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

Подождите немного. Документ загружается.

In February 2005, a new Danish Meteorological Institute suspended

fluxgate magnetometer and a Gem GSM-90 Overhauser magnetometers

were installed. These will become the prime recording instruments after

a suitable period of comparison with the existing instruments.

Apia observatory

In November 1899 at a meeting of the council entrusted with the pre-

paration of the German South-Polar Expedition, Adolf Schmidt pro-

posed that a magnetic observatory should be set up in Samoa at the

same time when the stations were to be set up by the expedition in

Antarctica, to enable simultaneous records. Dr. Otto Tetens sailed to

Samoa taking with him two Eschenhagen variometers, a Z-balance

and a magnetic theodolite for absolute measurements. He arrived in

June 1902 (Angenheister, 1974).

When World War I broke out, Samoa was occupied by 1500 sol-

diers from New Zealand. Dr. Gustav Heinrich Angenheister, who

was serving as director, managed to convince the English commander

that the observatory was a scientific institution and needed to be con-

tinued. He remained at the observatory during and after the war until

1920, working under conditions of extreme hardship and financial dif-

ficulties. At the end of the war, the League of Nations mandated con-

trol of the German Samoan Islands to New Zealand.

Funding for the observatory was provided by the New Zealand govern-

ment and the Carnegie Institution, assisted by an annual grant form

the British Admiralty (Sommerville, 1923). Eventually, the observatory

came under the control of the New Zealand Department of Scientific and

Industrial Research (DSIR). New La Cour variometers were installed by

Lawrie in 1958.

Control of the observatory remained with the DSIR until a few years

after independence in 1962 and was then handed over to the Samoan

government. The DSIR still provided some support by way of techni-

cal advice and with the processing and publication of the data.

In 1990, Samoa was devastated by tropical cyclone Ofa which

removed the roof from the variometer house. This turned out to be a

blessing in disguise because repairs to the roof would have cost more

than a new digital system. As part of New Zealand’s aid program, the

New Zealand Department of Foreign Affairs and Trade commissioned

the DSIR to supply and install a digital system as used at Eyrewell.

The installation of the digital system in December 1991 allowed Apia

to become an INTERMAGNET observatory in 1998.

Scott base observatory

Originally Scott Base was to be at Butter Point on the western side of

McMurdo Sound, but it was considered unsuitable for the Trans-Antarctic

Expedition. The present site on the eastern side of Hut Point Peninsula

on Ross Island was chosen instead. It is not a good site for a magnetic

observatory as Ross Island is composed of highly magnetic basalt

scoria (Cullington, 1961). However there was no alternative.

Two prefabricated buildings were erected in 1957, one each for abso-

lute observations and the La Cour variometer systems. Most buildings

have been replaced over the years but these two buildings are still used

for their original purpose.

A digital fluxgate magnetometer was installed at Scott Base in December

1990 and it became an INTERMAGNET observatory in 1996. The instru-

ments will be upgraded at the end of 2005 as has been done at Eyrewell.

Lester A. Tomlinson

Bibliography

Angenheister, G.G., 1974. Geschichte des Samoa-Observatoriums von

1902 bis 1921. In Zur Geschichte der Geophysik. Berlin: Springer-

Verlag, pp. 43–66.

Cullington, A.L., 1961. Scott Base Observatory Magnetic Results for

International Geophysical Year 1957 –58. Wellington: Government

Printer.

Farr, C., Coleridge, 1916. A Magnetic Survey of the Dominion of New

Zealand. Wellington: New Zealand Government Printer.

Schmidt, A., 1897. On the distribution of magnetic observatories.

Terrestrial Magnetism, XI, p. 30.

Sommerville, D.M.Y., 1923. Editorial note. In Apia Observatory

Report 1921. Wellington: Government Printer.

OBSERVATORIES IN NORDIC COUNTRIES

The Nordic countries comprise Denmark, the Faeroe Islands, Finland,

Greenland, Iceland, Norway, and Sweden. All but the Faeroe Island

houses one or more geomagnetic observatories. The observatories in

operation as of 2005 are listed in Table O8.

They are all equipped with modern fluxgate magnetometers—the

majority with the suspended Danish FGE type—for the continuous

recording, and are using standard D/I-flux theodolite along with proton

precession magnetometer for absolute calibration (see Observatories,

instrumentation). Digital recording was introduced during the 1980s

and 1990s at all of them.

Denmark and Greenland

The geomagnetic observatories in Denmark and Greenland have been

part of the Danish Meteorological Institute in Copenhagen ever since

the Copenhagen Observatory was established in 1891. Already in

1907 the Copenhagen Observatory was moved to Rude Skov 20 km

to the north due to increasing magnetic disturbance in the city. The

observatory remained here until 1984 when relocation to its present

site at Brorfelde 40 km west of Copenhagen took place.

The first magnetic observatory at Greenland was established in

Godhavn (today Qeqertarsuaq), at Greenland’s west coast in 1926,

where it is still found. The history of the North Greenland Observatory

dates back to the Second International Polar Year (IPY) 1932/1933,

when a magnetic observatory was operated in Thule. A permanent

observatory was set up there in 1947, but had to be abandoned after

only a few years due to the establishment of the US Air Force base

at Thule. Finally, operation was resumed in 1956 at its present site

of Qaanaaq (previously called Thule) 100 km to the north. The South

Greenland Observatory of Narsarsuaq was added in 1967.

Finland

The history of geomagnetic observatories in Finland dates back to

Göttingen Magnetic Union (see Geomagnetism, history of ) when

an observatory was set up in Helsinki by the University of Helsinki.

Regular operation commenced in 1844 (Nevanlinna, 2004). In 1912

it was relocated within Helsinki, and finally in 1953 it was replaced

by the present observatory at Nurmijärvi 40 km north of Helsinki. It

is operated by the Finnish Meteorological Institute.

The observatory of northern Finland in Sodanlylä has a precursor

in the First IPY station of 1882/1983. In 1914, the Finish Academy

of Science and Letters set up a permanent geophysical observatory,

a magnetic observatory being part of it (Kataja, 1973). Today the

Sodankylä Geophysical Observatory is organized as a branch of the

University of Oulu.

Iceland

The magnetic observatory at Iceland started recording in 1957. It is

located at Leirvogur 12 km ENE of Reykjavik, and is maintained by

the University of Iceland.

Norway

Similar to Finland, the first Norwegian magnetic observatory originated

in the activities of the Göttingen Magnetic Union. The astronomical

736 OBSERVATORIES IN NORDIC COUNTRIES

observatory in Oslo was then furnished also as a magnetic observatory

and began regular observations in 1842. The University of Oslo kept

it running until 1932 (Wasserfall, 1950). In the meantime, a magnetic

station had been set up at Dombås 300 km north of Oslo in 1916, but

not until 1950 was this station upgraded to a magnetic observatory. It

is today run jointly by the universities in Bergen and Troms.Themag-

netic observatory in Troms was established in 1930 as part of the

newly founded Auroral Observatory . From 1972 it is part of the Univer-

sity of Troms.

Bear Island was the site of a Polish research station including a

magnetic observatory during the Second IPY 1932/1933. The mag-

netic observations were continued by the Auroral Observatory in

Troms , but not until 1951 were the calibration and stability good

enough to produce reliable annual means. Today it is run by the Uni-

versity of Troms. Hornsund is a Polish research station maintained

since the International Geophysical Year in 1957/1958 by the Polish

Academy of Science in the southern part of Svalbard. From 1978 the

station has been manned all year and includes a magnetic observatory.

