Listofabbreviations
aa asimplifiedindexdescribingglobalgeomagneticactivity.Itisderivedfromthe K indicesfromtwoapproximately antipodalobservatoriesandhasunitsof1nT(nanotesla).CurrentobservatoriesusedareHartlandintheUnited Kingdom,operatedbyBGS,andCanberrainAustralia,operatedbyGeoscienceAustralia.Theobservatoriesused havechangedoverthetimespanoftheseries.
AD AnnoDomini,meaning‘intheyearoftheLord’(MedievalLatin).IntheJuliancalendaritcountsthenumber yearsfromthebirthofJesusChrist.Traditionallythe‘AD’abbreviationisplacedbeforetheyearnumber,e.g.AD 2019.
AE anauroralelectrojet(refertotheSpecificterminology)indexobtainedfromanumber(usuallygreaterthan10)of stationsdistributedinlocaltimeinthelatituderegionthatistypicaloftheNorthernHemisphereauroralzone.
Ap ameasureofthegenerallevelofgeomagneticactivityovertheglobeforagivenday.Itisderivedfrommeasurementsofthevariationofthegeomagneticfieldduetocurrentsflowingintheearth’sionosphere(andtoalesser extentinEarth’smagnetosphere),madeatanumberofstationsworldwide.
Aura-MLS microwavelimbsounder(MLS)experiments,aboardtheAurasatellite,measured(naturallyoccurring) microwavethermalemissionfromthelimb(edge)ofEarth’supperatmospherelaunchedin2004.Thedataare usedtocreateverticalprofilesofatmosphericgases,temperature,pressure,andcloudice.
BC BeforeChrist,traditionallyBCisplacedaftertheyearnumber(e.g.1200BC).
CCM chemistryclimatemodelscouplethestratosphericchemicalmodelswithclimatemodelsinonemodel, representingbothstratosphericchemistryandatmosphericclimate.Couplingbothprocessesinasinglemodel allowstheinvestigationofthefeedbackprocessesbetweenthesetwocomponents(e.g.addressingthequestion ofhowglobalclimatechange,associatedwiththeproductionofanthropogenicgreenhousegases,willinterfere withtheanticipatedozonerecoveryinthe21stcentury).However,couplingthesetwoprocessesintoasingle modelcomplicatestheinterpretationofresults,comparedtomodelsthattreattheprocessesseparately(e.g.chemistrytransportmodelsandgeneralcirculationmodels).
CMB core–mantleboundaryoftheEarth;thisliesbetweentheplanet’ssilicatemantleanditsliquidiron-nickelouter core.Itisobservedviathediscontinuityinseismicwavevelocitiesatthatdepth.Theboundaryisthoughttoharbourtopography,muchlikeEarth’ssurface,thatissupportedbysolid-stateconvectionwithintheoverlying mantle.
CRs cosmicraysarehighlyenergeticatomicnucleiorotherparticlestravellingthroughspaceataspeedapproaching thatoflight.Whenoriginatingfromthegalaxyorbeyondit,theyareknownasgalacticcosmicrays(GCRs).
CRM chemicalremanentmagnetizationisamagnetizationformedbyphasechange,physicochemicalchanges(such asoxidationorreduction),dehydration,recrystallization,orprecipitationofnaturalelementsatlowtemperatures.
ERA40,ERAInterim,andERA20C productsoftheEuropeanCentreforMedium-RangeWeatherForecasts (ECMWF)providinggridded4Ddataforagreatvarietyofatmosphericparameters,basedonthereanalysisof multidecadalseriesofpastobservations,aimedforuseinstudiesofclimatevariability.
eV anelectronvoltistheamountofkineticenergygained(orlost)byasingleelectron,whichisacceleratedfromrest, throughanelectricpotentialdifferenceofonevoltinvacuum.
GCRs galacticcosmicrays.
GeV agigaelectronvoltisequalto109 electronvolts.
HOx thefamilyofhydrogenoxideradicals(HOx H+OH+HO2),influencingtheozonedensityinmesosphere.
IGRF theInternationalGeomagneticReferenceFieldisastandardmathematicaldescriptionofthelarge-scalestructureofEarth’smainmagneticfield,anditssecularvariation.Itwascreatedbyfittingparametersofamathematical modeltomeasuredmagneticfielddatafromsurveys,observatories,andsatellitesacrosstheglobe.
IMF theinterplanetarymagneticfield,orheliosphericmagneticfield,isthecomponentofthesolarmagneticfield thatisdraggedoutfromthesolarcoronabythesolarwindflowtofillthesolarsystem.
INTERMAGNET InternationalReal-timeMagneticObservatoryNetwork.
IPCC theIntergovernmentalPanelonClimateChange,establishedin1988bytheWMOandtheUnitedNationsEnvironmentProgramme(UNEP).Itsreportscoverthescientific,technical,andsocio-economicinformationrelevant tounderstandingthescientificbasisofriskofhuman-inducedclimatechange,itspotentialimpactsandoptionsfor adaptationandmitigation.
IPDP IntervalsofPulsationswithDiminishingPeriodarecontinuousstructurednarrow-bandgeomagneticpulsations,whosefrequencyresemblePc1pulsationandinadditionincreaseswithtime(typicallyfrom0.2to1 2Hz). Thesepulsationsoccurintheeveningsector,inassociationwiththeexpansionphaseofasubstorm(refertothe Specificterminology).
K indexquantifyingdisturbancesinthehorizontalcomponentofEarth’smagneticfield,measuredinagivenmagneticobservatory.Itsrangeofvariabilityisbetween0and9—where1denotescalm,and5ormoreindicatesa geomagneticstorm.Itisaquasilogarithmiclocalindexofthegeomagneticactivity,derivedfromthemaximum fluctuationsof horizontal componentsobservedonamagnetometerduringa3hinterval.
ka akiloannus,abbreviated ka,isaperiodof1000Julianyears,equalto365,250days.Itisderivedfromtheprefixkilo (inSIsystem)andtheLatinforyear, annus; ka isequivalentto ky
keV akiloelectronvoltisequalto1000electronvolts(103 eV).
Kp theindexoftheglobalgeomagneticactivity,basedon3-hmeasurementsfromground-basedmagnetometers aroundtheworld.EachstationiscalibratedaccordingtoitslatitudeandreportsaboutacertainK-index,which dependsonthegeomagneticactivitymeasuredatthelocationofthemagnetometer.
LS thelowerstratosphereisthelowestpartofstratosphericlayerborderingthetropopause.Itisthinnerinequatorial regionsandcomparativelythickerathighlatitudes.Dominantcharacteristicsofthelowerstratospherearethe presenceofozoneanditsextremelylowtemperature,withintherange 40to 80°C.
Ma anabbreviationfromtheLatin mega-annum,i.e.millionyears.
MeV onemegaelectronvoltisequalto1millionelectronvolts(106 eV).
NAO NorthAtlanticOscillationisaweatherphenomenonintheNorthAtlanticOcean,manifestingitselfasfluctuationsinatmosphericsealevelpressurebetweentheIcelandicLowandtheAzoresHigh.Itcontrolsthestrength anddirectionofwesterlywindsandlocationofstormtracksacrosstheNorthAtlantic.
NH NorthernHemisphere.
NM neutronmonitor.
NMDB theneutronmonitordatabase,areal-timedatabaseforhigh-resolutionneutronmonitormeasurements.
NOAA theNationalOceanicandAtmosphericAdministrationisanAmericanscientificagencywithintheUnited StatesDepartmentofCommercethatfocusesontheconditionsoftheoceans,majorwaterways,andthe atmosphere.
NOx thefamilyofatmosphericnitrogenoxides(NOx NO+NO2)influencingthemesospheric,stratosphericand troposphericozone.
