Treatise on Geophysics: Geomagnetism

524 Pages·2015·88.215 MB·English

by  Masaru Kono, Gerald Schubert (eds.)

5.01 Geomagnetism: An Introduction and Overview
MKono,TokyoInstituteofTechnology,Tokyo,Japan
ã2015ElsevierB.V.Allrightsreserved.
5.01.1 GeomagnetisminPerspective 1
5.01.1.1 EarlyHistory 1
5.01.1.1.1 Attractiveforceofthemagnets 2
5.01.1.1.2 EarlyChinesecompasses 2
5.01.1.1.3 MagneticcompassinEuropeandocuments 4
5.01.1.1.4 EpistolaofPetrusPeregrinus 4
5.01.1.1.5 Europeanrecognitionofdeclination 5
5.01.1.1.6 Inclination 6
5.01.1.1.7 DeMagneteofWilliamGilbert 7
5.01.1.2 FurtherDevelopmentsinObservations 8
5.01.1.2.1 Secularvariation 8
5.01.1.2.2 Short-termvariations 8
5.01.1.2.3 Magneticcharts 8
5.01.1.2.4 Measurementofintensity 9
5.01.1.2.5 Magneticobservatories 10
5.01.1.3 DecipheringthePastUsingRemanentMagnetization 10
5.01.1.3.1 Earlyobservationoftheremanenceofrocks 11
5.01.1.3.2 Polarityreversals 11
5.01.1.3.3 Seafloorspreading 13
5.01.1.3.4 Long-termbehaviorsofthegeodynamo 13
5.01.1.4 DescriptionsoftheEarth’sMagneticField 15
5.01.1.4.1 Sphericalharmonicanalysis 15
5.01.1.4.2 Internationalgeomagneticreferencefield 15
5.01.1.4.3 Satellitemeasurements 15
5.01.1.4.4 Geomagneticspectrum 16
5.01.1.4.5 Inverseproblem 18
5.01.1.5 ObservationalConstraints 19
5.01.1.5.1 Geometricattenuation 20
5.01.1.5.2 Accuracyofdata 20
5.01.1.5.3 Signalandnoiseinmagneticfieldmeasurements 20
5.01.1.6 MagneticFieldsofthePlanetsandSatellites 21
5.01.1.6.1 Exampleofaflybyobservation 22
5.01.1.6.2 Magneticmappingofplanets 22
5.01.1.6.3 Planetarymagneticfields:Acomparativestudy 22
5.01.1.6.4 ThemagneticfieldoftheSun 24
5.01.1.7 IntroductiontotheOtherChapters 26
5.01.1.7.1 Thepresentfieldanditscomponents 26
5.01.1.7.2 Datasourcesandtheirreliability 27
5.01.1.7.3 Fieldbehaviorsinvarioustimewindows 28
5.01.1.7.4 Excursions,reversals,andpolarwander 29
5.01.1.8 Conclusions 29
Acknowledgments 30
References 30
5.01.1 GeomagnetisminPerspective observationofthegeomagneticfieldhasbeencarriedoutonly
inthelastfewcenturies,butwithindirectmeasurements,wecan
5.01.1.1 EarlyHistory
understandthefieldbehaviormillionsofyearsbackintime.In
The Earth has its own magnetic field (the geomagnetic field), thisextendedtimeframe,thereisevidencethatthepolarityof
whichisconfinedbytheactionofthesolarwindintoavolume the magnetic field reversed frequently and that the magnetic
called the magnetosphere. This field is not steady, but varies dipole axis in very ancient times was significantly displaced
withtimeduepartlytotheinteractionwiththesolarwind,but fromthepresentrotationalaxis(thenorthandsouthgeographic
more importantly by its own physical processes. Direct poles).
TreatiseonGeophysics,SecondEdition http://dx.doi.org/10.1016/B978-0-444-53802-4.00095-6 1
2 Geomagnetism:AnIntroductionandOverview
It is of considerable interest how such knowledge was AncientChinesepeoplemadesimilarobservations,butthe
acquired over several centuries. We will take a brief tour of recordsaresomewhatlaterthanthecorrespondingGreekones.
thehistorical eventsthatprovidedimportant stepsinformu- In 呂氏春秋 Lu¨ Shih Chhun Chhiu (Master Lu¨’s Spring and
latingourunderstandingofthegeomagneticfield.Indoingso, Autumn Annals), written in the late third century BC, it is
we have to rely solely on the written records, which is the saidinChapter45that
reason why only the European and Chinese histories are
referred. There are many works on this topic; some of the 慈石召鐵.或引之也.
important references are Mitchell (1932–46), Harradon Thelodestonecallstheirontoitself,orattractsit.
