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Pedro Reinel's compass rose from 1504 map, from Patricia Seed MIT
Illustration courtesy of Patricia Seed

Geomagnetism Frequently Asked Questions

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  • 1. What is the Earth's magnetic field?
    • 1a. What are the basics?
      • The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. This magnetic field changes both with time and with location on the Earth and resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth's magnetic field is characterized by a direction and intensity which can be measured. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z. From these elements, all other parameters of the magnetic field can be calculated.
    • 1b. Is the magnetic field different in different places of the Earth?
      • Yes, the magnetic field is different in different places. In fact, the magnetic field changes with both location and time and changes the way it is changing. It is so irregular that it must be measured in many places to get a satisfactory picture of its distribution. This is done using satellites, at the approximately 200 operating magnetic observatories worldwide, and at several more temporary sites. However, there are some regular features of the magnetic field. At the magnetic poles, a dip needle stands vertical (dip=90 degrees), the horizontal intensity is zero, and a compass does not show direction (D is undefined). At the north magnetic pole, the north end of the dip needle is down; at the south magnetic pole, the north end is up. At the magnetic equator the dip or inclination is zero. Unlike the Earth's geographic equator, the magnetic equator is not fixed, but slowly changes.
  • 2. What are the magnetic elements?
    • To measure the Earth's magnetism in any place, we must measure the direction and intensity of the field. The Earth's magnetic field is described by seven parameters. These are
      declination (D),
      inclination (I),
      horizontal intensity (H),
      the north (X) and east (Y) components of the horizontal intensity.
      vertical intensity (Z), and
      total intensity (F)

      The parameters describing the direction of the magnetic field are declination (D) and inclination (I). D and I are measured in units of degrees, positive east for D and positive down for I. The intensity of the total field (F) is described by the horizontal component (H), vertical component (Z), and the north (X) and east (Y) components of the horizontal intensity. These components may be measured in units of gauss but are generally reported in nanoTesla (1nT * 100,000 = 1 gauss). The Earth's magnetic field intensity is roughly between 25,000 - 65,000 nT (.25 - .65 gauss). Magnetic declination is the angle between magnetic north and true north. D is considered positive when the angle measured is east of true north and negative when west. Magnetic inclination is the angle between the horizontal plane and the total field vector, measured positive into Earth. In older literature, the term "magnetic elements" often referred to D, I, and H.

  • 3. What is the Main Field?
    • The geomagnetic field measured at any point on the Earth's surface is a combination of several magnetic fields generated by various sources. These fields are superimposed on and interact with each other. More than 90% of the field measured is generated INTERNAL to the planet in the Earth's outer core. This portion of the geomagnetic field is often referred to as the Main Field. The Main Field varies slowly in time and can be described by Mathematical Models such as the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM).

      The Main Field creates a cavity in interplanetary space called the magnetosphere, where the Earth's magnetic field dominates in the magnetic field of the solar wind. The magnetosphere is shaped somewhat like a comet in response to the dynamic pressure of the solar wind. It is compressed on the side toward the sun to about 10 Earth radii and is extended tail-like on the side away from the sun to more than 100 Earth radii. The magnetosphere deflects the flow of most solar wind particles around the Earth, while the geomagnetic field lines guide charged particle motion within the magnetosphere.

      The differential flow of ions and electrons inside the magnetosphere and in the ionosphere form current systems, which cause variations in the intensity of the Earth's magnetic field. These EXTERNAL currents in the ionized upper atmosphere and magnetosphere vary on a much shorter time scale than the INTERNAL Main Field and may create magnetic fields as large as 10% of the Main Field.

      It is the Main Field component that is modeled by the International Geomagnetic Reference Field (IGRF) and World Magnetic Model (WMM). Other important sources are the fields arising from electrical currents flowing in the ionized upper atmosphere, and the fields induced by currents flowing within the Earth's crust. The Main Field component varies slowly in time and can be grossly described as that of a bar magnet with north and south poles deep inside the Earth and magnetic field lines that extend well out into space. The Earth's magnetic field varies both in space and time.

  • 4. Where are the magnetic poles?
    • 4a. What is a magnetic pole?
      • The magnetic poles are defined as the area where dip (I) is vertical. You can compute this area using magnetic field models, such as the World Magnetic Model (WMM) and the International Geomagnetic Reference Field (IGRF). You can also survey for the magnetic pole, using instruments that measure the magnetic field strength and direction. In practice, the geomagnetic field is not exactly vertical at these poles, but is vertical on oval-shaped loci traced on a daily basis, with considerable variation from one day to another, and approximately centered on the dip pole positions. Magnetic declination (D) is unreliable near the poles.

