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P1:GLQ/GLE P2:FQPFinalPages Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 Space Plasma Physics Larry Lyons UniversityofCalifornia,LosAngeles I. Introduction II. BasicConcepts III. SolarWindandInterplanetaryMagneticField IV. SolarWindandInterplanetaryFieldInteractions withtheGeomagneticField V. ParticleAccessto,andTransportwithin, theMagnetosphere VI. AurorasandAuroralCurrents VII. GeomagneticDisturbances VIII. Conclusions GLOSSARY sunthatiscarriedthroughoutinterplanetaryspaceby thesolarwind. Aurora Emissions from the upper atmosphere by con- Ionosphere Region of enhanced ionization that sur- stituentsthathavebeenexcitedbytheimpactofener- roundstheearthataltitudesbetween∼75and500km geticparticlesfromthemagnetosphere. altitude. Auroral oval The prime region of visible auroral emis- Magnetopause Current layer that to a large extent sep- sions,whichconsistsofapproximatelycircularzones aratestheinterplanetarymagneticfieldfromthegeo- surroundingeachgeomagneticpolethatareafewde- magneticfield. greesinlatitudewideandcenterednear70◦ geomag- Magnetosphere Region of space within the magne- neticlatitude. topausethatisdominatedbythegeomagneticfield. Convection Flow of plasma throughout the magneto- Plasma An ionized gas in which electric forces main- spherethatisdrivenbythesolarwind. tainapproximatechargeneutrality(theexcessofneg- Geomagneticlatitude Latitudebasedontheearth’smag- atively or positively charged particles is everywhere neticaxis. muchsmallerthanthetotaliondensity). Gyroradius Radiusofthecircularmotionofchargedpar- Plasmasheet Energeticplasmaregionthatoccupiesthe ticlesaboutamagneticfield. outerportionsofthemagnetosphere. Interplanetary magnetic field Magnetic field from the Precipitation Loss of magnetospheric particles to the 577 P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 578 SpacePlasmaPhysics atmosphere by collisions at the low-altitude ends of hasrecentlybeentermed“spaceweather.”Spaceplasma magneticfieldlines. physics also includes the interaction of the solar plasma Radiationbelts Regionofhighfluxesofveryenergetic withotherplanets,themixingofsolarandplanetaryplas- electronsandionsthatencirclestheearthintheinner mas, and a wide range of wave modes associated with portionofthemagnetosphere. plasmaoscillationsinspace. Solarwind Plasmathatflowsoutwardfromthesunand fillsinterplanetaryspace. I. INTRODUCTION SPACE PLASMA PHYSICS is the study of the plas- Thesuncontinuouslyemitsastreamofionizedparticles, masthatoriginatefromthesunandfromtheplanetsand whichisreferredtoasthesolarwindandistheprimary moons within the solar system. These plasmas occupy componentoftheplasmawhichfillsinterplanetaryspace. interplanetary space and the magnetospheres of planets. The average speed of this stream in the ecliptic plane is Thisarticlegivesanoveralldescriptionoftheplasmapro- ∼400km/sec,sothatittakesabout4daysforparticlesto cesseswhichcontrolthelarge-scalestructureanddynam- reachtheearth.Solarwindspeeds,however,canbequite icsofthenear-earthspaceplasmaenvironment.Thisin- variable.Theytypicallyrangefrom∼300to∼800km/sec, cludestheformationofthesolarwindandinterplanetary with speeds exceeding 1000 km/sec being occasionally plasmadisturbances.Italsoincludestheinteractionofthe observed. The earth’s internal magnetic field is approx- solar wind plasma and magnetic field with the magnetic imately that of a dipole. However, the interaction of the fieldoftheearthandhowthisinteractionleadstothein- solarwindparticleswiththeearth’smagneticfieldcom- teresting and dynamic space plasma environment which pressestheearth’sfieldonthedaysideanddrawsthefield exists in the vicinity of the earth. Topics include energy outintoalongtailonthenightside.Thisinteractionalso transfertoandwithintheearth’smagnetosphere,forma- confines most of the magnetic field of the earth to a re- tionofmagnetosphericstructure,anddisturbancesofthe gion referred to as the magnetosphere (see Fig. 1, which magnetosphere–ionospheresystemwhichconstitutewhat is a sketch of the magnetosphere in