GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
EARTH’S MAGNETIC FIELD
Susan Macmillan
British Geological Survey, Edinburgh, UK.
Keywords:geomagnetism,Earth’smagnetic field, ompass, navigation, magnetic storms,
space weather
Contents
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1. Introduction
2. Geomagnetic Field Observations
3. Characteristics of Earth’s Magnetic Field
4. Earth’s Magnetic Field as Both a Tool and a Hazard in the Modern World
Glossary
Bibliography
Biographical Sketch
To cite this chapter
Summary
The geomagnetic field has its main source in the fluid outer core of Earth. Near the
surface of the planet it is observed to vary in space and in time on a huge range of
scales. These variations result from processes in regions ranging from the deep interior
of Earth, the crust, ionosphere, magnetosphere, all the way to the Sun. Observing
Earth’s magnetic field provides valuable insights on all these processes. These
observations are made by geomagnetic observatories, extensive surveys made on land,
at sea, and from aircraft and satellites, and from magnetic properties of rocks.
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1. Introduction
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Earth’s magnetic field is generated in the fluid outer core by a self-exciting dynamo
process. Electrical currents flowing in the slowly moving molten iron generate the
magnetic field. In addition to sources in Earth’s core, the magnetic field observable at
the planet’s surface has sources in the crust and in the ionosphere and magnetosphere.
The geomagnetic field varies on a range of scales, and a description of these variations
is now made—ordered from low- to high-frequency variations—in both the space and
time domains. The final section describes how Earth’s magnetic field can be both a tool
and a hazard to the modern world. First of all, however, methods of observing the
magnetic field are described.
2. Geomagnetic Field Observations
2.1. Definitions
The geomagnetic field vector, B, is described by the orthogonal components X
(northerly intensity), Y (easterly intensity), and Z (vertical intensity, positive
downwards); total intensity F; horizontal intensity H; inclination (or dip) I (the angle
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
between the horizontal plane and the field vector, measured positive downwards); and
declination (or magnetic variation) D (the horizontal angle between true north and the
field vector, measured positive eastwards). Declination, inclination, and total intensity
can be computed from the orthogonal components using the following equations:
D = arctan
Y
X
I = arctan
Z
H
F = H2 + Z2
where H is given by
X 2 +Y 2 .
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H=
The International System of Units (SI) unit of magnetic field intensity, strictly flux
density, most commonly used in geomagnetism is the Tesla. At Earth’s surface the total
intensity varies from 24 000 nanotesla (nT) to 66 000 nT. Other units likely to be
encountered are the Gauss (1 Gauss = 100 000 nT), the gamma (1 gamma = 1 nT), and
the Ørsted (1 Ørsted = (103/4π) A m–1).
2.2. Observatories
A geomagnetic observatory is a location where absolute vector observations of Earth’s
magnetic field are recorded accurately and continuously, with a time resolution of one
minute or less, over a long period. The site of the observatory must be magnetically
clean and remain so for the foreseeable future. The earliest magnetic observatories
where continuous vector observations were made began operation in the 1840s.
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There are two main categories of instruments at an observatory. The first category
comprises variometers that make continuous measurements of elements of the
geomagnetic field vector, but in arbitrary units, for example millimeters of photographic
paper in the case of photographic systems or electrical voltage in the case of fluxgates.
A fluxgate sensor comprises a core of easily saturable material with high permeability.
Around the core there are two windings: an excitation coil and a pick-up coil. If an
alternating current is fed into the excitation coil so that saturation occurs and if there is a
component of the external magnetic field along the fluxgate element, the pick-up coil
outputs a signal not only with the excitation frequency but also other harmonics related
to the intensity of the external field component. Both analog and digital variometers
require temperature-controlled environments and installation on extremely stable
platforms (though some modern systems are suspended and therefore compensate for
platform tilt). Even with these precautions they can still be subject to drift. They operate
with minimal manual intervention and the resulting data are not absolute.
The second category comprises absolute instruments that can make measurements of the
magnetic field in terms of absolute physical basic units or universal physical constants.
