Gubbins D., Herrero-Bervera E. Encyclopedia of Geomagnetism and Paleomagnetism

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CARNEGIE INSTITUTION OF WASHINGTON,

DEPARTMENT OF TERRESTRIAL MAGNETISM

When the Carnegie Institution of Washington (CIW) was established

in 1902 with a large bequest from Andrew Carnegie, one of its goals

was to encourage research in the “borderlands” between the scientific

disciplines. In its first years, the CIW established two departments that

became stages for research in the geosciences: the Geophysical

Laboratory and the Department of Terrestrial Magnetism (DTM).

While the Geophysical Laboratory moved quickly in the direction of

geochemical investigation and toward developing laboratory techni-

ques to study how rocks behave at great depth within the Earth, the

DTM focused on a series of problems related to geomagnetism and

geo-electricity during its first several decades. The DTM branched

out starting in the 1920s to investigate the atomic nucleus, cosmic

rays, and even cosmology. These two departments have now existed

for over a century and today are located on a single campus where

their researchers can more easily interact (Trefil and Hazen, 2002).

The geophysicist Louis Agricola Bauer (q.v.), who had been engaged

in geomagnetic research for over a decade, established the DTM in

1904 with the general goal of studying Earth’s magnetic and electric

phenomena, but with a definite project to prosecute “quickly” a mag-

netic survey of the planet. This project was to be completed within a

generation in order to provide data which could be more easily reduced

to a common epoch than the data then available. Magnetic data had only

been gathered since the mid-19th century with any degree of consis-

tency, and even then, it was concentrated in just a few regions and

was spread over decades. Entire continents were uninvestigated and

charts available for magnetic declination in the world’s oceans were

frequently in error by 1



and 2



, while inclination measurements were



–3



off. Moreover, outside the main shipping lanes very few readings

had been made at all (Good, 1994b). Bauer proposed a World Magnetic

Survey to the Carnegie Institution of Washington because, as a nongo-

vernmental agency, it would enjoy greater credibility for mounting expe-

ditions in Africa, Asia, South America, and other places where the

institutions of major governments might be looked upon with suspicion.

Moreover, free of any government connection, the DTM could directly

approach scientific officials in other countries without going through

their supervising bureaucracies. As he put it in his mission statement

for the new department, the DTM would survey the magnetism of areas

not being studied by others. This was most of the globe.

The DTM was to have a second purpose too: to coordinate and stan-

dardize magnetic measurements then being undertaken by dozens of

governments and agencies around the world. That is, it would be the

international standardizing bureau. It would carefully compare the

great variety of geomagnetic instruments (see History of Instrumenta-

tion) and promote the best methods of investigation and measurement.

To this end, for decades the DTM had its traveling “magneticians” visit

major magnetic observatories around the world to standardize instru-

ments and to learn and teach different working methods. DTM pro-

moted data exchange and uniform methods of reduction. In these

regards the DTM acted in the way that various commissions of the

International Association for Geomagnetism and Aeronomy or IAGA

(q.v.) and Intermagnet do today.

Bauer set the DTM’s first task as evaluation of existing data for

secular variation studies and he contracted Adolph Schmidt to deter-

mine the status of data on magnetic storms. The second over-arching

goal was to “establish the facts” and to convince theoreticians that

the data and mathematical tools could become available for decisive

testing among different theories.

Bauer also brought his journal Terrestrial Magnetism and Atmo-

spheric Electricity (since 1948 the Journal of Geophysical Research)

to the DTM, where he and his associate director John A. Fleming

(q.v.) used it to provide a disciplinary and international identity for

geomagnetic researchers. In its pages, Bauer published critical studies

of various instruments, theoretical discussions, and “news of the pro-

fession,” including capsule biographies of geomagnetic role models

like Edmond Halley (q.v.), Alexander von Humboldt (q.v.), and Carl

Friedrich Gauss (q.v.).

Bauer’s most dramatic project, however, was the global magnetic

survey. Between 1905 and World War II, the DTM employed two hun-

dred magnetic observers on land expeditions. Counting the number of

expeditions is a little difficult, since many of them split and rejoined

and they varied from a few weeks to many months in duration. The

DTM reported in 1928 that 178 land expeditions had thus far occupied

5685 magnetic stations. This averaged 271 stations per year. By 1928,

when Fleming was taking over supervisory duties for the DTM, the

main land survey was completed. Dwindling numbers of expeditions

from then until the mid-1940s concentrated on secular variation mea-

surements at a smaller number of repeat stations (q.v.).

These troops of magneticians were a tightly organized, well-trained

army of observers. They learned the physics of electricity and magnet-

ism, the use of magnetometers, induction coils, and sine galvanometers

(see History of Instrumentation) and the techniques of data reduc-

tion. They also were resourceful, independent travelers. Unlike other

expeditionaries, these scientists often traveled alone and rarely in

groups of more than three. They transported hundreds of kilograms

of instrumentation and other gear and frequently traveled by camel or

canoe, as well as by steamer or locomotive. The magnetic data was

hard won and surprisingly, not one observer died of disease or accident

in all of those travels into the Polar Regions, Amazonia, and bandit-

ridden China (Figure C1).

