C.T. Russell - The Magnetosphere Q3

8/13/2019 C.T. Russell - The Magnetosphere Q3

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THE MAGNETOSPHERE

C.T. Russell

Institute of Geophysics and Planetary Physics (IGPP) and Department of Earth and

Space Science (ESS), University of California, Los n!eles California

In "he Solar #ind and the Earth, edited by S. -I. Akasofu and Y. Kamide, pp 73-100.Copyrigt by !erra S"ientifi" #ub$ising Company %!&''A#()*, !okyo, 1+7

Abstract

!e magneti" ie$d of te eart to a $arge etent sie$ds it from te "ontinua$ supersoni"outf$o/ of te suns ionied upper atmospere. 2o/eer, some of te mass, momentumand energy of tis so$ar /ind gains entry into te magnetospere, po/ering "urrentsystems, geomagneti" storms and aurora$ disp$ays. 4ter p$anets too ae

magnetosperes. Some of tese magnetosperes are due to intrinsi" magneti" fie$ds asin te "ase of te eart. 4ters are due so$e$y to te intera"tion of te p$anet /it teso$ar /ind as in te "ase of a "omet.

Introduction


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If tere /ere no so$ar /ind, te pysi"a$ pro"esses o""urring far aboe te eart in tetenuous upper atmospere and ionospere migt be /e$$ understood and te sub5e"t of$itt$e interest. 2o/eer, tere is a "ontinua$ supersoni" outf$o/ of ionied gas or p$asmafrom te sun tat "onfines and distorts te terrestria$ magneti" fie$d into a "aity /it a$ong tai$ etending undreds of eart radii in te antiso$ar dire"tion. !is "aity, "a$$ed

te magnetospere, is ery responsie to "anging so$ar /ind "onditions. #ysi"a$pro"esses o""urring terein modu$ate te energy f$o/ "arried by te so$ar /ind to teeart. At times te magneti" "aity a"ts as a sie$d def$e"ting te in"ident energy6 atoter times it a"ts as an a""e$erator, drien by te so$ar /ind, "reating "arged parti"$ebeams tat it te neutra$ upper atmospere, "ausing it to $igt up /it te bri$$iant formsof te po$ar aurora.

ig.1

!at te eart as a magneti" fie$d as been kno/n for "enturies. i$bert so/ed asear$y as 1800, in is treatise De $a!nete, te predominate$y dipo$ar "ara"ter of teterrestria$ magneti" fie$d. As igure 1so/s, e /ise$y aoided etrapo$ating ismagneti" fie$d $ines into te 94rbis :irtutis,9 /i" /e no/ kno/ as te magnetospereproper, and kno/ to be ig$y distorted from a dipo$e at ig a$titudes. !e $o/er borderof te magnetospere, te ionospere, /as not dis"oered unti$ 13 by )a$fourSte/art. It /as re"ognied ear$y tat te terrestria$ magneti" fie$d /as not "onstant. ;oton$y /ere tere se"u$ar "anges o""urring /it time s"a$es of "enturies, /i" /ere

presumab$y due to sour"es /itin te eart, but a$so "anges on sort time s"a$es ofdays to ours, /i" /ere presumab$y due to "urrent sour"es in te upper atmospere.

A$toug te idea tat so$ar "orpus"u$ar radiation, tat is, radiation due to ionied so$arparti"$es rater tan potons, /as responsib$e for tis sort-term a"tiity in tegeomagneti" fie$d and in some /ay "aused te aurora in te ig-$atitude regions, it /asnot unti$ 1+30 tat a satisfa"tory teory /as proposed.
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ig. eart radii de"ayed from te be$ts mu" more =ui"k$ytan tey sou$d ae due to parti"$e-parti"$e "o$$isions. It /as soon rea$ied tate$e"tromagneti" noise generated by $igtning %"a$$ed /ist$ers* /ou$d propagate uptroug te magnetospere a$ong fie$d $ines and resonate /it te gyromotion of tese

parti"$es "ausing tem to trae$ more near$y para$$e$ to te magneti" fie$d. &$e"tronstrae$ing more a$ong te fie$d go furter before tey are ref$e"ted. 2en"e tey are more$ike$y to it te atmospere and be $ost. !us, for a /i$e it /as tougt tat te amountof terrestria$ $igtning "ontro$$ed te $ee$ %or f$u* of energeti" e$e"trons in te radiationbe$ts. igure i$$ustrates te penomenon of $igtning-generated /ist$ers.
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ig.