The most common types of absolute instruments are the fluxgate theodolite for
measuring D and I and the proton precession magnetometer for measuring F. In the
former, the basic unit is an angle. The fluxgate sensor mounted on the telescope of a
nonmagnetic theodolite is used to detect when it is perpendicular to the magnetic field
vector. Collimation errors between the fluxgate sensor and the optical axis of the
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theodolite, and within the theodolite, are minimized by taking readings from four
telescope positions. With the fluxgate sensor operating in null-field mode, the stability
and sensitivity of the sensor and its electronics are maximized. True north is determined
by reference to a fixed mark of known azimuth. This can be determined astronomically
or by using a gyro attachment. In a proton precession magnetometer, the universal
physical constant is the gyromagnetic ratio of the proton, and the basic unit is time
(frequency). Measurements with a fluxgate theodolite can only be made manually,
whereas a proton magnetometer can operate automatically.
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Figure 1. Locations of currently operating geomagnetic observatories
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The locations of currently operating magnetic observatories are shown in Figure 1. It
can be seen that the spatial distribution of the observatories is rather uneven, with a
concentration in Europe and a dearth elsewhere in the world, particularly in the ocean
areas.
2.3. Satellites
Since the 1960s, Earth’s magnetic field has been observed intermittently by satellites.
The first satellites measured only the strength of the magnetic field, sometimes using
only a nonabsolute instrument, but beginning in 1999 there have been a number of
satellite missions attempting to measure the full field vector, using star cameras to
establish the direction of a triaxial fluxgate sensor. An absolute intensity instrument is
normally also carried, and both magnetic instruments are kept remote from the main
body of the satellite by mounting them at the end of a nonmagnetic boom. Satellites
provide an excellent global distribution of data, but generally only last for a short period
(i.e., months to a few years).
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Satellites which have provided valuable vector data for geomagnetic field modelling are
Magsat, which flew in 1979 and 1980, and Orsted and CHAMP which were launched in
1999 and 2000 respectively and are still returning data in 2006. The Orsted satellite
takes just over 2 years to sample all local times and flies in the altitude range 640-850
km. The CHAMP satellite samples all local times in 4-5 months and flies in the altitude
range 350-450 km.
2.4. Other Direct Observations
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Earth’s magnetic field is observed in a number of other ways. These are repeat stations
and surveys made on land, from aircraft and ships. Repeat stations are permanently
marked sites where high-quality vector observations of Earth’s magnetic field are made
for a few hours, or sometimes a few days, every few years. Their main purpose is to
track changes in the core-generated magnetic field.
Most aeromagnetic surveys are designed to map the crustal field. As a result, they are
flown at altitudes lower than 300 m, and they cover small areas, generally once only,
with very high spatial resolution. Because of the difficulty in making accurately
oriented measurements of the magnetic field on a moving platform, these kinds of
aeromagnetic surveys generally comprise total intensity data only. However, between
1953 and 1994 the Project MAGNET program collected high-level three-component
aeromagnetic data specifically for modeling the core-generated field. The surveys were
mainly over the ocean areas of Earth, at middle to low latitudes. A variety of platforms and
instrumentation were used; the most recent set-up included a fluxgate vector magnetometer
mounted on a rigid beam in the magnetically clean rear part of the aircraft, a ring laser gyro
fixed at the other end of the beam, and a scalar magnetometer located in a stinger
extending some distance behind the aircraft’s tail section.
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Modern marine magnetic surveys are also invariably designed to map the crustal field,
but with careful processing it is possible to obtain information about the core-generated
field from the data. In a marine magnetic survey, a scalar magnetometer is towed some
distance behind a ship, usually along with other geophysical equipment, as it makes
either a systematic survey of an area or traverses an ocean.
Prior to the establishment of observatories and an absolute method of measuring
magnetic intensity by Gauss in the 1830s, magnetic observations were madeby mariners
engaged in merchant and naval shipping, as well as by others. These measurements
were mainly of declination and serve to extend the global historic data set back to the
beginning of the seventeenth century.
2.5. Indirect Observations
Prior to the seventeenth century, indirect observations of Earth’s magnetic field are
possible from archaeological remains and rocks using paleomagnetic techniques. The
subject of rock magnetism and paleomagnetism is covered elsewhere in this
encyclopedia (see Rock Magnetism and Paleomagnetism).