Sweden

The observatory of southern Sweden dates back to 1928 when the Lov

Observatory in Stockholm was built. A new observatory near Uppsala is

replacing Lov and is effective from 2005. In northern Sweden, the

magnetic station at Abisko was set up by the Swedish Academy of

Science in 1921, but before 1945 absolute calibrations were quite

scarce. Both are operated today by the Swedish Geological Survey.

Truls Lynne Hansen

Bibliography

Magnetic yearbooks

Descriptions of the observatories, their results, instruments, and meth-

ods are found in the so-called magnetic yearbook. Over the years they

have been issued under changing forms and titles. Below the most

recent issues are listed. Today the distribution of magnetic data by

the Internet is important and this is done through INTERMAGNET

(see Observatories, INTERMAGNET ) and the World Data Centers as

well as through the institutes ’ own Web sites.

Leirvogur Magnetic Results 2004, Rauvísindastofnun Háskólans,

Reykjavík 2005.

Magnetic Results 2000 Brofelde, Qeqertarsuaq, Qaanaaq and Narsar-

suaq Observatories, Danish Meteorological Institute, Data report

02–01, Copenhagen, 2002.

Magnetic Results Nurmijärvi Geophysical Observatory 2002, Finnish

Meteorological Institute, Helsinki, 2003.

Magnetic Results Sodankylä 2002, Sodankylä Geophysical Observa-

tory Publication No. 94, Oulu, 2003.

Geomagnetic Observatory Data 2002: Lovö and Abisko, Geofysiska

meddelanden Cb31, Sveriges geologiska undersökning, Uppsala,

2004.

Magnetic Observations 1998, The Auroral Observatory—University

of Troms, Troms, 2000.

The Magnetic Station at Dombås, Observations 1998, Institute of

Solid Earth Physics, University of Bergen, Bergen, 2000.

Others

Kataja, Eero, 1973. The Sodankylä Geophysical Observatory in 1973.

Veröffentlichungen des geophysikalischen Observatoriums der fin-

nischen Akademie der Wissenschaften, No 56/1.

Nevanlinna, H., 2004. Results of the Helsinki magnetic observatory

1844–1912. Annales de Geophysicae, 22: 1691–1704.

Wasserfall, K.F., 1950. A study of the secular variation of magnetic

elements based on data for D, I, and H for Oslo 1820–1948. Jour-

nal of Geophysical Research, 55: 275–299.

Cross-references

Geomagnetism, History of

Magnetometers

Observatories, Overview

Observatories, Instrumentation

Observatories, INTERMAGNET

OBSERVATORIES IN RUSSIA

The first regular measurements of the geomagnetic field in Russia

were started in St. Petersburg in 1829. Over the course of the next cen-

tury geomagnetic observatories equipped with the most modern, by

contemporary standards, instruments, were established in Ekaterinburg

(1836), Tiflis (1844), Pavlovsk (1878), Irkutsk (1886), and Kazan

(1909). The first polar observatory in the world was open in 1924 in

Table O8 Magnetic observatories in the Nordic countries operating in 2005

Observatory

name

IAGA

code

Country

Geographic

latitude

Geographic

longitude

First

annual

mean

Qaanaaq THL Greenland 77 29



N6910



W 1956

Hornsund HRN Norway 77 00



N1533



E 1979

Bear Island BJN Norway 74 30



N1900



E 1951

Troms TRO Norway 69 40



N1856



E 1930

Qeqertarsuaq GDH Greenland 69 15



N5332



W 1926

Abisko ABK Sweden 68 22



N1849



E 1946

Sodankylä SOD Finland 67 22



N2632



E 1914

Leirvogur LVR Iceland 64 11



N2142



W 1958

Dombås DOB Norway 62 05



N0907



E 1952

Narsarsuaq NAQ Greenland 61 11



N4526



W 1967

Nurmijärvi NUR Finland 60 31



N2439



E 1953

(1844)

Uppsala UPS Sweden 59 54



N1721



OBSERVATORIES IN RUSSIA 737

strait Matochkin Shar (close to modern Amderma). The next step,

aimed at economic development of the Russian Arctic, was made in

connection with the Second Polar Year (1932– 1933). The observa-

tories Yakutsk, Srednekan (Magadan), Dixon, Tikhaya Bay, Cape

Chelyuskin, and Wellen, were set up in this period. The International

Geophysical Year (1957–1958) and the subsequent beginning of the

period of space research played a crucial role in building the present

network of geomagne tic observations. About 50 magnetic observa-

tories and stations were operated on the territory of former Soviet

Union by 1985 (we define an observatory or a station by availability

or absence of absolute geomagnetic measurements, respectively).

The situation changed dramatically in connection with the disintegra-

tion of the Soviet Union and economic problems after 1991.

At present geomagnetic observations in Russia are carried out by

about 30 observatories and stations, but quality of observations, deter-

mined by the type of instruments, is quite variable. Some observatories,

equipped with modern instruments, provide digital data of high resolu-

tion, whereas others, using outdated devices, continue to record the

magnetic variations in analog format or with insufficiently precise abso-

lute measurements. In Ta b l e O 9 we list only those geomagnetic observa-

tories (GO) and stations (S), which provide the geomagnetic data in

digital form. The table contains the name of observatory, abbreviation,

geographical coordinates, the data resolution (in nT) and sampling rate

(in s), and years covered by digital data. The observatories and stations

are operated by various institutions of the Russian Hydrometeorological

Service and Russian Academy of Science. These institutions are listed

below and the last column of the table (Aff.) corresponds to the appro-

priate number in the list of affiliations. Letter R indicates that the data

are presented in real time at the institution Web sites:

1. Arctic and Antarctic Research Institute (AARI), St. Petersburg.

Web site: http://www.aari.nw.ru/clgmi/geophys/index _ru.htm

2. Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy

(IKFIA), Siberian Branch of RAS, Yakutsk; http://ikfia.ysn.ru,http://

denji102.geo.kyushu-u.ac.jp/denji/obs/cpmn/cpmn_obs_e.html

3. Institute of Space Research and Radiowave Propagation (IKIR),

Petropavlovsk at Kamchatka; ikir.kamchatka.ru

4. Institute of Solar-Terrestrial Physics, Irkutsk; iszf.irk.ru

5. Polar Geophysical Institute (PGI), Murmansk; http://pgi.kolasc.net.ru

6. Institute of Earth Magnetism, Ionosphere, and Radiowave Propaga-

tion (IZMIRAN), Moscow; http://www.izmiran.ru/magnetism/

mos_data.htm

7. Schmidt ’s Institute of Earth Physics (IFZ), Moscow; http://geo-

data.borok.ru

8. Altay-Sayan Branch of the Geophysical Survey of RAS,

Novosibirsk; http://www.gs.nsc.ru/russian/ionka.html

9. Institute of Geophysics, The Ural Branch of RAS, Ekaterinburg

10. Siberian Physics-Technical Institute, Tomsk; http://sosrff.tsu.ru

11. V.I.Il ’ichev Pacific Oceanological Institute (POI), Far Eastern

Branch of RAS, Vladivostok; http://poi.febras.ru

Observations at stations KTN, TIX, CHD, ZYK, PTK, MGD, and PPI

are carried out in the framework of the Australian-Japan-Russian pro-

ject “Circum-pan Pacific Magnetometer Network ” (CPMN). Data from

stations PBK, TIK, CCS, NOR, and DIK form the basis for calculation

of the AE index, characterizing the intensity of magnetic disturbances

in the auroral zone.