the noon–midnight Bow shock V E B Magnetopause current sheet Open-closed field boundary Lobe Plasma sheet E Cross-tail current sheet E R Plasma sheet 0 4 V V V ~ Radiation belts and ring and ring Radiation belts current current E E Auroral zone (Λ ~ 63-73°) Lobe ~10 RE ~5-12 RE ~5 RE FIGURE1 Schematicillustrationofthemagnetosphereinthenoon–midnightmeridianplane. P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 SpacePlasmaPhysics 579 meridian plane). The outer boundary of the magnetosphere is called the magnetopause, which typically lies ∼10R (earthradii)abovethenoonequatorand∼15R E E awayfromtheearthwithinthedawn–duskmeridianplane. Onthenightside,themagnetosphereflaresoutwardwith increasing distance away from the earth, eventually be- coming approximately cylindrical with a diameter of 40–50 R . Solar wind speeds are supersonic, so that a E FIGURE2 Themotionofelectronsandpositiveionsinuniform shock(calledthe“bowshock”)liesseveral RE upstream andconstantelectricandmagneticfields. ofthedaysidemagnetopause. Plasmaparticlesmoveincirclesaroundmagneticfield lines and thus can become trapped within the magneto- isbasedonconsideringtheforceF=q(E+v×B)acting sphere.Majorregionsoftrappedenergeticparticleswithin onaparticleofchargeq,massm,andmovingwithave- the magnetosphere are the plasma sheet and the radia- locityvinanelectricfieldEandmagneticfieldB.Particle tion belts. As illustrated in Fig. 1, the plasma sheet on motionisgenerallyseparatedintocomponentsv(cid:8)parallel the nightside is displaced from the high-latitude magne- toBandv⊥perpendiculartoB.WithE=0andauniform, topausebywhatarereferredtoasthetaillobesandextends time-independentmagneticfield,v(cid:8)isaconstantandv⊥is alongtheentiremagnetospherictailinwardtoanequato- circularmotionaboutBwithafrequency|qB/m|,which rialradialdistancefromthecenteroftheearthr≈5–12 isreferredtoasthegyrofrequency,andradiusmv⊥/|qB|, R . The plasma sheet also extends around the earth to E whichisreferredtoasthegyroradius.Thedirectionofgy- otherlocaltimesandhasanouterboundarythatliesadja- ration is right (left)-handed with respect to the direction centtothemagnetopauseonthedayside.Typicalenergies of B for electrons (positive ions) as illustrated in Fig. 2. of plasma sheet particles are ∼1–50 keV for ions and Except very near current sheets, particle gyroradii are gen- ∼0.2–10keVforelectrons.Earthwardoftheplasmasheet erally very much less than the scale length for field and lies a population of more energetic electrons and ions plasmavariationsinspaceplasmas.Also,particlegyrope- which encircle the earth and are referred to as the “ra- riodsaregenerallyverymuchlessthanspaceplasmatime diation belts.” These particles form a current encircling scales for transport and for changes in plasma and field theearthwhichisreferredtoasthe“ringcurrent.” properties. Particlesfromthemagnetospherecanmovealongmag- Figure 2 shows the motion of electrons and positive netic field lines and strike the upper atmosphere. Those ionsinuniformandconstantelectricandmagneticfields. that reach an altitude of ∼100–200 km undergo colli- The acceleration by the perpendicular component of the sions with the neutral atmosphere resulting in a loss of electricfieldE⊥alternativelyincreasesanddecreasesthe theirenergytotheneutralatmosphereandtheirlossfrom particlegyroradiusonceeachgyration,sothat,inaddition themagnetosphere.Suchlossofenergeticmagnetospheric tothegyromotionaboutB,particlesmovewithanaver- particles is referred to as precipitation. The energy from agevelocityV =(E×B)/B2,whichisreferredtoasthe E precipitating particles excites constituents of the upper electricfielddriftspeed.Thisrelationcanberewrittenas atmosphere,andtherelaxationoftheupperatmospheric constituentsbacktotheirgroundstategivesoffemissions, E⊥ =−VE ×B. (1) whichwhensufficientlyintensearereferredtoastheau- rora. Such precipitation is most intense from the plasma When|V |(cid:13)|v|,asisthecasethroughoutmostofspace, E sheet,leadingtoanapproximatelycircularregionofemis- wecanseparatetheparticlemotionintoitsgyrationabout sions surrounding