(1943–45), Needham (1962), and Yamamoto (山本義隆)
(2003). Among them, Needham’s work is the most compre- Afterthat,referencestomagnetsappearabundantlyinthe
hensiveaboutChinesecontributionsintheearlyhistory.Such Chinese literature. Examples of old documents with such
mattersasthediscoveryofthenorth(orsouth)-pointingprop- descriptions are 淮南子 Huai Nan Tzu (The Book of Huai
ertyofthemagnet,themagneticdeclination,andthecreation Nan) in the first century BC and 論衡 Lun Heˆng (Discourses
oftheoldestmagneticcompassareverythoroughlydiscussed WeighedintheBalance)inAD83.
there. In this seminal monograph, Needham convincingly Theattractiveforcethatmagnetsexertonironwasawonder
concludedthatthesemattersweredescribedinChinesedocu- inancienttimes,anditwasoftenattributedtomagicalpower.
ments considerably earlier than in the corresponding Euro- Itsfullunderstandinghadtowaituntilthenineteenthcentury
pean documents. Accordingly, the following descriptions whenthemagneticforcewasexplainedbyphysicaltheorems
related to the Chinese old history are taken from Needham such asAmpe`re’s andGauss’s laws in theframework ofelec-
(1962).Thus,theEnglishtranslationsofChineseliteratureand tromagnetictheory.
thealphabeticalexpressionsofthenamesofpersonsandbooks
areasgiveninNeedham(1962).ButsincetheChineseoriginal 5.01.1.1.2 EarlyChinesecompasses
sentences were not given there, they were taken from the Thefactthatmagnetshavethepropertytoaligninthenorth–
Japanesetranslationofthisbook(Needham(ニーダム),1977). southdirectionwasdiscoveredbytheancientChinese.From
Whenwetalkabouttheearliestrecognitionofthemagne- about the second century AD, there are many Chinese texts
tism of the Earth, we should be careful to discriminate two referring to ‘south-pointing carriage,’ which, in many
separateissues,thatis,theattractiveforceexertedbyamagnet instances,weredescribedasguidingthesoldiersinthickfogs
on iron and the north (or south)-seeking property of the to the right direction to beat the enemies. Many people
magnet. The former can be taken as the forerunner to the thoughtthatthiswasadevicethatusedthepropertyofmag-
scienceofmagnetism,whilethelatteristhebasisforappreci- nets. However, it is now considered to be some mechanical
ationofthemagneticfieldassociatedwiththeEarth.Ourmain devicemadeupofgearsandaxlesratherthananinstrument
interestisinthegeomagneticfield,butitisnecessarytolook similar to a magnetic compass (Needham, 1962). A more
intomagnetsfirst. interesting description appears in the earlier-mentioned
論衡 Lun Heˆng, written by 王充 Wang Chhung in AD 83. In
Chapter52,itissaidthat
5.01.1.1.1 Attractiveforceofthemagnets
Theearliestobservationofthenaturalmagnets(lodestoneor 司南之杓、投之於地、其柢指南.
loadstone) is attributed to the Greek philosopher Thales of
When the south-controlling ladle is thrown upon the ground, it
Miletus (624–546 BC). Thales did not leave any writings of comestorestpointingatthesouth.
his own, but Aristotle (384–332 BC) wrote about him in De
Anima(‘Onthesoul’)abouttwocenturieslater.Accordingto 王振鐸WangChen-To(1948)suggestedthatthefirsttwo
this, Thales taught that the lodestone has a soul, because it letters 司南south-controlling werechangedintheprocess of
could set another body (iron) in motion. Diogenes Laertius hand copying from the original 指南 south-pointing, the
(c.AD200–250)alsowroteinhiswork‘LivesandOpinionsof fourthletter杓ladlemeansaspoonworkedoutfromalode-
Eminent Philosophers’ that Thales admitted that souls exist stoneintothatshape,andtheeighthletter地groundactually
eveninnonlivingmatterbasedontheobservationthatmag- indicates式(or栻)diviner’sboard.Now,adiviner’sboardwas
nets and ambers can attract things. This suggests that Thales usedinancientChinaforthepurposeoftellingfortunes,andit
knewnotonlyaboutthe attractive forceofmagnetsbut also isinscribedwiththeconstellationofGreatBearinthecenter
abouttheforceduetothestaticelectricityofamber,whichcan andthenamesof24directionsonthecirclearoundit. With
beseenwhentheyarerubbedbyclothes(Mitchell,1937). theseinterpretations,thesentencecanbetakentodescribean
References to the attractive force of magnets appear quite instrument for seeking south using a magnet! Note that the
ofteninGreekmanuscripts(e.g.,Plato,Aristotle,Democritus, GreatBearisthesymbolofthenorthpoleandthespoonalso
andLucretius),andthereisnodoubtthatthisforcewaswell hasashapereminiscentofitsform.
knowntotheancientGreeks.Thismaybebecausetheattrac- Wang (1948) went further to show the credibility of his
tiveforceappearedtothemasaveryremarkablephenomenon interpretation, by actually remaking a model of this instru-
sinceitcanactonmaterialsthatarenotincontact.Inthese,the ment,withabronzebaseplateandaspooncutfromlodestone
magnetswerereferredtomostlyastherockofMagnesia(liyοB (seeFigure1).Aphotooftheactualinstrumentisshownin
magn(cid:1)si(cid:1)). Magnesia is the name of a place in either Needham (1962). When Needham visited China, he was
Macedonia, Crete, or Asia Minor. The names of magnetism shownbyWanghimselftheexperimentinwhichthelodestone
andmagnetite(Fe O )werederivedfromthisGreekword. spoongraduallyrotatedtothesouthwarddirectionandsettled
3 4
Geomagnetism:AnIntroductionandOverview 3
Thisistheearliestwrittenrecordaboutthemagneticcom-
passusingamagnetic(magnetized)needle.