Oleg Troshichev

Table O9 List of Russian geomagnetic observatories (GO) and stations (S)

Station (abbr.) Latitude (j),

Longitude (l

)

Year

found

Resolution (nT)

rate (s)

Available 1-min data

Aff.

GO Heiss Island (HIS) 80.62; 58.05 1959 0.1 nT; 1 s 1997–1999, 2005 1

GO Dixon (DIK) 73.54; 80.56 1933 0.1 nT; 1 s 1990, 1993–2005 1R

GO Cape Chelyuskin (CCS) 77.72; 104.28 1935 0.1 nT; 1 s 1997–2005 1R

GO Tiksi (TIK) 71.6; 129.0 1944 0.1 nT; 1 s 1995–2005 1R

GO Pebek (PBK) 70.1; 170.9 2000 0.1 nT; 1 s 2000–2005 1R

S Vize Island (VIZ) 79.5; 77.0 1982 1 nT; 1 min 1997–2005 1

GO Amderma (AMD) 69.5; 61.4 1924 0.1 nT; 1 s 1997–1998, 2005 1

GO Vostok (VOS) –78.45; 106.87 1958 0.1 nT; 1 s 1995–2002, 2004 –05 1R

GO Mirny (MIR) –66.55; 93.02 1956 0.1 nT; 1 s 1995–2005 1R

GO Novolazarevskaya (NVL) –70.77; 11.83 1960 0.1 nT; 1 s 2002–2005 1R

S Kotelny (KTN) 76.0; 137.9 1994 0.1 nT; 1 s 1994– 2000, 2002–04 2

S Tiksi (TIX) 71.6; 128.9 1992 0.1 nT; 1 s 1992– 2004 2

S Chokurdakh (CHD) 70.6; 147.9 1992 0.1 nT; 1 s 1992–2004 2

S Zyryanka (ZYK) 65.8; 150.8 1994 0.1 nT; 1 s 1994– 2004 2

S Zhigansk (ZGN) 66.8; 123.4 1994 0.1 nT; 1 s 2005 2

GO Yakutsk (YAK) 62.0; 129.6 1932 0.1 nT; 1 s 2005 2

GO Paratunka (PTK) 52.90; 158.43 1967 0.1 nT; 1 s 1992–2004 3

GO Stekolny (MGD) 60.12; 151.02 1932 0.1 nT; 1 s 1992–2004 3

GO Irkutsk (IRT) 52.46; 104.04 1886 0.01 nT, 1 s 1996–2004 4

GO Norilsk (NOK) 69.40; 88.10 1965 0.1 nT; 1 s 2002–2005 4R

S Loparskaya (MMK) 68.63; 33.25 1953 1.5 nT; 10 s 1993–2005 5

GO Lovozero (LOZ) 67.97; 35.02 1957 1.5 nT; 10 s 1996–1999 5R

0.005 nT; 0.1 s 1999 –2005

GO Moscow (MOS) 55.48; 37.31 1944 1 nT; 1 min 1986–2005 6R

GO Borok (BOX) 58.03; 38.97 1957 0.1 nT; 5 s 1998–2005 7R

GO Novosibirsk (NVS) 54.85; 83.23 1967 0.01 nT, 1 s 2003–2005 8R

GO Ekaterinburg (ARS) 56.43; 58.56 1836 0.1 nT; 1 s 1997–2005 9

S Tomsk (TMK) 56.47; 84.93 1958 0.2 nT; 0.05 s 1997–2005 10R

S Popov Island (PPI) 43.7; 132.0 2000 0.1 nT; 1 s 1997– 2004 11

738 OBSERVATORIES IN RUSSIA

OBSERVATORIES IN SOUTHERN AFRICA

The Early history of 1600 till 1932

This overview of geomagnetism and its historical development in

southern Africa covers a period of approximately 400 years till pre-

sent. During the era of exploration between 1600 and 1700, visiting

seamen from Europe have been responsible for the early magnetic

observations in South Africa. In fact, the earliest recorded observation

of magnetic declination on land was made at Mosselbay in 1595 by

Cornelis Houtman (Beattie, 1909), commander of a Dutch fleet on

its way to India, who obtained a value of 0



. The first systematic

observations in South Africa resulted from the establishment, in

1841, of a worldwide network of observation stations. One of these

stations was built in the grounds of the Royal Observatory at the Cape

of Good Hope, with the Royal Artillery detachment responsible for all

measurements. After the magnetic observatory was destroyed by fire

in 1852, a period of 80 years elapsed before a magnetic observatory

worthy of name was again established in South Africa. During that

period, observations of magnetic declination had been made by sur-

veyors in different parts of the interior of the country. The first mag-

netic survey of the Union of South Africa, comparable with that of

developed countries in the Northern Hemisphere, was carried out by

Beattie (1909), assisted by Prof. J.T. Morrison of the University of

Stellenbosch between 1898 and 1906.

The Period 1932 till present

The Polar Year of 1932–1933 prompted the establishment of a magnetic

observatory at the University of Cape Town in 1932 under the leader-

ship of Prof. Alexander Ogg (Trigonometrical Survey Office, 1939).

International organizations provided grants for buildings and the loan

of instruments for standard observations. The subsequent electrification

of the suburban railway network however created a disturbing influence,

to the extent that accurate observations became almost impossible. A

new site was identified in Hermanus, because it was sufficiently remote

from electric railway disturbances and had been proven by a magnetic

survey to be suitable in other respects. The Hermanus Magnetic Obser-

vatory officially commenced operation on January 1, 1941. One of the

most recent major contributions to geomagnetism in southern Africa

was the institution of a long-term magnetic secular variation program

in 1938–1939. The program consisted initially of 44 permanent field

stations covering, with a fairly uniform distribution, the whole of

South Africa, Namibia, and Botswana, but has since been expanded

to Zimbabwe as well, with a total of 75 stations. To ensure exact reoc-

cupation during subsequent surveys, concrete beacons were erected

at each field station to mark the position of the instruments during

observations.

A major undertaking during the period 1960–1975 was the estab-

lishment of other magnetic observatories in southern Africa. In 1964,

an opportunity arose to establish a magnetic observatory on the pre-

mises of the “Forschungsstation Jonathan Zenneck,” a permanent

ionospheric observation station of the Max Planck Institut für Aerono-

mie outside Tsumeb in Namibia. Two more recording stations were

also established in South Africa—one at Hartebeesthoek in 1972,

and the other at Grahamstown in 1974. Problems at Grahamstown

eventually led to the closure of the station in 1980. Since then both

Hermanus and Hartebeesthoek became INTERMAGNET Observa-

tories, with Tsumeb obtaining the same status in November 2004.

The data from Hermanus are used together with those of Honolulu,

San Juan, and Kakioka to derive the Dst index, with Hermanus being

the only Southern Hemisphere magnetic Observatory of these four

stations.

Since the first observatory was founded in Cape Town in 1841, geo-

magnetism has grown in stature where it is now part of a research pro-

gram at the Hermanus Magnetic Observatory, exploiting ground

observations as well as data from low-earth orbit satellites like Ørsted

and CHAMP to investigate the peculiar behavior of the field over

southern Africa.

Pieter Kotzé

Bibliography

Beattie, J.C., 1909. Report of a Magnetic Survey of South Africa.

London: Cambridge University Press.

Trigonometrical Survey Office, 1939. Results of Observations Made at

the Magnetic Observatory. University of Cape Town.