each magnetic pole that is referred to BandadriftofthegyratingparticlewithvelocityV .Any E astheauroraloval.Theovalistypicallyafewtoseveral electric field parallel to B simply gives constant particle degreesinlatitudewideandiscenterednear70◦geomag- accelerationalongB. neticlatitude. As shown in Fig. 2, electric fields perpendicular to B cause all charged particles to drift with the same veloc- ity,whichdoesnotleadtocurrents.Spatialvariationsin II. BASIC CONCEPTS magneticfieldalsogiveparticledrifts.However,thedrifts frommagneticfieldvariationsareoppositelydirectedfor Spaceplasmaphysicsoftenrequiresthatdynamicsbean- negativelyandpositivelychargedparticles,sothatacur- alyzedintermsofboththemotionofindividualparticle rentisformed.Thiscurrentisazimuthalintheregionof andintermsofmacroscopicmomentssuchastemperature the radiation belts, giving rise to the ring current in this T,density n,andpressure P.Individualparticlemotion region. Such a current also flows within the tail plasma P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 580 SpacePlasmaPhysics sheetandisdirectedacrossthetailfromthedawnsideto greater than the magnetic pressure in the plasma sheet, theduskside. wheretheplasmapressureishigh. The current formed by an individual charged parti- Equation(3),withε ∂E/∂t neglected,relatescurrents 0 cle gyrating about B can be represented as a magnetic andmagneticfieldstructure.Forexample,ittellsusthata dipolewithmagneticmomentµ=K⊥/B,wheretheper- changeinmagneticfieldstrength(cid:11)Bacrossaplaneper- pendicular energy K⊥=(mv⊥2)/2. As long as magnetic pendiculartoBmustbeassociatedwithacurrentwithin field changes experienced by a particle are small during the plane across which B changes. Such a planar cur- thecourseofonegyrationaboutthemagneticfield,µis rentisreferredtoasacurrentsheetandhasamagnitude conservedforparticlesundergoingelectricandmagnetic per unit distance normal to the current direction of driftsperpendiculartoBandmotionparalleltoB.Thisis I=(cid:11)B/µ [A/m].Forthemagnetospherictail,thiscur- 0 importantbecauseitgenerallymeansthatparticleenergies rent is directed in the dawn-to-dusk direction across the increase(decrease)whenparticlesundergoadriftacross tail and is referred to as the cross-tail current sheet (see magnetic field lines into regions of increasing (decreas- Fig. 1). ing) B. On the other hand, motion along magnetic field lines results in a conversion of parallel energy to (from) perpendicularenergyas B increases(decreases)withthe III. SOLAR WIND AND INTERPLANETARY totalparticleenergybeingconserved. MAGNETIC FIELD When we think of the plasma as whole, we deal with macroscopicvariablesthataredefinedperunitvolume.We The sun is a large ball of gas held together by its own considertheforcesactingontheplasmaperunitvolume, gravity.Thegasesareabout90%hydrogenand10%he- whichwewriteas liumwithminoramountsofotherconstituents.Duetothe high temperature of the sun, the solar gases are mostly ρdV/dt =−∇P +J×B, (2) ionized.Thesunappearstohaveavisiblesurfacebecause where ρ is the total plasma mass per unit volume, V is thesteepradialgradientinsolardensitygivesasharptran- themass-averagedvelocityforallparticleswithinaunit sitionbetweenlowerregions,wherephotonsareabsorbed volume,Pistheplasmapressure,andJisthecurrentden- and reemitted by solar gases without traveling very far, sity(currentperunitareanormaltothecurrent).Equation andhigherregions,wheremostphotonsmoveawayfrom (2)assumeschargeneutrality(essentiallyequalnumbers thesunalongstraighttrajectorieswithoutcollisions.So- ofpositiveandnegativecharges),anassumptionwhichis lar radiation appears to arise from this narrow transition nearlyalwaysvalidforspaceplasmas,andneglectsgrav- region (only a few hundred kilometers thick), which is ity,anassumptionwhichisvalidformostspaceplasmas referredtoasthephotosphere. (neglectofgravityisnotvalid,forexample,nearthesun). Solar gases extend outward well beyond the photo- Whenplasmaandfieldchangesaresmallinthedirec- sphere with very high temperatures, forming the solar tionofB,(2)canberewrittenusingtheMaxwellequation corona.Thesolarcoronaissufficientlyhotthatmanycoro- nalparticleshaveoutward-directedvelocitieswithamag- J=∇×B/µ −ε ∂E/∂t (3) 0 0 nitudethatexceedsthespeedforgravitationalescapefrom toobtain thesun.Suchparticlesstreamoutwardfromthesunform- ingthesolarwind.Thesolarwindescapestowardthenear ρdV/dt =−∇(P +B2/2µ ). (4) 0 vacuumofinterstellarspaceatsupersonicspeeds,filling Here the constants are µ =4π×10−7H/m and ε = theentiresolarsystemwithcoronalplasma. 