(nfortunate$y, te fre=uen"y spe"trum of /ist$ers /as not ea"t$y rigt for "ausing teobsered e$e"tron $osses. !ere ad to be anoter sour"e of "o$$isions. !e sour"eturned out to be oter e$e"tromagneti" /aes, not generated in te atmospere butgenerated by te parti"$es temse$es. !ese /aes ad been dete"ted by re"eiers onte ground for many years. 4ne of te types of emissions /as "a$$ed te da/n "orus,/i" sounded mu" $ike te "irping of birds. igure +and igure 10i$$ustrate tefre=uen"y-time beaior of some of tese signa$s. !is emission and many oters /eresoon dete"ted deep in te magnetospere /en sate$$ites /ere instrumented to $istenfor tem. @e no/ ae een re"orded su" emissions in te magnetospere of Dupiter.
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ig. +

ig. 10

An eamination of te means by /i" tese emissions are generated e$ps to i$$ustrateo/ te magnetospere /orks to regu$ate itse$f. If a rigtanded "ir"u$ar$y po$ariede$e"tromagneti" /ae propagation a$ong a fie$d $ine en"ounters an e$e"tron gyratingabout te fie$d and moing to/ard te /ae, te e$e"tron "ou$d eperien"e a fie$d at its

period of gyration if te fre=uen"y of te /ae and te e$o"ity of te e$e"tron a$ong tefie$d are 5ust rigt. epending on te pase of te intera"tion, te e$e"tron /i$$ eiterbe"ome more para$$e$ to te fie$d or more perpendi"u$ar. If it be"omes more para$$e$, it$oses energy and te /ae gains energy6 if it be"omes more perpendi"u$ar, it gainsenergy and te /ae $oses energy. &iter "an appen and does appen 5ust by "an"e.2o/eer, if tere are more e$e"trons moing main$y perpendi"u$ar to te fie$d tan tereare moing main$y para$$e$, ten tis s"attering pro"ess /i$$ gie a net diffusion to/ardmotion a$ong te fie$d. !us, tere /i$$ be a net $oss of energy by te e$e"trons and a net


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gain by te /ae. !erefore gien a spe"ifi" distribution of e$e"tron motions, "ertain/ae types and "ertain /ae fre=uen"ies /i$$ be amp$ified. Some of tese /aespropagate out of te magnetospere into te atmospere and are $ost, and some ref$e"tba"k and get amp$ified again. If te parti"$e f$ues are strong enoug, tey /i$$ amp$ifyte /aes enoug to oer"ome te $oss due to propagation into te atmospere. If not,

ten te /aes 5ust de"ay a/ay and te parti"$es are stab$y trapped. !e f$u of "argedparti"$es aboe /i" tis amp$ifi"ation /i$$ take p$a"e and "ause te remoa$ of teparti"$es is "a$$ed te stab$e trapping $imit and te pro"ess is "a$$ed a /ae-parti"$einstabi$ity6 spe"ifi"a$$y ere, it is "a$$ed te e$e"tron-"y"$otron resonan"e instabi$ity, or te/ist$er $oss-"one instabi$ity. In genera$, te magnetospere attempts to regu$ate itse$fby be"oming unstab$e to some pro"ess if its parameters e"eed some $imit. !us, temagnetospere attempts to keep itse$f "$ose to te norm despite =uite aried eterna$"onditions.

The Plas!as"hereAn important sour"e of "arged parti"$es for te magnetospere is te eartsionospere, but in genera$ tese parti"$es are not near$y so energeti" as tose parti"$estat "ome from te so$ar /ind. !e ionospere is "reated prin"ipa$$y by te u$traio$etradiation from te sun being absorbed by te earts upper atmospere and terebybeing ionied. !e temperature of te ionospere is "oo$ %E


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$ines did not "ross. 2o/eer, as /e dis"uss be$o/, te so$ar /ind e$e"tri" fie$d does"oup$e to te magnetospere and does "ontro$ "o$d parti"$e drift. !e e$e"tri" fie$d tatdoes tis is "a$$ed te "one"tion e$e"tri" fie$d. !e e$e"tri" fie$d tat is asso"iated /itte parti"$es /i" at sma$$ radia$ distan"es drift around te eart is "a$$ed te "orotatione$e"tri" fie$d. !e p$asmapause o""urs approimate$y /ere te "orotation e$e"tri" fie$d

e=ua$s te "one"tion e$e"tri" fie$d. !e e$e"tri"a$ potentia$ differen"e app$ied to temagnetospere by te so$ar /ind is about 0,000 :.