OBSERVATORIES IN SPAIN

History

The history of magnetic observatories in Spain, as elsewhere in the

world, started in the last quarter of the 19th century. Some measure-

ments of the geomagnetic field at earlier times have been recorded,

for example the geomagnetic chart of the Iberian Peninsula made by

Lamont in 1868, but nothing indicates continuous observations.

San Fernando Observatory (astronomical and geophysical observa-

tory maintained up to the present time by the Spanish Navy;

SFS—36



27.7

N, 6



12.3

W) received a set of instruments for absolute

measurements around 1842. But the first records of regular measure-

ments at this place were not till 1877. An Adie variometer was

received on 1875, but continuous recording did not start till 1891.

The Madrid Astronomical Observatory (today named Observatorio

Astronómico Nacional; 40



24.5

N, 3



41.2

W) started the publication

of daily measurements of the geomagnetic field in 1879. They were

stopped in 1901 due to the installation of electric tramways in Madrid.

At the same time, Jesuits founded magnetic sections in two meteor-

ological observatories in Spanish colonies, Belen (from 1862—



8.24

N, 82



21.30

W), in Havana (Cuba), and Manila (from

1889—14



34.7

N, 120



58.5

E) in the Philippine Islands. Both contin-

ued after the independence of these countries in 1898.

At the beginning of the 20th century, in 1904, the Jesuits founded

Ebre (or Ebro) Observatory (EBR-40



49.2

N, 0



29.6

E). The observa-

tion program was extensive, including telluric currents, solar activity,

seismology, and atmospheric electricity. It was used as the reference

observatory for the first Spanish geomagnetic chart completed in 1927.

In 1934, the state founded Toledo Observatory (TOL—39



53.0



2.8

W). It was designated as the new central geophysical observa-

tory in Spain. Geomagnetic measurements started in 1934, but the

Spanish Civil War delayed the final set-up of the instruments, and

regular observations were not undertaken till 1947.

As a contribution to the International Geophysical Year in

1957–1958, Spain decided to install some new geomagnetic observa-

tories. Two were placed in the Iberian Peninsula: Almeria (1955–

1989; ALM; 36



51.2

N, 2



27.6

W) and Logroño (1957–1980; LOG;



27.5

N, 2



30.3

W); another, Las Mesas (1961–1992; TEN;



28.6

N, 16



15.7

W), in Tenerife Island and the last one, Moca

(1958–1971; MOC; 3



20.6

N, 8



39.6

E), in Equatorial Guinea, at that

time a Spanish colony.

Present status

As elsewhere in the world, magnetic perturbations due to human activ-

ity have forced magnetic observatories to move or to shut down.

Present status is as follows:

San Fernando observatory (SFS)

Toward the end of the 1970s (1978), the electrification of the Sevilla-

Cádiz railway forced the movement of the station 8 km northeast of

the original location. Present instrumentation consists of a variometric

OBSERVATORIES IN SOUTHERN AFRICA 739

station with feedback torsion photoelectric sensors, a declinometer/

inclinometer (YOM MG2KP), and a Geometrics G-856 proton mag-

netometer. Since April 1998, to minimize the scatter, observations

are restricted to periods when the railway is nonelectric. Two nighttime

absolute measurements are made per week.

Although averaged data are not affected, new anthropogenic high

frequency (some 2 nT rms) interferences are detected in the records.

Thus, since 1997, tests have been made at different sites in order to

select a new location. This has been chosen at 40 km distance from

the present one. Works have already started. The new instrumentation

envisaged for deployment there, consists of a fluxgate triaxial tilt-

compensated variometer (model FGE) plus an Overhauser scalar mag-

netometer. For the absolute instrument the previously mentioned YOM

MG2KP declinometer/inclinometer will be used.

Bulletins from this observatory have been published for the period

1891–1978 and from 1991 onward.

San Pablo observatory (SPT)

Located at 50 km southwest of Toledo, in 10 ha grounds free from dis-

turbances, close to San Pablo de los Montes town, at 922 m above sea

level (39



32.8

N, 4



20.9

W), it started operations in 1981. This was

when Toledo Observatory closed due to the railway electrification.

The variometric instrumentation consists of a tilt-compensated FGE

station and a Geomag M390, both equipped with Overhauser GSM90

scalar magnetometers. As backup equipment there is also a vector

magnetometer equipped with Geometrics G856AX sensors and there

is a plan to incorporate a new dIdD sensor recently acquired from

Gem Systems. Absolute observations are performed weekly with a

couple of declinometer/inclinometer 010B Zeiss theodolites with

Bartington fluxgate sensors. Bulletins from Toledo were published

from 1947 to 1981 and those from San Pablo from 1982 onward.

imar observatory (GUI)

This observatory is located at 27 km south of Santa Cruz de Tenerife

(Canary Islands) and at 4 km from Güimar town, at 868 m above sea

level (28



19.3

N, 16



26.4

W). It has been operating since February

1993 after the closure of Las Mesas due to contamination from Santa

Cruz city, located only 1 km away. Its pavilions are located in a 20 ha

area of public mountain, free from human-made disturbances. The

volcanic character of the terrain means that there is a large crustal bias

for this observatory and that large gradients are present across the site.

It is equipped with two tilt-compensated FGE stations with Overhauser

GSM90 and GSM19 scalar magnetometers. The absolute instruments

and the observation program are the same as at SPT. Published bulletins

from Las Mesas cover the period from 1961 to 1992 and those of

Güimar from 1992 onward.

Ebre observatory (EBR)

Ebre observatory started operation in 1904 but since 1973 the records

have been disturbed because of the electrification of the railway. As a

result, the rapid run magnetograph recordings had to be abandoned.

Recently, a new location away from the disturbance has been selected

and new instrumentation deployed. This comprises a tilt-compensated

FGE and an Overhauser GSM90 scalar magnetometer. Continuous

data are downloaded regularly via telephone and the site is visited

once per week for the absolute measurements. These are done with a

declinometer/inclinometer 015B Zeiss theodolite with an ELSEC

810A fluxgate sensor. For backup equipment there is a fluxgate triaxial

tilt-compensated Geomag M390 station (also equipped with an Over-

hauser GSM90 scalar magnetometer) and an ARGO system, devel-

oped by the British Geological Survey (BGS), all located at the old

site. The ARGO system comprises a dDdI semiabsolute vector mag-

netometer and a triaxial fluxgate sensor. Absolute measurements are

also performed 5 days a week at the old site using another ELSEC

810A DI-flux, but with a 010B Zeiss theodolite.

This observatory has developed a procedure to semiautomatically

digitize old classic records on photographic paper. Bulletins are avail-

able from the period 1910–1937 and from 1995 onward.

Livingston Island observatory (LIV)

Recently Ebre Observatory started a research program in the Antarctic

that included the installation of a new geomagnetic observatory (LIV)

at the Spanish Antarctic Station, Juan Carlos I (Livingston Island,

South Shetland Islands, 62



39.7

S, 60



23.7

W). The main instrument

is again a BGS dDdI vector magnetometer running all year round,

although absolute measurements (with 015B Zeiss theodolite and an

ELSEC 810A sensor) are only made during the 3 months of the sum-

mer survey. Data have been transmitted via the METEOSAT data col-

lection system to the Edinburgh INTERMAGNET GIN, and presently

through GOES-E to the Ottawa GIN. Data have been published since

December 1996.

Miquel Torta and Josep Batlló

Cross-references

Crustal Magnetic Field

Jesuits, Role in Geomagnetism

Observatories in Antartica

OCEAN, ELECTROMAGNETIC EFFECTS

The oceans play a special role in electromagnetic induction, due to

their relatively high conductivity and the dynamo effect of ocean cur-

rents. Typical crystalline rocks of the Earth’s crust have electrical con-

ductivities in the range of 10

–6

–10

–2

1

, (Siemen [1/O]). In

comparison, seawater, with 2.5–6Sm

1

, is a very good conductor.

