Illustration courtesy of Patricia Seed |
Geomagnetism Frequently Asked Questions
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- 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
surrounded by a magnetic field. This
magnetic field changes both with time and with location on the Earth and
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 and time, 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 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. In fact, the
magnetic field changes with both location and time and changes the way
it is changing. It is so irregular that it must be measured in many places
to get a satisfactory picture of its distribution. This is done using
satellites,
at the approximately 200 operating
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.
- 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 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).
The 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.
- 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 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
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 (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 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.
- 4. Where are the magnetic poles?.
- 4a. 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 undefined at the
poles and unreliable near the poles.
- 4b. Where are they?
-
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 current WMM model, the 2005 location of
the geomagnetic north pole is 79.74°N and 71.78°W and the
geomagnetic south pole is 79.74°S and 108.22°E.
The magnetic poles or dip pole are computed from
all the Gauss coefficients using an iterative method. Based
on the current WMM model, the 2005 location of the north magnetic pole
is 83.21°N and 118.32°W and the
south magnetic pole is 64.53°S and 137.86°E.
The location of the center of the eccentric dipole, sometimes known as the magnetic center,
computed using the first eight Gauss coefficients for 2005.0, is at approximately (r, φ´, λ) =
(552 km, 22.2°N, 141.6°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
(map of North
and South polar wander - courtesy
of Dr. John Quinn, U.S. Geological Survey retired). The Geological
Survey of Canada keeps track of the North Magnetic Pole, which is slowly
drifting across the Canadian Arctic, by periodically carrying out magnetic
surveys to redetermine the Pole's location. The most recent survey, completed
in May, 2001, determined an updated position for the Pole and established
that it is moving
approximately northwest at 40 km per year. The observed position for 2001
and estimated positions for 2002 to 2005 are:
| Year |
Latitude (°N) |
Longitude (°W) |
| 2001 |
81.3 |
110.8 |
| 2002 |
81.6 |
111.6 |
| 2003 |
82.0 |
112.4 |
| 2004 |
82.3 |
113.4 |
| 2005 |
82.7 |
114.4 |
| Observed position of the South Magnetic Pole |
| 2001 |
64.7° S |
138.0° E |
| Source: Canadian Geologic Survey |
- 4c. What is the magnetic equator?.
5. How does a magnetic 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
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. NGDC has an
on-line 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.
- 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 and the horizontal intensity (H) is weak). The strength
and direction tend to "tilt" the compass needle up or 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.
There are established zones around the north and south magnetic poles where compass behavior
is deemed to be "erratic" and "unusable". These zones are defined where
H (the horizontal intensity) is between 3000 nT -
6000 nT (erratic zone) and H is less than 3000 nT (unusable zone). Experts in
the field claim that if you have a good compass and are careful, you can get decent results
through the "erratic" zone. However, when H is small (H < 2000nT), the daily
variation in D can easily be greater than 10 degrees. The
Canadian Geological Survey has excellent information on their web site concerning magnetism
and the north magnetic pole.
- 5c. 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 the 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.
- 5d. 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
east magnetic declination and a
west magnetic declination.
- 5e. What influences the magnetic field measured by my compass?
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 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 (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.
- 6d. How do I get the latest model?
-
NGDC/WDC for Solid Earth Geophysics 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.
Go to /geomag/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
on-line from our web site.
7. What are the Space Weather Scales?
-
The NOAA Space Weather Scales were introduced as a way to communicate the current and
future space weather conditions and their possible effects on people and systems. The scales
describe the environmental disturbances for three event types: geomagnetic storms,
solar radiation storms, and radio blackouts. The scales have numbered levels,
analogous to the Saffir-Simpson hurricane and Fujita tornado scales that convey severity and
list possible effects at each level. The scales also show how often such events happen, and
give a measure of the intensity of the physical causes. Visit NOAA's Space Environment Center
to learn more about Space Weather
Scales.
8. Will the magnetic field reverse?
- 8a. 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).
- 8b. 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.
- 8c. 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 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.
- 8d. What causes the magnetic field to reverse?
-
There are special challenges to understanding a phenomena that occurs so rarely and originates thousands of kilometers below the surface of the Earth.
There are no specific causes driving the magnetic field reversals, rather these occur as part of the on-going chaotic behavior of the Earth's dynamo.
- 8e. 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 thus, possible
that a reversal of geomagnetic field affect 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, no immediate concern is required.
9. What are some other uses of geomagnetic measurements?
-
Magnetic field measured on the surface of the Earth is a composite of the main magnetic field generated in the 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 the 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 under ground.
10. Where can I find out more about geomagnetism?
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