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3. Characteristics of Earth’s Magnetic Field
3.1. Reversals
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When a rock is formed, it usually acquires a magnetization parallel to the ambient
magnetic field (i.e., the core-generated field). From careful analyses of directions and
intensities of rock magnetization from many sites around the world, it has been
established that the polarity of the axial dipole has changed many times in the past, with
each polarity interval lasting several thousand years. These reversals occur slowly and
irregularly, and for a period of roughly 30 million years centered approximately 100
million years before present, there were no reversals at all. In addition to full reversals,
there have been many aborted reversals when the magnetic poles are observed to move
toward the equator for a while but then move back and align closely with Earth’s spin
axis. The solid inner metal core is thought to play an important role in inhibiting
reversals. At the present time we are seeing a 6% decline in the dipole moment per
century. Whether this is a sign of an imminent reversal is impossible to say.
3.2. The Present Magnetic Field
In a source-free region near the surface of Earth the magnetic field is the negative
gradient of a scalar potential that satisfies Laplace’s equation. A solution to Laplace’s
equation in spherical coordinates is called a spherical harmonic expansion, and its
parameters are called Gauss coefficients. There are internal or external coefficients,
modeling the field generated inside or outside the planet, respectively. A separation of
the core and crustal fields, both internal, is not perfect. The internal field is often called
the main field.
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The main-field coefficients change with time as the core-generated field changes, and in
commonly used spherical harmonic models, for example the International Geomagnetic
Reference Field (IGRF) and the World Magnetic Model, this secular variation is
assumed to be constant over five-year intervals. It has only been possible to accurately
determine the small but persistent field generated outside Earth, largely by the ring
current, since satellite data have become available.
At the Earth's surface the main field can be approximated by a dipole placed at the
Earth's centre and tilted to the axis of rotation by about 11°.. However, significant
deviations from a dipole field exist. Figures 2-6 show maps of declination, inclination,
horizontal intensity, vertical intensity and total intensity at 2005.0, and their predicted
secular variations for the period 2005.0 to 2010.0, derived from the 10th Generation
IGRF model.
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Figure 2 - Contour maps of declination (degrees) at 2005.0 and secular variation of
declination (arc-minutes/year) for 2005.0-2010.0.
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Figure 3 - Contour maps of inclination (degrees) at 2005.0 and secular variation of
inclination (arc-minutes/year) for 2005.0-2010.0.
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Figure 4 - Contour maps of total intensity (nT) at 2005.0 and secular variation of total
intensity (nT/year) for 2005.0-2010.0.
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Figure 5 - Contour maps of horizontal intensity (nT) at 2005.0 and secular variation of
horizontal intensity (nT/year) for 2005.0-2010.0.
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Figure 6 - Contour maps of vertical intensity (nT) at 2005.0 and secular variation of
vertical intensity (nT/year) for 2005.0-2010.0.
3.3. Westward Drift
Using direct observations of the magnetic field over the past 400 y, the pattern of
declination seen at Earth’s surface appears to be moving slowly westward. This is
particularly apparent in the Atlantic hemisphere at mid- and equatorial latitudes. This
may be related to the motion of fluid at the core surface slowly westward, dragging with
it the magnetic field lines.
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3.4. Geomagnetic Jerks
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The rate of change of declination at Lerwick, Eskdalemuir, and Greenwich–Abinger–
Hartland observatory series in the UK is shown in Figure 7. It can be seen from this plot
that there have been a number of changes in the general trend of secular variation in the
past, in particular at about 1925, 1969, 1978 and 1992. These sudden changes are
known as jerks or impulses and, at the present time, are not well understood and are
certainly not predictable. Some researchers have found evidence for a correlation with
length-of-day changes.
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Figure 7-Rate of change of declination at Greenwich (GRW), Abinger (ABN), Hartland
(HAD), Eskdalemuir (ESK) and Lerwick (LER) observatories 1900-2005.
3.5. Crustal Magnetic Field
The field arising from magnetic materials in Earth’s crust varies on all spatial scales and
is often referred to as the anomaly field. Knowledge of the crustal magnetic field is
often very valuable as a geophysical exploration tool for determining the local geology.
