Earth’s Magnetic Field - EOLSS Chapters/C01/E6-16-04-01.pdf · UNESCO - EOLSS SAMPLE CHAPTER GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan theodolite, and

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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 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. 1. Introduction 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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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:

22arctanarctan ZHFHZI

XYD +===

where H is given by

22 YXH += . 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. 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.

Figure 1. Locations of currently operating geomagnetic observatories 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 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. 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 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. 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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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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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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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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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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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 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.

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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invaluable to the theory of plate tectonics developed in the 1960s (see Tectonic Processes). 3.6. Field Variations at Quiet Times 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 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 comet-shaped 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. 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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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 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 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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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. Glossary Aurora: Emissions of mainly red and green light seen at high latitudes

caused by interaction of charged particles with constituents of the upper atmosphere.

Crustal field: Magnetic field arising from magnetized material in Earth’s crust, often referred to as the anomaly field or magnetic anomalies.

Declination: The horizontal angle between true north and the field vector, measured positive eastward. In navigation, often called magnetic variation.

Diurnal variation: A daily variation in Earth’s magnetic field arising from the Sun’s effect on the ionosphere.

Fluxgate sensor: a sensor for measuring the intensity of the magnetic field in the direction of its core.

Fluxgate-theodolite: A nonmagnetic theodolite with a fluxgate sensor mounted on telescope, used to determine absolute values of declination and inclination.

Geomagnetic observatory:

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.

IGRF: 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.

Inclination: The angle between the horizontal plane and the field vector, measured positive downward. In nongeophysical applications, sometimes called magnetic dip.

Ionosphere: The ionized region of the upper atmosphere. Interplanetary magnetic field (IMF):

Magnetic field carried by the solar wind.

Jerk: An impulse, estimated to last a few months, in the third time derivative of the main field.

Magnetic storm: A disturbance in Earth’s magnetic field caused by interaction of the solar wind, and disturbances therein, with Earth’s


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magnetic field. Magnetosphere: Bounded, comet-shaped region surrounding Earth, created by

the pressure of the solar wind on Earth’s magnetic field. Main field: Earth’s magnetic field with origins inside Earth. Proton magnetometer:

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.

Secular variation: The slow change of the main field. Solar cycle: The approximately 11-year quasiperiodic variation in

frequency or number of solar activity events. Solar wind Stream of charged particles continuously emitted by the Sun. Space weather: 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.

Westward drift: The phenomenon of the pattern of the main field moving slowly westward in many parts of the world.

World Magnetic Model:

Spherical harmonic model of the present main field produced for the US and UK governments and used widely in navigation.

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.]

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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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. 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]


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Earth’s Magnetic Field - EOLSS Chapters/C01/E6-16-04-01.pdf · UNESCO - EOLSS SAMPLE CHAPTER GEOPHYSICS AND GEOCHEMISTRY-Earth’s Magnetic Field- Susan Macmillan theodolite, and