NRM naturalremnantmagnetizationisthepermanentmagnetismofarockorsediment.Itpreservesarecordofthe Earth’smagneticfieldatthetimewhenthemineralwaslaiddownassediment,orcrystallizedinmagma,andalso thetectonicmovementoftherockovermillionsofyearsfromitsoriginalposition.
nT ananoteslaisaunitofmeasurementofamagneticfield(inSIunitssystem),equaltoonebillionthofatesla(T),i.e. 10 9 T.
SAGEII StratosphericAerosolandGasExperiments(SAGE)aresatellite-basedsolaroccultationinstrumentsspanningover26yearsthathavebeenacornerstoneinstudiesofstratosphericchange.
SH SouthernHemisphere.
SHA sphericalharmonicanalysisistheprocedureofrepresentingapotentialfunctionbyasumofsphericalharmonicfunctions.
SpH atmosphericspecifichumidity,definedasthemassofwatervapourperunitmassofthemoistair,usuallyin kg/kg.
T2m airtemperatureat2mabovethesurfacefromERAreanalyses(productsoftheECMWF—theEuropeanCentre forMedium-RangeWeatherForecasts).
TIM/SOURCE abbreviationoftheTotalIrradianceMonitor(TIM),launchedinJanuary2003ontheNASAEarth observingsystem—SolarRadiationandClimateExperiment(SORCE).TheTIMmeasuresthetotalsolarirradiance,thespatiallyandspectrallyintegratedsolarradiationincidentatthetopoftheEarth’satmosphere.
TOZ thetotalozoneatanylocationontheglobeisdefinedasthesumofalltheozoneintheatmospheredirectly abovethatlocation.Mostozoneresidesinthestratosphericozonelayerandasmallpercentage(about10%)is distributedthroughoutthetroposphere.
TRM thermoremanentmagnetizationisthemagnetizationthatanigneousrockacquires,usuallyfromthemagnetic fieldinwhichitislocated,whenthetemperatureofthemagmaorlavafromwhichitformsfallsbelowtheCurie pointduringthecoolingandsolidificationprocess.
TSI totalsolarirradianceisameasureoftheelectromagneticradiationemittedfromtheSunandincidentonthe Earth’supperatmosphere—perunitarea,averagedoverallwavelengths.
UARS-HALOE theHalogenOccultationExperiment(HALOE)hasbeencollectingprofilesofmiddleatmosphere compositionandtemperatureonboardtheUpperAtmosphereResearchSatellite(UARS),withintheperiod 1991–2005.
UTLS theuppertroposphereandlowerstratospherelayerisbroadlydefinedastheregion 5kmaroundthetropopause,whichisthetraditionalboundarybetweenthetroposphereandthestratosphere.Thedynamical,chemical,andradiativepropertiesoftheUTLSareinmanywaysdistinctfromboththelowertroposphereandthe middlestratosphere.
UV ultravioletiselectromagneticradiationwithwavelengthsfrom10to400nm,shorterthanthoseofvisiblelight butlongerthanX-rays.UVradiationispresentinsunlight,andconstitutesabout10%ofthetotalelectromagnetic radiationoutputfromtheSun.
VADM thevirtualaxialdipolemomentdescribestheintensityofanimaginaryaxial(alongtheEarth’srotationaxis) centric(locatedinthecentreoftheEarth)dipolethatwouldproducetheestimatedarchaeo-/palaeointensityatthe samplingsite.
VGP virtualgeomagneticpoleisthepointontheEarth’ssurfaceatwhichamagneticpolewouldbelocated iftheobserveddirectionofremanenceataparticularlocationwasduetoamagneticdipoleatthecentreof theEarth.
VIRGO/SOHO VIRGO(VariabilityofsolarIRradianceandGravityOscillations)isanexperimentontheESA/ NASASolarandHeliosphericObservatory(SOHO)mission,investigatingtheirradiance(particularlytheTSI) andgravityoscillationsoftheSun.
VRM viscousmagnetizationisremanencethatisacquiredbyferromagneticmaterialsbysittinginamagneticfield forsometime.Thenaturalremanentmagnetizationofanigneousrockcanbealteredbythisprocess.
WMO WorldMeteorologicalOrganization.
Γw wetadiabaticlapserateofatmospherictemperature,knownalsoasmoistorsaturatedlapserate.
Specificterminology
adiabaticinvariant propertyofaphysicalsystemthatstaysapproximatelyconstantwhenchangesoccurslowly. Thismeansthatifasystemisvariedbetweentwoendpointsandthetimeforthevariationbetweenthemisclose toinfinity,thevariationofanadiabaticinvariantbetweenthetwoendpointsgoestozero. Antarcticconvergencezone knownalsoastheAntarcticPolarFront,acurvecontinuouslyencirclingAntarctica(varyinginlatitudeseasonally),wherethecoldequatorward-flowingAntarcticwatersmeettherelativelywarmerwaters ofthesub-Antarcticregion.Antarcticwaterspredominantlysinkbeneaththewarmersub-Antarcticwaters. atmosphericstaticstability(alsocalledhydrostaticstabilityorverticalstability) measuresthegravitationalresistanceofatmospheretoverticaldisplacements.Itisdeterminedbytheverticalstratificationofdensityorpotential temperature.
atmosphericwindow wavelengthsoftheelectromagneticspectrumthatcanbetransmittedthroughtheEarth’satmosphere.Atmosphericwindowsoccurinthevisible,infrared,andradioregionsofthespectrum. auroralelectrojet thelargehorizontalcurrents(Hallcurrents)thatflowintheDandEregions(i.e.100–150km)ofthe auroral ionosphere,flowingfromnoontowardsnights.
autocatalyticcycle asetofchemicalreactionsproducingcatalysts,stimulatingtheentiresetofchemicalreactions andensuringitsself-sustainability,givenaninputofenergyandfoodmolecules.
α-particles alphaparticles,alsocalledalpharayoralpharadiation,whichconsistoftwoprotonsandtwoneutrons boundtogetherintoaparticleidenticaltoahelium-4nucleus.
Brewer–Dobsoncirculation meanmeridionaloverturningcirculationinthestratosphere,characterizedbytwo-cell structureinthelowerstratosphere,i.e.ascendanceofairinthetropics,itspolewardpropagation,andits descendanceinthemiddleandhighlatitudesinbothhemispheres.Thetwo-cellstructurewasfirstproposed
byDobsonandBrewertoexplaintheobservationsofozoneandwatervapourinthestratosphere.Athigheraltitudesasingle-cellcirculationexistswithairascendinginthesummerhemisphere,crossingtheequator,and descendinginthewinterhemisphere.
cation ionicspecieswithapositivecharge.Acationhasmoreprotonsthanelectrons,givingitanetpositivecharge. chronozone(orchron) timeintervalinchronostratigraphy,definedbyeventssuchasgeomagneticreversals (magnetozones),orbasedonthepresenceofspecificfossils(biozoneorbiochronozone). climaticmode repeatingpatternsoftime-spacevariabilityoftheclimatesystem. climatology(inmeteorology) long-termmeanofagivenclimatevariable. cosmicrayspallation(innuclearphysics) theprocessinwhichaheavynucleusemitsnumerousnucleonsasaresult ofbeinghitbyahigh-energyparticle,thusgreatlyreducingitsatomicweight. cosmogenicisotopes(i.e.cosmogenicnuclides) rareisotopescreatedwhenahigh-energycosmicrayinteractswith atomicnucleus,causingnucleons(protonsandneutrons)tobeexpelledfromtheatom.TheseisotopesareproducedwithinEarthmaterialssuchasrocksorsoil,inEarth’satmosphere,andinextra-terrestrialitemssuchas meteorites.Therearebothradioactiveandstablecosmogenicisotopes.
crosssection measureofprobabilitythataspecificprocesswilltakeplaceinacollisionoftwoparticles.Iftheparticlesinteractthroughsomeaction-at-a-distanceforce,suchaselectromagnetismorgravity,theirscatteringcross sectionisgenerallylargerthantheirgeometricsize.