Regarding the oldest compass, the existence of an even
earlierrecordwaspointedoutalsobyWang(1948).Thetext
wasfoundin武經總要WuChingTsungYao(Collectionofthe
MostImportantMilitaryTechniques),whichisacompendium
ofmilitarytechnologyeditedby曾公亮TseˆngKung-Liangand
completedin1044.InChapter15,itissaidthat
若遇天景噎霾. 夜色瞑黒. 又不能辨方向. 則當縦老馬前行. 令識道
路. 或出指南車及指南魚. 以辨所向. 指南車世法不傳. 魚法以薄鐡
葉剪裁.長二寸闊五分.首尾鋭如魚形.
Whentroopsencounteredgloomyweatherordarknights,andthe
directionsofspacecouldnotbedistinguished,theyletanoldhorse
go on before to lead them, or else they made use of the south-
pointingcarriage,orthesouth-pointingfishtoidentifythedirec-
tions.Nowthecarriagemethodhasnotbeenhandeddown,butin
thefishmethodathinleafofironiscutintotheshapeofafishtwo
incheslongandhalfaninchbroad,havingapointedheadandtail.
Afterthat,howtomakethisfishisdescribed.
置炭火中焼之.候通赤.以鐡鈐.鈐魚首出火.以尾正對子位.蘸水盆
中.沒尾數分則止.以密器收之.用時置水碗於無風處.平放魚在水面
令浮.其首常南向午也.
This is then heated in a charcoal fire, and when it has become
thoroughlyred-hot,itistakenoutbytheheadwithirontongsand
placed so that its tail points due north. In this position, it is
quenchedwithwaterinabasin,sothatitstailissubmergedfor
Figure1 AnancientChinesecompasswiththelodestonecutinthe
severaltenthsofaninch.Itisthenkeptinatightlyclosedbox.Touse
shapeofaspoon,restoredbyWang(1948).ReproducedfromNeedham
it,asmallbowlfilledwithwaterissetupinawindlessplace,andthe
J(1962)ScienceandcivilisationinChina,PhysicsandPhysical fishislaidasflataspossibleuponthewater-surfacesothatitfloats,
Technology,Part1Physics,vol.4,pp.434.Cambridge:Cambridge whereuponitsheadwillpointsouth.
UniversityPress.
Apparently,thismagneticpointer(fish)isgivenathermo-
remanentmagnetization(TRM)byquenchingfromhightem-
there.Althoughthiseffortisveryimpressive,itisratherdoubt- perature and keeping it in the north–south direction. The
fuliftheChineseatthisearlyagereallyusedaninstrumentthat record is very convincing as the description is detailed and
canbeidentifiedastheancestorofthemagneticcompass.The correct.Thiscommunicationcanbetakenasthefirstdescrip-
interpretation, as suggested by Wang, is not completely con- tion of the magnetic compass of wet type. Figure 2 shows
vincing.Moreover,thereisaconspicuousabsenceofreferences Wang’sreconstructionofthiscompass.
to compass-like instruments for about a thousand years The dry pivoted compass was described in 事林廣記 Shih
afterward. LinKuangChi(GuidethroughtheForestofAffairs),anency-
Awell-knownearlyrecordaboutthemagneticcompassis clopedia compiled around 1150 by 陳元靚 Chhen Yuan-
in夢溪筆談MeˆngChhiPiThan(DreamPoolEssays)writtenby Ching.InChapter10,Chhenwroteaboutthewetcompass:
沈括 Shen Kua at about AD 1088. In this book, it is said in
Chapter24that 以木刻魚子位一个.如拇指大.開腹一竅.陥好磁石一塊子.却以﨟塡
滿.用針一半.僉従魚子口中鈎入令沒.放水中.自然指南.以手撥轉.
又復如初.
方家以磁石磨針鋒.則能指南.然常微偏東.不全南也.
They(themagicians)cutapieceofwoodintotheshapeofafish,as
Magiciansrubthepointofaneedlewiththelodestone;then,itis
bigasone’sthumb,andmakeaholeinitsbelly,intowhichthey
abletopointtothesouth.Butitalwaysinclinesslightlytotheeast
neatlyfitapieceoflodestone,fillingupthecavitywithwax.Intothis
anddoesnotpointdirectlyatthesouth.
wasaneedlebentlikeahookisfixed.Thenwhenthefishisputin
thewateritwillofitsnaturepointtothesouth,andifitismoved
Thetextfurtherexplainshowtomakeamagneticneedleby withthefingeritwillreturnagaintoitsoriginalposition.
rubbing a needle with the lodestone, its south-seeking prop-
erty, and moreover the fact that there is a slight difference Butinthiscase,alodestonemagnetisinsertedinawooden
between the true south and its pointing direction (i.e., the fish that is put in the water. After that, Chhen continues to
firstmentionofthedeclination).ShenKuafurthersaysthat describeadifferentcompass:
以芥子許蝋綴於針腰.無風處懸之.則針常指南. 以木刻龜子一个.一如前法製造.但於尾邊敲針入去.用小板子.上安
以竹針子如箸尾.大龜腹下.微陥一穴.安針子上.撥轉常指北.須是
Itisbesttosuspenditbyasinglecocoonfiberofnewsilkattached
釘尾後.
