reversals is, quite simply, a ‘solution’ to a non-problem. Reversals would happen anyway.

11. The strength of the magnetic field has been decreasing lately, does this mean that we are about to have a reversal?

Almost certainly not. Direct historical measurements of the intensity of the geomagnetic field have been possible ever since Gauss invented the magnetometer in the 1830’s. Since then the average intensity of the field at the Earth’s surface has decreased by about ten percent. And we know, from paleomagnetic records, that the intensity of the field does indeed decrease, by as much as ninety percent, at the Earth’s surface during a reversal. But those same paleomagnetic records also show that the field intensity has often exhibited significant variation, with both decreases and increases in intensity, without there always being a coincident reversal. So, an intensity low does not necessarily mean that a reversal is about to occur. Moreover, the recent decrease in intensity is not really that dramatic of a departure from normality, and for all we know the field may actually get stronger at some point in the not-so-distant future. It's worth remarking that predicting the occurrence of a reversal based upon a knowledge of the current state of the magnetic field is about as easy as predicting the next bull market on Wall Street; you don’t know it’s happening until it’s half over!

12. Could the mass extinctions observed in the paleontological record be correlated with magnetic reversals?

The magnetic field of the Earth does protect us from fast-moving charged particles streaming from the Sun, but so does the atmosphere. It is not clear whether or not the radiation that would make it to the Earth’s surface during a polarity transition, when the magnetic field is relatively weak, is sufficient to affect evolution, either directly or indirectly, and cause extinctions, such as that of the dinosaurs. But it seems that the radiation is probably insufficient. This conclusion is supported by the fact that reversals happen rather frequently, every million years or so, compared to the occurrence of mass extinctions, every hundred million years or so. In other words, many reversals and, in fact, most reversals, appear to be of no consequence for extinctions.

13. Are variations in the geomagnetic field somehow associated with earthquakes or vice versa?

The USGS supports an important National Earthquake Program. As a small part of that effort there have been studies attempting to correlate magnetic variations, or more precisely, electro-magnetic variations, with earthquakes. It is worth acknowledging that geophysicists would actually dearly love to demonstrate a causal relationship between electro-magnetic variations and earthquakes, especially if they could be used for predicting earthquakes! Unfortunately, no convincing evidence of a correlation has been found, despite decades of work. And it should be emphasized that isolated coincidences are not sufficient to demonstrate a relationship. What is needed to confirm an extraordinary claim is, of course, an extraordinary amount of evidence, which in this case would mean many repeated correlations of earthquakes with specific and identifiable field variations. Such evidence simply doesn’t exist in this case.

14. Does the Earth’s magnetic field affect human health?

Not directly, no. High-altitude pilots can experience enhanced levels of radiation during magnetic storms, but the hazard is due to the radiation, not the magnetic field itself. Direct effects on human health by the magnetic field at the Earth’s surface are, quite frankly, insignificant. The primary effects of geomagnetism are on the health of electrically-based technological systems that are critically important to the modern civilization of humanity, not the humans themselves.

15. What about other magnetic fields, such as those from power lines, do they affect human health?

This is, of course, not a question about geophysics. Nonetheless, it is a question we are often asked, and so, we refer the curious reader to the following authoritative articles:

Bennett, W. R., April 1994. Cancer and power lines, Physics Today, 47, 23-29.

Report on Health Effects from Exposure to Power-Line Frequency Electric and Magnetic Fields, 1999, NIH Publication No. 99-4493, The National Institute of Environmental Health Sciences.

Alternatively, the curious reader can visit the following websites:

The National Institute for Safety and Health, Topic Electric and Magnetic Fields.

The International World Health Organization, EMF Project.

16. Do animals use the magnetic field for orientation?

Yes. There is evidence that some animals, probably most notably sea turtles, have the ability to sense the Earth’s magnetic field (although probably not consciously) and to use this sense, along with their several other senses, for purposes of orientation. We acknowledge that this is an interesting subject, and inquisitive acquaintances have posed this question to us on many occasions. However, the issue of magnetic orientation by animals is really more a matter of biophysics rather than geophysics, and we will, therefore, refer the curious reader to the following authoritative articles:

Lohmann, K. J., Hester, J. T. & Lohmann, C. M. F., 1999. Long-distance navigation in sea turtles, Ethology Ecology & Evolution, 11, 1-23.

Skiles, D. D., 1985. The geomagnetic field: Its nature, history and biological relevance, In Magnetite Biomineralization and Magnetoreception by Living Organisms: A New Biomagnetism, Ed: Kirschvink, J. L., Jones, D. S. & MacFadden, B. J., Plenum Publishing Corporation, New York.

Walker, M. M., Dennis, T. E. & Kirschvink, J. L., 2002. The magnetic sense and its use in long-distance navigation by animals, Current Opinion in Neurobiology, 12, 735-744.

Wiltschko, R. & Wiltschko, W., 1995. Magnetic orientation in animals, Zoophysiology, 33, Springer Verlag, Berlin.