driftmotion movementofchargedparticles(confinedbyamagneticfield)inadirectionperpendiculartoboththe magneticfieldlineandtheappliedforce(electricfield,magneticgradient,orcurvatureofmagneticfieldlines)due totheactionofLorentzforce.
effectivetemperature thetemperatureofanobjectcalculatedfromtheradiationitemits,assumingblack-body behaviour.
floodbasalt theresultofagiantvolcaniceruptionorseriesoferuptionsthatcoverslargestretchesofland(orthe oceanfloor)withbasaltlava.
geomagneticexcursion likeageomagneticreversal,itmanifestswithasignificantchangeinEarth’smagneticfield. Unlikereversals,however,anexcursiondoesnotpermanentlychangethelarge-scaleorientationofthefield,but ratherrepresentsadramatic,typicallyshort-livedchangeinfieldintensity,withavariationinpoleorientationof upto45degreesfromthepreviousposition.Theseevents,whichtypicallylastafewthousandtoafewtensof thousandsofyears,ofteninvolvedeclinesinfieldstrengthbetween0%and20%ofnormal. glaciation formation,existence,ormovementofglaciersoverthesurfaceoftheearth. guidingcentre thecentreofthevastcircularmotionofchargedparticlearoundmagneticfieldline,whichisdrifting slowlyinadirectionperpendiculartothefieldlinesinthecaseofspatiallyheterogeneousortemporaryvarying magneticfields.
interstadial relativelywarmperiodduringaglacialepoch,whenglacierstemporarilystoporretreat. knock-onelectron secondaryelectron(ejectedbyhighspeedparticlesthroughitsinteractionwithmatter)having enoughenergytoescapeasignificantdistanceawayfromtheprimaryradiationbeamandproducefurther ionization.
lithosphere solid,outerpartofEarth,includingthebrittleupperportionofthemantleandthecrust. losscone solidangledefiningtheminimumanglebetweenvelocityvectorofarrivingchargedparticleandmagnetic fieldline,ensuringparticlereflectionbythemagneticmirror,i.e.itsconfinementbythemagneticfield.Particles approachingthemagneticfieldatloweranglesarelostinthesurroundingenvironment.
magneticlensing focusingordeflectionofmovingchargedparticles,suchaselectronsorions,duetotheactionof themagneticLorentzforce. magneticpolarity theorientationofmagneticfieldpolesinspace. magneticrigidity measureofthemomentumofchargedparticleinmagneticfield.Itreferstothefactthatahigher momentumparticlewillhaveahigherresistancetoadeflectionbymagneticfield.Itisdefinedas R ¼ p/q,where p istheparticlemomentumand q isitscharge.
Matuyama–Brunesborder isEarth’slatestmagneticfieldreversalevent.Itisanimportantcalibrationpointonthe geologicaltimescale,connectingsedimentsandvolcanicrocks,andhasthereforebeenthefocusofanumberof palaeomagneticstudies.
Maunderminimum periodaround1645–1715duringwhichsunspotsbecameexceedinglyrare. meanfreepath theaveragedistancetravelledbyamovingparticle(i.e.atom,molecule,photon,etc.)betweencollisionswithotherparticles—modifyingitsdirection,energy,orotherparticleproperties.
meson hadronic(i.e.large,massive)subatomicparticlescomposedofonequarkandoneantiquark,boundtogether bystrong(i.e.nuclear)interactions.
Milankovitchcycles describethecollectiveeffectsofchangesintheEarth’srotationarounditsaxis,andrevolution aroundtheSun(duetothegravitationalinteractionswithotherbodiesinthesolarsystem)onitsclimateover thousandsofyears.
modeofvariability climatepatternwithidentifiablecharacteristics,specificregionaleffects,andoftenoscillatory behaviour.
muon anelementaryparticlesimilartotheelectron,withanelectricchargeof 1 e andaspinof1/2,butwithamuch greatermass(105.66MeV/c2,whichisabout207timesthatoftheelectron).
obliquity(oraxialtilt) theanglebetweenEarth’srotationalaxisanditsorbitalaxis,or,equivalently,theanglebetweenitsequatorialplaneandorbitalplane,varyingbetween 22.1and24.5degrees.
OlderDryas stadial(cold)periodbetweentheBøllingandAllerødinterstadials(warmerphases),about14,000years BP,towardstheendofthePleistocene.
planetaryalbedo percentageofsolarirradiancethatisreflectedimmediatelybackintospacebyclouds,aerosols,the Earth’ssurface,etc.TheEarth’splanetaryalbedoisapproximately30%. platetectonics scientifictheorydescribingthestructureofEarth’scrustandmanyassociatedphenomenaas resultingfromtheinteractionofrigidlithosphericplates,movingslowlyovertheunderlyingmantle. potentialfunction mathematicalfunctionwhosevaluesareaphysicalpotential(scalarorvectorpotential).Ingeomagnetism,themagneticvectorfieldBispresentedasagradientofascalarfield P,i.e. B ¼ rP ¼
calledmagneticpotential,whichiswelldescribedbysphericalharmonicfunctions.Thusaleast-squaresfittothe magneticfieldmeasurementsgivestheEarth’sfieldasthesumofsphericalharmonics,eachmultipliedbythe best-fittingGausscoefficient gm ‘ or hm‘ primarycosmicrays stablechargedparticlesthathavebeenacceleratedtoenormousenergiesbyastrophysical sourcessomewhereinouruniverse(theMilkyWayordistantgalaxies,thesolaratmosphereandheliomagnetic field,andevenEarth’sradiationbelts).UponimpactwiththeEarth’satmosphere,cosmicrayscanproduce showersofsecondaryparticlesthatsometimesreachthesurface.
Quaternary currentandmostrecent(startingabout2.5millionyearsago)ofthethreeperiodsoftheCenozoicErain thegeologictimescale(accordingtotheInternationalCommissiononStratigraphy).TheQuaternaryPeriodistypicallydefinedbythecyclicgrowthanddecayofcontinentalicesheetsassociatedwithMilankovitchcycles,andthe associatedclimateandenvironmentalchangesthatoccurred.Inthisperiod,modernhumansappeared.
Regener–Pfotzermaximum themaximumoftheloweratmosphericionizationlayer,consistingofso-called‘secondary’cosmicradiation.Itisproducedbythemultipleinteractions(nuclear-electromagnetic-muonic-pionic,etc.)of primarycosmicrayswithatmosphericatomsandmolecules.
secondaryelectrons electronsgeneratedasionizationproducts.Theyarecalled‘secondary’becausetheyaregeneratedbytheprimaryradiation(i.e.ions,electrons,orphotonswithenergyexceedingtheionizationpotentialof thetargetatom/molecule).
secularvariation geomagneticsecularvariationreferstochangesinEarth’smagneticfieldwithperiodsofayearor more,reflectingchangesintheEarth’score.
stadialsandinterstadials phasesdividingtheQuaternaryPeriod,orthelast2.6millionyears.Stadialsareperiodsof colderclimatewhileinterstadialsareperiodsofwarmerclimate. subduction ageologicalprocessthattakesplaceatconvergentboundariesoftectonicplates,wheretheheavierplate (usuallyoceanic)issinkinggravitationallyunderthelighterone(usuallycontinental)intothemantle.Thesubductionratesaretypicallymeasuredincentimetresperyear,withtheaveragerateofconvergencebeingapproximately2–8cmperyearalongmostplateboundaries.Regionswherethisprocessoccursareknownassubduction zones.
substorm(alsosometimesmagnetosphericsubstormorauroralsubstorm) abriefdisturbanceintheEarth’smagnetospherethatcausesenergytobereleasedfromthe‘tail’ofthemagnetosphereandinjectedintothehighlatitude ionosphere.Visually,asubstormisseenasasuddenbrighteningandincreasedmovementofauroralarcs. superchron timeinterval(chron)lastingmorethan10millionyearsbetweenevents,especiallyreversalsofthepolarityoftheEarth’smagneticfield. teleconnection(inatmosphericsciences) causalconnectionorcorrelationbetweenmeteorologicalorotherenvironmentalphenomenawhichoccuralongdistanceapart.