to the centre of the needle by a piece of wax the size of a
mustard-seed – then, hanging in a windless place, it will always Theyalsocutapieceofwoodintotheshapeofaturtle,andarrange
pointtothesouth. itinthesamewayasbefore,onlythattheneedleisfixedatthetail
4 Geomagnetism:AnIntroductionandOverview
NatureofThings)writtenabout1190,hedescribedtheuseof
themagneticneedleinnavigationtoindicate northandthat
theneedleisputonapivotthatmaybetheformofaprimitive
compass (Mitchell, 1932). Guyot de Provins of France
(1184–1210) wrote a poem called La Bible around 1205, in
whichhedescribedafloatingcompass.JacquesdeVitryofthe
Kingdom of Jerusalem (1165–1240) left a similar document
(c. 1218). These people were all monks or priests, and they
only referred to the compass as having the noble property
(topointalwaystothesamedirection).Itisthereforenatural
to think that the properties of the compass were known to
mariners well before it became popular so that the priests
coulduseitforallegoryinthesewritings(Mitchell,1932).
5.01.1.1.4 EpistolaofPetrusPeregrinus
PetrusPeregrinus(RomannameforPierrePe`lerin,activeinthe
thirteenthcentury)wroteEpistola(EpistolaPetriPeregrinideMar-
icourt ad Sygerum de Foucaucourt, militem, de magnete; Letter of
PeterPeregrinusofMaricourttoSygerusofFoucaucourt,Soldier,
ontheMagnet)in1269,whilehetookpartinthesiegeofthe
southern Italian town of Lucera, by the army of Charles, the
CountofAnjou.MaricourtandFoucaucourtarenamesoftowns
inPicardy,France.Ithastheformofaletter(Epistola)toasoldier
calledSygerusofFoucaucourt.Althoughitwasnotpublishedin
printed form until 1558, many hand copies were circulated
widely in western Europe in medieval times. In this booklet
(Figure4),Peregrinusexplainedvariouspropertiesofthemag-
Figure2 IllustrationofaChinesefishcompassbyWang(1948). net(lodestone)basedonexperimentalobservations.Infact,this
ReproducedfromNeedhamJ(1962)ScienceandcivilisationinChina, canberegardedasthefirstscientifictreatisedescribingobserva-
PhysicsandPhysicalTechnology,Part1Physics,vol.4,pp.434. tionsandexperimentscarriedoutforthepurposeofclarifyinga
Cambridge:CambridgeUniversityPress. natural phenomenon. The conclusions were derived logically
basedonobservationsandexperiments.
TheEpistolaiscomposedoftwoparts.Thefirstpartismade
end.Abamboopinaboutasthickastheendofachopstickissetup
on a small board, and sustains the turtle by the concave under- up of ten chapters, in which the properties of magnets are
surfaceofitsbody,wherethereisasmallhole.Thenwhentheturtle discussed. The second part covers the technical use of the
isrotated,itwillalwayspointtothenorth,whichmustbeduetothe magnets, such as the construction of a magnetic compass.
needleatthetail.
The second part also contains a discussion of a perpetual
motionmachineusingmagnets.Thisiscertainlyinvalidfrom
In this compass, a wooden turtle has a tail made of a
thepresent-dayknowledgeofphysics,butitcannotbeblamed
magnetic needle. A thin bamboo stick stands from the base-
asanerror,becauseitwaswrittenlongbeforetheconceptsof
board and holdsthe turtle atthehole made initsbelly. The
thermodynamicsorenergyconservationwereformed.
turtlerotatesandpointstothenorthbecauseofthemagnetic
All of the material written in Epistola may not have been
needle(Figure3).
discovered by Peregrinus himself (Harradon, 1943), but this
doesnotdecreasetheimportanceofEpistolaasthefirstscien-
tificpaperinhumanhistory.Themostimportantpropertiesof
5.01.1.1.3 MagneticcompassinEuropeandocuments
magnetsdescribedbyPeregrinuswereasfollows:
It is not clear when the knowledge of the magnetic compass
reached Europe and when it was first used in navigation. 1. Finding the two magnetic poles: Toshow that amagnet has
Gilbert wrote that it was brought to Europe by the Venetian two poles, grind and polish the magnet into a spherical
MarcoPolo,butthereisevidencethatthecompasswasused shape.Next,putasmallneedle-shapedirononthissphere
well before his return to Europe in 1295. It is often thought andwritealinealongitsdirectiondividingthesphereinto
thattheknowledgeofthecompasscamefromChinathrough twoequal parts (this defines a magneticmeridian). Move
theintermediaryoftheIslamcivilization.Thereisnowritten the needle to a different position and write the second
evidence,however,andtheappearanceofthecompassisear- divisionline.Peregrinusnotedthat,evenifthepreviously
lier in European documents than in Islamic ones (Mitchell, mentionedprocessisrepeatedatmanyplaces,allthelines
1932;Needham,1962). meetatthesamepointonthesphere.Theintersectionof
Theearliestrecordofthenorth-andsouth-seekingproperty great circles defines two poles, the north and south mag-
of the compass in Europe appears to be that of Alexander neticpoles.