0 0 8.85×10−12F/m, and the term ε ∂E/∂t in (3) was ne- 0 glectedin(4)becauseitissmallforthelarge-scalephe- A. GeneralStructure nomena discussed here. Assuming steady state and no changesinthedirectionofV,weobtainfor(4) If it were not for the solar magnetic field, the escaping plasma from the solar corona would form an approxi- P +B2/2µ =const. (5) 0 matelysphericallysymmetricsolarwindflowingradially Equation (5) is referred to as pressure balance and is outwardfromthesunat ∼750km/sec.However,thees- usually applied to regions, such as the geomagnetic tail, caping plasma is strongly affected by the magnetic field wherechangesinthedirectionofBaresmall.Thequan- of the sun because of the tendency for particles to move tity B2/2µ can be thought of as magnetic pressure, so moreeasilyalongmagneticfieldlinesthanacrossthem. 0 that(5)statesthatthetotalpressure(plasma+magnetic) Thesolarmagneticfieldishighlyvariable,whichmakes is constant. This tells us, for example, that the magnetic thesolarwindandthemagneticfielditcarrieswithitinto pressure in the lobes, where plasma pressure is low, is interplanetaryspacehighlyvariableinspaceandintime. P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 SpacePlasmaPhysics 581 FIGURE3 Sketchofcoronalmagneticfieldandplasmastructureduringsolarminimum(upperpanel)andsolar maximum(lowerpanel). To a first approximation, the magnetic field near the aloweraveragespeedof∼350km/sec.Theearth,whichis visiblesolarsurfacecanberegardedasadipolelikethat neartheeclipticplane,isexposedmoreoftentothisslow oftheearth.Thisfieldiscarriedoutwardintointerplane- solarwindthantothefastsolarwindthatcoversmostof taryspacefromthesunbythesolarwind,givingasolar the region away from the ecliptic plane. magneticfieldconfiguration(sketchedinaplaneperpen- The magnetic field and solar wind flow configuration dicular to the ecliptic plane in the upper panel of Fig. 3) illustrated in the upper panel of Fig. 3 corresponds to pe- whichislikeadipolenearthesun,butishighlystretched riods when the solar magnetic field is more dipolar than away from the sun. At radial distances of more than a at other times. During such periods, which are referred fewsolarradii,thestretchedsolarmagneticfieldreverses toassolarminima,magneticandsunspotactivityonthe direction across a narrow region near the ecliptic plane sunislow.Solarminimaoccurevery11years.Afterso- formingacurrentsheetsurroundingthesun.Intheregions lar minimum, the dipolar magnetic field structure of the wherethemagneticfieldlinesareapproximatelydipolar sungraduallyisdestroyed.Thisprocesstakesafewyears, anddonotextendwellawayfromthesun,plasmaaccu- leavingthesolarmagneticfieldinamuchmoredisorga- mulatesgivingregionsofhigh-densitycoronalplasmaas nized state, as illustrated in a plane perpendicular to the illustrated in Fig. 3. Such regions are clearly visible in ecliptic plane in the bottom panel of Fig. 3. Magnetic and images of the solar corona. For the magnetic field con- sunspot activity on the sun is high during these periods, figuration shown in the upper panel of Fig. 3, a region of which are referred as solar maxima. At solar maximum, high-density corona extends around the sun in the vicinity localizedregionsofmagneticfieldlinesthatreturntothe of the equator. Free escape of plasma only occurs well solarsurfacewithoutextendingfarintospaceandcontain withintheregionswheremagneticfieldlinesextendlarge highcoronaldensitiescanoccuroveralmostanyportion distancesintointerplanetaryspace,wherethesolarwind ofthesun.Aftersolarmaximum,thedipolefieldofthesun speedreaches∼750km/sec.Neartheboundariesbetween