Electrical currents are induced in the oceans by two different effects:

Induction by time-varying external fields and induction by motion of

the seawater through the Earth ’s main field (see Main field).

Ocean conductivity

While fresh water has a very low conductivity, a salt concentration of

about 35 g L

1

turns the oceans into good conductors. In contrast to

the electron and electron-hole conductivity of solid-Earth materials,

the carriers of charge in the oceans are the hydrated ions of the dis-

solved salts. Positive ions are called anions and negative ions are

called cations. The conductivity of seawater depends on the number

of dissolved ions per volume (i.e., salinity) and the mobility of the

ions (i.e., temperature and pressure). Conductivity increases by the

same amount with a salinity increase of 1 g L

1

, a temperature in-

crease of 1



C, and a depth (i.e., pressure) increase of 2000 m. The

change in conductivity is dominated by temperature, with a range

of about 2.5 S m

1

for cold, deep water to about 6 S m

1

for warm

surface water.

Motional induction

When the ions are carried by ocean flow through the Earth’s magnetic

field, they are deflected by the electromagnetic Lorentz force. The

direction of the Lorentz force on the anions is given by the right-hand

rule: thumb pointing in the direction of ocean flow, index finger in

the direction of the magnetic field, then the middle finger indicates the

direction of the deflection of the anions. The cations are deflected into

the opposite direction. This separation of charge sets up large-scale elec-

tric fields. Depending on the conductivity structure of the ocean and

solid Earth, these fields drive electric currents, which in turn generate

secondary magnetic fields.

740 OCEAN, ELECTROMAGNETIC EFFECTS

Governing equations

Motional induction is governed by the following equations: The local

current density J in a conductor moving with velocity u is given by

J ¼ sðE þ u  B

main

Þ; (Eq. 1)

where s is the local conductivity and B

main

is the ambient main mag-

netic field (see Main field). The u  B

main

term is the driver of the

motionally induced electric current, while the induced electric field E

counteracts this driver and results in the actual current J. This current

generates a magnetic field B

ocean

rB

ocean

¼ m

J: (Eq. 2)

Combining Eqs. (1) and (2), and rearranging , yields an equation for

the induced electric field:

E ¼

rB

ocean

 u  B

main

: (Eq. 3)

While the irrotational (divergent) part of this electric field is main-

tained by an induced charge, the rotational part is induced by the tem-

poral variations of the ocean magnetic signal as

]

ocean

¼rE: (Eq. 4)

Taking the curl of Eq. (3) and combining with Eq. (4) gives the

motional induction equation:

]

ocean

¼r u  B

main



rB

ocean



; (Eq. 5)

which is valid inside the oceans and the solid Earth. The fields above

the surface are then determined by Laplace’s equation r

V ¼ 0 for

the scalar potential V of the magnetic field B

ocean

¼rV in the

current-free region outside of the Earth.

Electric fields of motional induction

Already in 1832, Faraday predicted a motionally induced electric field

for the river Thames but failed to detect it, due to a lack of adequate

instrumentation. Such fields were then observed in 1851 by Wollaston

in a telegraphic cable across the English Channel. Young demon-

strated in 1920 the recording of electric fields in the oceans by ship-

towed electrodes. Submarine cables and towed electrodes are still the

principal methods of measuring ocean flow by electromagnetic meth-

ods. Overviews of the theory and instrumentation are given by Sanford

(1971) and by Filloux (1973).

Magnetic fields of motional induction

Motionally induced magnetic fields can be divided into toroidal

(strictly horizontal) and poloidal (both horizontal and vertical) parts.

The toroidal magnetic field is generated by electric currents closing

in vertical planes and is estimated to reach 100 nT in amplitude. It is

confined to the oceans and solid Earth and is therefore only observable

by subsurface measurements, for example by ship-towed or ocean-

bottom magnetometers. Indeed, Lilley et al. (2001) showed that a

magnetometer lowered by a cable from a ship can be used to determine

vertical profiles of the ocean velocity.

The much weaker poloidal field, with amplitudes up to 10 nT,

results from electric currents closing horizontally. It has a significant

vertical component and reaches remote land and satellite locations.

Much attention has been given to the periodic magnetic signals of

ocean flow, which is driven by the lunar tides. In interpreting tidal

magnetic signals one has to take into account that lunar tidally driven

winds in the upper atmosphere induce tidal electric fields and currents

in the ionosphere that also contribute to the magnetic signal at these

periods. However, the strength of the ionospheric currents depends

on the conductivity of the ionosphere, which varies with the time of

the day. This additional solar-time modulation of the ionospheric lunar

tidal fields can be used to separate them from the oceanic lunar tidal

signals (Malin, 1970). The dominant M2 lunar tidal ocean flow mag-

netic signal has also been identified in CHAMP (see CHAMP) satellite

data (Tyler et al., 2003). A map of this signal at surface level is dis-

played in Figure O14.

Using high-resolution ocean circulation models predictions of the

magnetic signal of nontidal ocean flow have been made by several

authors. Amplitudes at surface level are predicted to be comparable

to those of tidal ocean flow signals. However, the magnetic signal of

the steady ocean circulation is impossible to distinguish from the crus-

tal magnetic field (see Crustal magnetic field). Observations must

therefore concentrate on the much weaker time-varying part of the sig-

nal. For example, the annual variation in the Indian Ocean is predicted

to generate a signal with only about 0.3 nT at satellite altitude. It is a

great challenge to separate this weak ocean signal from ionospheric

fields, which have similar periodicities and may even share some of

the atmospheric driving forces. At this point of time, nontidal ocean

currents have not yet been convincingly identified in land observatory

or satellite magnetic measurements.

Induction by time-varying magnetic fields

External magnetic fields, generated in the ionosphere (see Ionosphere)

and the magnetosphere (see Magnetosphere of the Earth), vary

strongly in time, exhibiting regular daily variations (see Geomagnetic

spectrum, temporal) and occasional strong magnetic storms (see

Storms and substorms) caused by coronal mass ejections from the

sun. These time-varying magnetic fields induce electric fields and cur-

rents in the oceans, generating secondary, induced magnetic fields.

Governing equations

A time-varying magnetic field induces an electric field E in the

oceans, given by

]

B ¼rE; (Eq. 6)

where B is the sum of the inducing and the secondary induced mag-

netic field. The induced electric field drives electric currents

J ¼ sE; (Eq. 7)

generating a secondary-induced magnetic field:

J ¼rB

induced

: (Eq. 8)

Figure O14 Predicted amplitude of the vertical component of the

magnetic field at the Earth’s surface generated by ocean flow due to

the lunar M2 tide. (Reproduced from Kuvshinov and Olsen, 2005b.)

OCEAN, ELECTROMAGNETIC EFFECTS 741

Combining Eqs. (6), (7), and (8) yields

]

B ¼r

rB

induced

; (Eq. 9)

which is the equation for induction by time-varying magnetic fields

inside the oceans. In contrast to the motional induction discussed

above, the oceans are assumed here to be stationary in the Earth-fixed

reference frame. The field in the source-free region above the Earth ’ s

surface and below the external current systems is again given by

Laplace ’s equation.