VanAllenradiationbelt layerofchargedandenergeticparticleswhichisheldbytheplanetarymagneticfield aroundtheplanet.TheVanAllenbeltspecificallyreferstotheradiationbeltsaroundtheEarth. virtualgeomagneticpole pointontheearthsurfaceatwhichamagneticpolewouldbelocatediftheobserveddirectionofremanence—ataparticularlocation—wasduetoamagneticdipoleatthecentreoftheEarth.
YoungerDryas periodaround12,900–11,700yearsBP,characterizedbyareturntoglacialconditionsaftertheLate GlacialInterstadial.ItreversedtemporarilythegradualclimaticwarmingaftertheLastGlacialMaximum(LGM) startedaround20,000BP.TheYoungerDryaswasthemostrecentandlongestofseveralinterruptionstothegradualwarmingoftheEarth’sclimatesincethesevereLGM,about27,000–24,000yearsBP.
1.1Geomagneticfieldstructure—Dipole
1.2Directandindirectobservationsof
Earth’smagneticfieldplaysanimportantroleinmanyaspectsofEarthsciences.Itisoneof thekeycomponentsofthecomplexintegratedsystemofourplanet,becauseitinteractswith allEarth’sshells—theatmosphere,thebiosphere,Earth’scrust,mantleandcore—shielding thelifeontheplanetfromtheharmfuleffectsofcosmicradiation.Therefore,themagnetic field‘contains’informationaboutboththestateofnear-Earthouterspaceandtheinternal structureofthedeepEarth’sinterior.Thischapterconsidersthestructure,properties,nature, andmethodsforinvestigationofEarth’smagneticfield.
1.1Geomagneticfieldstructure—Dipoleandnondipolecomponents;temporal variability
TheobservedgeomagneticfieldonEarth’ssurfaceisavectorsumofthemagneticfieldsof severalsources(Fig.1.1)locatedindifferentareasinsidetheplanetandinnear-planetary space(Parkinson,1983; Yanovsky,1978;etc.):
FIG.1.1 SourcesofEarth’smagneticfieldcomponents.
where F0 isthedipolarcomponentofgeomagneticfield, Fm isthefieldofworldanomalies associatedwiththeheterogeneityofthedeepEarth’sinterior(nondipolefield), Fa istheremanentmagnetizationoftherockswithintheupperpartofEarth’scrust(anomalousfield), Fe is thefieldofexternalsources,and δF isthefieldofvariation,alsoassociatedwithexternal causes(Fig.1.1).Thesumofthedipoleandnondipolefieldsissometimescalledthe main magneticfieldoftheEarth,i.e. F ¼ F0 + Fn.
ThegeomagneticfieldcanbedescribedonEarth’ssurfacebyitsthreeorthogonalcomponents: X (pointingtothegeographicnorthdirection), Y (pointingeastward),and Z (pointing downwardintheNorthernHemisphere).Thetwohorizontalcomponents X and Y canbe combined,yieldingthehorizontalcomponent H,whichisalignedinthedirectionofthecompassneedle:
Thesumofallthreecomponentsdefinesthetotalfieldintensity,directedtowardsthecentreoftheplanet:
Thedeclination D isdefinedastheanglebetween H andgeographicnorth,whiletheinclination I istheanglebetweenthehorizontalplaneandthevectoroftotalfieldintensity F.In theinternationalSIsystem,themeasurableunitsofgeomagneticfieldstrengthareTesla(T) anditssubunits: μT ¼ 10 6 T and nT ¼ 10 9 T
Thespatial-temporalvariabilityofEarth’smagneticfieldisoneofitsmostcharacteristic features.Theavailableinformationhasadifferentphysicalbasis,accuracy,andresolution, andcoversdifferenttimeranges.ToobtainthemostcompleteinformationaboutEarth’smagneticfield,datafromallsourcesareused(referto Table1.1).
1.1Geomagneticfieldstructure—Dipoleandnondipolecomponents;temporalvariability
TABLE1.1 Geomagneticfieldvariationsfromvarioustypemeasurements.
№ Geomagneticvariations
Period (1 < n < 10)
Amplitude (1 < n < 10)
Measurement accuracy Dataacquisition methodsa
1Steadyandirregular pulsations Minutes n 10 1 (nT)0.1–1.0(nT) О,S
2Disturbedandundisturbed variations Hours n 10(nT)1.0–5.0(nT) О,S
3MagneticstormsHours-days n 10–n 102 (nT)10(nT) О,S
4Secularvariations n 10–n 103 years Morethan (n 102n 103)(nT) 1–3degrees О,H, А,P
5Episodesandexcursions n 102–n 104 years >50degrees (excursions) 10degreesP
6Reversals n 103–n 104 years – 10degreesP
7Intervalsbetweenreversals n 105–n 106 years – 10–20degreesP
a A,archaeomagnetic; H,historical; O,ground-basedobservations; P,palaeomagneticmethods; S,satellitemeasurements. FromBakhmutov,V.G.,2006.PaleosecularVariationsofGeomagneticField,Kiev.NaukovaDumka,p.9(inRussian).
Temporalvariationsofgeomagneticfieldcoverabroadrangeoftimescales(Table1.1). Short-termchanges(e.g.variationswithNos.1–3)arecausedbytheexternalsources—i.e. theelectromagneticcurrentsinthemagnetosphereandionosphere,whicharestudiedby theuseofdirect(instrumental)observations.Long-periodchanges(Nos.4–6in Table1.1) arecausedbytheinternalsourcesinEarth’scoreandarestudiedbyusingboth—directobservationsandtheresultsofindirect(i.e.archaeomagneticandpalaeomagnetic)methods. Geomagneticpulsations(No.1in Table1.1)areveryshort-lastingoscillationsofgeomagnetic field.Theoriginofthesefluctuationsisultra-low-frequencyhydromagneticwaves,whichare excitedinsolarwindandEarth’smagnetosphere.Theyaredividedintotwoclasses:irregular pulsationsPi(individualburstslastingseveralminutes),andmorestablecontinuouspulsationsPc(lastingseveralhourswithaquasisinusoidalshape).AmongallPcs,Pc1pulsations aredistinguishedwithaperiodof0.2–5s(alsocalled‘pearls’)anddurationoftheseriesfrom halfanhourtoseveralhours.Themaximumoftheiroccurrenceisobservedintheearlymorninglocaltimehours.InthePi1range,severaltypesofpulsationsareobserved,inparticular IntervalsofPulsationswithDiminishingPeriod(IPDPs)associatedwiththedevelopmentof themagnetosphericsubstorm.IPDPsaremostoftenobservedintheafternoonandevening sectorsintheformofaseriesofseparatewavepackets,similartoPc1oscillations,butwitha graduallydecreasingperiod,i.e.increasingfrequency.
Theperturbedandunperturbedgeomagneticvariations(Nos.2–3in Table1.1)arechanges intheEarth’smagneticfieldovertimeundertheinfluenceofvariousfactors. Unperturbed are thesmallamplitude( tensofnT) annual variationsofthemonthlyaveragevaluesofEarth’s magneticfield,and diurnal variations,whichareassociatedwithchangesinsolaractivityand themoonphase.Theyhaveamaximumduringthedaytimehours,andwhenthemoonisin
opposition.Thesearesmoothperiodicvariationswithintensitiesreaching200nT,increasing fromtheequatortothepoles.