Meanwhile, Bauer carefully executed a parallel set of oceanic expe-

ditions. He consulted in 1904 and 1905 with experts in geomagnetism

(e.g., Arthur Schuster) and hydrographers (e.g., George W. Littlehales

and Ettrick W. Creak) (Good, 1994b). In 1906, 1907, and 1908 the

brig Galilee followed three, spiraling loops in the northern Pacific

Ocean, providing magnetic measurements within each 5



quadrangle

in the region, or one station every 200 miles. The DTM constantly

improved sea-going magnetic instruments, so that by 1908, magnetic

observers read declination to within 0.5



, compared with 0.1



for land

observers. By this last cruise, errors due to iron on board ship was a

larger source of error than the instruments.

Hence, Bauer obtained funding from the CIW for construction of a

nonmagnetic yacht, the Carnegie (q.v.), which was launched in 1909.

This yacht was magnificent, if peculiar. Its dominant peculiarity was

the deck house, a glass enclosed galley with a glass observing dome

at each end. Protected within, magnetic observers could make mea-

surements even in quite rough seas. From 1909 until its demise in

1929, the Carnegie conducted seven cruises, including a celebrated

fastest-ever circumnavigation of Antarctica for a sailing vessel in 118

days; all the while the observers continued their scientific work.

A map published by the DTM in 1929 shows the globe enmeshed in

the observations of Bauer’s remarkable World Magnetic Survey (Fig-

ure C2). Bauer had imagined an ambitious plan to make thousands

of measurements around the globe in a single generation and he found

the funding, personnel, and instruments to do it. The effort required to

produce this first geomagnetic “snap-shot” was nothing short of

remarkable.

Figure C1 Magnetic observer Frederick Brown traveled around

Asia by camel and sedan chair. This photo is from April 1915.

Photo courtesy of Carnegie Institution of Washington. Photo DTM

5769.

Figure C2 A map showing all of the expeditions undertaken by the DTM for the first World Magnetic Survey, 1904 to 1929. Each dot is

a discreet station. The lines on the oceans indicate the tracks of the Galilee and the Carnegie between 1905 and 1929. Map courtesy

of Carnegie Institution of Washington.

CARNEGIE INSTITUTION OF WASHINGTON, DEPARTMENT OF TERRESTRIAL MAGNETISM 57

Although the DTM still exists, its focus on expeditionary, geomag-

netic work ended with World War II. The department published two

volumes in 1947, coordinated by E.H. Vestine, in which the data of the

enterprise were summarized and analyzed (Vestine et al., 1947a,b).

Although the DTM has continued to conduct geophysical research since

then, its character has been much more varied, including research in

seismology and nuclear physics.

Gregory A. Good

Bibliography

Good, Gregory A. (ed.), 1994a. The Earth, the Heavens and the Car-

negie Institution of Washington. History of Geophysics, volume 5.

Washington, D.C.: American Geophysical Union, 252 pp.

Good, Gregory A., 1994b. Vision of a global physics: The Carnegie

Institution and the first world magnetic survey. History of Geophy-

sics, 5:29–36.

Trefil, J., and Hazen, M.H., 2002. Good Seeing: a Century of Science

at the Carnegie Institution of Washington. Washington, D.C.:

Joseph Henry Press.

Vestine, E.H., Laporte, L., Cooper, C., et al.,1947a. Description of the

earth’s main magnetic field and its secular change, 1905–1945.

Carnegie Institution of Washington, Washington, D.C., Publication

No. 578. 532 p.

Vestine, E.H., Laporte, L., Lange, I., and Scott, W.E., 1947b. The Geo-

magnetic Field, its Description and Analysis. Carnegie Institution

of Washington, Washington, D.C., Publication No. 580.

Cross-references

Bauer, Louis Agricola (1865–1932)

Carnegie, Research Vessel

Fleming, John Adam (1877–1956)

Gauss, Carl Friedrich (1777–1855)

Halley, Edmond (1656–1742)

Humboldt, Alexander von (1759–1859)

IAGA, International Association of Geomagnetism and Aeronomy

Repeat Stations

CARNEGIE, RESEARCH VESSEL

When Louis Agricola Bauer (q.v.) established the Department of Ter-

restrial Magnetism (DTM) at the Carnegie Institution of Washington

(cv) in 1904, he intended to focus the DTM’s efforts strongly on an

oceanic magnetic survey. The oceans represented both the largest

and the most accessible gap in geomagnetic data. Bauer felt that most

of the oceanic data available was not of high enough quality and that it

was mainly restricted to coastal waters and trade routes. From 1905 to

1908, the DTM leased a wooden sailing vessel, the brig Galilee. Bauer

had the steel rigging replaced with hemp and stripped away as much

other iron as possible, but bolts and nails had to stay. The Galilee

sailed over 70000 miles and established 442 magnetic stations, but

its most important result was this: The Galilee demonstrated that a

completely nonmagnetic ship was necessary to carry on this research.