returnswiththedirectionofthedipolereversedfromwhat theapproximatelydipolarfieldlinesandthefieldlinesthat iswasduringtheprevioussolarminimum.Thisgivesan extentoutlargedistances,thesolarwindescapes,butwith 11-yearsolarcyclefromonesolarminimumtothenext, P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 582 SpacePlasmaPhysics with the direction of the magnetic field reversing every nitude, is most important for activity within the earth’s cycle. While the solar cycle is 11 years, the total period magnetosphere.Thiscomponentofthemagnetic fieldis forthismagneticfieldvariationofthesunis22years. referred to as the southward component. Thus the magnetic field enhancements associated with stream–stream interactionsandcoronalmassejectionsmoststronglyaf- B. SolarWindDisturbancesandTheirRelation fectthemagnetospherewhentheenhancedmagneticfield toSolarMagneticStructure happenstobedirectedsouthward. Boththedynamicpressureofthesolarwindandtheinter- planetarymagneticfield(IMF)areimportantfortheinter- actionofthesolarwindwiththeearth’smagnetosphere, IV. SOLAR WIND AND INTERPLANETARY andbothofthesearegenerallyquitevariable.Inadditionto FIELD INTERACTIONS WITH THE avarietyofwavephenomena,therearetwotypesoflarge- GEOMAGNETIC FIELD scaledisturbancesofthesolarwindplasmathatareknown tohavelargeeffectsonthemagnetosphere.Thefirsttype A. TheMagnetopause isrelatedtotheshapeoftheinterplanetarycurrentsheet. Thesolarwindcanbegenerallyviewedashighlyconduct- Ratherthanbeingpreciselyadiscwithintheeclipticplane, ing,sothattoafirstapproximationtheinterplanetaryand thecurrentsheetisgenerallytiltedsomewhatwithrespect geomagneticfieldsdonotmix.Thisrequiresthatthesolar totheeclipticplaneandalsohasawavystructureasfunc- windbedivertedaroundacavitythathasanouterbound- tionofazimuthalanglearoundthesun.Withinazimuthal ary which approximately separates the geomagnetic and regionswherethetiltand/orwavystructuredisplacesthe interplanetaryfields.Thisboundaryisreferredasthemag- currentsheetsufficientlyfarfromtheeclipticplane,fast netopause,whichisacurrentsheetofappropriateintensity solarwindcanbeemittedneartheeclipticplane.Dueto therotationofthesun(withan∼27-dayperiod)locations toseparatetheinterplanetaryandgeomagneticfields. Thelocationofthemagnetopausecanbeestimatedby neartheeclipticplanecanthusbeexposedtoperiodsof balancingthedynamicpressureoftheincomingsolarwind slowsolarwindfollowedbyfastsolarwind.Thisisillus- (n m V2,wheren isthesolarwinddensity,m isthe trated in the ecliptic plane in the upper panel of Fig. 4. In sw p sw sw p proton mass, and V is the solar wind speed) with the this plane, the solar rotation also imparts a spiral shape sw pressureofthegeomagneticfield,giving to magnetic field lines. Within azimuthal regions where fastsolarwindfollowsslowwind,thefastsolarwindwill n m V2 cos2ψ = B2/2µ . (6) catch up with the slow solar wind. The interaction of the sw p sw in 0 fast solar wind with the slow solar wind (referred to as Thisneglectstheinterplanetarymagneticpressure,which “stream–streaminteractions”)causesacompressionofthe is generally a reasonable assumption. In (6), B is the in solarwindplasmaandmagneticfieldwithintheinterface magneticfieldjustinsidethemagnetopause,ψ isthean- regionbetweenthefastandslowstreams.Suchcompres- glebetweenthesolarwindvelocityvectorandthenormal sionsgiveregionsofgreatlyenhancedsolarwinddensities tothemagnetopause,andthepressureoftheIMFandof andmagneticfieldstrengthswhichcansignificantlyaffect themagnetosphericplasmaareneglected.Fortypicalsolar themagnetosphere.Thesestream–streaminteractionsare wind parameters (n =5×106m−3;V =400 km/sec), sw sw particularlyimportantduringtheperiodaftersolarmaxi- andadipolegeomagneticfield,(6)placesthenoon,equa- mum,butbeforethenextsolarminimum,whenthesolar torialmagnetopause(alocationreferredtoasthe“nose” dipolarmagneticfieldisreforming. of the magnetosphere) at a distance r=10R from the E The second large-scale disturbance of the solar wind centeroftheearth,whichagreesverywellwithitsaver- plasmaisassociatedwiththelocalizedhighcoronalden- age observed position. [The earth’s