The perturbed variationsare magneticstorms and substorms,associatedwithactiveprocesses ontheSun,andirregularprocessesinthesolarwind,whichaffectEarth’sionosphereand magnetosphere.Thedurationofgeomagneticstormsrangesfromseveralhourstoseveral days.Theyareinitiatedbythedisturbedsolarwind,whenarrivingatEarth’smagnetopause. Theintensificationoftheequatorialringcurrent(constantlyexistingintheregionofEarth’s radiationbelts)inducesamagneticfieldopposingthedirectionofthemaingeomagneticfield. Asaresult,theground-basedobservatoriesdetectasuddendropinthehorizontalgeomagneticfieldcomponent(e.g. Rastogi,2005; Maksimenkoetal.,2008).
Magnetospheric substorms aredisturbancesdetectedinpolarregions,associatedwiththe interactionofasurgingsolarwindwithEarth’smagnetosphere.Theiramplitudecanreach 1000nT,graduallydecreasingtowardstheequator.Thedurationofthesubstormsisupto1h. Disturbancesaredevelopedinthemagnetosphere,ionosphere,andatmosphere,andmanifestthemselvesinperturbationsofcurrentsandmagneticfield,accelerationofenergeticparticles,andaurora.Incontrasttomagneticstorms,whicharemainlyassociatedwithchanges intheringcurrentnearthegeomagneticequator,andleadtoalmostglobal(exceptforregions nearthepolarregions)geomagneticdisturbances,substormsarelocalinnatureandcover mainlythenightsideofpolarregions.Allthesephenomenacharacterizetheexternallyforced geomagneticactivity,fortheassessmentofwhichvariousindicesareused(see,e.g. Parkinson,1983).
ThisbookisfocusedonthevariationsassociatedwithchangesinEarth’sdeepinterior(i.e. variationsNos.4–7in Table1.1). Secular variationsofgeomagneticfield(i.e.No.4in Table1.1) coverlongperiodsoftens,hundreds,andeventhousandsofyears.Theyleadtosignificant changesintheannualmeanvaluesofterrestrialmagneticfield.Theinvestigationsofthesecularvariationsarebasedontheground-basedobservations,aswellasonhistorical, archaeomagnetic,andpalaeomagneticdata.Accordingtorecentknowledge,theyareassociatedwithprocessesatthecore–mantleboundary.
VariationsNos.5–7in Table1.1 aredistinguishedbypalaeomagneticdata.Theyare discussedin Section1.3.4.
1.2Directandindirectobservationsofgeomagneticfield
Directobservationshavecontributedtoourknowledgeaboutgeomagneticfieldvariability overthepastfewcenturies.Informationaboutthelonger-scalevariations,however,isaccessibleonlyfromarchaeomagneticandpalaeomagneticdata.Directinstrumentalmeasurementsofgeomagneticfield,anditsvariationsonEarth’ssurface,beganabout400years ago.Duringthistime,agreatdealofdatahasbeenaccumulatedaboutthefielddeclination, butmuchlessaboutitsinclination.Inthemedievalera,mostgeomagneticmeasurements werecarriedoutonboardships,aspartoftheirnavigation.
Duringthe20thcentury,manymeasurementsofgeomagneticfieldcomponentswerecollectedduringmagneticsurveys:ground-based,oceanic,andaeromagnetic.Untilthe1960s, thelackofobservatorydataovertheoceans,andsomelandareas,wascompensatedfor
byrepeatedmeasurementsonspeciallyequippedships,oratpreliminaryselectedpointsfor determinationofgeomagneticsecularvariations.
Thefirstmagneticobservatorieswereestablishedinthe1820s,andbythemid-1980s,the globalgeomagneticnetworkhadexpandedto180observatories.Theaccumulatedglobaldata allowedscientiststoinvestigatenotonlytheshort-termdisturbancesofmagnetosphericionosphericorigin,butalsothegeomagneticsecularvariations.ManydigitalmagneticobservatoriesareintegratedintotheglobalnetworkInternationalReal-timeMagneticObservatory Network(INTERMAGNET).DatacollectedbyINTERMAGNETmeetuniformaccuracy requirementsof0.1nTforthemagnetometerresolution(INTERMAGNET,2012).Moreover, thepreliminaryminutedatamustbetransmittedtoageomagneticinformationnodewithin 72hofacquisition.AccesstothefinaldataisopenboththroughtheINTERMAGNET networkandthroughglobaldatabases.In2019,theINTERMAGNETportalprovidedgeomagneticdatafrom150geomagneticobservatoriesfrom42countries(Fig.1.2).
Withtheonsetofthesatelliteerainthe1970s,satellitemeasurementshavebeeninvolvedin globalstudiesofthegeomagneticfieldanditsvariations.Todate,however,theobservatory datahavebeenandremainthemostaccurateandreliablesourceofinformation.
Fundamentalknowledgeaboutthespatial-temporalstructureofEarth’spalaeomagnetism canbeobtainedusingindirectdata,derivedbyarchaeomagneticandpalaeomagnetic methods.Thephysicalfoundationofarchaeomagnetismandpalaeomagnetismaresimilar, sothearchaeomagneticmethodcanbeconsideredakindofpalaeomagneticresearch. Palaeomagneticstudiesarebasedonthefollowingfundamentalassumptions:
1. Ageomagneticfieldaveragedoverarelativelysmallinterval(onageologicaltimescale)is thefieldofthecentralaxialmagneticdipole,theaxisofwhichcoincideswithEarth’s rotationalaxis.
2. Rockscanbemagnetizedinthedirectionofanappliedexternalmagneticfield,and obtainedmagnetization,callednaturalremanentmagnetization(NRM),canpersistupto thepresentday(Tauxe,2010).
Theironmineralswithferromagneticproperties(i.e.magnetiteanditsvarieties,maghaemite, haematite,haemo-ilmenite,ironhydroxides,andothers)areofparamountimportancefor palaeomagneticstudies.Ferromagneticsubstancesarecharacterizedbymagnetichysteresis, denotingtheirreversibilityofthecurveofnormalmagnetizationduringtheprocessofrock demagnetization.Therefore,aftertheterminationoftheappliedpermanentmagneticfield, therockswillhavearemanentmagnetization,whichisnotequaltozero.
Rocksmagnetizationisacomplexphenomenondependingonavarietyoffactorssuchas conditionsoftheirformation,tectonicevolution,exposuretoambientinfluencesetc.Partof therocksmagnetizationhasthememoryaboutthestrengthanddirectionofthepastmagnetic fieldatthetimeoftheirformationorchemicalalteration.Magneticcomponentrepresenting thismemoryisknownasa naturalremanentmagnetization (NRM).Itisthevectorsumofseveralmagnetizationsgeneratedoverthegeologicalhistoryoftherocks.Theydependsnotonly ontherockspropertiesandthemagnitudeoftheappliedconstantmagneticfield,butalsoon manyfactors,suchastime,temperature,mechanicalstress,chemicaltransformations,and others.Consequently,therocksNRMcouldbearseveralcomponents’imprinted’onitbythe temporaryvaryinggeomagneticfieldindifferentepochs.Themagnetizationacquiredatthe timeofrockformation—i.e.coolingbelowtheblockingtemperatures(i.e.theCuriepoint)
FIG.1.2 Globalnetworkofgeomagneticobservatories. Blackdots,accompaniedbytheIAGA(InternationalAssociationofGeomagnetismandAeronomy)code,denotethepositionofeachobservatory.AllobservatoriesaremembersoftheINTERMAGNETnetwork. Basedondatafrom http://www. intermagnet.org/imos/imotblobs-eng.php.
ofvolcanicorintrusiverocks,depositionofsedimentsormetamorphicevents—isreferredas primary magnetization.Allothercomponentsacquiredatlatertimesrepresentthe secondary components.Consequently,themainquestionstandinginfrontofpaleomagneticstudiesis todistinguishbetweenprimaryandsecondarycomponentsinNRM.Thisisthefoundation oftheaccuracyandreliabilityofpaleomagneticinformation.