@i$e it as $ong been appre"iated tat te "o$d p$asma in te p$asmaspere ad itsorigins in te ionospere, it as on$y be"ome re"ent$y appre"iated tat te ionospere isan important sour"e of te more energeti" "omponent of te radiation be$ts /itenergies of a fe/ ke:. Some pro"ess, peraps e$e"tri" fie$ds a$ong te magneti" fie$d orintense /ae-parti"$e resonan"es in te ionospere, is energiing te "o$d e$ium andoygen ions and a""e$erating tem to te e=uatoria$ regions. !us, te "emi"a$"omposition of mu" of te magnetospere is not so mu" $ike te "omposition of teso$ar /ind as origina$$y tougt but is more dominated by eay ions in te $o/er

ionospere.

Ma#netos"here $%na!ics

4ne of te ear$iest ep$anations for te sour"e of te aurora /as made sort$y after teadent of te spa"e era by D. @. ungey /o proposed tat te magnetied p$asma inte so$ar /ind merged /it te magnetied p$asma in te terrestria$ magnetospere sotat te magneti" fie$d of te eart and te sun be"ame "onne"ted, or re"onne"ted,sin"e tis pro"ess /as supposed to start and stop. !is re"onne"tion mode$ proided a

simp$e ep$anation for te "one"tion e$e"tri" fie$d dis"ussed aboe, proided a sour"efor te energiation of aurora$ parti"$es, and predi"ted intimate "ontro$ ofmagnetosperi" pro"esses by te orientation of te magneti" fie$d in te so$ar /ind, i.e.,te interp$anetary magneti" fie$d. As so/n in igure 11, tis mode$ predi"ts a neutra$point or $ine on te front of te magnetospere at /i" magneti" energy is re$eased tote p$asma and one in te magnetotai$ again at /i" p$asma is energied. @i$e in fa"ttis mode$ /as proposed as a steady state mode$ /i" "an be treated simp$y in termsof e$e"tri" fie$ds and p$asma drifts, it is "onenient to tink in terms of magneti" fie$d$ines moing. !us, in ungeys mode$, fie$d $ines in te so$ar /ind of dire"tion oppositeto te terrestria$ fie$d 5oin /it te earts fie$d /en tey are "one"ted against it.

!is ear$y insigt into magnetosperi" beaior did not a"iee uniersa$ a""$aim and itis on$y re"ent$y tat it as be"ome genera$$y a""epted. !e mode$ most easi$y ep$ainste entry and beaior of so$ar f$are parti"$es in te magnetotai$ and po$ar "aps6 teresponse of geomagneti" a"tiity to te so$ar /ind e$o"ity and te magnitude anddire"tion of te interp$anetary magneti" fie$d6 te ariations in magnetosperi"parameters su" as te sie of te po$ar "ap, position of te po$ar "usp and fie$dstrengt in te magnetotai$6 and te stru"ture of te magnetopause.


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ig.11

A$toug te ungey re"onne"tion mode$ is "ou"ed in steady state terms, te rea$magnetospere is se$dom in a steady state. irst, te dire"tion of te interp$anetarymagneti" fie$d is ig$y ariab$e and te re"onne"tion rate is etreme$y sensitie to tisdire"tion. @en te interp$anetary magneti" fie$d is een s$igt$y nort/ard,re"onne"tion seems to "ease. Se"ond$y, te magnetospere as diffi"u$ty keeping terates of a$$ its pro"esses in step. !us /en re"onne"tion is initiated on te dayside,magneti" f$u is opened and transported to te tai$ before more "$osed f$u "an "one"tin from te tai$ and te magnetopause as to moe in to/ard te eart. !e magneti"f$u in te tai$ /i$$ ten bui$d up unti$ re"onne"tion in te tai$ starts up to "onert te openfie$d $ines to "$osed fie$d $ines. If tis "onersion pro"ess is too effi"ient, more "$osed f$uis supp$ied tan needed by te magnetospere and te re"onne"tion point or neutra$point moes off do/n te tai$. In sort, te magnetospere, be"ause of re"onne"tion, isery dynami". !is "y"$i" pro"ess of f$u transfer to te tai$ and ba"k to temagnetospere is manifested in te aurora$ one as a magnetosperi" substorm /itbrigtening and motion of te aurora as /e$$ as energiation and in5e"tion ofmagnetosperi" parti"$es.