dipole field strength sity regions that form during solar maximum. These re- is3.1×10−5/r3T.However,twicethisvalueistypically gionscanbecomebuoyantandbreakawayfromthesun, used for B in Eq. (6) in order to include contributions in carrying the high-density coronal plasma and associated fromthemagnetopausecurrent.]Equation(6)alsoshows magnetic fields radially away from the sun as illustrated thatthelocationofthemagnetopausemovesfurtherfrom in the bottom panel of Fig. 4. These ejections of coronal the earth with increasing distance from the nose. This is material, referred to as coronal mass ejections, can have becausecos2ψ decreasesawayfromthenosesothat B in dramaticaffectsontheearth’smagnetosphere. mustalsodecrease. Asdiscussedlater,thecomponentoftheIMFdirected Ifonlytheearth’sdipolefieldandthemagneticfields paralleltotheearth’smagneticdipole(whichisdirected of the magnetopause current sheet were included, the from the northern polar cap to the southern polar cap), magnetopausewouldnotextendsignificantlytailwardof rather than the total interplanetary magnetic field mag- the earth, and there would not be a magnetospheric tail. P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 SpacePlasmaPhysics 583 FIGURE4 Sketchoftwoimportantlarge-scaledisturbancesofthesolarwindplasma,stream–streaminteractions (upperpanel)andcoronalmassejections(lowerpanel). However, the large plasma pressures of the tail plasma whichiswellearthwardofitsaverageposition.Suchan sheetformthecross-tailcurrentsheetwhichisidentified earthwarddisplacementofthemagnetopausecorresponds in Fig. 1, and the magnetic field of this current allows tomorethanafactorofthreeincreaseinthemagnitudeof thetailmagnetopausetoextenduptoseveralhundred R themagnetopausecurrent.Thisillustratesoneimportant E awayfromtheearthintheanti-sunwarddirection. way in which the solar wind disturbances mentioned in On the dayside, B varies asr−3. With this variation, theprevioussectioncancauselargedynamicchangesto in Eq. (6) shows that the distance to the dayside magne- theearth’smagnetosphere. topause is proportional to (n )1/6. Solar wind densities sw canbequitevariable,andalargesolarwinddisturbance B. ClosedandOpenFieldLines canincreasen to∼50×106m−3.Adisturbanceofthis sw magnitude compresses the magnetosphere significantly, Theabovepressurecalculation,withtheinclusionofthe and brings the nose of the magnetosphere to r<7R , earth’s dipole magnetic field and of the magnetic fields E P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 584 Space Plasma Physics fromthemagnetopauseandcross-tailcurrentsheets,gives spheredowntotheionosphere.(Theionosphereisaregion anaccuratedescriptionoftheshapeofthemagnetosphere, ofionizedupper-atmosphericconstituentsthatsurrounds which is illustrated in Fig. 1. Because the calculation as- theearthataltitudesbetween∼75and∼500km.Forthe sumes that the interplanetary and geomagnetic fields do purposeshere,theionosphereisatthelow-altitudeendsof notmix,thecalculationgivesgeomagneticandinterplan- the magnetic field lines shown in Fig. 1 and marks the low- etarymagneticfieldsthatareparalleltothemagnetopause est altitude to which the interplanetary electric field has at all locations directly adjacent to the magnetopause. significanteffects.)FortheorientationoftheIMFshown However, this is not strictly valid because the magne- in Fig. 1, the mapping of the interplanetary electric field topause current sheet has large, but finite, conductivity, into the magnetosphere gives an electric field through- which allows a small portion (∼10–20%) of the IMF to out the open field line region of the magnetosphere that crossthemagnetopauseandconnectwiththegeomagnetic points from the dawn side of the magnetosphere toward field.SuchpenetrationoftheIMFintothemagnetosphere thedusk.ForotherorientationsoftheIMF,themapping connectstheinterplanetaryandgeomagneticfieldsandis intothemagnetosphereissimilar,buttheorientationofthe critical to magnetospheric dynamics, though it does not electricfieldcanhavesomedifferencesfromthatshown