Neckam(1157–1217)ofSt.Albans,England.Intwotreatises, 2. Determinationofthepolarityofthepoles:Todeterminewhich
DeUtensibibus(OnInstruments)andDeRerumNaturis(Onthe is the north pole and which is the south pole, place a
Geomagnetism:AnIntroductionandOverview 5
Figure3 IllustrationofaChineseturtlecompassbyWang(1948).Clockwisefromtopleft:planview,lengthwisesection,transversesection,and
sideview.ReproducedfromNeedhamJ(1962)ScienceandcivilisationinChina,PhysicsandPhysicalTechnology,Part1Physics,vol.4,434pp.
Cambridge:CambridgeUniversityPress.
magnetonawoodenplatethatfloatsonthewatersurface (insteadofamagnetizedneedle).Inaddition,theinstrument
in a large-enough container. After some time, the magnet hasarulerwithapinateachend.Todeterminethedirectionof
settles into a north–south direction, and thus, the north theSunortheMoon,therulerisrotateduntiltheshadowofa
andsouthmagneticpolescanbedetermined.Thismethod pin falls along its length. Peregrinus also showed another
isquitesimilartothemethodofusinganaturalmagnetasa instrumentinwhichanironneedlemagnetizedbyamagnet
compass (to put on water a wooden fish containing a isusedinplaceofthemagnetinthisfigure.
naturalmagnet)inChinesedocuments(seeFigure2).Per- Many of the previously mentioned findings have usually
egrinusconcludedthatthemagnetrotatessothatthetwo beenattributedtoGilbert.Forinstance,points1,2,and4are
polesareinthesamedirectionasthecelestialpoles. described very similarly to the description in Epistola but in
3. Forcesbetween two magnetic poles:Using two magnets with more detail in Part I, Chapters 3–5 of De Magnete (Gilbert,
polesmarkedasintheprecedingtext,onemagnetfloating 1600). As an example, Figure 6 shows an illustration from
onthewaterandanotherheldbyahand,itcanbeshown De Magnete that describes point 4: cut a magnet in half, and
that the two poles attract each other if the S pole of the the poles of opposite polarities appear at the new edges.
secondisbroughtneartheNpoleofthefirst,orviceversa. ArathercuriousfactisthatGilbertdidnotrefertoPeregrinus
Ontheotherhand,theyrepeleachotherifthetwopolesare inthesedescriptions.TheneglectofthecontributionofPere-
ofthesamepolarity(NtoNorStoS). grinus in later years is perhaps caused by the popularity of
4. Amagneticpolecannotbeisolated:Toshowthis,Peregrinus Gilbert’sworkandthelackofpropercitationinit.
described an experiment of cutting the magnet into two
halves.Then,newpolesappearatthecutend.Thepolarities
5.01.1.1.5 Europeanrecognitionofdeclination
ofthesenewpolesareoppositetotheoneattheotherend
Before Needham (1962) showed the precedence of Chinese
ofthecutpieces,whichcanbeseenbyrepeatingthethird
discovery, itwas widely believed that Christopher Columbus
experimentwithhalvedmagnets.Thisisthefirstrealization
discovered the declination on his first journey to the West
of the dipolar nature of the magnet and the fact that
Indies in 1492. According to the journal of the ship, quoted
monopolesdonotexist.
inabookbyLasCasasandinthebiographyofColumbusby
Figure 5 is the illustration of an instrument to determine hissonFernando,magneticdirectionchanged‘fromnorthwest
the azimuth of the Sun and others shown by Peregrinus. to northeast’ on September 13. However, Mitchell (1937)
A floating magnet (Magnes) determines the north direction. argued convincingly that the declination must have been
This part is a magnetic compass using a natural magnet known in Europe at the time of the voyages of Columbus.
6 Geomagnetism:AnIntroductionandOverview
Figure6 Experimentofcuttinglodestonetoshowtheappearanceof
magneticpoleatthenewedgesasdescribedinDeMagnete.
totheeastoftruenorth,whichshowsthatthedeclinationin
westCentralEuropeatthattimewascompensatedinthelatter
compass. From the speed of information transfer in those
times, it is deemed not probable that this knowledge is the
outcome of the first journey of Columbus. The artisans of
Flandersmusthaveknownthedifferencebetweentruenorth
andmagneticnorthforsometime.
Itremainstrue,however,thatthedescriptionbyColumbus
is the first scientific record of declination measurement and
thatimportantdiscoveriesweremadeof(1)thevariabilityof
declination from place to place and (2) the position of the
agonicline(thelineconnectingplaceswhereD¼0(cid:2))attheend
ofthefifteenthcentury.
Figure4 ThetitlepageofEpistola.