Skin depth

Low-frequency electromagnetic waves penetrate deeper into an electri-

cal conductor than high-frequency waves. For a periodic external field

with frequency n, Eq. (9) demands that the amplitude in a uniform con-

ductor decays exponentially with depth. The depth at which the ampli-

tude is reduced to 1/e is called the skin depth d , given by

d ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ



270m

ﬃﬃﬃ

(Eq. 10)

where the frequency n is given in hertz and a seawater conductivity of

3.5 S m

 1

is assumed. Thus, to effectively penetrate through a sea-

water column of 5 km, magnetic disturbances have to have periods

longer than about 6 min. However, even up to periods of several days,

induced fields are strongly influenced by the highly conducting

oceans.

Ocean induct ion by magneti c storms

While daily variations during solar quiet conditions generate signifi-

cant induction in the oceans, a much stronger effect is caused by mag-

netic storms. Strong magnetic storms generate ocean-induced magnetic

fields reaching magnitudes of more than 100 nT. An overview of

induction in the oceans by solar quiet and storm time fields is given

by Fainberg (1980). The induced current s are concentrated near the

coasts, the horizontal boundaries of the conductor. Their magnetic sig-

nal is also known as the coast effect (see Coast effect of induced cur-

rents). The ocean-induced fields can be predicted for a given external

storm signature using a 3D conductivity model of the oceans, crust,

and mantle. The induction equation (9) is solved either directly in

the time domain, or separately for all contributing frequencies with a

subsequent inverse Fourier transform to the time domain. The result

of a simulation using the latter approach (Kuvshinov and Olsen,

2005a) for a strong magnetic storm is shown in Figure O15.

Stefan Maus

Bibliography

Fainberg, E.B., 1980. Electromagnetic induction in the world ocean.

Geophysical Surveys, 4: 157–171.

Filloux, J.H., 1973. Techniques and instrumentation for study of nat-

ural electromagnetic induction at sea. Physics of the Earth and

Planetary Interiors, 7: 323–338.

Kuvshinov, A., and Olsen, N., 2005a. Modelling the ocean effect of

geomagnetic storms at ground and satellite altitude. In Earth

Observation with CHAMP. Berlin: Springer, 353–358.

Kuvshinov, A., and Olsen, N., 2005b. 3-D Modelling of the magnetic

fields due to Ocean tidal flow. In Earth Observation with CHAMP.

Berlin: Springer, 359–365.

Lilley, F.E.M., White, A., and Heinson, G.S., 2001. Earth’s magnetic

field: ocean current contributions to vertical profiles in deep

oceans. Geophysical Journal International , 147: 163–175.

Malin, S.R.C., 1970. Separation of lunar daily geomagnetic variations

into parts of ionospheric and oceanic origin. Geophysical Journal

of the Royal Astronomical Society, 21: 447–455.

Sanford, T.B., 1971. Motionally induced electric and magnetic fields

in the sea. Journal of Geophysical Research, 76: 3476–3492.

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

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

Cross-references

CHAMP

Coast Effect of Induced Currents

Crustal Magnetic Field

Geomagnetic Spectrum, Temporal

Ionosphere

Magnetosphere

Main field

Storms and Substorms

OLDHAM, RICHARD DIXON (1858–1936)

A British geologist and seismologist, born on July 31, 1858 in Dublin

and died on July 15, 1936 in Llandrindod Wells. He was the third son

of Thomas Oldham (1816–1878), Prof. of Geology at Trinity College,

Dublin, and director of the Geological Surveys of Ireland and India.

Richard Dixon Oldham was educated in England at first in Rugby,

later at the Royal School of Mines. In 1879, he followed his father’s

footsteps by moving to India and working at the Geological Survey

of India. During his time in India he wrote over 40 publications related

to geology and tectonics of India, many of them about earthquakes and

their observations in India. After moving back to England in 1903, he

worked for sometime together with John Milne (1850 –1913) on the

Isle of Wight. Later, because of his ill health, he spent the winters in

Southern France studying the history of the Rhône delta and the sum-

mers in Llandrindod Wells working on seismological problems. In

1908, he was honored with the Lyell Medal by the Geological Society

and in 1911 he was elected a Fellow of the Royal Society. In geomag-

netism, Oldham is known for his establishment of the Earth’s core.

Oldham started his career in India with the publication of his

father’s collection of macroseismical and geological observations in

the context of the large 1869 earthquake in Silchar (Assam). With this

background, he was predestined for a similar investigation of the huge

Assam earthquake of June 12, 1897. The results of his study are pub-

lished in a famous monograph of 1899 and numerous additional

Figure O15 Magnetic field induced in the oceans by the

Bastille-day magnetic storm of July 15, 2000. Shown is the

vertical component of the induced magnetic field at 21:30

universal time at the Earth’s surface. (Reproduced from Kuvshinov

and Olsen, 2005a.)

742 OLDHAM, RICHARD DIXON (1858–1936)

smaller publications. Since the first teleseismic observations of seismic

waves by Ernst von Rebeur-Paschwitz (1861–1895), surface and body

waves were known not only to exist but also to have different propa-

gation velocities (e.g., Rebeur-Paschwitz, 1895). Oldham’s most

important result for modern seismology was that he for the first time

could identify onsets in seismograms belonging to both body-wave

types as proposed by the theory of elastic waves, which propagate with

different velocities through the Earth: compressional waves (later

named P-waves) are faster than shear waves (later named S-waves)

(Oldham, 1899, 1900).

In the last decade of the 19th century, Emil Wiechert (1861–1928)

deduced from moment of inertia, ellipticity, and mean density of the

Earth the need for dividing the Earth in two principle parts: a less

dense mantle of rock material and a much denser core of compressed

iron (Wiechert, 1896, 1897; Brush, 1980, 1982). However, the seismo-

logical proof could not be given in these early days of seismology.

During his first years back in England, Oldham analyzed and compiled

teleseismic observations from the collection available at Shide,

Milne’s Observatory on the Isle of Wright. In 1906, he published a

paper in which he claimed to have observed and modelled the Earth’s

core by delayed P- and S-phase observations due to lower seismic

velocities inside the core. Oldham himself was not very convinced

by his delayed P observations because of their scatter (Oldham,

1906). More important for his argumentation was the delay of S onsets

at distances beyond 120



epicentral distance. From these data, Oldham

concluded that the Earth’s core has a size of about 0.4 of the Earth’s

radius and that both P- and S-wave velocities are much smaller than

in the Earth’s mantle. As Wiechert (1907) pointed out, Oldham’s

delayed S-phase observations are mostly SS phases, that is, the reflec-

tions of S phases at the Earth’s surface, an interpretation later accepted

also by Oldham (Oldham, 1919).

The correct deciphering of the main structural elements of the

Earth’s interior was then the work of the next generation of seismolo-

gists: Beno Gutenberg (1889–1960) could estimate the depth of the

core-mantle boundary (q.v.) at 2890 km, calculate travel-time curves

for core phases, and correctly identify in seismograms the P phases,

which had been reflected from, or which had traversed the core

(Gutenberg, 1913, 1914). Harold Jeffreys (1891–1989) showed that

the Earth’s core must have the rheological behavior of a fluid, and that

therefore no S phases can pass it (Jeffreys, 1926) and in 1936 Inge

Lehmann (1888–1993) discovered the inner core of the Earth

(Lehmann, 1936, 1987).

However, Oldham was the first who discovered and published about

the shadow effect of the Earth’s core due to lower seismic velocities.

Together with his work on seismic phase types, Oldham is one of

the pioneers for the application of seismic-wave phenomena to inves-

tigate the structure of the Earth’s interior.