Bauer convinced the Carnegie Institution’s Board (and Andrew

Carnegie) that the DTM needed its own yacht, specially designed for

magnetic research. The shipyard in Brooklyn, New York that had built

Kaiser Wilhelm’s Meteor (and the winner of the 1906 Atlantic cup race)

designed and built a most peculiar ship: the Carnegie (Figure C3). It was

155 feet long and carried 13000 square feet of sails. Bauer stationed a

magnetic investigator at the shipyard for 7 months to guarantee that no

magnetic materials were used in its construction. The planking was all

wood, the nails were of locust tree, and the bolts were copper or bronze.

Its bronze anchors each weighed 5500 lb and instead of anchor chains,

the ship used 11-in. hemp cable. At $115000 (perhaps ten million dollars

today), the Carnegie cost more than any other scientific instrument in

history.

The most important part of the ship for geomagnetic research was

the observation deck, a special glass-enclosed room with a circular

observation dome at each end (Figure C4). The domes were of glass

panels with a brass framework. Magnetic observations were made in

foul weather and fair, in order to obtain the density of measurements

that was required.

A crowd of 3500 people turned out to launch the ship in 1909. It sailed

the world’s oceans for 20 years, collecting many thousands of magnetic

measurements, and sailed on seven cruises, the equivalent distance of

circumnavigating the globe twelve times. It met its demise in 1929 in

the harbor at Apia, Samoa, when a gasoline engine exploded during

refueling. The Carnegie Institution never again operated its own research

vessels. Although the British government proposed a new “non-

magnetic” research vessel in the 1930s, the Research, World War Two

Figure C3 The Carnegie under full sail. Photo courtesy of

Carnegie Institution of Washington. Photo H-112r.

Figure C4 A full complement of Carnegie observers in the two

observing domes. Captain W.J. Peters is on the deck above. The

photo was taken in the Indian Ocean in 1911. Courtesy of

Carnegie Institution of Washington. Photo H-118.

58 CARNEGIE, RESEARCH VESSEL

prevented it being placed in service. The Soviet government did launch

the nonmagnetic schooner Zarya in the 1950s and continued to use it for

several decades. (The name Zarya is now attached to the Russian com-

ponent of the International Space Station.)

Simultaneously with the oceanic surveys of the Galilee and the

Carnegie, the DTM (see Carnegie Institution of Washington, Depart-

ment of Terrestrial Magnetism) conducted an ambitious magnetic land

campaign. The goal was to gather enough magnetic data in a genera-

tion to provide a much better “snapshot” of Earth’s magnetism than

was then available. While this survey was completed by about 1930,

the DTM continued to send observers to selected “repeat stations”

(q.v.) to obtain data useful for studying secular variation.

Gregory A. Good

Bibliography

Good, Gregory A., 1994. Vision of a Global Physics: The Carnegie

Institution and the First World Magnetic Survey. History of

Geophysics, 5:29–36.

Cross-references

Bauer, Louis Agricola (1865–1932)

Carnegie Institution of Washington, Department of Terrestrial Magnetism

Repeat Stations

CHAMP

Challenging Minisatellite Payload (Figure C5) is a satellite mission

dedicated to improving gravity and magnetic field models of the

Earth. CHAMP was proposed in 1994 by Christoph Reigber of

GeoForschungsZentrum Potsdam in response to an initiative of the

German Space Agency (DLR) to support the space industry in the

“New States” of the united Germany by financing a small satellite mis-

sion. The magnetic part of the mission is lead by Hermann Lühr.

CHAMP was launched with a Russian COSMOS vehicle on July 15,

2000 onto a low Earth orbit. Initially planned to last 5 years, the mis-

sion is now projected to extend to 2009 (Figure C6). The official

CHAMP website is at http://op.gfz-potsd am.de/champ/.

Satellite and orbit

A limiting factor for low Earth satellite missions is the considerable

drag of the atmospheric neutral gas below 600 km altitude. This

brought down Magsat (q.v.) within 7 months, despite of its elliptical

orbit, and necessitated the choice of a higher altitude orbit for Ørsted

(q.v.). To achieve long mission duration on a low orbit, CHAMP was

given high weight (522 kg), a small cross section (Figure C5), and a

stable attitude. It was launched onto an almost circular, near polar

(i ¼ 87.3



) orbit with an initial altitude of 454 km. While Magsat

was on a strictly sun synchronous dawn/dusk orbit, CHAMP advances

one hour in local time within eleven days. It takes approximately

90 min to complete one revolution at a speed of about 8 km s

1

Magnetic mission instrumentation

Magnetometers

At the tip of the 4 m-long boom, a proton precession Overhauser magnet-

ometer (q.v.), measures the total intensity of the magnetic field (q.v.)

once per second. This instrument, which was developed by LETI,

Grenoble, has an absolute accuracy of <0.5 nT. Its measurements are

used in the absolute calibration of two redundant vector magnetometers,

located mid-boom on an optical bench. These fluxgate magnetometers

were developed and supplied by DTU Lyngby, Denmark. They sample

the field at 50 Hz with a resolution <0.1 nT.