The anomalies seen at midocean spreading ridges are of particular interest. At these
locations, molten mantle comes to the surface and solidifies to form new oceanic crust,
preserving in it the strength and direction of the contemporary ambient magnetic field.
As new material is extruded, the existing crust is pushed away on either side of the
ridge, with the direction of the ambient magnetic field at time of formation frozen into
it. Marine and aeromagnetic surveys reveal a series of stripes in the total intensity
anomalies that run parallel to and symmetrically about the central ridge, and these are
interpreted as alternating blocks of normal and reversely magnetized oceanic crust. This
discovery of a crude tape recorder of Earth’s magnetic field and its reversals was
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
invaluable to the theory of plate tectonics developed in the 1960s (see Tectonic
Processes).
3.6. Field Variations at Quiet Times
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The geomagnetic field has a regular variation with a fundamental period of 24 hours.
This regular variation is dependent on local time, latitude, season, and solar cycle. It is
caused by electrical currents in the upper atmosphere, at altitudes ranging 100–130 km
above Earth’s surface. At these altitudes the atmosphere is significantly ionized by the
Sun’s ultraviolet and X-radiation, and the ions are moved by winds and tides arising
from the heating effects of the Sun and the gravitational pull of the Sun and the Moon.
This creates the required conditions for a dynamo to operate (i.e., motion of a conductor
in a geomagnetic field), and two current cells are formed, one in the sun-lit northern
hemisphere going counterclockwise, the other in the sun-lit southern hemisphere going
clockwise. The magnetic effect of these current systems is observed on the ground at
observatories as solar quiet-day variation.
3.7. Field Variations at Disturbed Times
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As well as the regular daily variation, Earth’s magnetic field also exhibits irregular
disturbances, and when these are large they are called magnetic storms. These
disturbances are caused by interaction of the solar wind, and disturbances therein, with
the Earth’s magnetic field. The solar wind is a stream of charged particles continuously
emitted by the Sun, and its pressure on Earth’s magnetic field creates a bounded cometshaped region surrounding the planet called the magnetosphere (see Magnetosphere and
its Coupling to Lower Layers). When there is a disturbance in the solar wind, the current
systems existing within the magnetosphere are enhanced and cause magnetic
disturbances and storms. Figure 8 shows a schematic picture of the solar wind and
Earth’s magnetosphere.
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The amplitude of magnetic disturbances is larger at high latitudes because of the
presence of the oval bands of enhanced currents around each geomagnetic pole called
auroral electrojets. Some charged particles get trapped at the boundary of the
magnetosphere and, in the polar regions, are accelerated along the magnetic field lines
towards the atmosphere and finally collide with oxygen and nitrogen molecules. These
collisions result in sometimes-spectacular emissions of mainly red and green light
known as aurora borealis at northern latitudes and aurora australis at southern latitudes.
The prevailing conditions in the solar–terrestrial environment that are a consequence of
the emission of charged particles from the Sun and their interaction with Earth’s
magnetic field is called space weather (see Magnetosphere and Its Coupling to Lower
Layers). There is presently some considerable interest in forecasting space weather, and
data from satellites observing the Sun and the solar wind are extremely valuable for this.
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Figure 8 Artist's rendition of solar wind and Earth's magnetosphere (credit: NASA)
Figure 9 The annual number of magnetic storms is represented by each bar of the
histogram. Superimposed is the smoothed sunspot number.
The dashed lines indicate solar minima; the dotted lines indicate solar maxima.
Note the correlation of magnetic activity with solar activity and the apparent increase in
magnetic activity with time.
Although irregular, magnetic disturbances exhibit some patterns in their frequency of
occurrence. The main pattern is the correlation with the 11-year solar cycle, and Figure
9 shows the number of magnetic storms per year and the corresponding number of
sunspots for the period 1868 to present. Another important pattern is the 27-day
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
recurrence of some storms, which are related to the 27-day rotation of the Sun as seen
from Earth.