Johannes Schweitzer

Bibliography

Brush, S.G., 1980. Discovery of the Earth’s core. American Journal of

Physics, 48: 705–724.

Brush, S.G., 1982. Chemical history of the Earth’s core. EOS, Trans-

actions, American Geophysical Union, 63 : 1185–1186, 1189.

Gutenberg, B., 1913. Über die Konstitution des Erdinnern, erschlossen

aus Erdbebenbeobachtungen. Physikalische Zeitschrift, 14:

1217–1218.

Gutenberg, B., 1914. Über Erdbebenwellen. VII A. Beobachtungen an

Registrierungen von Fernbeben in Göttingen und Folgerungen über

die Konstitution des Erdkörpers. Nachrichten von der Königlichen

Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-

physikalische Klasse, 1914: 125–177.

Jeffreys, H., 1926. The rigidity of the Earth ’s central core. Monthly

Notices of the Royal Astronomical Society, Geophysical Supple-

ment, 1: 371–383.

Lehmann, I., 1936. P’. Publications du Bureau Central Séismologique

International, Série A, Travaux Scientifique, 14:87–115.

Lehmann, I., 1987. Seismology in the days of old. EOS, Transactions,

American Geophysical Union, 68:33–35.

Oldham, R.D., 1899. Report on the great earthquake of 12th June

1897, Chapter 25, The unfelt earthquake. Memoirs of the Geologi-

cal Survey India, 29: 226–256.

Oldham, R.D., 1900. On the propagation of earthquake motion to great

distances. Philosophical Transactions of the Royal Society of

London, 194: 135–174.

Oldham, R.D., 1906. The constitution of the interior of the Earth, as

revealed by earthquakes. The Quarterly Journal of the Geological

Society of London, 62: 456–473.

Oldham, R.D., 1919. The interior of the Earth. The Geological Maga-

zine, New series 6, Decade 6, 56:18

–27.

Rebeur-Paschwitz, E.V., 1895. Horizontalpendel-Beobachtungen auf

der Kaiserlichen Universitäts-Sternwarte zu Strassburg 1892–

1894. Beiträge zur Geophysik, 2:211–536.

Wiechert, E., 1896. Über die Beschaffenheit des Erdinnern. Schriften

der Physikalisch-ökonomischen Gesellschaft zu Königsberg in

Preußen, Sitzungsberichte, 37:4–5.

Wiechert, E., 1897. Über die Massenvertheilung im Innern der Erde.

Nachrichten von der Königlichen Gesellschaft der Wissenschaften

zu Göttingen, Mathematisch-physikalische Klasse, 1897: 221–243.

Wiechert, E., 1907. Über Erdbebenwellen. Theoretisches über die Aus-

breitung der Erdbebenwellen. Nachrichten von der Königlichen

Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-

physikalische Klasse, 1907: 413–529.

Cross-references

Core Composition

Core Density

Core-Mantle Boundary

Core Viscosity

Inner Core Seismic Velocities

Lehmann, Inge (1888–1993)

Seismic Phases

ØRSTED

This geomagnetic research satellite (Figure O16/Plate 5b), named after

the Danish scientist Hans Christian Ørsted (1777–1851), is the first

satellite mission after Magsat (q.v.) (1979–1980) for high-precision

mapping of the Earth’s magnetic field. It was launched with a Delta-

II rocket from Vandenberg Air Force Base (California) on February

23, 1999 into a near polar orbit. Being the first satellite of the Inter-

national Decade of Geopotential Research (IAGA, 1997), it has been

a model for other missions, like CHAMP (q.v.) and Swarm (Friis-

Christensen et al., 2006). Since optimal utilization of magnetic data

for improved field modeling requires firm knowledge of all contributing

sources, there was an endeavor to bring together experts in internal

sources (core and crustal field, electromagnetic induction) and in exter-

nal sources (current systems in the ionosphere (q.v.) and magnetosphere

(q.v.)) in the Ørsted International Science Team (OIST) very early in the

planning phase of the project (Friis-Christensen and Sktt, 1997). Sixty-

four research groups from 14 countries joined the OIST, giving impetus

to the mission (Stauning et al., 2003). Ørsted has also had an impact

on the collection of ground data, as is evident from the significantly

increased number of geomagnetic observatories that deliver data to the

World Data Centers in support of the mission. Ørsted external field

science is coordinated by the Danish Meteorological Institute (DMI),

internal field science is coordinated by the Danish National Space Center

(DNSC, formerly Danish Research Institute, DSRI). Satellite control is

managed by the industrial company Terma A/S (Birkerd, Denmark).

ØRSTED 743

Satellite and orbit

Ørsted was built in a joint effort of various Danish research institutions

and companies with significant contribution from space agencies in

the United States and Europe. The satellite weighs 62 kg, measures

34  45  72 cm and contains an 8 m long boom, deployed shortly

after launch, carrying two magnetometers (q.v. ). The satellite is gravity

gradient stabilized; attitude maneuvers are performed using magnetic

torquers. The orbit has an inclination of 96.5



, a period of 100.0 min,

a perigee at 643 km and an apogee at 881 km (decreased to respectively

99.7 min, 632 km, and 860 km after 7 years in space). The orbit

plane is slowly drifting, the local time of the equator crossing decreases

by 0.91 min day

– 1

, starting from an initial local time of 1426 on

February 23, 1999 for the north-going track, cf. the upper part of

Figure O17 . Nominal lifetime of the mission was 14 months (2 months

commissioning phase þ 12 months science phase), but after almost

8 years in space the satellite is still health y and provides high-precision

magnetic data.

Instru mentatio n

A proton precession Overhauser magnetometer (OVH), measuring the

magnetic field intensity with a sampling rate of 1 Hz, is mounted at the

top of the deployable 8 m long boom. This instrument, developed by

LETI (Grenoble/F), has an absolute accuracy better than 0.5 nT. Two

meters away, at a distance of 6 m from the satellite body, is the optical

bench with the compact spherical coil (CSC) fluxgate vector magnet-

ometer mounted closely together with the star im ager (SIM); these

instruments were developed by the Danish Technical University and

DNSC. The CSC samples the magnetic field at 100 Hz (burst mode,

at polar latitudes) or 25 Hz (normal mode) with a resolution better than

0.1 nT and is calibrated using the field intensity measured by the OVH

(Olsen et al., 2003) . After calibration, the agreement between the two

magnetometers is better than 0.33 nT rms.

A copy of the Ørsted boom and payload (but with a scalar helium

magnetometer provided by NASA/JPL instead of the Overhauser

magnetometer), called Ørsted-2, was launched in November 2000

onboard the Argentinean satellite SAC-C. However, due to a broken

connection in a coaxial cable, no high-precision attitude data (and

hence no reliable vector data) are available.

Dat a accuracy and av ailability

Ørsted data are available at the Ørsted Science Data Centre (http://

www.dmi.dk/projects/oersted/SDC) and at www.spacecenter.dk/data/

magnetic-satellites. Scalar measurements (from the OVH if available,

and from the CSC otherwise) are provided at 1 Hz sampling rate in

the MAG-F files. The overall accuracy of the scalar data is estimated

to be better than 0.5 nT. Vector data (at the sampling rate of 1.135 s

of the SIM) are distributed as MAG-L files. The biggest limitation of

the vector data is the star imager attitude accuracy. It is highly aniso-

tropic: determination of the SIM bore-sight direction (pointing) is

more accurate than determination of the rotation around bore-sight

(rotation), resulting in a relatively higher noise of the SIM rotation

angle. In addition, the rotation angle is more sensitive to distorting

effects like instrument blinding (for instance by the moon). This atti-

tude anisotropy results in correlated errors between the magnetic com-

ponents, which have to be taken into account when deriving field

models, as demonstrated by Holme (2000). Let n be the unit vector

of the SIM bore-sight, and let B be the observed magnetic field vector.