    • 4b. Where are they?
      • The geomagnetic poles or geocentric dipole, can be computed from the first three Gauss coefficients from a main field model, such as the WMM or IGRF. Based on the WMM2010 coefficients for 2010.0 the geomagnetic north pole is at 72.21°W longitude and 80.08°N latitude, and the geomagnetic south pole is at 107.79°E longitude and 80.08°S latitude. The axis of the dipole is currently inclined at 9.98° to the Earth's rotation axis. The same dipole is the basis of the simple geomagnetic coordinate system.

        The magnetic poles or dip pole are computed from all the Gauss coefficients using an iterative method. Based on the current WMM model, the 2010 location of the north magnetic pole is 84.97°N and 132.35°W and the south magnetic pole is 64.42°S and 137.34°E.

        The location of the center of the eccentric dipole, sometimes known as the magnetic center, computed using the first eight Gauss coefficients for 2010.0, is at approximately (r, φ´, λ) = (563 km, 22.49°N, 140.22°E).

        The task of locating the principal magnetic pole by instrument is difficult for many reasons; the large area over which the dip or inclination (I) is nearly 90 degrees, the pole areas are not fixed points, but move tens to hundreds of kilometers because of daily variations and magnetic storms, and finally, the polar areas are relatively inaccessible to survey crews . Natural Resources Canada (NRCan) tracked the North Magnetic Pole, which is slowly drifting across the Canadian Arctic, by periodically carrying out magnetic surveys to reestablish the Pole's location from 1948 to 1994. An international collaboration, led by a French fundraising association, Poly-Arctique, and involving NRCan, Institut de Physique du Globe de Paris and Bureau de Recherche Geologique et Miniere, added two locations of the North Magnetic Pole in 2001 and 2007. The most recent survey determined that the Pole is moving approximately north-northwest at 55 km per year. For more details visit our page on polar wandering. The observed position for 2001 and estimated positions for 2002 to 2005 are:

        Year Latitude (°N) Longitude (°W)
        2001 81.3 110.8
        2002 81.6 111.6
        2003 82.0 112.4
        2004 82.3 113.4
        2005 82.7 114.4
        Observed position of the South Magnetic Pole
        2001 64.7° S 138.0° E
        Source: Canadian Geologic Survey
    • 4c. What is the magnetic equator?
      • The magnetic equator is where the dip or inclination (I) is zero. There is no vertical (Z) component to the magnetic field. The magnetic equator is not fixed, but slowly changes. North of the magnetic equator, the north end of the dip needle dips below the horizontal, I and Z are positive. South of the magnetic equator, the south end dips below the horizontal, I and Z are measured negative. As you move away from the magnetic equator, I and Z increase. This image shows the magnetic equator in green.

        If you find this topic interesting, we suggest the following sites for more information:

      • The Geological Survey of Canada excellent site on geomagnetism and magnetic poles
      • The Australian Antarctic Division discussion of poles and projections
      • NASA's Earth's Inconsistent Magnetic Field story about Earth's magnetic field
  • 5. How does a magnetic compass work?
    • 5a. Does the compass needle point toward the magnetic pole?
      • No. The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination (the angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A mile or two away the magnetic declination may be considerably different, requiring a different correction. NGDC has an on-line magnetic declination calculator where you can enter your location (or zip code for the USA) and get the Declination value. Remember: east declination is positive, west negative.
    • 5b. What happens to my compass at the magnetic pole?
      • A magnetic compass needle tries to align itself with the magnetic field lines. However, at (and near) the magnetic poles, the fields of force are vertically converging on the region (the inclination (I) is near 90 degrees and the horizontal intensity (H) is weak). The strength and direction tend to "tilt" the compass needle up or down into the Earth. This causes the needle to "point" in the direction where the compass is tilted regardless of the compass direction, rendering the compass useless.

        There are established zones around the north and south magnetic poles where compass behavior is deemed to be "erratic" and "unusable". These zones are defined where H (the horizontal intensity) is between 3000 nT - 6000 nT (erratic zone) and H is less than 3000 nT (unusable zone). Experts in the field claim that if you have a good compass and are careful, you can get decent results through the "erratic" zone. However, when H is small (H < 2000nT), the daily variation in D can easily be greater than 10 degrees. The Canadian Geological Survey has excellent information on their web site concerning magnetism and the north magnetic pole.