Rocksarecharacterizedbyvarioustypesofremanentmagnetization.Volcanogenicand intrusiverockshavethermoremanentmagnetization(TRM),whichisformedwhentheferromagneticmaterialiscooledbelowtheCuriepointinEarth’smagneticfield(theCuriepoint formagnetiteis580°С,pyrrhotite300°С,haematite675°С).Archaeologicalobjects(furnaces, bricks,ceramics,etc.)arealsocharacterizedbythesamemagnetization,fixingthemagnitude anddirectionofgeomagneticfieldinthetimeoftheirfiring.
Timeplaysanimportantroleinmagnetizationofferromagneticmineralsinaweak magneticfield.Therefore,duetothemagneticviscosity(i.e.time-laginmagnetizationof magnetizablematerialundertheinfluenceofgeomagneticfield),theprocessof‘viscous’ magnetizationofrockscontinueswithtime.Viscousmagnetization(VRM)isformedunder favourableconditions;therocksareexposedtotheinfluenceofEarth’smagneticfieldfor millionsandhundredsofmillionsofyears.
Chemicalremanentmagnetization(CRM)isformedasaresultofchemicalorother changesinthegrainsofmagneticmineralsplacedinamagneticfield,attemperaturesbelow theCurietemperature.Itspropertiesdependbothonthemagneticcharacteristicsofinitial minerals,andonthenewlyformedchemicalproducts,suchasduringthechemicaltransition ofhaematitetomagnetite,magnetitetomaghaemite,etc.Chemicalmagnetizationiswidespreadinsedimentaryrocks,manifestedasdepositional/postdepositionalremanent magnetization.
Duringthesedimentationofparticlesonthebottomofthewaterpool,onthelandsurface, orinthelayerofunconsolidatedsediments(inthepresenceofacertainamountofwaterinit), theclayfractions(likesmallmagnets)areorientedinthedirectionofgeomagneticfields(or closetoit).However,theprimarymagnetizationoftherockeventuallydecays,duetothe influenceofvariousfactorslikeoccurrenceofsecondarymagnetization,notbeingidentical inmagnitudeanddirectiontotheprimaryone.Theresistanceoftheprimarymagnetization tothesubsequentlyimposedinfluenceofothermagneticfieldsischaracterizedbyits palaeomagnetic stability (describingitsabilitytopreservetheprimaryrockmagnetization). Ithasbeenexperimentallyprovedthateachtypeofrockmagnetizationischaracterizedby acertainresistancetotheeffectsofanalternatingmagneticfield,temperature,andotherfactors,leadingtothedestructionofmagnetization.Thedifferenceinstabilityisthebasisofthe methodsof‘magneticcleaning’ofrocksamples.Theessenceofthesemethodsisthatasample placedinanonmagneticspaceisheatedorexposedtoalternatingmagneticfieldsthatgrow sequentially.Atcertaintemperaturesoramplitudesofanalternatingmagneticfield,thesecondary,lessstableformsofmagnetizationaredestroyed.Agoodindicationthatananalysed sampleiscleanedfromthesecondarymagnetizationisthestabilizationofthedirectionsofthe residualmagnetization.Theremainingmagnetizationisconsideredcharacteristic,butitis stillnecessarytoprovethatitisprimary.Toachievethisgoal,specialtechniqueshavebeen developed,inparticularvariousgeologicalfieldtests.
Inresume,todatethereisafullsetofmethodsallowingcollectionofgeomagneticfielddata inallrangesofitsvariations.However,asarule,thesedataareunevenlydistributedinspace
andtime.Toimprovethespatialandtemporalmappingofgeomagneticcomponents, differenttypesofmodelsofthecontemporaryandancientEarth’smagneticfieldhavebeen developed(see Section1.5).
1.3CharacteristicsofEarth’smainmagneticfield
1.3.1Descriptionofthepresentmagneticfield
AnalysisofEarth’smagneticfieldvariationsoverthepastcentury(frominstrumentalobservations),andoverthelastseveralhundredsandthousandsofyears(fromhistoricaland archaeomagneticdata),showsthatevenonthescalesofseveralyearstoseveraldecades,the fieldcomponentscanvarysignificantly.Formostofthe20thcentury,thedriftvelocityofgeomagneticpoleswasabout 10km/year.Inthe1990s,however,thespeedofthepoleinthe NorthernHemisphereincreasedsharply,reachingspeedsof40–60km/year(Olsenand Mandea,2007).Moreover,since1840themagnitudeofthegeomagneticdipolemoment hasdecreasedbyabout5%–7%percentury(Gubbinsetal.,2006; MandeaandPurucker, 2005).ThestrongestchangesintheintensityoftheEarth’smagneticfieldoverthepast fewcenturieshaveoccurredinSouthAmericaandtheSouthAtlantic(Finlayetal.,2010).
TworegionswithastrongergeomagneticfieldcouldbedistinguishedintheNorthern Hemisphere.OneofthemisknownastheCanadiananomaly(containingtherecentposition ofthegeomagneticSouthPole),whiletheotherisknownastheSiberianone.IntheSouthern Hemisphere,however,thereisasinglemaximumingeomagneticfieldintensity,placedinthe SouthernOcean,betweenAustraliaandAntarctica(Fig.1.3,upperpanels).Overthe20thcentury,theintensityofgeomagneticfieldintheCanadiananomalyhassignificantlydecreased, anditscentrehasbeenshiftedinanorth-westdirection.Incontrast,theSiberiananomalyhas strengthenedanditscentrehasbeenshiftedsouth-west.ChangesintheCanadiananomaly couldaffectthemotionofthegeomagneticSouthPole,whosevelocityhasactuallyincreased since1990(OlsenandMandea,2007).Temporalvariationsofgeomagneticfieldintensityalso differinbothhemispheres.Thus,thegeomagneticfieldstrengthweakensconsiderablymore slowlyintheNorthernHemispherethanintheSouthernHemisphere(Fig.1.3,lowerpanels). Morespecifically,forthepast120years,thehemisphericalmeanintensityhasdecreasedby 1140nTintheNorthernHemisphereandby 5530nTintheSouthernHemisphere.
1.3.2Secularvariations
Secularvariations,asindicatedabove,arelong-termchangesintheelementsofEarth’s magnetismovertime.Althoughtheperiodicitiesofthesevariationsarestillunderinvestigation,someofthemaremoreorlessestablished.Forexample,variationswithperiods60–80, 500,2000,5000yearsormorehavebeenreported.Thegeomagneticsecularvariationsare calculatedasadifferencebetweenthefieldintensityinthefinalandinitialmomentsofan examinedperiod,dividedbythenumberofyearsinthatperiod,asfollows:
FIG.1.3 Spatialstructureofthecentenniallyaveragedgeomagneticfieldintensity (top) anddynamicsofthespatiallyaveragedgeomagneticfield strength (bottom) intheNorthern (left) andSouthern (right) Hemispheres. Datasource:InternationalGeomagneticReferenceFieldmodel(https://www.ngdc. noaa.gov/geomag/geomag.shtml).
where Fsv isthesecularvariation, Fi+n isthefinalyearoftheperiod, Fi istheinitialyearofthe period,and n isthenumberofyearswithintheperiod.
Datafromgeomagneticobservatoriesarethemostreliablesourceofinformationabout changesinEarth’smagneticfield,andtheyshowthatintheNorthernHemisphere,geomagnetic fieldintensitydecreasesintheWesternHemisphere(Fig.1.4A)andincreasesintheEastern Hemisphere(Fig.1.4B).Moreover,ground-basedmeasurementsalsorevealthattheSouthern HemisphericfieldisweakeningmuchfasterthantheNorthernHemisphericone.
