Consult these frequently asked questions for information about NCEI's geomagnetic data and products, descriptions of the Earth's magnetic field, and answers to common questions about the fundamentals of geomagnetic science.
What is Earth's magnetic field?
Earth acts like a large spherical magnet: it is surrounded by a magnetic field that changes with time and location.The field is generated by several sources inside and outside Earth. It is very similar to the field that a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of Earth would produce. The axis of the dipole is offset from the axis of 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 in time, 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.
What is the Main Field?
The geomagnetic field measured at any point on 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 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). Earth's Main Field dominates over the interplanetary magnetic field in the area called the magnetosphere. 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 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 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.
What are the magnetic elements?
To measure Earth's magnetism in any place, we must measure the direction and intensity of the field. 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). 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.
Does the magnetic field change in different locations?
Yes, the magnetic field is different in different places. In fact, the magnetic field changes with both location and time. It is so irregular that satellites and approximately 200 operating magnetic observatories must measure it in many places to get a satisfactory picture of its distribution.However, the magnetic field has some regular features. 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 Earth's geographic equator, the magnetic equator is not fixed, but slowly changes.
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. More information is available at wandering poles.
Where are the magnetic poles?
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 WMM2020 coefficients for 2020.0 the geomagnetic north pole is at 72.68°W longitude and 80.65°N latitude, and the geomagnetic south pole is at 107.32°E longitude and 80.65°S latitude. The axis of the dipole is currently inclined at 9.41° to Earth's rotation axis. The same dipole is the basis of the simple geomagnetic coordinate system.
The magnetic poles or dip poles are computed from all the Gauss coefficients using an iterative method. Based on the current WMM model, the 2020 location of the north magnetic pole is 86.5°N and 164.04°E and the south magnetic pole is 64.07°S and 135.88°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.
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.
How do I calculate the geomagnetic coordinates?
There are many different definitions of geomagnetic coordinates. The simplest is to take the location of the geomagnetic dipole and then do a coordinate transformation from coordinates centered on the geographic pole to coordinates centered on the dipole. Longitude 0 is defined as the imaginary line from the geographic north pole to the geomagnetic north dipole. A software to calculate geomagnetic coordinates from geographic coordinates is available here.
Has Earth's magnetic field changed significantly in the last several years?
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. Sampling these rocks and using radiometric dating techniques has made it possible to reconstruct the history of Earth's magnetic field for the last approximately 160 million years. 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 reveal a correct picture of the nature of the magnetic field and 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 Earth's magnetic field strengthening, weakening, and often changing polarity (North and South magnetic poles reversing).
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 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.
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 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.
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 possible that a reversal of the geomagnetic field affects 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, there is no immediate concern.
What are some other uses of geomagnetic measurements?
Magnetic field measured on the surface of Earth is a composite of the main magnetic field generated in 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 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 underground.
What are magnetic models and why do we need them?
Because 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 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 often are new models developed?
The 2020.0 - 2025.0 World Magnetic Model (WMM) developed by the National Centers for Environmental Information (NCEI) and the British Geological Survey, was made available in December 2019. A new one will be released December 2024. A new International Geomagnetic Reference Field (IGRF) is adopted every five years. The International Association for Geomagnetism and Aeronomy (IAGA) developed the IGRF for 2015.0 through 2020.0 in autumn of 2014. Models need to be revised at least every five years because of the changing nature of the magnetic field. Existing models predict the magnetic field based on the rate of change in the several years preceding the model generation. Since the rate of change itself is changing, using models beyond 5 years introduces progressively greater errors in the field parameters calculated.
What is the actual model range if the decimal year range is between 2020.0 and 2025.0?
1/1/2020 00:00:00 AM to 1/1/2025 00:00:00 AM (so the day 1/1/2025 is not included).
Where do I get the old WMM coefficients?
Please note that these models are outdated and that we do not recommend its use for any purposes other than software evaluation. Write to us at email@example.com for old coefficient files. For earlier than 2020.0, we recommend using the IGRF model (which are updated retrospectively with the newer data sets). Technical reports for the older versions of WMM are available in the WMM Code Changes accordion under the Software tab of the World Magnetic Model page.
What is the difference between IGRF and WMM models?
The WMM and the International Geomagnetic Reference Field (IGRF) are estimated from the most recent data and are of comparable quality. The differences between IGRF and WMM are within expected model inaccuracy. The WMM is a predictive-only model and is valid for the current epoch (2020.0 to 2025.0). The IGRF is retrospectively updated and the latest update, IGRF-12 is valid for the years 1900.0 - 2020.0. While IGRF is produced by the voluntary research of the scientific community under the banner of the International Association of Geomagnetism and Aeronomy (IAGA), the WMM is produced by NCEI and the British Geological Survey (BGS) for the US and UK defense agencies with guaranteed quality, user support, and updates. For the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO), the International Hydrographic Organization (IHO) and the Federal Aviation Administration (FAA) the standard is WMM. For other users, the choice between WMM and IGRF is arbitrary.