significantlyaffecttheshapeofthemagnetosphere. inthefigure. Theconnectionoftheinterplanetaryandgeomagnetic Figure 5 shows the mapping of the interplanetary elec- fields is illustrated in Fig. 1 for an IMF that is directed pri- tric field into the magnetosphere along open field lines marilysouthward(i.e.,nearlyparalleltotheearth’smag- in the dawn–dusk meridian plane, where the coordinate netic dipole). The figure shows how the penetration of system used has x directed from the earth to the sun, y a small portion of the interplanetary field into the mag- directedfromthedawntothedusksideoftheearth,and netospheremodifiesthegeometryofmagneticfieldlines zdirectedfromthesouthtothenorthmagneticpole.This emanatingfromthepolarregionsoftheearth.Withouta mapping gives an anti-sunward flow of plasma all along penetratingfield,allmagneticfieldlineswouldleavethe the open field region of the polar caps. At the boundary earthandreturntotheearthaftercrossingtheequatorial betweenopenandclosemagneticfieldlines,themapped plane.Suchfieldlinesarereferredtoas“closed.”Witha interplanetaryelectricfieldandtheanti-sunwardflowter- penetratingfield,closedmagneticfieldlinesdonotextent minate.Thisboundarythusbecomeschargedasindicated all the way to the magnetic pole. Instead there is an ap- inFig.5,givinganelectric fieldthatextendsintotheclosed proximatelycircularregioncenteredneareachmagnetic fieldlineregionofthemagnetosphere.Thiselectricfield polewherefieldlinescrossthemagnetopauseandenterin- isorientedsoastogivesunwardflowonclosedfieldlines. terplanetaryspace.Suchpolar-capfieldlinesarereferred to as “open.” For the earth, the boundary between open andclosedfieldlinesisat∼73◦ magneticlatitude.Open fieldlinesallowforatappingofenergydirectlyfromthe flowingsolarwindplasma,andsuchenergydrivesawide varietyofphenomenawithinthemagnetosphere. C. MappingofInterplanetaryElectricField intotheMagnetosphere The solar wind flows radially outward from the sun car- ryingtheIMFwithit.Ingeneralthemagneticfieldisnot paralleltothesolarwind,sothatthereisanelectricfield ininterplanetaryspacethatisrelatedtothesolarwindand IMFby E=−V ×B. (7) sw Sinceparticlescanmoveeasilyalongmagneticfieldlines, it is generally appropriate to assume that magnetic field linesaresohighlyconductingthattherecanbenoelectric fields parallel to the magnetic field lines. This assump- FIGURE 5 Schematic illustration of the magnetosphere in the dawn–dusk meridian plane. Plus and minus signs indicate tion implies that magnetic field lines are equipotentials chargesalongtheboundarybetweenopenandclosedmagnetic sothattheinterplanetaryelectricfieldgivenby(7)maps field lines, and shading identifies the region of open, polar-cap along open polar-cap field lines through the magneto- fieldlines. P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 SpacePlasmaPhysics 585 FIGURE6 Electricfieldsandplasmaflowsdrivenbythemappingoftheinterplanetaryelectricfieldintothemagne- tosphereasseenlookingdownontotheionospherefromaboveoneofthepolarcaps. The total electric field pattern drives a three-dimen- mappingoftheinterplanetaryelectricfieldintothemag- sional circulation of plasma, which is illustrated in the netosphere also depends significantly on the orientation noon–midnight meridian plane in Fig. 1. This flow is re- oftheIMF,theefficiencyincreasingastheorientationbe- ferredtoasmagnetosphericconvection.Thexcomponent comesincreasinglysouthward(i.e.,increasingtowardthe of the flow is anti-sunward across open, polar-cap field negative-zdirection).Thusvariationsinthezcomponent linesandtheflowreturnsinthesunwarddirectionwithin of the IMF when this component is negative (and thus theclosedfieldlineregion.Theflowalsomovespoleward antiparallel to the equatorial magnetospheric field) have on the dayside and equatorward within the tail, complet- the largest effects on the strength of convection, though ingtheconvectivecirculation.Thismagnetosphericcon- variations in the magnitude of the y component are also vectionpatternisoftenviewedbylookingattheelectric important. field, or equivalently the plasma flow, since the two are related by