Star cameras

The orientation of the optical bench in space is given by a star camera,

which was developed and supplied by DTU Lyngby, Denmark. Attitude

uncertainty is the largest source of error in satellite vector magnetic data.

Star cameras are often blinded by the sun or moon, and provide unreli-

able attitude with regard to rotations about their direction of vision

(boresight). For this reason, CHAMP was equipped with a dual head star

camera, improving relative attitude by an order of magnitude to 3 arc

seconds, corresponding to around 0.5 nT accuracy for the vector compo-

nents. Since this high accuracy is only achieved in dual head mode (62%

of CHAMP data), future magnetic field missions (e.g., the European

Space Agency mission swarm, scheduled for launch in 2010) will have

triple head star cameras. A further, redundant dual head star camera on

the body of CHAMP is of limited utility for the magnetic field measure-

ments, due to the flexibility of the boom.

Electric field, electron density, and temperature

A digital ion drift meter (DIDM) and a planar Langmuir probe (PLP)

were provided by the Air Force Research Laboratory, Wright-Patterson

Air Force Base, Ohio. The DIDM, designed to measure the electric field

from ion velocities, partly failed due to frictional overheating during the

launch phase of CHAMP. The PLP, which was not damaged, provides

Figure C5 Front view of the CHAMP satellite (courtesy GFZ Potsdam). The acronyms POD and CoG stand for Precise Orbit

Determination and Center of Gravity, respectively.

CHAMP 59

the spacecraft potential, electron temperature, and electron density, once

in 15 s. As discussed below, these quantities can be used to correct mag-

netic field measurements for the diamagnetic effect of the plasma sur-

rounding the satellite. The PLP measurements have thus, turned out to

be essential for accurate geomagnetic field modelling.

GPS receiver

Apart from providing the accurate position of CHAMP, the Black Jack

GPS receiver (supplied by NASA) has the important task of providing

an absolute time frame. A pulse delivered every second is used to syn-

chronize all of the instruments on board. Furthermore, it provides a

stable reference frequency for the proton precession magnetometer

readings, giving them absolute accuracy.

Data pro ducts

CHAMP ’s standard science products are labelled from level-0 to level-4,

according to the amount of preprocessing applied to the original

data. Scientific utility starts with level-2 products, which are calibrated,

flagged, and merged with accurate orbits and are supplied as daily files

in common data format (CDF). Level-3 products comprise the final pro-

cessed, edited , and calibrated data, as well as the rapid delivery

products. Derived products such as the initial CHAMP main field model

CO2 (Holme et al., 2003) are classified as level-4. The level-2 to level-4

products are archived and distributed by the Information System and

Data Centre (ISDC) at GFZ Potsdam (http://isdc.gfz-potsdam.de/

champ/).

Early science results

Main field models

Combined analysis of measurements from the three magnetic satel-

lite missions (CHAMP, Ørsted (q.v.) and SAC-C) has to a break-

through in main field model accuracy (Holme et al., 2003). While

models from ground-based observations are limited in resolution to

about 5000 km wavelength, the new combined satellite models resolve

the main field (q.v.) and its secular variation (q.v.) to 3000 km. Shor-

ter wavelengths of the main field are masked by the crustal magnetic

field (q.v.).

Crustal field models

With its low altitude, circular, polar orbit, CHAMP is particularly sui-

ted for mapping the field caused by the magnetic minerals in the

Earth’s crust. The first CHAMP crustal field model (Maus et al.,

2002)—extending to spherical harmonic degree 80, corresponding to

a wavelength of 500 km—shows that the Earth’s crustal magnetic field

is weaker than indicated by earlier observations from Magsat.

Magnetic signal of ocean flow

The flow of electrically conducting seawater through the Earth’s mag-

netic field acts as a global dynamo, inducing electric fields which drive

electric currents and thus, give rise to secondary magnetic fields.

Focussing on the dominant semidiurnal lunar tide, periodic ocean flow

magnetic signals were clearly identified in CHAMP satellite data

(Tyler et al., 2003).

Diamagnetic effect of ionospheric plasma

A diamagnetic substance responds to an applied magnetic field by

circular currents that produce a magnetisation opposing the applied

field. Such a diamagnetic response, which is a well-known property

of plasma in the outer magnetosphere and the solar wind, was until

recently, thought to be negligible in the ionosphere. However, bands

of enhanced plasma density on both sides of the magnetic equator have

been found to cause depressions of the order of 5 nT in the geomag-

netic field intensity (Lühr et al., 2003). With the electron density and

temperature acquired by the PLP instrument, CHAMP’s magnetic field

readings can be corrected for this effect. A related phenomenon is the

formation of surface currents on plasma cavities. The CHAMP satellite

has made it possible for the first time to directly observe these currents

in their magnetic field signature (Lühr et al., 2002).

