|| The Geomagnetic Field -
Frequently Asked Questions
What is the Earth's magnetic field?
The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field . The Earth's magnetic field 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, 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 these elements, all other parameters of the magnetic field can be calculated.
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 parameters describing the direction of the magnetic field are declination (D), inclination (I). D and I are measured in units of degrees. 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 Oersted (1 oersted=1gauss) but are generally reported in nanoTesla (1nT * 100,000 = 1 0ersted). The Earth's magnetic field intensity is roughly between 25,000 - 65,000 nT (.25 - .65 oersted). 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. In older literature, the term 'magnetic elements' often referred to D, I, and H.
Is the magnetic field different in different places of the Earth?
Yes, the magnetic field is different in different places. It is so irregular that it must be measured in many places to get a satisfactory picture of its distribution. This is done at the approximately 200 operating magnetic observatories world-wide 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.
What are the magnetic poles?
The magnetic poles are defined as the area where dip is vertical. There are several definitions of magnetic pole. Two common uses are the "surveyed" magnetic dip pole where the magnetic field is measured to be vertical and the "modeled" magnetic dip pole based on a model of Earth's magnetic field where inclination is calculated to be 90 degrees. In reality, the surveyed magnetic pole is not a single point, but more likely an area where many 'magnetic poles' exist. The task of locating the principal magnetic pole 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. Based on recent (~1990) surveys carried out by the Canadian Geological Survey and the U.S. Naval Oceanographic Office (among others), the current location of the surveyed magnetic poles are approximately:
The current model dip pole based on the IGRF 1995, computed for mid-1996 is:
- 78.5 N and 103.4 W degrees, near Ellef Ringnes Island, Canada
- 65 S and 139 E degrees, in Commonwealth Bay, Antarctica
Another pole position is the geomagnetic dipole (geocentric dipole). This is the pole positions based on the first three terms of the current International Geomagnetic Reference Field, a model of the Earth's main magnetic field. Using the IGRF, and computing the symmetric positions where the dipole would intersect the Earth's surface, the pole positions are 79.3N, 71.5W for the north pole and 79.3S, 108.5E for the south pole. These positions are frequently used to generate the geomagnetic coordinate system.
- 79.0 N and 105.1 W degrees
- 64.7 S and 138.6 E degrees
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 (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.
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). The strength and direction tend to "pull" the compass needle 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.
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.
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%);
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.
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), and
e. ocean current effects
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.
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! To find out more about the limitations of the IGRF, see the IGRF Health Warning.
How often are new models adopted?
A new International Geomagnetic Reference Field (IGRF) is adopted every five years. The IGRF for 2000 through 2005 was adopted in the fall of 1999 by the International Association for Geomagnetism and Aeronomy (IAGA) at the General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Birmingham, England. The 2000-2005 World Magnetic Model (WMM) developed by the U.S. Geological Survey and the British Geological Survey, was made available in January 2000. 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.
How do I get the latest model?
NGDC/WDC-A 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. 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.
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 which emerges from the paleomagnetic record shows the Earth's magnetic field strengthening, weakening and often changing polarity (North and South magnetic poles reversing). During the past 100 million years, the reversal rates vary considerably. Recent rock records indicate reversals occuring on time scales of about 200,000 years. The last time the magnetic field reversed was about 750,000 - 780,000 years ago.
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.
Where can I find out more about geomagnetism?
NGDC has a brief description of the Earth's magnetic field as well as answers to some commonly asked questions. The following references are just a few of the books on magnetism available from your university library. We also recommend library searches for recent articles published in Science magazine, Scientific American, and other popular scientific magazines. Ask your librarian for help, or try searching the Web!
- Campbell, Wallace H, Introduction to Geomagnetic Fields Cambridge University
Press, ISBN 571936, 1997.
- Langel, R.A., The Main Field, in Geomagnetism, Volume
1, J. Jacobs (Editor), Academic Press, 1987.
- Chapman, S., and J. Bartels, Geomagnetism, Oxford University
Press (Clarendon), London and New York, Volumes 1 and 2, 1940.
- Nelson H. J., L. Hurwitz and D. Knapp, Magnetism of the Earth,
Publication 40-1 U.S. Department of Commerce, United States
Government Printing Office, Washington, 1962.
- United States Department of Commerce, Magnetic Poles and
the Compass, Serial 726, Washington, 1962 (out of print, reproductions
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Last Modified on:
Tuesday, 14-Nov-2000 11:44:05 MST