Eq. (1), as mapped to the ionosphere. Such a V. PARTICLE ACCESS TO, AND mapping, illustrated in Fig. 6, gives a complete picture of TRANSPORT WITHIN, THE magnetospheric convection because magnetic field lines MAGNETOSPHERE are approximately equipotentials. Figure 6 shows electric fields and plasma flow streamlines as seen looking down A. Access ontotheionospherefromaboveoneofthepolarcaps.The figureshowshowtheflowmovesintheanti-sunwarddi- Particles within the magnetosphere come from both the rectionoverthepolarcaps,crossestheboundarybetween solar wind and the ionosphere. While ionospheric parti- open and closed field lines, and returns to the dayside clesmakeimportantcontributions,thesolarwindsource withintheclosedfieldlineregion. generallydominatesthroughoutmostregionsofthemag- Thestrengthofmagnetosphericelectricfieldsandthe netosphere. resulting convection depends upon the magnitude of the Theentryofsolarwindparticlesandtheirtransportto interplanetary electric field, which varies with the mag- and within the tail plasma sheet is illustrated in Fig. 7. Be- nitude of the y and z, components of the IMF and with causethegeomagneticandinterplanetarymagneticfields thesolarwindspeed.Generally,variationsintheIMFare are connected, particles with a finite v(cid:8) are able to flow much greater than are variations of solar wind speed, so across the magnetopause. This is most effective across thatvariationsintheIMFaregenerallymoreimportantin thedaysidemagnetopause.Theresomeofthesolarwind modifyingthestrengthofconvection.Theefficiencyofthe particles,whichhavebeenheatedaftercrossingthebow P1:GLQ/GLE P2:FQPFinalPages EncyclopediaofPhysicalScienceandTechnology EN015A-716 August2,2001 14:57 586 SpacePlasmaPhysics FIGURE7 Illustrationoftheentryofsolarwindparticlesintothemagnetosphereandtheirtransporttoandwithin thetailplasmasheet. shock, cross the magnetopause and enter the portion of tail. As they flow down the tail, the electric field across the open field line region that is near noon. These parti- thetailcausesthemtoalsodrifttowardtheplasmasheet. clesinitiallyflowprimarilyalongmagneticfieldlinesto- Thiscausesasignificantfractionoftheparticlestocross wardtheionosphere,buttheyalsodriftslightlypoleward theboundarybetweenopenandclosedmagneticfieldlines acrossmagneticfieldlinesduetothemagnetosphericelec- andentertheregionofthedistantplasmasheet.Whenthey tricfield.Astheparticlesflowtowardtheionosphere,the reachthecross-tailcurrentsheet,theyaresignificantlyen- magneticfieldstrengthstronglyincreases,sothatconser- ergizedbythecross-tailelectricfieldandbecomeanim- vationofµrequiresthatparallelenergyisconvertedinto portant contributor to the energetic particle population of perpendicular energy. For the vast majority of particles, theplasmasheet.Theparticlesarethencarriedearthward all parallel energy is lost before the particles reach the byelectricfielddrift,gainingenergyastheymoveearth- ionosphere.Theparallelcomponentofvelocityforthese ward into regions of increasing magnetic field strength. particlesthenreverses,andtheparticlesmovealongfield Thisinwardmotioncontinuesuntilthespatialvariationin lines in the direction away from the earth. At the same magnetic field becomes sufficiently strong to deflect the time’theparticlesalsocontinuetomovepolewarddueto particlesaroundtheearthtowardthedaysideportionofthe theelectricfielddriftacrossfieldlines.Thereversalofthe plasma sheet, which is identified in Fig. 1. This deflection particles’parallelvelocityastheyflowtowardincreasing formstheinneredgeofthenightsideplasmasheet,which Bisreferredtoas“magneticmirroring.” typicallyliesatr≈5–12R . E B. TransporttoandwithinthePlasmaSheet C. FormationoftheRadiationBelts andStormtimeRingCurrent Aftermirroring,thesesolarwindparticlesformamagne- tosphericlayerthatliesadjacenttothemagnetopausethat Thelocationoftheinneredgeoftheplasmasheetdepends isreferredtoasthe“mantle.”Mantleparticlesflowaway onthestrengthofconvectionaswellasonthestrengthof fromtheearthalongopenfieldlinesofthemagnetospheric magnetic drift. As the strength of convection increases,