Stefan Maus

Bibliography

Holme, R., Olsen, N., Rother, M., and Lühr, H., 2003. CO

—ACHAMP

magnetic field model. In Reigber, C., Lühr, H., and Schwintzer, P.

(eds.), CHAMP Mission Results I. Berlin: Springer, pp. 220–225.

Lühr, H., Maus, S., Rother, M., and Cooke, D., 2002. First in-situ obser-

vation of night-time F region currents with the CHAMP satellite.

Geophysical Research Letters, 29(10):10.1029/2001GL013845.

Lühr, H., Maus, S., Rother, M., Mai, W., and Cooke, D., 2003. The

diamagnetic effect of the equatorial Appleton anomaly: its charac-

teristics and impact on geomagnetic field modelling. Geophysical

Research Letters, 30(17): 1906, doi:10.1029/2003GL017407.

Maus, S., Rother, M., Holme, R., Lühr, H., Olsen, N., and Haak, V.,

2002. First scalar magnetic anomaly map from CHAMP satellite

data indicates weak lithospheric field. Geophysical Research

Letters, 29(14): 10.1029/2001GL013685.

Tyler, R., Maus, S., and Lühr, H., 2003. Satellite observations of mag-

netic fields due to ocean flow. Science, 299: 239–241.

Cross-references

Crustal Field

Magnetometer

Magsat

Main Field

Ørsted

Secular Variation

Figure C6 The decay of CHAMP’s orbital altitude (left scale)

depends on the neutral gas density, which is enhanced by solar

activity (right scale). CHAMP’s orbit has been raised thrice to

prolong the mission, which is predicted to end after the solar

activity minimum in 2009 (courtesy of F.-H. Massmann, GFZ

Potsdam).

60 CHAMP

CHAPMAN, SYDNEY (1888–1970)

Sydney Chapman received 17 degrees (mostly honorary doctorates),

published seven books and over 400 scientific papers, received nine

major medals (including the Royal and Copley medals of the Royal

Society), held ten visiting chairs, was president of eight professional

societies, and fellow of many more. Rather than try to encompass a

career of such breadth and depth, it is mainly his contribution to

geomagnetism that is considered here, though he also contributed

massively to, inter alia, solar-terrestrial physics, ionospheric physics,

meteorology, aurorae, and the kinetic theory of gasses.

Chapman was born in Eccles, Lancashire, on January 29, 1888.

He was set to enter an engineering firm, but, before doing so, he won

the last of 15 university scholarships offered by Lancashire and went

on to take an engineering degree at Manchester University. He stayed

on for a further year and added a mathematics B.Sc. On the advice of

Horace Lamb, he then tried for and obtained a Cambridge mathematics

scholarship. He completed the examinations after two years at Trinity

College, but was required to stay on for a further year before graduating.

During this time he received a visit from Frank Dyson, the Astronomer

Royal, offering him the prestigious and well-paid post of Chief Assistant

at the Royal Observatory, Greenwich. (The other Chief Assistant at the

time was the cosmologist Arthur Eddington, with whom Chapman

devised the cycling performance measure n, which is the number of occa-

sions on which n or more miles have been cycled in a day. Eddington

achieved n ¼ 77, and Chapman rather more.)

Chapman was at Greenwich from 1911 to 1914, and again from

1916 to 1918 to help out when many of the staff were at the war.

He was a conscientious objector during the World War I—a far from

easy option—but had revised his views by the time of World War II.

While at Greenwich, Chapman was introduced to geomagnetism.

He organized the re-instrumentation of the magnetic observatory, took

part in the routine magnetic observations (on his own admission he

was not a good observer), and set about the interpretation of geomagnetic

variations.

In 1919 he moved from Trinity College, Cambridge, to succeed

Lamb in the chair of mathematics and natural philosophy at Manchester.

In the same year he was elected to Fellowship of the Royal Society at

the early age of 31. This was largely for his work on the theory of solar

and lunar daily geomagnetic variations.

In 1924, Chapman became chief professor of mathematics at Imperial

College, London, where he remained until 1946. While there, he won

the John Couch Adams essay prize, the subject of which (chosen at

the suggestion of Dyson, with Chapman in mind) was the theoretical

interpretation of geomagnetic phenomena. One of the conditions of

the award was that the work should be published, which Chapman

satisfied with a number of papers. But he also felt obliged to turn

the essay into a book. In this he collaborated with the young German

scientist Julius Bartels (q.v.) to eventually produce the two-volume

Geomagnetism in 1940. This became the “bible” for geomagneticians

for half a century. Though the instrumental part and the bibliography

are now outdated, the mathematical sections are still of sufficient value

to make this book an important part of any geomagnetic library.