17. What is space weather?

Space weather is the state of the environment in space near the Earth, including the solar wind and the Sun’s magnetic field, the outer part of the Earth’s magnetic field called the magnetosphere, and the electrically conducting part of the Earth’s atmosphere called the ionosphere. All of these different parts of the near-Earth space environment can interact with each other dynamically, giving rise to occasional rapid variation in space-weather conditions, manifest at the Earth's surface as magnetic-field variation. The analogy with atmospheric weather, or meteorology, is a very loose one, but space weather, just like the weather we experience on the Earth’s surface, can change over time. There are periods of calm and there can be stormy periods as well. Working in partnership with other Federal Government agencies, the USGS Geomagnetism Program is an integral part of the National Space Weather Program as outlined in its Strategic Plan. The nation’s official information source for space weather is the National Oceanic and Atmospheric Administration’s Space Environment Center, an agency that is an important customer of the USGS Geomagnetism Program.

18. What is a magnetic storm?

A magnetic storm is period of time during which the magnetic field displays rapid temporal variation. The causes of magnetic storms are explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, magnetic storms have two basic causes. First of all, let us be reminded that the Sun is always emitting a wind of charged particles that flows outward into space away from the Sun itself. Occasionally the Sun emits a strong surge of solar wind, something called a coronal mass ejection. When this gust of solar wind impacts upon the outer part of the Earth’s magnetic field, the magnetosphere, the field is disturbed and it undergoes a complex oscillation. This causes the generation of associated electric currents in the near-Earth space environment, which, in turn, generate additional magnetic-field variations -- all of which constitute a 'magnetic storm'. The second cause of magnetic storms is the occasional direct linkage of the Sun’s magnetic field with that of the Earth’s. This direct magnetic connection is not the normal state of affairs in the space environment, but when it occurs, charged particles, traveling along magnetic-field lines, can easily enter the magnetosphere, generate currents, and cause the magnetic field to undergo time-dependent variation. On occasion, the Sun emits a coronal mass ejection at a time when the magnetic-field lines of the Earth and Sun are directly connected. Then we can experience a truly large magnetic storm, which can be easily measured by magnetic observatories on the Earth’s surface.

19. What are the hazardous effects of magnetic storms?

The infrastructure and activities of our modern technologically-based society can be adversely affected by rapid magnetic-field variations generated by electric currents in the near-Earth space environment, particularly in the ionosphere and magnetosphere. This is especially true during so-called ‘magnetic storms’. Because the ionosphere is heated and distorted during storms, long-range radio communication, which relies on sub-ionospheric reflection, can be difficult or impossible and global-positioning systems (GPS), which relies on radio transmission through the ionosphere, can be degraded. Ionospheric expansion can enhance satellite drag and thereby make their orbits difficult to control. During magnetic storms, satellite electronics can be damaged through the build up and subsequent discharge of static-electric charges, and astronaut and high-altitude pilots can be subjected to increased levels of radiation. There can even be deleterious effects on the ground: pipe-line corrosion can be enhanced, and electric-power grids can experience voltage surges that cause blackouts. The reason why space-based effects can have consequences down here on the Earth’s surface is related, at least in part, to our answer to the first question, ‘What is a magnetic field?’. Electric currents in one place can induce electric currents in another place, this action at a distance is accomplished via a magnetic field. So, even though rapid magnetic-field variations are generated by currents in space, very real effects, such as unwanted electric currents induced in electric-power grids, can result down here on the Earth’s surface. More generally, the hazardous effects associated with geomagnetic activity, which are discussed more fully in the Further Reading page of this website, are one reason why the USGS Geomagnetism Program is part of the Central Region Geohazards Team.

20. Why measure the magnetic field at the Earth’s surface? Wouldn’t satellites be better suited for space-weather studies?

Both satellites and ground-based magnetometers are important for making measurements of the Earth’s magnetic field. They are not redundant, but are, instead, complementary. After executing several orbits of the Earth, satellites can provide good geographical coverage for data collection. On the other hand, ground-based magnetometers are much less expensive than satellites, they are much easier to install and control than satellites, and, with an array of magnetometers, they can provide coverage from numerous locations simultaneously. Another consideration is that satellites orbit the Earth either inside or above the ionosphere, the electrically conducting part of the Earth’s atmosphere. Since currents in the ionosphere contribute to the magnetic field, this means that the field measured by a satellite is somewhat different than the field measured at the surface. Finally, don’t forget that it is at the surface of the Earth, where we live, that many of the effects of space weather are most important, so measurements from ground-based observatories will always play a critical role in space-weather studies.

21. What are Aurorae?

Aurorae are a luminous glow of the upper atmosphere caused by energetic particles descending from the Earth’s magnetosphere or coming directly from the Sun. These energetic particles are mostly electrons, but protons can also be involved, and their energetic rain into the atmosphere is greatest during magnetic storms. As the particles descend, they collide with molecules in the atmosphere, causing an excitation of the oxygen and nitrogen molecular electrons. The molecules can return to their original, unexcited state by emitting a bit of light, a photon. This light, a photograph of which appears in the banner of this website, is the aurora that we see. Since electrically-charged particles tend to follow magnetic-field lines, and since magnetic-field lines are oriented in and out of the Earth and its atmosphere, near the magnetic poles, aurorae tend to be seen at high latitudes.