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Geology
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Declination
Horizontal Intensity
Vertical Intensity Secular Variation
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Declination Component
Horizontal Intensity
Vertical Intensity Secular Variation
 

Article

  Geomagnetism:  The Magnetic Field of the Earth
downThe Magnetic Field Model
downThe Earth as Dynamo
downComponents of the Magnetic Field
downVariations in the Earth's Magnetic Field
downInfluence of the Sun
  The Magnetic Field Model
 

Geomagnetic models form the foundation of traditional, compass-based navigational systems. These models provide a picture of the Earth's magnetic field and how it varies from one point on the Earth's surface to another.

The primary world model is the International Geomagnetic Reference Field (IGRF), compiled from magnetic measurements collected by national observatories in many countries, as well as readings made from ships, airplanes, and satellites. The model, derived through mathematical analysis of a vast amount of data, represents the magnetic field generated in the Earth's core, with small-scale variations at the surface and solar effects filtered out of the basic data. Even in an age of Global Positioning System (GPS) navigation, when finding your position on the Earth's surface is just a click away, the geomagnetic model still plays a vital role, it is built into GPS navigation systems as a backup. The geomagnetic field model is also vital to various kinds of magnetic surveys, such as those used in mineral exploration and the mapping of hazardous earthquake faults.

  DOD World Magnetic Chart
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  The Earth as Dynamo
 

The earth as dynamo.  Lines show a possible configuration of fluid flow and magnetic field in the liquid outer core
Diagram courtesy of Scientific American (December 1989)
The Earth's magnetic field is generated within its molten iron core through a combination of thermal movement, the Earth's daily rotation, and electrical forces within the core. These elements form a dynamo that sustains a magnetic field that is similar to that of a bar magnet slightly inclined to a line that joins the North and South Geographic Poles. A compass placed in this magnetic field thus does not point due north, declination measures the angle between the compass reading at any point on the Earth's surface and true north (measured in degrees). The geomagnetic reference model is the basis for establishing the declination and its variation across the surface of the globe.

  This map shows lines of equal declination in the contiguous United States. A compass placed anywhere along the 12° E contour, for example, would read that (magnetic) north is 12° E of true north. True north is displayed with longitudinal reference lines.
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  Components of the Magnetic Field
 

Magnetic field components The direction and strength of the magnetic field can be measured at the surface of the Earth and plotted. The total magnetic field can be divided into several components:

  • Declination (D) indicates the difference, in degrees, between the headings of true north and magnetic north.
  • Inclination (I) is the angle, in degrees, of the magnetic field above or below horizontal.
  • Horizontal Intensity (H) defines the horizontal component of the total field intensity.
  • Vertical Intensity (Z) defines the vertical component of the total field intensity.
  • Total Intensity (F) is the strength of the magnetic field, not divided into its component parts.
 
Declination
US Map - Declination

Inclination
US map - Inclination

Total Intensity
US map - Total Intensity
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  Variations in the Earth's Magnetic Field
 

Graph showing Magnetic Declination Observations at San Francisco, California (1783-1991) and IGRF Models (1965-1995)
Courtesy of John Hillhouse, USGS
The intensity and structure of the Earth's magnetic field are always changing, slowly but erratically, reflecting the influence of the flow of thermal currents within the iron core. This variation is reflected in part by the wandering of the North and South Geomagnetic Poles. Because a wide range of commercial and military navigation and attitude/heading systems are dependent on models of the magnetic field, these models need to be updated periodically. The magnetic field's strength and direction and their rates of change are predicted every 5 years for a 5-year period.


  Charts of secular variation
  Charts of secular variation document the predicted yearly changes in each of the components of the magnetic field, providing the information necessary to update the field strength and direction information during the 5 year periods that separate publication of new models. Older models continue to be of use for such necessary processes as the establishment of property boundaries. Compass readings for points located in the past were likely to have been made using a geomagnetic model with a declination value different from that in use today. Re-surveying boundaries requires access to the geomagnetic model in use at the time of the original survey.
  DOD World Magnetic Chart
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  Influence of the Sun
  Electrical particles streaming from the sun cause the "solar wind" which warps Earth's geomagnetic field lines, flattening them on the sun-ward side and stretching them out on the downstream side. The influence of this distortion of the geomagnetic field is quite small near Earth's surface (except during solar eruptions associated with sunspots) and becomes larger with increasing distance from Earth.
  Solar wind, as depicted in this artist's illustration, travels from the Sun and envelops the Earth's magnetic field. High-energy pulses of solar wind from sunspot activity ("solar bursts" or "plasma bubbles") travel from the Sun to the Earth at speeds exceeding 500 miles per second. The pulses distort the Earth's magnetic field and produce geomagnetic storms that disrupt the Earth's environment.
Illustration by K. Endo, Nikkei Science Inc. - Japan
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