There seem to be many models listed here, WMM, IGRF, EMM, USHistoric, etc. What are the differences between them?
NCEI geomagnetic models can be split into two broad categories: predictive models designed to give magnetic field values for future dates, and historic models designed to give magnetic field values for past dates. Our predictive models are the WMM, Enhanced Magnetic Model (EMM), High Defintion Geomagnetic Model (HDGM), and HDGM-Real Time. These differ in what they predict; the WMM predicts only the main magnetic field generated by the Earth’s internal dynamo, while the EMM, HDGM and HDGM-RT include contributions from Earth’s crust. The HDGM also includes a basic model of the external field. The HDGM-RT includes a real-time model of Earth's external field. They also differ in how often they are updated; the WMM and EMM are updated once every five years, while the HDGM is updated every year.
Historic models that we use are the IGRF, the gUFM, the USHistoric model, as well as the IGRF+. The IGRF is the accepted international scientific model of Earth’s field going back to the year 1900. The gUFM is a research model created by Jackson, Jonkers, and Walker (Jackson et al., 2000, Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. Lond. A, 358, 957- 90.); it goes back to 1590. The IGRF+ is a combination of these models that uses an interpolation from 1890 to 1900 to ensure a smooth transition. Finally, the USHistoric is a model based on a polynomial interpolation of early magnetic data in the continental United States (This is different from all of our other models which use a spherical harmonic expansion to model the magnetic field).
Why do some of the geomagnetic models (e.g. WMM, IGRF) carry a valid time range and some (e.g. EMAG, MF6, NGDC720) do not?
The crustal-fields-only models (MF7 and EMAG2 available here) do not carry "start" and "expiration" dates. This is because the crustal field varies over geological time scales (thousands of years). These models get updated as new data arrives. Therefore, you should always use the latest model as it contains the most data. The WMM and IGRF series of models are core-field-only models and they do come with a start and expiration dates. This is because the magnetic field from Earth’s core changes (roughly by up to 200 nano-Tesla per year) over time due to the liquid flow in Earth’s core.
Is it possible to calculate declination for dates before 1900 outside of the United States?
Our calculators now support dates before 1900. The new range for the IGRF+ is 1590-2020. Based primarily on magnetic data from ship logs, the GUFM model (Jackson et al., 2000, Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. Lond. A, 358, 957- 90.) covers the period 1590 - 1990.
How accurate are the magnetic field models? What is the error of the declination estimates on the online calculators and models?
Actual measurements of the geomagnetic field at any location on Earth can be substantially different from the model outputs. These errors primarily arise from the omission of numerous magnetic signal sources while modeling. Contributions from magnetized crustal rocks and man-made magnetic noise sources (such as a buried iron pipe) are the major sources of error. Additionally, magnetic signals from near-Earth space also contribute to the total error.
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 magnetic anomalies exceeding 10 degrees of declination, 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!
The second type of error originates from errors in the data and methods used for developing the magnetic models. These are called errors of commission and are much smaller than the errors of omission. Figure 3 on the Limitations tab of the World Magnetic Model shows the inaccuracy estimated for the year 2020.0. However, the declination error is different in different parts of the Earth - but mostly less than 0.3°. For more information see the WMM technical report (p. 55) and the IGRF "Health Warning".
Do the models include magnetic fields from power lines or other manmade sources?
No, stray magnetic fields from power lines or similar sources are not included in the model. In general manmade structures are too small to be seen with our models, which have resolutions of at least 30 km.
Is there a simple formula or lookup table to calculate the declination of the magnetic field?
There is no simple equation connecting latitude and longitude to declination. Each time you access our online calculator, we run the model in the background. The model consists of a C driver program and a set of model coefficients. In the geomagnetic model, the information about the magnetic fields (varying with time and location) is stored as a set of "spherical harmonic coefficients". The C program reads these coefficients and computes the field elements for any location on Earth - declination one among them. The equations are given in the accompanying WMM technical report. The program and coefficients are available for download on the WMM Page under Data Access.
Is there a MATLAB wrapper file for World Magnetic Model or IGRF?
No. We do not offer MATLAB gateway / wrapper files for our models. MATLAB offers WMM as part of their aerospace toolbox (function “wrldmagm” for WMM and function “igrf11magm” for IGRF).
Does your agency still publish the hard copy magnetic declination charts?
We do not print paper copies of the declination charts anymore, but the charts are available online in pdf form. While they are designed to print in "poster size", 2'x3', they do scale down well to page size. See the WMM maps tab for examples.
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. NCEI has an online 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.
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 Earth. This causes the needle to "point" in the direction where the compass is tilted regardless of the compass direction, rendering the compass useless.