It was also during his time in London that Chapman started his col-

laboration with V.C.A. Ferraro on the theory of magnetic storms,

which was to be superseded only when satellite observations became

available. With another of his students, A.T. Price (q.v.), he examined

the currents induced in the Earth by Sq and their implication for mantle

conductivity.

In 1946, Chapman moved to Oxford University to become Sedleian

Professor of Natural Philosophy, a post he held until approaching com-

pulsory retirement at 65. Rather than retire, he moved to the United

States, where he already held visiting appointments. This was during

the lead-up to the 1957/8 International Geophysical Year (IGY) and

Chapman, as president of the IGY Special Committee, took the oppor-

tunity to travel widely, soliciting support for the project. As well as his

theoretical work, he had undertaken many analyses of vast quantities

of observational data and was well aware of the need for such data

on a world-wide basis.

In the final, though far from inactive, part of his career (more than

150 papers were published after his “retirement”), Chapman divided

his time between the Universities of Colorado and Alaska. In Alaska

he worked with S.-I. Akasofu on geomagnetic disturbance phenomena

and in Colorado with several colleagues, largely updating and extend-

ing earlier work in the light of recently acquired data, including IGY

data. He was still actively working until a few days before his death

on June 16, 1970.

Chapman’s personal qualities were as remarkable as his scientific

ability. He was slightly built, but very athletic, though his feats were of

endurance rather than speed. Some mention has already been made

of his cycling prowess. He gave up cycling at about 70, mainly because

American highways were not cyclist-friendly, as he and his wife dis-

covered experimentally. He swam half a mile a day throughout his life,

going to considerable lengths to avoid missing a day. The nearest he

came to owning a car was a half-share in one with Bartels, but only

briefly. He much preferred to walk. He was a gentleman with strong

moral principles and was always greatly considerate of the feelings

of others. His many colleagues and students valued him as much as a

friend as for his teaching.

Stuart R.C. Malin

Bibliography

Akasofu, S.-I., Fogle, B., and Haurwitz, B. (eds.), 1968. Sydney Chap-

man, Eighty, From his Friends. Privately published by University

of Alaska, University of Colorado and University Corporation for

Atmospheric Research.

Chapman, S., and Bartels, J., 1940. Geomagnetism. Oxford: Clarendon

Press.

Cowling, T.G., 1971. Sydney Chapman 1888–1970. Biographical

Memoirs of Fellows of The Royal Society, 17:53–89.

Cowling, T.G., and Ferraro, V.C.A., 1972. Obituary Sydney Chapman.

Quarterly Journal of the Royal Astronomical Society, 13: 464–467.

Malin, S.R.C., 1988. Sydney Chapman 1888–1970: a centennial celebra-

tion. Eos, Transactions, American Geophysical Union, 69:57–67.

Cross-references

Bartels, Julius (1899–1964)

Geomagnetism, History of

Price, Albert Thomas (1903–1978)

COAST EFFECT OF INDUCED CURRENTS

Introduction

When changes in Earth’s magnetic field are measured on timescales of

about 1 hour, the vertical component near most coastlines is abnor-

mally large and correlates with the onshore horizontal component.

This phenomenon is known as the geomagnetic coast effect. It is

caused by electromagnetic induction occurring naturally in the Earth’s

surface layer.

The “primary” currents in the induction process are electric currents,

which flow external to the Earth and change with time. As the solid

Earth and its oceans conduct electricity, “secondary ” currents are

induced to flow in them. At Earth’s surface, observations of magnetic

and electric variations record the effects of the primary and secondary

fields combined. Because in many instances the primary fields are uni-

form and spatially uncorrelated from one induction event to the next,

CHAPMAN, SYDNEY (1888–1970) 61

spatial patterns occurring systematically reflect spatial patterns in the

secondary fields due to structure in the electrical conductivity of the

Earth.

The main electrical conductivity contrast at Earth’s surface is

between land and seawater. Seawater is a relatively homogeneous

material, with its electrical conductivity, s, depending mainly on salt

content and temperature. A typical value for s throughout the world’s

oceans is 3.3 S/m, in a range of 3 S/m for cold, less saline water to

6 S/m for warm, salty water (Bullard and Parker, 1970).

Land generally consists of inhomogeneous rock material and its

conductivity, in contrast to that of seawater, varies by orders of magni-

tude (Sheriff, 1999). However the electrical conductivity of rock mate-

rial generally is less than that of seawater. Further, the electrical

conductivity of rock material is generally less than 1 S/m, leading to

the common use of electrical resistivity r (the reciprocal of conductiv-

ity s) when describing rock material. Thus a resistivity of 1 O mis

equivalent to a conductivity of 1 S/m and a resistivity of 10 O mis

equivalent to a conductivity of 0.1 S/m.