The magnetic residual vector (observation minus model field) d B ¼

( dB

, d B

) can be transformed such that the first component,

d B

, is in the direction of B (and therefore to first order is equal

to the scalar residual), the second component, d B

, is aligned with

n  B , and the third component, d B

, is aligned with B  ( n  B).

The last two components are perpendicular to the magnetic field. Atti-

tude noise may result in significant noise in the magnetic vector com-

ponents: since the nominal bore-sight direction is almost East-West,

typical rotational attitude noise will result in noise in B

of up to

10 nT amplitude at low latitudes. Whenever possible, the user should

be aware of this effect and use data in the ( dB

, d B

)-frame rather

than (B

, B

The boom experiences occasional jerks associated with thermal

expansion/c ontraction, especially near the terminator. The resulting

boom oscillations, mainly around the boom axis, have a frequency

close to 0.5 Hz and are seen in the magnetic vector components as

occasional damped oscillations with amplitudes of a few nanoteslas.

These technically induced signals should not be confused with geo-

magnetic pulsations of natural origin.

Accuracy of dB

, dB

, and d B

is estimated to be better than 1, 2, and

8 nT, respectively. Vector data in an Earth-Centered-Earth-Fixed frame

( B

, B

) and the SIM bore-sight direction n are available together

with time and position in the MAG-L files. Data at higher sampling

rates are available upon request. The lower part of Figure O17 shows

the availability of scalar data (MAG-F, black) and of vector data

(MAG-L, light grey). Since attitude data (dark grey) are essential for

providing vector data, drop outs of the SIM (for instance due to thermal

problems, or due to blinding of the instrument by the Sun, Moon, or

Earth) limit the availability of Ørsted vector data. The satellite was fully

illuminated by the Sun from July to November 2000, from August

2002 to February 2003, and from January to May 2005 (shaded area

in Figure O17 top). Since the satellite was not designed for this situation

(nominal lifetime of 14 months ended in April 2000), thermal problems

resulted in decreased data availability during these periods, as shown in

the bottom panel of Figure O17.

As secondary data products, magnetic field models that have been

derived using Ørsted data are provided at www.spacecenter.dk/

projects/oersted/mode ls /.

Scientific results

After almost 8 years of operation, about 150 scientific papers using

Ørsted data have been published. The satellite and first science results

were presented in an overview article in EOS (Neubert et al., 2001).

Figure O16/Plate 5b The Ørsted satellite.

744 ØRSTED

A parade of magnetic field (q.v.) models of increasing complexity

and accuracy have been derived from Ørsted observations, starting

from models that describe a snapshot of the field at a specific epoch,

like the IGRF, International Geomagnetic Reference Field (q.v.) 2000

or the Ørsted Initial Field Model OIFM (Olsen et al., 2000) to models

that include the temporal changes of the field (Olsen, 2002, Oslen

et al., 2006). Spherical harmonic coefficients estimated using more

than 6 years of Ørsted observations are robust up to degree n ¼ 40

for the static terms, and up to at least n ¼ 15 for the linear time

terms (see Secular variation models). Comparing two models esti-

mated from Ørsted data (epoch 2000) and Magsat data (epoch 1980),

respectively, Hulot et al. (2002) derived core-flow models from the

observed mean secular variation, down to length scales previously

inaccessible.

Ørsted’s altitude makes an investigation of the crustal magnetic

field (q.v.) difficult, but combination with other satellite (POGO

(q.v.), Magsat (q.v.), CHAMP (q.v.)) and ground data allows for

an improved separation of internal and external sources, and thereby

better crustal field determination. This was shown by Sabaka et al.

(2004) in their Comprehensive Model, CM4, which determined crustal

field contributions that are reliable at least up to degree n ¼ 50.

Although the main objective of Ørsted is the precise mapping of

Earth’s internal field, the satellite has contributed to improved under-

standing of ionospheric and magnetospheric current systems. A much

more detailed determination of their variability with season and local

time is possible due to the greatly improved data distribution compared

to previous missions. By taking the curl of the magnetic field residual

(observation minus field model), Christiansen et al. (2002) derived

mean field-aligned currents (FAC) in dependence on season, local

time, and interplanetary magnetic field (solar wind) conditions and

found that the entire polar caps are filled with currents even during

very quiet solar wind conditions. The influence of these quiet time

polar cap currents on core and crustal field models is still an unsolved

problem. Embedded in the large-scale average current systems are

small-scale (few hundreds of meters) current structures (filaments),

especially in the cusp region, as found by Neubert and Christiansen

(2003).

Nils Olsen

Bibliography

Christiansen, F., Papitashvili, V.O., and Neubert, T., 2002. Seasonal

variations of high latitude field-aligned current systems inferred

from Ørsted and MAGSAT observations. Journal of Geophysical

Research, 107: doi:10.1029/2001 JA900104.

Friis-Christensen, E., and Sktt, C., 1997. Ørsted compendium-

contributions from the International Science Team. DMI Scientific

Report 97-1, Copenhagen.

Friis-Christensen, E., Lühr, H., and Hulot, G., 2006. Swarm: a constel-

lation to study the Earth’s magnetic field. Earth, Planets and

Space, 58: 351–358.

Holme, R., 2000. Modelling of attitude error in vector magnetic data:

application to Ørsted data. Earth, Planets and Space, 52: 1187–1197.

Hulot, G., Eymin, C., Langlais, B., Mandea, M., and Olsen, N., 2002.

Small-scale structure of the geodynamo inferred from Ørsted and

Magsat satellite data. Nature, 416: 620–623.

International Association of Geomagnetism and Aeronomy (IAGA),

1997. Resolution No. 1, 8th IAGA Scientific Assembly, Uppsala,

Sweden.

Neubert, T., and Christiansen, F., 2003. Small-scale, field-aligned cur-

rents at the top-side ionosphere. Geophysical Research Letters,

30(19): 2010, doi:10.1029/2003GL017808.

Neubert, T., Mandea, M., Hulot, G., von Frese, R., Primdahl, F.,

Jrgensen, J.L. Friis-Christensen, E., Stauning, P., Olsen, N., and

Risbo, T., 2001. Ørsted satellite captures high-precision geomag-

netic field data. EOS, Transactions of the American Geophysical

Union, 82(7): 81, 87, and 88.

Olsen, N., 2002. A model of the geomagnetic field and its secular var-

iation for epoch 2000. Geophysical Journal International, 149(2):

454–462.

Oslen, N., Luehr, H., Sabaka, T.J., Mandea, M., Rother, M., Tffner-

Clausen, L., and Choi, S., 2006. CHAOS —A Model of Earth’s

Magnetic Field derived from CHAMP, Ørsted, and SAC-C mag-

netic satellite data, Geophys. J. Int., 166:67–75.

Olsen, N., Holme, R., Hulot, G., Sabaka, T., Neubert, T., Toeffner-

Clausen, L., Primdahl, F., Jrgensen, J., Legèr, J.-M., Barraclough,

D., Bloxham, J., Cain, J., Constable, C., Golovkov, V., Jackson, A.,

Kotze, P., Langlais, B., Macmillan, S., Mandea, M., Merayo, J.,

Figure O17 Local time evolution of the Ørsted orbit (top) and data availability (bottom), as of July 2006.

ØRSTED 745