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Geomagnetic Field Frequently Asked Questions

[8]1. What is the Earth's magnetic field?

[9]1a. What are the basics?
[10]1b. Is the magnetic field different in different places of
the Earth?

[11]2. What are the magnetic elements?

[12]3. What is the Main Field?
[13]4. Where are the magnetic poles?

[14]4a. What is a magnetic pole?
[15]4b. Where are they?
[16]4c. What is the magnetic equator?

[17]5. How does a magnetic compass work?

[18]5a. Does the compass needle point to the magnetic pole?
[19]5b. What happens to my compass at the magnetic pole?
[20]5c. How do I correct my compass bearing to true bearing?
[21]5d. What influences the magnetic field measured by my
compass?

[22]6. What are geomagnetic models?

[23]6a. What are magnetic field models and why do we need them?
[24]6b. How accurate are the magnetic field models?
[25]6c. How often are new models adopted?
[26]6d. How do I get the latest model?

[27]7. Will the magnetic field reverse?

[28]7a. Has the Earth's magnetic field changed significantly in
the last several years?
[29]7b. Is the Earth's magnetic field going to reverse?

[30]8. Where can I find out more about geomagnetism?

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
[31]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 [32]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.

1b. 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 [33]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.

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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 [35]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.

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=gauss) but are
generally reported in nanoTesla (1nT * 100,000 = 1 Oersted). 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.

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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
[37]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 [38]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 ([39]IGRF) and World Magnetic Model
([40]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.

[41]top

4. Where are the magnetic poles?

_4a. What is a magnetic pole?_
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.

4b. Where are they?
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:

78.5 N and 103.4 W degrees, near Ellef Ringnes Island, Canada
65 S and 139 E degrees, in Commonwealth Bay, Antarctica

The current model dip pole based on the IGRF 1995, computed for
mid-1996 is:

79.0 N and 105.1 W degrees
64.7 S and 138.6 E degrees

Another pole position is the geomagnetic dipole (geocentric dipole).
This is the pole position based on the first three terms of the
current [42]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.

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 [43]image shows the
magnetic equator in green.

[44]top

5. How does a 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 [45]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.

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). 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.

5c. 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 [46]east magnetic declination and a [47]west magnetic
declination.

5d. 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.

[48]top

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 [49]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 ([50]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 ([51]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.

6d. How do I get the latest model?
NGDC/WDC-A archives and distributes the IGRF, WMM and other models.
You can [52]download the software and latest model available at no
charge or order the software, model and documentation package for a
slight fee. Go to http://www.ngdc.noaa.gov/seg/potfld/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
[53]on-line from our web site.

[54]top

7. Will the magnetic field reverse?

7a. 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).

7b. 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.

During the past 100 million years, the reversal rates vary
considerably. Recent rock records indicate reversals occurring on time
scales of about 200,000 years. The last time the magnetic field
reversed was about 750,000 - 780,000 years ago.

[55]top

8. Where can I find out more about geomagnetism?

There are plenty of great places on the Web to get more information
about geomagnetism. We have compiled a list of useful links. Go to:
[56]http://www.ngdc.noaa.gov/seg/potfld/servers.shtml

[57]top

blue horizontal line

[58]Magnetic Declination On-Line   |   [59]F A Q   |   [60]Models and
Software
[61]Images of Earth's Magnetic Field   |   [62]Today's Space Weather
|   [63]Web Links
[64]Space Physics Interactive Data Resource

blue horizontal line

Questions? Please contact:
[65]Susan McLean - Main Field
[66]Eric Kihn - Space Weather
[67]Dan Metzger - Marine Magnetics
URL: http://www.ngdc.noaa.gov/seg/potfld/faqgeom.shtml
_________________________________________________________________

Last Modified on: Tuesday, 24-Sep-2002 09:49:48 MDT

References

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67. mailto:%20Dan.R.Metzger at noaa.gov