Sincethemid-1990s,however,thedecreasingtendencyoftheSouthernHemispheregeomagneticfieldhas‘sloweddown’(Fig.1.5).Thisisverynoticeableincentralandeastern Antarcticaandadjacentterritories(Fig.1.5B).Datafromgeomagneticobservatoriesshowthat duringtheperiod1957–2010,thefieldintensityatwesternAntarcticshoresdecreasedfaster (Fig.1.5A),especiallyattheAIAobservatory(Bakhmutovetal.,2006; Melnyketal.,2014).The decreaseoffieldintensityissloweratobservatorieslocatedinthecentreoftheSouthAtlantic anomaly(inRussianliteratureknownastheBraziliananomaly)andatthegeomagneticpole, comparedtootherobservatoriesforthesameperiod(e.g.VSSandDRVin Fig.1.5).Positive trendsofthefieldintensity,sincethe1990s,havebeenobservedatobservatoriesintheIndian Ocean(PAF,TAN,CZT,andAMSin Fig.1.5B).
FIG.1.4 ChangesintheNorthernHemispheregeomagneticfieldintensity(F)registeredinvariousgeomagnetic observatoriesduringthe20thand21stcenturies:(A)inNorthAmericaand(B)inEuropeandAsia. Datasource:British GeologicalSurvey(http://www.geomag.bgs.ac.uk/data_service/data/annual_means.shtml).
FIG.1.5 ChangesoftheSouthernHemispheregeomagneticfieldintensity(F),duringthe20thand21stcenturies, derivedfromthedataofindividualgeomagneticobservatories:(A)inwesternAntarctica,SouthAmerica,and(B)in centralandeasternAntarcticaandadjacentterritories. Datasource:BritishGeologicalSurvey(http://www.geomag.bgs.ac. uk/data_service/data/annual_means.shtml).
Consequently,thevariousregionsoftheAntarcticcontinenthavedifferentfielddynamics, withmuchfasterweakeninginthewesternthaninthecentralandeasternregionsofthecontinent.Since1920,anegativefocusofgeomagneticcentennialvariationshasbeenfoundin westernAntarctica(Fig.1.6),whichexplainstherapiddecreaseoffieldintensitynotedin theAIAmagneticobservatory.
Thespatialstructureofthecentennialevolutionofgeomagneticfieldintensityisillustrated onthemapofits secular variations.Analysisofthedecadalmaps,basedontheIGRFmodel data(Fig.1.6),revealstheexistenceofthreemainperiodsinthegeomagneticsecularvariations:(i)uptothe1940s;(ii)fromthemid-1940stothe1970s;and(iii)fromthemid-1970sto thepresent(Bakhmutovetal.,2014).Thefirstperiodischaracterizedbythedominanceofa fewmajorfocuses(i.e.areaswithmaximalfieldchanges)inthesecularvariationsmaps.Duringthesecondperiod,multiplefocusesappearwhichtendtogroupagaininafewmainones sincethe1970s,i.e.duringthethirdperiod.Thefocusesinthesecularvariationsemergeand decayoveraperiodofabout60years.
Theanalysisofgeomagneticfieldevolution,basedontheIGRFmodel,led Xuetal.(2000) totheconclusionthattherapidchangesofgeomagneticfieldintensityduringthe20thcentury
FIG.1.6 Mapsofgeomagneticfieldsecularvariations,accordingtotheIGRF-12model,foreachdecadeofthe period1900–2010.
couldhintatupcomingreversalofgeomagneticfieldpolarity.Within100years,thedipole momentdecreasedby6.5%,from8.32 1022 Am2 in1900to7.78 1022 Am2 in2000.At thesametime,thefieldquadrupoleandoctupolecomponents(membersoftheserieswith n ¼ 2,3;refertoEq. 1.4)increasedby95%and74%,respectively,whichmaybeevidence ofanapproachingreversal.
1.3.3Westwarddrift
Sincethetimesof Halley(1692),ithasbeenknownthattheEarth’smagneticfieldchanges significantlyoverdecadestocenturies.Perhapsthemoststrikingaspectofthesesecularvariationsisthewestwardmotionofthenondipolarfieldfeatures,knownas westwarddrift.This driftcanbeseenwhencomparingmapsofthenondipolefieldanditssecularvariationsfor successiveeras.Introducedby Halley(1692),theconceptforwestwarddrifthasbeendevelopedfurtherby Vestineetal.(1947), VestineandKahle(1968), YukutakeandTachinaka (1969),etc.Theunderstandingofthemechanismofwestwarddriftgenerationevolvesfrom theideaaboutalarge-scaleconvectioninthemoltenironcore(Bullardetal.,1950)tothe hypothesizedpropagationofmagnetohydrodynamicwavesintheliquidEarth’score,as proposedindependentlyby Hide(1966) and Braginsky(1967).
Laterworks,basedontheempiricalmeasurementsofsecularvariationsandmodelexperiments,haveshownthattheconceptofthewesterndriftistoosimplistic.Theyshownotonly thatthewestwarddriftisclearlyvisible(mainlyoverEuropeandNorthAmerica),butalso thatinsomeregions,thefieldisdriftingnorthwardoreveneastward(GubbinsandHerreroBervera,2007,p.994).Thesituationiswell-illustratedin Fig.1.7AandB,whichshows:(i)the trajectoriesofmovementofthecentresofglobalgeomagneticanomalies(Thompson,1984); and(ii)thedriftofthefocalpointsofthesecularvariations(Tretyaketal.,2002),calculatedby theuseofsphericalharmonicanalysis(SHA)coefficientsaccordingto Benkovaetal.(1974) Thearrowsindicatedirectionsofmovement.Despiteminordiscrepanciesbetweenbothpapers,thefigureillustratesfairlywellthemainfeaturesofthetemporallychanginggeomagneticfield.Italsoshowsthatsomeofthefocuses(e.g.No.8,nearMongolia)havemoved slowly,whileothers(e.g.anomalyNo.7)havedriftedataspeedof0.4degrees/year.Note thatunlikethedominantwestwardmovement,someofthefocuseshavemovedeastward.
Thelifespanofthesefocuseshasbeenestimatedtobeapproximately500years(Yukutake, 1979).Someoftheanomalieshavedisappearedovertime(e.g.No.13)andnewoneshave arisenneartheothers(e.g.No.5).AneastwarddriftrecordedattheSitkaObservatory (Alaska),overthepast70years,hasbecomeaclassicalexampleofthevaryingdirections ofgeomagneticsecularvariations(Skiles,1970).
Analysisofthesecularvariationsonthecore–mantleboundary(CMB),capturedbythe archaeomagneticfieldmodelCALS7K.2overthepast3000years,hasshownthatthemost pronouncedazimuthalmovementsareobservedatmiddleandhighlatitudesintheNorthern Hemisphere,wherebotheastwardandwestwardmotionshavebeendetected.SimilarmovementshavenotbeenobservedintheSouthernHemisphere,whichcouldbeattributedtopoor datacoverage(DumberryandFinlay,2007).AccordingtotheCALS7K.2empiricalmodel,the globallyaverageddrifthasbeeneastwardbetweenAD1000and1400andwestwardafter
FIG.1.7 (A)TracksofnondipolefocusesatEarth’ssurface,at50-yearintervals(Thompson,1984),and(B)thedrift ofthefocalpointsofthesecularvariations(Tretyaketal.,2002),calculatedbytheuseofSHAcoefficientsaccordingto Benkovaetal.(1974);full(open)circlesrepresentthedynamicofpositive(negative)focusesforeach50yearsinterval, innT. (A)ModifiedfromThompson,R.,1984.Geomagneticevolution:400yearsofchangeonplanetEarth.Phys.EarthPlanet. Inter.,SpecialIssue:OriginofMainFieldsandSecularChangesoftheEarthandPlanets36,61–77. https://doi.org/10.1016/ 0031-9201(84)90099-2 page66,fig.4.(B)BasedonTretyak,A.,Yaremenko,L.,Bakhmutov,V.,2002.Geomagneticsecular variationinAntarctica.Ukr.Antarct.Cent.Bull.,4,83–89.Page87,fig.4(inRussian).
that.Ontheotherhand,theequatorialmotionsofthemainfieldfeaturesaredirected westwardformostofthepast3000years(DumberryandFinlay,2007).