Then rock material typically has a resistivity of some hundreds of

O m in the crust and possibly thousands of O m in ancient cratonic

rocks. In sedimentary basins the resistivity may be less, say tens of

O m. In ore bodies, and in crustal concentrations of the good conduc-

tor graphite, the resistivity may be only small fractions of an O m;

however such good conductors form only a minor part of the total

volume of crustal rocks.

The land-ocean conductivity contrast exerts a strong effect on electro-

magnetic induction at Earth’s surface. Induction at places of such conduc-

tivity contrast is characterized by the generation of a vertical magnetic

field component. Near a land-ocean contrast, this vertical component is

seen as the geomagnetic coast effect.

Discovery of the coast effect

The coast effect was first observed by Parkinson (1959, 1962) and the

discovery demonstrated that conductivity contrasts at and near Earth’s

surface could be detected by using natural electromagnetic induction at

periods of about 1 hour. Parkinson’s early results for Australia are

shown in Figure C7 (see Parkinson, Wilfred Dudley).

At about the same time detailed observations of magnetic fluctua-

tion patterns were also made in other continents, for example by

Schmucker (1963); Rikitake (1964); Price (1967). The advent of digi-

tal recording and time-series analysis using electronic computers led to

numerical analysis methods replacing Parkinson’s graphical methods

and the phenomenon was investigated as a function of frequency (Ever-

ett and Hyndman, 1967; Schmucker, 1970). One consequence of this

development was then to demonstrate in-phase and out-of-phase com-

ponents of the coast effect. Generally the in-phase component is stronger

than the out-of-phase component (a result of the high conductivity of

seawater) and often the effect is described and interpreted simply in

terms of the in-phase component. If the out-of-phase component is being

studied, care must be taken with the definition of phase (see Lilley and

Arora, 1982). See also induction arrows (q.v.).

Observation of the coast effect

The review by Parkinson and Jones (1979) gives a global survey of the

observation of the coast effect, which is still geographically represen-

tative. The authors note that the few coastal locations where the coast

effect is absent are tectonically anomalous. In a regular coast effect,

the strength of the phenomenon, as reckoned by the response in the

vertical component of the fluctuating field, reduces to about half its

coastal strength at a distance of some hundreds of kilometers inland.

Figure C7 Early Parkinson arrows for Australia, after Parkinson (1964). Small magnetic vector changes at an observatory tend to lie in

a “preferred plane.” The horizontal projection of the downwards normal to this plane gives arrows as shown which, generally

pointing to the high conductivity side of a nearby conductivity contrast, point to the deep ocean.

62 COAST EFFECT OF INDUCED CURRENTS

The authors also note that the coast effect varies only slightly with

period, reaching a broad maximum in the period range 30–90 min.

An important quantity is the electromagnetic “skin depth,” d,

d ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃ

mso

(Eq. 1)

where o denotes angular frequency and m denotes the permeability of

the material (usually taken as that of free space). For the period range

30–90 min, the skin depth of seawater is some 20 km, which is greater

than the ocean depth. Thus an external magnetic fluctuation signal

reaches the ocean floor and penetrates the seafloor rock material and

the ocean may be regarded as an electromagnetic “thin sheet.”

An example of induction arrows for the coast effect of southeast

Australia, observed on both land and seafloor, is shown in Figure C8

(White et al., 1990). The site is favorable for a clear demonstration

of the effect, as the coastline runs relatively straight for hundreds of

kilometers, the continental shelf is narrow, and at the foot of the con-

tinental slope the seafloor is deep (almost 5000 m) and flat, comprising

the Tasman Abyssal Plain. Included in the compilation are sites spaced

down the continental slope. As can be seen from the induction arrows,

the coast effect is at a maximum about halfway down the slope into the

deep ocean.

An example of the data recorded in this particular case is shown in

Figure C9. Displayed are the horizontal and vertical components of

magnetic variation at five stations. It is in the vertical components that

the coast effect is evident and the signal can be seen to be a maximum

at station 4 (the trace marked CS4Z).

Fluctuations in the geomagnetic field occur over a wide frequency

band, and the coast effect has also been investigated at shorter and

longer periods. At shorter periods, the effect has been demonstrated

to occur in “pulsations” of period order 1 min and less. At these short

periods the skin depth of seawater no longer is greater than the depth

of the deep ocean, and the effect takes place in the upper layer of

the ocean. The relatively shallow seawater of the continental shelf

becomes important.

At longer periods, a natural frequency band at which to examine the

coast effect is that of the magnetic daily variations (termed “Solar

quiet,” and denoted Sq). However, for these signals different observa-

tion and analysis techniques are required due to the presence of a sig-

nificant vertical component in the Sq source field.

Generally there is agreement that major “electric current gyres” are

induced to flow in the oceans by the Sq source fields (Beamish et al.,

1983) and these also have a measurable effect at observatories near

coast lines. Bennett and Lilley (1973) showed how a coast-effect ver-

tical component could be separated from an observed record by sub-

tracting an inland record observed simultaneously, which was taken

to be a “normal” (i.e., background) component.