5.01.1.1.6 Inclination
The fact that the magnetic needle deflects not only from the
horizontalnorth–southdirectionbutalsofromthehorizontal
plane was first recognized by Georg Hartmann and Robert
Norman. Hartmann’s findings were written only briefly in a
letter(1544)thatwasmostlyunknownuntilitwasrediscov-
eredinthenineteenthcentury,andhisvalueoftheinclination
(9(cid:2))istooshallowforEuropeatthattime.Ontheotherhand,
Norman published his measurement of the inclination and
discussion of the results in 1581 as The Newe Attractive (The
Newe Attractive, Containyng a short discourse of the Magnes or
Lodestone:andamongstotherhisvertuesofanewediscoveredsecret
and subtill propertie, concernyng the Declinyng of the Needle,
touchedtherewithundertheplaineoftheHorizon.Nowfirstfounde
outbyRobertNormanHydrographer).
Normanknewaboutthemagneticdevicesquitewellsince
hewasasailorfor20years,andhislaterjobwastheconstruc-
tionandsalesofnavigationalinstruments.Hewasawareofthe
tendencyofthemagneticneedletomakesomeanglewiththe
horizontalplanefromhisvastexperienceinmakingmagnetic
compasses.Heusuallycorrectedthisbyputtingasmallweight
Figure5 ThecompassusingamagnetshowninEpistola.
on the needle, but at some time, he decided to study this
phenomenoninmoredetail.Heconstructedaspecialinstru-
In the biography written by Fernando, it is said that on the menttomeasurethedeflectionfromthehorizontalplaneby
second voyage (1493), Columbus carried compasses from preparingamagneticneedlefromcarefullybalancedironand
Genoa and fromFlanders. In the Genoan type, the magnetic makingitfreelyrotatearoundthehorizontalpivot(Figure7).
needlewasfixedintheN–Sdirectionofthecompasscard,but Thisisthefirstdipcircle,andsimilarinstrumentswereusedin
intheFlemishtype,theneedlewasfixedatonepoint(11.25) observatoriesuntilveryrecenttimes.Byrepeatingexperiments,
Geomagnetism:AnIntroductionandOverview 7
90 80
70
60
E 50
DL 40
E
E 3
N 0
2
0
1
0
THE HORIZON
0
1
0
2 E
03 TH
0
4
05
0607
08 09
Figure8 ThetitlepageofDeMagnete.
Figure7 ThedipcircleofRobertNorman.
he found out that this angle (the inclination) was 71(cid:2)500 in
London.Asthetitleofthebookindicates,Normannotonly
carriedoutthemeasurementbutalsogavediscussionsabout
thenatureofthisforce.Init,hedeniedtheoldideathatthere
are attractive points in heaven (such as the celestial poles of
PetrusPeregrinus)orintheEarth(themagneticmountainin
theArctic,etc.),whichisimportantinthedevelopmentofthe
notionofremotelyactingforces(Yamamoto,2003).
Figure9 Illustrationofversorium,apivotedironneedleusedby
Gilberttomeasurethedirectionofthefieldaroundterella.
5.01.1.1.7 DeMagneteofWilliamGilbert
William Gilbert (1544–1603) was a well-established medical
about magnetism. His most important contributions may be
doctor and was the president of the Royal College of Physi-
summarizedasfollows:
cians. He was famous in his time because he became the
physiciantoQueenElizabethfortheperiod1601–03,buthe 1. Magnetsandironarethesame:Thismaysoundabitqueer,
is now considered as one of the founders of the physical butlodestoneandironwereconsideredtobelongtodiffer-
sciencesbythepublicationofDeMagnete(DeMagnete,magne- ent kinds of matter (‘rock’ and ‘metal’) from the time of
ticisquecorporibus,etdemagnomagnetetellure;Physiologianova, Aristotle.Gilbertconcludedthattheyarethesamebecause
plurimis&argumentis,&experrimentisdemonstrata;Onmagnets, most of the best magnets were found in an iron mine,
magnetic materials, and the Earth as a large magnet; New magnets heated in the furnace produced good-quality
natural philosophy proved by arguments and experiments). iron,etc.
This book (Figure 8) had a profound influence in Europe 2. Similarity of the spherical magnet and the Earth: Gilbert
and prepared for Newton’s theory of gravity by establishing formed a lodestone intoasphere and calleditterella. He
thefactthatthemagneticforceisaremotelyworkinginterac- used a small magnetic needle suspended at the pivot
tion (Yamamoto, 2003). Gilbert also founded the science (versorium;seeFigure9)tofindthedirectionofmagnetic
of electricity by the experiments described in De Magnete. force on this sphere. Like Peregrinus, he identified poles,
Here, we will only discuss what was made clear by him meridians,andtheequatoronthesphere.Fromtheresults
8 Geomagnetism:AnIntroductionandOverview
ofthismodelexperiment,heconcludedthattheEarthisa large error in Borough’s measurement. Henry Gellibrand
magneticbodyandthusamagnet(Globusterraeestmag- (1597–1636), Gunter’s successor at Gresham College, went
neticus&magnes). back in 1635 to Deptford, repeated the measurement, and
3. Inclination is determined by the (magnetic) latitude: Gilbert foundavalueofabout4(cid:2)E.Thismarksthediscoveryofsecular
arguedthatifterellasaremadefromlodestonesofdifferent variation.