Fromtheforegoing,theambiguityinestimationofthenondipolefielddriftvelocity becomesobvious,asdoesthedirectionofdriftmotionsindifferentregionsovertheworld. Dependingontheanalysedtimeintervals,theamountofdatafromdifferentregions, methodsofdataprocessing,andsubjectivityoftheinterpretation,theestimationsofthe westwarddriftvelocityvaryfrom0.08eastwardperyear,upto0.733westward(forasummary,see Langel,1987).Today,theconceptofwestwarddrifthasbeenlargelyreplacedby moresophisticatedmodelsofsecularvariations,andmapsoftheevolutionofthefieldatthe core–mantleboundary(seebelow).However,theconceptisstillusedforinterpretationof archaeologicalandpalaeomagneticdata.
1.3.4Geomagneticreversal
TheoreticalestimatesshowthatEarth’smagneticfieldhasexistedformorethan3billion years.Palaeomagneticstudiesshowevidenceformultiplereversalofthefielddirectionduringthisperiod,inwhichtheNorthernandtheSoutherngeomagneticpolesinterchangetheir places.Therecentunderstandingofgeomagnetismandcontemporarynumericalmodesare abletoreproducethereversalsofgeomagneticpolarity.However,manyfundamentalquestionsremaintobeclarified.Forthisreason,theproblemofgeomagneticfieldreversalsisstill oneofthemaintasksforgeophysicists,particularlyforthoseinvolvedintheproblemsof palaeomagnetism.
Informationaboutthereversalsis‘recorded’inrocksandorescontainingferromagnetic minerals(e.g.magnetite,haematite,titanomagnetite).Theyareabletopreservetheremanent magnetization,storingitinsuchawaythatinformationaboutthestateofEarth’smagnetic fieldatthetimeoftherocks’formationisretained.Thestudyoftheremanentmagnetization inrocksofdifferentagesisthebasisforcompilingatimescaleofgeomagneticfieldreversals.
Duringthegeomagneticpolarityreversal,themagneticfieldstrengthdropsdramatically, leadingtoasevereweakeningoftheplanetarymagneticshielding,whichprotectsliving organismsfromtheharmfulcosmicradiation.Severalauthorshavedemonstratedthatinsuch periods,whenthemagneticfieldisdominatedbyitsquadrupole,octupole,etc.moments,the highlyenergeticcosmicrays(withhundredsMeVandGeVenergies)areabletoreachthelowlatitudetropospherealongtheopenmagneticfieldlines.Thisisthemotivationforsome authorstoassumethatgeomagneticfieldreversalscouldbeconsideredasoneoftheprobable causesfortheepisodesofmassextinctionsthatoccurredduringtheevolutionofterrestrialbiota (StarchenkoandShcherbakov,1991; Vogtetal.,2007; ChannellandVigliotti,2019).
Thetimeintervalsduringwhichthegeomagneticfieldmaintainsconstantpolarity(normal orreversed)arecalled chrons.Eachchronmaylasthundredsofthousandstomillionsofyears. Palaeomagneticdatashowthatgeomagneticreversalsoccuratrandomseparations,sothata sequenceofreversalsdefinespolaritychronsofquitevariablelengths.Thefirstscaleofgeomagneticpolarityalterationwasbuiltinthe1960sby Coxetal.(1963).Themostcomplete anddetailedrecordofgeomagneticpolarityreversalsisstoredinsequencesoflineatedmarine magneticanomalies,foundinoceanbasins(VineandMatthews,1963).Thesequenceofpolarityreversalintervalshasbeencarefullystudiedandusedtoconstructmagnetostratigraphic
timescaleslinkingbiostratigraphy,isotopestratigraphy,andgeochronologicalagedeterminations(seereviewby OpdykeandChannell,1996).
Forthelastmillionyears,thegeomagneticfieldhaschangeditspolarityfourtimes.This lasthappenedabout780,000yearsago.Intheentirehistoryoftheplanet,atleastseveral hundredreversalsofthemagneticfieldhaveoccurred.Nevertheless,nosystematicpattern inreversalsoccurrencehasbeenfound,andtheyareconsideredtobeastochasticprocess.
Thefrequencyofpolaritychangesoverthepast160millionyearsisnotconstantintime. Onaverage,thereisonereversalpermillion,withamaximumvalueofsixreversalspermillionyears.Thereareperiodswhenthefielddidnotchangeitspolarityfortensofmillionsof years.Forexample,theCretaceousnormalsuperchronoccurredbetween120and83million years(Ma);Kayamareversedpolaritysuperchron(fromlateCarboniferoustomiddlePermian(i.e.between310and260Ma);andMoyeroreversedpolaritysuperchroninthefirsthalfof theOrdovician(PavlovandGallet,2005).
Informationaboutthegeometryanddynamicsofthegeomagneticfieldduringpolarity reversalcanbeobtainedfromhigh-resolutionpalaeomagneticrecordsofbothsedimentary andigneousrocks.Thekeyissueinanalysisofthesedataisthelengthofthetransitionalintervalofchangingfromonepolaritytoanother,inordertodeterminethebeginningandend ofanychron.Theanalysisiscomplicatedbytheappearanceofshort-termbutstrongoscillations,precedingthepolarityreversal(HartlandTauxe,1996; Dormyetal.,2000).Suchoscillationscouldbeinterpretedasincreasedsecularvariationsinperiodsofdipolarfield weakening.Ontheotherhand,theycouldbeconsideredasconsecutiveattemptsofthegeomagneticfieldtochangepolarity.Thedeterminationofintermediatefielddirection,during thepolaritytransformation,couldalsobeaproblemobstructingtheformationofhigh-quality palaeomagnetictimeseries.
Despitethedifficultiesinobtainingreliablepalaeomagneticrecords,somegeneralconclusionscannowbedrawn.Regardingthefieldintensity,forseveralthousandyearsbeforethe polaritychange,theintensitystartsdecreasing.Itremainslowuntilthepolarityreversalis completed.Withfinalizationofthereversal,thegeomagneticfieldintensityincreasestoits normalvalues.Ingeneral,theprocesstakesplaceforseveralthousandyearsandvaries significantlyfromonereversaltoanother.Accordingto Valetetal.(2005),thedecayofaxial dipolepriortothepolarityreversallastsfor60,000–80,000years.Therestorationofitsintensityafterthereversal,however,occursmuchfaster—atmostforseveralthousandyears.
Thepalaeomagneticrecordsshowthatthereareshortperiodswithinthechrons(witha durationofseveralthousandyears),whenthefieldhasdepartedfromitsnear-axialconfiguration.Suchshort-termeventsarecalledgeomagnetic excursions.Investigationofthese eventsincreasesourknowledgeaboutthegeomagneticfieldoscillations.Thelatterisvery importantforunderstandingthenatureofthegeomagneticfieldandtheprocessesoccurring intheliquiduppercoreofEarth.Theyarewidelyusedaschronologicalandstratigraphic benchmarks,invariousfieldsofearthsciences:instratigraphyandgeochronology,insedimentologyandtectonics,inpalaeontologyandclimatology,etc.
Therearedifferentformulationsoftheterm‘excursion’.Itisusuallydefinedasadeviation ofthevirtualgeomagneticpole(VGP)equatorwardof45-degreelatitude(forcomparison,the secularvariationsoftheVGPcertainlydonotreach45-degreelatitude).
Accordingtootherdefinitions,geomagnetic‘excursion’isashort-termchangeinthe directionofageomagneticfield,whoseamplitudeisatleastthreetimesgreaterthanthe