Figure C8 Induction arrows (in-phase, for period 1.14 h) for

the coast of southeast Australia, for a line of stations from inland

to deep seafloor (Ferguson, 1988; Lilley et al., 1989; White

et al., 1990).

Figure C9 Examples of magnetic fluctuation records for one day,

showing the coast effect (White et al., 1990). The top five traces,

with suffix H, are the horizontal north components for stations

1 to 5, as marked on Figure C8. The middle set of traces, with

suffix D, are the horizontal east components. The lower five

traces, with suffix Z, are the vertical components. The strength of

the signal in the vertical component characterizes the coast effect,

as it is the result of the electrical conductivity structure at the

land-sea interface. Note this signal is strongest at station 4, half

way down the continental slope.

COAST EFFECT OF INDUCED CURRENTS 63

Analogu e modeling of the coast effect

The relative electrical conductivities of rock material and seawater

lend themselves to analogue model studies. Following the initial glo-

bal “terrella ” model of Parkinson (1964), model studies have been car-

ried out on geometries seen in coastlines worldwide (Dosso, 1973).

These analogue models have the ability to model 3D situations, such

as the islands of Tasmania (Dosso et al., 1985) and New Zealand

(Dosso et al., 1996).

Calcu lation of the coast effect

The basic calculation of the coast effect is a two-dimensional problem

and the forward problem was solved in one of the early achievements

of numerical modeling of electromagnetic induction in the Earth

(Jones and Price, 1970; Weaver, 1994). Given that the ocean bathyme-

try is generally known well, the calculation becomes an exercise in

modeling the rock material beneath the seawater. Figure C10 shows

such a 2D model for the observed data shown in Figures C8 and C9.

As abilities in 2D and 3D numerical modelling and inversion have

progressed, the coast effect has been a basic case for testing new meth-

ods and techniques. As an example, which may be compared with

Figures C7 and C8, the induction arrow response for a numerical

model of the Australian continent set in its surrounding seas (Corkery

and Lilley, 1994) is shown in Figure C11 .

Geoph ysical ocean-coa st transects

Geophysical studies now frequently address the tectonic boundaries,

which may be present at coastlines. By making observations along a

transect, which crosses the coast line from deep ocean to inland, infor-

mation may be obtained, which characterizes the structure and history

of the region. Electromag netic studies to obtain electrical conductivity

information have been foremost amongst such transects, which have

included subduction zones (Yukutake et al. , 1983; EMSLAB-Group,

1988) and passive margins (Wannamaker et al., 1996). These transects

Figure C11 The electromagnetic response of a thin-sheet model

of Australia presented as induction arrows, for period 1 h

(in-phase) from Corkery and Lilley (1994). (Scale: an arrow which

exactly spans a grid unit has a magnitude of 0.3).

Figure C10 A two-dimensional model for the coast effect shown

in Figures C8 and C9, with seawater layer and contrasting

electrical conductivity structure beneath continent and ocean

(Kellett et al., 1991).

Figure C12 Diagram showing, near a buried body of high

electrical conductivity, the “preferred plane” in which small

vector magnetic changes tend to lie, and the normal n to the

preferred plane. When the normal n is parallel or antiparallel to

the main field F, a total-field magnetometer such as a

proton-precession instrument will be insensitive to the small

vector changes (Lilley et al., 1999).

64 COAST EFFECT OF INDUCED CURRENTS

have given some of the most comprehensive observations of the coast

effect, with electric and magnetic field measurements on both land and

seafloor. Note also that Hitchman et al. (2000) made observations of

the coast effect offshore at the sea surface, using a floating total-field

magnetometer.

An applicatio n of the coast effect

Coast effects cover large regions and their influence may be wide-

spread. An example is the “amphidrome” effect, which, as a result of

the coast effect, exists across a wide swath of southern Australia (Lil-

ley et al., 1999). Here the coast effect causes the Parkinson preferred

plane to be approximately orthogonal to the main magnetic field vec-

tor. Thus fluctuation events, which generally lie in the preferred plane,

tend to be orthogonal to the main field and the component of them

measured by a “total field” magnetometer (as in an aeromagnetic sur-

vey) is much reduced.

Figure C12 illustrates the “amphidrome effect” in which magnetic

fluctuation signals, measured by a “total field” magnetometer, are sup-

pressed when the normal to the Parkinson preferred plane is parallel to

Earth’s main magnetic field vector.

Conclusion

The geomagnetic coast effect is regarded as being well understood,

and is primarily due to the electrical conductivity contrast between

land and seawater, controlling natural electromagnetic induction in

the environs of a coastline.

Ted Lilley

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Cross-references

Electromagnetic Induction (EM)

Geomagnetic Deep Sounding

Induction Arrows

Magnetotellurics

Mantle, Electrical Conductivity, Mineralogy

Parkinson, Wilfred Dudley

COAST EFFECT OF INDUCED CURRENTS 65