Based on the WMM military specification, we define "Blackout Zones" (BoZ) around the north and south magnetic poles where compass accuracy is highly degraded. The BoZ are defined as regions around the north and south magnetic poles where the horizontal intensity of Earth's magnetic field (H) is less than 2000 nT. In BoZs, WMM declination values are not accurate and compasses are unreliable.
We additionally define a "Caution Zone" (2000 nT <= H < 6000 nT) around the BoZ, where caution must be exercised while using a compass. Compass accuracy may be degraded in this region. The blackout (unreliable) and caution zones are displayed in Figures 1 and 2. The WMM online/offline software displays warnings when the user location is within these zones.
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 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.
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 that 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.
What influences the magnetic field measured by my compass?
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 following are the five most important geomagnetic sources:
- Earth's conducting, fluid outer core (~90%)
- Magnetized rocks in Earth's crust
- Fields generated outside Earth by electric currents flowing in the ionosphere and magnetosphere
- Electric currents flowing in Earth's crust (usually induced by varying external magnetic fields)
- 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 Earth's outer core. It is this portion of the geomagnetic field that is represented by magnetic field models, such as the WMM or IGRF.
How is altitude defined in the geomagnetic calculators and software? Is it above mean sea level or WGS84 ellipsoid?
The downloadable WMM software library provides functions to convert height above mean sea level (AMSL) to height above WGS84. Currently the online calculators use height or altitude above WGS84 ellipsoid. However, for most practical applications in geomagnetism, the difference between ellipsoidal height and sea-level-height is insignificant. A map of the difference in F between MSL and WGS 84 referencing is available on the WMM Maps tab.
What is a decimal year, and how do I convert the date to decimal year?
Decimal year is defined as year plus decimal fraction of year. This is obtained using the formula: Decimal_Year = Year + day_of_the_year/number_of_days_in_the_year.
For example, 2020-Jul-03 has decimal representation as 2020.5. An Excel spreadsheet "Date" entry (example: 12/31/2022), may be converted to decimal year using the function =YEAR(A1)+(A1-DATE(YEAR(A1),1,1))/(DATE(YEAR(A1),12,31)-DATE(YEAR(A1),1,0)) for a date value in A1.
Does the latitude/longitude input require geodetic coordinates or geocentric coordinates?
Input should be given as geodetic coordinates
How do I access your calculators programmatically for my app?
Our calculators provide an easy way for you to get results in HTML, JSON, XML, or CSV programmatically. To use the programmatic interface, you just have to url-encode the form parameters into a GET request including the result format you want. Every calculator is provided with instructions on how to access it programmatically. These instructions can be found by navigating to the calculator page of your choice and clicking on the "instructions" link in the upper righthand corner. Scroll the pop-up window down to "Using the Programmatic Interface".
Are there any limits to programmatic access on the calculators?
A maximum of 50 connections/sec from all the users is allowed at any time. If this is exceeded, the calculator stops taking requests for 10 seconds. Bulk calculations should be requested serially and not in parallel. If the requests start returning with errors then your script should go to sleep for 5-10 minutes before trying the next request.
I do not use C, how do I calculate magnetic declination for my application in a different programming language?
There are two ways to accomplish this. Use programmatic access as described above. Port NCEI geomagnetic software to your language of choice. Some developers share their successful ports, which are available in the "Third Party Software" section of the WMM Data Access tab.
Do you have software for calculating the magnetic field or declination at multiple points?
If the locations are on a regular grid, use our grid calculators. Otherwise, use the "wmm_file" software. Get the WMM2020_Windows.zip (or linux) file.
usage: wmm_file f input_file output_file
or for help: wmm_file h
Look at the sample_coords.txt for an example on how to write the xyz data. The output can then be imported to a spreadsheet program.
Are the magnetic models or software copyrighted?
No. The WMM source code is in the public domain and not licensed or under copyright. The information and software may be used freely by the public. As required by 17 U.S.C. 403, third parties producing copyrighted works consisting predominantly of the material produced by U.S. government agencies must provide notice with such work(s) identifying the U.S. Government material incorporated and stating that such material is not subject to copyright protection.
May I include an image from your webpage in my publication?
The images and maps created and posted online by NCEI are free to be used. You do not require our permission to do so. Though not required, we would appreciate you giving credit to NCEI for its products.
Earth's magnetic field is described by seven parameters. These are declination (D), inclination (I), horizontal intensity (H), vertical intensity (Z), total intensity (F) and the north (X) and east (Y) components of the horizontal intensity. By convention, declination is considered positive when measured east of north, inclination and vertical intensity positive down, X positive north, and Y positive east. The magnetic field observed on Earth is constantly changing.
The geomagnetic field measured at any point on 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 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 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 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 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 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 Earth and magnetic field lines that extend well out into space. Earth's magnetic field varies both in space and time.
There are several good sites on the web providing information on Earth's geomagnetic field. Some of the sites we think are worth a visit include Scientific American's Ask the Experts, the Geological Survey of Canada, and the U.S. Geological Survey. The AGU site includes several articles from scientists.