4. Earth’s Magnetic Field as Both a Tool and a Hazard in the Modern World
4.1. Navigation
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The earliest writings about compass navigation are credited to the Chinese and date
from the eleventh century. There is evidence of compasses being used in Europe
approximately 100 years later, but the first observations of declination were not
recorded until the sixteenth century. In 1700, Halley produced the first magnetic chart,
which covered the Atlantic Ocean.
Contemporary values of declination, or for some maps the difference between grid north
and magnetic north, are depicted on the majority of modern sea charts, topographic
maps, and aeronautical charts. These values must be kept up-to-date by regular revision
of the models from which they are derived. This is an example of use of Earth’s
magnetic field as a tool.
4.2. Directional Drilling
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A specialist form of navigation is directional drilling in the petroleum industry. There
are two main methods of navigation available for drilling deviated wells towards
targets, which are often small, in the oil reservoirs. One method makes use of gyro
tools, but this can be expensive. The other method makes use of magnetic tools. As
accuracy is critical for economic reasons and to avoid well collisions, the accuracy
requirements on Earth’s magnetic field values, which are used to correct the direction of
the well from magnetic bearings to true bearings, are typically 0.1° in direction and 50
nT in total intensity. To attain these accuracies, account must be taken of the crustal
field, daily variations, and magnetic storms. This application is another example of
Earth’s magnetic field as a tool in the modern world, but it can also be a hazard if no
account is taken of these other sources.
4.3. Geomagnetically Induced Currents
The most severe magnetic storm in recent times occurred in March 1989, and this had a
number of serious impacts on technological systems by generating damaging
geomagnetically induced currents. In particular, the power transmission system in
Quebec, Canada, was shut down for over nine hours. Other effects such as increased
corrosion in pipelines are also likely. This is an example of Earth’s magnetic field as a
hazard in the modern world.
4.4. Satellite Operations
When a magnetic storm is underway, Earth’s atmosphere expands because of heating,
and increases the atmospheric drag on satellites at altitudes below about 1000 km. The
orbit of the satellite can be changed, and maneuvers that are sometimes expensive must
be made to compensate. Other effects on satellites are caused by radiation hits, which
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
can interfere with onboard computers. The prediction of magnetic activity, as a monitor
of conditions at satellite altitude, is therefore of great interest to satellite operators. This
is another example of Earth’s magnetic field as a hazard in the modern world.
4.5. Exploration Geophysics
Total intensity traverses and surveys over an area can aid understanding of the
underlying geology and, in the case of iron ore deposits, can indicate very clearly their
locations. This is an example of Earth’s magnetic field as a tool, but it may turn into a
hazard, especially at high latitudes, if care is not taken to remove the daily variations
and magnetic storm effects from the data before interpretation.
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Glossary
Aurora:
Crustal field:
Declination:
Diurnal variation:
Fluxgate sensor:
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Fluxgate-theodolite:
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Geomagnetic
observatory:
IGRF:
Inclination:
Emissions of mainly red and green light seen at high latitudes
caused by interaction of charged particles with constituents of
the upper atmosphere.
Magnetic field arising from magnetized material in Earth’s
crust, often referred to as the anomaly field or magnetic
anomalies.
The horizontal angle between true north and the field vector,
measured positive eastward. In navigation, often called
magnetic variation.
A daily variation in Earth’s magnetic field arising from the
Sun’s effect on the ionosphere.
a sensor for measuring the intensity of the magnetic field in
the direction of its core.
A nonmagnetic theodolite with a fluxgate sensor mounted on
telescope, used to determine absolute values of declination and
inclination.
A location where absolute observations of the complete
Earth’s magnetic field vector are recorded accurately and
continuously, with a time resolution of one minute or less,
over a long period of time.
International Geomagnetic Reference Field, a series of
spherical harmonic models of the main field, revised, on
average, every five years by a working group of the
International Association of Geomagnetism and Aeronomy.
The angle between the horizontal plane and the field vector,
measured positive downward. In nongeophysical applications,
sometimes called magnetic dip.
The ionized region of the upper atmosphere.
Magnetic field carried by the solar wind.
Ionosphere:
Interplanetary
magnetic field (IMF):
An impulse, estimated to last a few months, in the third time
Jerk:
derivative of the main field.
