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Geomagnetism: The Magnetic Field of the
Earth |
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The
Magnetic Field Model
The
Earth as Dynamo
Components
of the Magnetic Field
Variations
in the Earth's Magnetic Field
Influence
of the Sun
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The Magnetic Field Model |
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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.
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The Earth as Dynamo |
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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.
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Components of the Magnetic Field |
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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.
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Declination
Inclination
Total Intensity
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Variations in the Earth's Magnetic Field |
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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.
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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. |
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Influence of the Sun |
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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.
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Illustration by K. Endo, Nikkei Science Inc. - Japan
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