strength, theinclinationmightbelargerinonebecauseof
thestrongerattractionofthemagnet,yettheactualinclina- 5.01.1.2.2 Short-termvariations
tionswerethesame.Thisobservationstrengthenedhiscon- GeorgeGraham(1673–1751),aLondonclockmaker,observed
clusionthattheEarthisalargesphericalmagnet(Figure10). declination very frequently in 1722–23 using a very sensitive
(12in.) compass. He discovered that the declination changed
Hisexperimentusingasphericalmagnetwasverysimilarto
evenin1day,andsometimes,themagnitudeofchangereached
theonedescribedbyPeregrinus.However,theconclusionwas
up to half a degree. Following this, Anders Celsius and Olof
quitedifferent:Peregrinusfoundcorrespondencebetweenthe
Hjorter performed the same experiments in Uppsala in
sphericalmagnetandthecelestialglobe.Gilbert,ontheother
1740–47. They not only confirmed Graham’s results but also
hand,concludedthatitisthesameastheEarth,ortheEarthis
foundout thatthe activityofthe northernlights(aurora bor-
asphericalmagnet.Thewaytoreachthisideabasedonexper-
ealis)wasaccompaniedbyalargechangeindeclination.Later,
iments is not quite perfect in the standards of present-day
theycooperatedwithGrahaminLondontomakesimultaneous
science, but Gilbert reached the correct conclusion for the
measurementsandfoundoutthatlargedisturbancesoccurredat
first time that the Earth has a dipole field that is aligned
the same time in two places on 5 April 1741. These are the
roughlyparalleltotherotationalaxis.
discoveriesofmagneticstormsandsolarquiet-dayvariations.
5.01.1.2.3 Magneticcharts
5.01.1.2 FurtherDevelopmentsinObservations
AfterthevoyageofColumbus,thepracticalutilityofmagnetic
Isaac Newton (1642–1727) published his Philosophiae Natur- measurementswasrecognizedbecauseoftheirimportancein
alisPrincipiaMathematica(MathematicalPrinciplesofNatural navigation.Apreciseknowledgeofthepositionoftheshipand
Philosophy)in1668,whichfirmlyestablishedmodernphys- the direction it is heading is very important for navigation.
ics.ItisclearthatNewton’sworkreceivedabiginfluencefrom The latitude part of the position and direction is relatively
Gilbert’sDeMagnete,especiallytheideaoftheforcesbetween easy.Theanglebetweenthehorizonandthepolestar(which
objectsthatarenotincontact(Yamamoto,2003).Geomagne- istheastronomicallatitudeifthediurnalmotionofthepole
tismasaphysicalsciencetookshapeataboutthesametime, starisadjusted)canbemeasuredwithenoughaccuracyusing
anditisremarkablethatprogresswasmadethroughobserva- sextants.Thepolestarshowsthenorthdirectionatnight,and
tionsevenfromtheseoldtimes. inthedaytime,themeanofthetwomeasurementsoftheSun’s
azimuthgivesthesouthdirection,iftheyareobtainedattimes
5.01.1.2.1 Secularvariation when the Sun’s height is the same (e.g., at the sunrise and
sunset).
In October 1580, William Borough (1536–99), who later
Ontheotherhand,thedeterminationoflongitudeisquite
served as a naval officer under Captain Drake, measured the
difficult.Ifaclockcankeepveryaccuratetimeaftertheshiphas
declinationatLimehouseinLondonandrecordedthatitwas
11½(cid:2)E. Edmund Gunter (1581–1626), professor of astron- leftaport,thedifferencebetweentheclocktimeandthelocal
timeasdeterminedfromtheposition oftheSun(e.g.,12:00
omy at Gresham College, measured the declination at Dept-
noonforsouthing)givesthelongitudedifferencebetweenthe
fordandLimehouseinJune1622andobtainedthevaluesof
6(cid:2)150 and 5(cid:2)560E (Limehouse value was the mean of eight currentship’spositionandtheport.Buttheneededaccuracy
couldnotbeachievedbytheclocksofthosetimes.Sinceitwas
observations). Gunter pointed out that temporal change was
foundbythevoyagesofColumbusthatthedeclinationchan-
very likely, but he was cautious to admit the possibility of a
gedfromNEtoNWastheshipcrossedtheAtlanticfromeastto
west,itwasconsideredthatthelongitudemayhaveasimple
relationwiththemagneticdeclination.
At the end of the seventeenth century, Edmund Halley
(1656–1742)organizedanexpeditiondevotedtothedetermi-
nation of the declination in various parts of the Atlantic. He
thuspublishedin1701thefirstisogonicmapoftheAtlantic
(GeneralChartoftheVariationoftheCompass(1701)).Thiswas
the first magnetic chart, and its importance in the study of
geomagnetismcannotbeoverstated.Ironically,however,this
mapdidnotsolvetheproblemofthelongitude,becauseofthe
unlucky coincidence that the isogonic lines (the traces on
which the declination is equal) ran mostly east–west in the
northernAtlanticatthattime(Figure11).
For further discussion of magnetic data obtained by the
measurementsatseaandtheiruseinthestudyofgeomagne-
Figure10 Gilbert’sterella,orthesphericalmagnet. tism,seeChapter5.12.
Geomagnetism:AnIntroductionandOverview 9
Figure11 Halley’s1701Atlanticchartshowingthelinesofequaldeclination.