A disturbance in Earth’s magnetic field caused by interaction
Magnetic storm:
of the solar wind, and disturbances therein, with Earth’s
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Magnetosphere:
Main field:
Proton
magnetometer:
Secular variation:
Solar cycle:
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Solar wind
Space weather:
magnetic field.
Bounded, comet-shaped region surrounding Earth, created by
the pressure of the solar wind on Earth’s magnetic field.
Earth’s magnetic field with origins inside Earth.
An instrument for the absolute measurement of the intensity of
the magnetic field. Protons in a liquid are first polarized.
When the polarizing field is switched off, the protons precess
around the magnetic field direction with a frequency related to
the magnetic field intensity.
The slow change of the main field.
The approximately 11-year quasiperiodic variation in
frequency or number of solar activity events.
Stream of charged particles continuously emitted by the Sun.
Conditions on the Sun and in the solar wind, magnetosphere,
ionosphere, and thermosphere that can influence the
performance and reliability of space-borne and ground-based
technological systems and that can endanger human life or
health.
The phenomenon of the pattern of the main field moving
slowly westward in many parts of the world.
Spherical harmonic model of the present main field produced
for the US and UK governments and used widely in
navigation.
Westward drift:
World Magnetic
Model:
Bibliography
Campbell W.H. (1997). Introduction to Geomagnetic Fields, 290 pp. Cambridge, UK: Cambridge
University Press. [This book is an introduction to geomagnetism suitable for those who are not familiar
with the field, with an emphasis on external fields.]
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Jankowski J. and Sucksdorff C. (1996). IAGA Guide for Magnetic Measurements and Observatory
Practice, 235 pp. Boulder, CO: International Association of Geomagnetism and Aeronomy. [This book
describes magnetic observatory instrumentation and procedures and is suitable for those setting up or
running an observatory and using observatory data.]
Jacobs J.A. (1987). Geomagnetism Volumes 1, 2, and 3, 627, 579, and 533 pp., respectively. London:
Academic Press. [Volume 1: history of geomagnetism, instrumentation, and overviews of main and
crustal fields. Volume 2: origins of Earth’s magnetic field and that of the moon and other planets. Volume
3: rock magnetism, paleomagnetism and the ancient magnetic field behavior, conductivity structure of
Earth, indices of geomagnetic activity, and magnetic field characteristics during solar quiet conditions.]
Jacobs J.A. (1991). Geomagnetism Volume 4, 806 pp. London: Academic Press. [This volume covers the
solar-terrestrial environment including the solar wind, the Earth’s magnetosphere, magnetopause,
magnetotail, ionosphere, pulsations, plasma waves, magnetospheric storms and substorms, and auroras.]
Merrill R.T., McElhinny M.W., and McFadden P.L. (1996). The Magnetic Field of the Earth, 531 pp.
London: Academic Press. [This book is an introduction to geomagnetism suitable for those who are not
familiar with the field, with an emphasis on internal fields and paleomagnetism.]
Parkinson W.D. (1983). Introduction to Geomagnetism, 433 pp. Edinburgh, UK: Scottish Academic
Press. [This book is an introduction to geomagnetism suitable for those who are not familiar with the
field.]
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GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan
Biographical Sketch
Dr Susan Macmillan ,is a senior geophysicist at the British Geological Survey. She acquired her doctoral
degree in 1989 and has since worked at the British Geological Survey (BGS) on a wide variety of projects
concerning Earth’s magnetic field. Her research interests include regional and global magnetic field
modeling using satellite and ground-based data, as well as crustal field modeling using aeromagnetic data.
She has also devoted much time to developing applications for the geomagnetic field in industry. She is
cochair of the International Association of Geomagnetism and Aeronomy Working Group V-MOD on
“Geomagnetic Field Modeling”. She is the author or coauthor of over 50 scientific publications and
reports.
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To cite this chapter
Susan Macmillan, (2004/Rev.2006), EARTH’S MAGNETIC FIELD, in Geophysics and Geochemistry,
[Ed. Jan Lastovicka], in Encyclopedia of Life Support Systems (EOLSS), Developed under the Auspices of
the UNESCO, Eolss Publishers, Oxford ,UK, [http://www.eolss.net]
©Encyclopedia of Life Support Systems (EOLSS)