    • 5c. What happens to my compass in the southern hemisphere?
      • For a compass to work properly, the compass needle must be free to rotate and align with the magnetic field. The difference between compasses designed to work in the northern and southern hemispheres is simply the location of the "balance", a weight placed on the needle to ensure it remains in a horizontal plane and hence free to rotate. In the northern hemisphere, the magnetic field dips down into the Earth so the compass needle has a weight on the south end of the needle to keep the needle in the horizontal plane. In the southern hemisphere, the weight needs to be on the north end of the needle. If you did not change the weight, the needle would not rotate freely, and hence would not work properly.
    • 5d. How do I correct my compass bearing to true bearing?
      • You can compute the true bearing from a magnetic bearing by adding the magnetic declination to the magnetic bearing. This works so long as you follow the convention of degrees west are negative (i.e. a magnetic declination of 10-degrees west is -10 and bearing of 45-degrees west is -45). Some example case illustrations are provided for an east magnetic declination and a west magnetic declination.
    • 5e. What influences the magnetic field measured by my compass?
      • The Earth's magnetic field is actually a composite of several magnetic fields generated by a variety of sources. These fields are superimposed on each other and through inductive processes interact with each other. The most important of these geomagnetic sources are:
        a. the Earth's conducting, fluid outer core (~90%);
        b. magnetized rocks in Earth's crust
        c. fields generated outside Earth by electric currents flowing in the ionosphere and magnetosphere
        d. electric currents flowing in the Earth's crust (usually induced by varying external magnetic fields)
        e. ocean current effects

        These contributions all vary with time on scales ranging from milliseconds (micropulsations) to millions of years (magnetic reversals). More than 90% of the geomagnetic field is generated by the Earth's outer core. It is this portion of the geomagnetic field that is represented by the Magnetic Field Models.

  • 6. What are geomagnetic models?
    • 6a. What are magnetic field models and why do we need them?
      • Because the Earth's magnetic field is constantly changing, it is impossible to accurately predict what the field will be at any point in the very distant future. By constantly measuring the magnetic field, we can observe how the field is changing over a period of years. Using this information, it is possible to create a mathematical representation of the Earth's main magnetic field and how it is changing. Since the field changes the way it is changing, new observations must continually be made and models generated to accurately represent the magnetic field as it is.
    • 6b. How accurate are the magnetic field models?
      • The accuracy of a model in calculating the magnetic field influencing a compass or other magnetic sensor is affected by many things, including where you are using the compass. In general, the present day field models such as the IGRF and World Magnetic Model (WMM) are accurate to within 30 minutes of arc for D and I and about 200 nanoTesla for the intensity elements. It is important to understand that local anomalies exceeding 10 degrees, although rare, do exist. Local anomalies of 3 to 4 degrees also exist in relatively limited spatial areas. One area in Minnesota has a mapped anomalous area of 16 degrees east declination with anomalies a few miles away of 12 degrees west!
    • 6c. How often are new models adopted?
      • A new International Geomagnetic Reference Field (IGRF) is adopted every five years. The IGRF for 2010.0 through 2015.0 was adopted in the fall of 2009 by the International Association for Geomagnetism and Aeronomy (IAGA). The 2010.0 - 2015.0 World Magnetic Model (WMM) developed by the National Geophysical Data Center (NGDC) and the British Geological Survey, was made available in January 2010. Models need to be revised at least every five years because of the changing nature of the magnetic field. Existing models forward predict the magnetic field based on the rate of change in the several years preceding the model generation. Since that rate of change itself is changing, to continue to use models beyond 5 years introduces progressively greater errors in the field parameters calculated.
    • 6d. How do I get the latest model?
      • NGDC/WDC for Solid Earth Geophysics archives and distributes the IGRF, WMM and other models. You can download the software and latest model available at no charge or order the software, model and documentation package for a slight fee. Go to /geomag/magmodel.shtml. We have software developed to run on an IBM compatible PC and on some UNIX machines. Alternatively, you can run the latest IGRF model on-line from our web site.
  • 7. What are the Space Weather Scales?
    • The NOAA Space Weather Scales were introduced as a way to communicate the current and future space weather conditions and their possible effects on people and systems. The scales describe the environmental disturbances for three event types: geomagnetic storms, solar radiation storms, and radio blackouts. The scales have numbered levels, analogous to the Saffir-Simpson hurricane and Fujita tornado scales that convey severity and list possible effects at each level. The scales also show how often such events happen, and give a measure of the intensity of the physical causes. Visit NOAA's Space Environment Center to learn more about Space Weather Scales.