5.01.1.2.4 Measurementofintensity strength of magnetic field increased. This property was used
Thedeterminationoftheintensityofthemagneticfieldstarted bytheexplorerAlexandervonHumboldt(1769–1859),who
muchlaterthanthatofthedirection.Firstcamerelativemea- measuredtheoscillationperiodofadipcircleatmanyplaces
surements.Itwasnoticedthatiftheneedleofadipcirclewas while he traveled in South America with Aime` Bonpland in
displacedfromitsrestposition,theneedleoscillatedaboutthe 1798. He could show that the intensity systematically
equilibrium position with a period that decreased as the increased as he went further south from the magnetic
10 Geomagnetism:AnIntroductionandOverview
equator, which was near Cajamarca in Peru. Actually, the Next,nearanothersuspendedmagnet(BinFigure12(b)),
periodofoscillation(T)dependsnotonlyonthestrengthof heplacedthemagnet(A)inthedirectionperpendiculartothe
themagneticfield(H)butalsoonthemomentofinertia(I) magneticmeridian.Themagneticfieldatthesuspendedmag-
andmagneticmoment(M)ofthemagneticneedle: netisrotatedbyanangleH0/H,whereH0isthemagneticfield
pffiffiffiffiffiffiffiffiffiffiffiffiffi duetothemagnet:
T¼2p I=MH [1] " (cid:3) (cid:4) #
Thus,itisdifficulttoassumethatthechangeintheperiodis H0¼M 1(cid:3)3 l 2+(cid:4)(cid:4)(cid:4) [2]
d3 8 d
entirelyduetothechangeinthefieldstrengthandnotdueto
changesinthepropertyofthemagnet.
wheredisthedistancebetweenthecentersofthetwomagnets
Carl Friedrich Gauss (1777–1855) devised a method of
andlisthelengthofthemagnetwiththemagneticmomentM.
absoluteintensitydeterminationin1832.Hismethodwasto
Observingtheangulardeflectionfordifferentvaluesofdand
use a suspended magnet (A in Figure 12(a)) and make it
again utilizing the method of least squares, it is possible to
oscillateinthehorizontalplaneonbothsidesofthemagnetic
obtain M/H. With the values of MH and M/H, Gauss could
meridian. As the moment of inertia of the magnet is not determine the horizontal field intensity in Go€ttingen as
known, he added weights (W) to the magnet so that the 17.8mT. Gauss’s method of intensity determination was
moment of inertia can be changed by a precisely known
quickly taken up by other people, such as Hansteen and
amount.ByobservingtheperiodTforvariousincrementsin
Sabine,formeasurementsinotherpartsoftheworld.Present
themomentofinertia,itispossibletodeterminetheproduct
knowledgeoftheintensityofthemagneticfieldisdiscussedin
MH (through least-squares method, which is another inven-
Chapter5.13.
tionofGauss).
5.01.1.2.5 Magneticobservatories
MoreorlesscontinuousobservationsofDstartedinLondon
about1652andinParisin1663.BythetimeofGauss,there
werealreadyanumberofsuchobservatories,anditwasnec-
T
essarytocoordinatethemtoutilizethemagneticdata.Inthe
F 1830s,GaussandHumboldtorganizedtheGo€ttingerMagne-
C
tischerVerein(GottingenGeomagneticUnion),whichcanbe
regarded as the first international geophysical organization.
S
About50observatories(amongthem15wereoutsideEurope)
took part in this collaboration to carry out standardized
M
observations, for example, daily observation at the assigned
W
time,andintensiveobservationsofDonthesixselecteddays
peryear.
R
A
W 5.01.1.3 DecipheringthePastUsingRemanent
(a) Magnetization
GeomagnetismhasauniquepositioninalltheEarthsciences
becauseitcangobackintimeandtreatchangesthatoccurred
T
over millions of years or an even longer timescales. In other
F branches of geophysics, there are some efforts to go back in
history, such as paleoseismology based on the estimation of
past movements of active faults uncovered by trench excava-
S
tion.Geologyisinasenseastudyofalltheprocessesoperating
M on the Earth since its formation. However, in all of these
studies, the quality ofdatais much inferior tothose thatare
obtainedforthepresentobservations.Therearemanyreasons
forthis.Forinstance,theamountofinformationusedinthe
B analysis of earthquakes today (e.g., waveforms recorded on
seismogramsattheobservatories)ismanyordersofmagnitude
largerthantheinformationfromactivefaulttraces.Geologic
A
processes themselves modify the original information into
quite different forms (e.g., the fossils are different from the
livingorganismsandirreversibleprocessessuchasmetasoma-
(b) tismchangethepropertiesofsediments).
Magneticfielditselfcannotbepreservedovertime,whichis
Figure12 (a,b)PrincipleofGauss’smethodofintensitydetermination. similartothecaseofseismicwaves.Onlytheremanentmagne-
ModifiedfromMalinSRC(1987)Historicalintroductionto tizationinrocksandothermaterialsmaysurvive,andtheyare
geomagnetism.In:JacobsJA(ed.)Geomagnetism,vol.1,pp.1–49. also subject to geologic processes that can destroy or change