  • 8. Will the magnetic field reverse?
    • 8a. Has the Earth's magnetic field changed significantly in the last several years?
      • The Earth's magnetic field is slowly changing and appears to have been changing throughout its existence. When the tectonic plates form along the oceanic ridges, the magnetic field that exists is imprinted on the rock as they cool below about 700 Centigrade. The slowly moving plates act as a kind of tape recorder leaving information about the strength and direction of past magnetic fields. By sampling these rocks and using radiometric dating techniques it has been possible to reconstruct the history of the Earth's magnetic field for the last 160 million years or so. Older "paleomagnetic" data exists but the picture is less continuous. An interlocking body of evidence, from many locations and times, give paleomagnetists confidence that these data are revealing a correct picture of the nature of the magnetic field and the Earth's plate motions. In addition, if one "plays this tape backwards" the continents, which ride on the tectonic plates, reassemble along their edges with near perfect fits. These "reassembled continents" have matching fossil floras and faunas. The picture that emerges from the paleomagnetic record shows the Earth's magnetic field strengthening, weakening and often changing polarity (North and South magnetic poles reversing).
    • 8b. Is Earth's magnetic field going to reverse?
      • While we now appear to be in a period of declining magnetic field strength, we cannot state for certain if or when a magnetic reversal will occur. Based on measurements of the Earth's magnetic field taken since about 1850 some paleomagnetists estimate that the dipole moment will decay in about 1,300 years. However, the present dipole moment (a measure of how strong the magnetic field is) is actually higher than it has been for most of the last 50,000 years and the current decline could reverse at any time. Even if Earth's magnetic field is beginning a reversal, it would still take several thousand years to complete a reversal. We expect Earth would still have a magnetic field during a reversal, but it would be weaker than normal with multiple magnetic poles. Radio communication would deteriorate, navigation by magnetic compass would be difficult and migratory animals might have problems.
    • 8c. How often does the magnetic field reverse?
      • During the past 100 million years, the reversal rates vary considerably. Consecutive reversals were spaced 5 thousand years to 50 million years. The last time the magnetic field reversed was about 750,000 - 780,000 years ago. While we now appear to be in a period of declining magnetic field strength, we cannot state for certain if or when a magnetic reversal will occur. Based on measurements of the Earth's magnetic field taken since about 1850 some paleomagnetists estimate that the dipole moment will decay in about 1,300 years. However, the present dipole moment (a measure of how strong the magnetic field is) is actually higher than it has been for most of the last 50,000 years and the current decline could reverse at any time.
    • 8d. What causes the magnetic field to reverse?
      • There are special challenges to understanding a phenomena that occurs so rarely and originates thousands of kilometers below the surface of the Earth. There are no specific causes driving the magnetic field reversals, rather these occur as part of the on-going chaotic behavior of the Earth's dynamo.
    • 8e. Will a reversal of the magnetic field affect animal behavior?
      • Many migratory animals use the geomagnetic field to orient themselves. However, the mechanism underlying this ability of animals remains unknown. Experiments show that migratory birds can sense the declination and inclination of the local geomagnetic field. Changing the polarity of the horizontal magnetic field is known to affect the hanging position of bats. Some migrating butterflies use the geomagnetic field for direction. In the ocean, spiny lobsters, dolphins, and whales are known to use geomagnetic field for directions. It is thus, possible that a reversal of geomagnetic field affect the migratory behavior of some animals. Since the chance of a reversal in the near future (in the next few hundred years) is very low, no immediate concern is required.
  • 9. What are some other uses of geomagnetic measurements?
    • Magnetic field measured on the surface of the Earth is a composite of the main magnetic field generated in the Earth's core and the crustal magnetic field dependent on the magnetization and iron content of the subsurface materials. Hence, magnetic exploration is a powerful tool to detect subsurface magnetic features. Magnetic surveys are typically carried out by ships or aircrafts, with magnetometers mounted on a boom - an extension from the body of the craft. Though less common, magnetic surveys are also carried out by foot. The strength of the magnetic signal from rocks is typically less than 1% of the strength of the Earth's main magnetic field. However with the use of a geomagnetic field model (e.g. the International Geomagnetic Reference Field - IGRF ), these tiny signals can be recovered from the measured data. Magnetic methods are used in oil exploration to determine depth to the basement rock, in mineral exploration to detect magnetic minerals or to locate a dike (dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks), and in archaeological surveys to detect buried artifacts, grave sites etc. Magnetic surveys can also help locate ferrous objects (drums, storage tanks, and in at least one well-publicized case a Cadillac car, etc.) that are buried under ground.
  • 10. Where can I find out more about geomagnetism?