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ig.1

?ast, tere are "omets. or our purposes "omets may be tougt of as i$$ustrated inigure 1>, /i" so/s "ontours of te $igt s"attered from te neutra$ ydrogen "$oudsurrounding "omet )ennett in 1+70. Superimposed on tese "ontours is te isib$e"oma and ion tai$ of te "omet /i" is mu" sma$$er.


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ig.18

igure 18so/s a s"emati" of o/ te so$ar /ind intera"ts /it su" a "omet. @ente so$ar /ind enters te "$oud or "orona surrounding te "omet, it pi"ks up "ometaryions and te so$ar /ind f$o/ be"omes eaier, and due to "onseration of momentum, its$o/s do/n. !is s$o/ing do/n bends te fie$d $ines as so/n. If te intera"tion isstrong enoug, a bo/ so"k /i$$ form to def$e"t te f$o/ around te "omet. :enus a$soadds ions from its p$anetary atmospere to te so$ar /ind in mu" te same /ay $eading

to a p$anetary magnetotai$. Studies of te intera"tion of te so$ar /ind /it :enus oerte "ourse of a so$ar "y"$e suggest tat :enus a"ts most $ike a "omet at so$ar maimum,and $east $ike a "omet at so$ar minimum.

The &uture


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@i$e our understanding of te earts magnetospere as epanded rapid$y oer te$ast t/o de"ades, tere remains mu" to be done. !e distant magnetotai$ and te po$ar"usp ae s"ar"e$y been ep$ored, and /e $a"k a detai$ed understanding of teseregions. At mid-a$titudes oer te aurora$ oa$ fie$d $ines appear to dee$op e$e"tri"a$potentia$ drops /ose sour"e /e do not understand. @e do not yet agree on a$$ te

arious me"anisms $eading to $oss of parti"$es from te radiation be$ts, parti"u$ar$y oerte aurora$ oa$. Some of te ans/ers to tese =uestions may $ie in data a"=uired duringte Internationa$ Fagnetosperi" Study period from 1+77-1+7+ but not yet ana$yed.Some of te =uestions re=uire ne/ data. !e Internationa$ So$ar !errestria$ #rogram isdesigned to a"=uire mu" of tis information. 4ne sate$$ite, @ind, /ou$d gater data inte so$ar /ind and monitor te p$asma density e$o"ity and temperature and teinterp$anetary magneti" fie$d strengt and dire"tion /i" "ontro$s magnetosperi"response. Simu$taneous$y, an e=uatoria$ spa"e"raft /ou$d measure te /aes andparti"$es at te magnetosperi" e=uator tat $ead to te aurora$ pre"ipitation /i$e a#o$ar spa"e"raft and a !ai$ spa"e"raft %4#&;-D* measure te ig-a$titude po$ar "uspand distant magnetotai$ respe"tie$y. !e simu$taneous probing of ea" of tese regionsof te magnetospere is essentia$ to "ontinued progress in magnetosperi" resear",for our spa"e"raft "an be in on$y one $o"ation at a time /i$e pro"esses in temagnetospere affe"t te entire "aity.

!ere is a$so a need to repeat te IS&&-1 and -< mission but in oter regions of spa"e.IS&&-1 and -< /ere $aun"ed into identi"a$ orbits in 4"tober 1+77 /it ariab$e spa"ingin tat orbit. !is ariab$e spa"ing permitted time-of-f$igt measurements of boundarymotions and /aes /i" ae proen ina$uab$e in fina$$y determining e$o"ities,ti"knesses and "urrent densities of te bo/ so"k and magnetopause and determining/ae modes in te so$ar /ind. 2o/eer, IS&&-1 and -< probe on$y te near- e=uatoria$magnetospere and not te ig-$atitude regions. urter, tere are some boundariesand /aes for /i" orientation "annot be determined from a sing$e spa"e"raft and for/i" four spa"e"raft f$ying in formation are needed. Su" a mission, "ode-namedC$uster, /i$$ a$so be in"$uded in te IS!# mission.

ina$$y, /e note tat our ep$oration of p$anetary magnetosperes is in te state ourep$oration of te terrestria$ magnetospere /as a de"ade ago. !oday in te terrestria$magnetospere /e are main$y attempting to determine o/ and /y pro"esses o""ur.In p$anetary magnetosperes /e are sti$$ dis"oering /at o""urs, but our eperien"e/it te terrestria$ magnetospere is speeding te understanding of tese pro"esses.!e ep$oration of te terrestria$ magnetospere as been a igorous and e"itingprogram. @it te prob$ems $eft before us and te adent of p$anetary ep$oration /e$ook for/ard to a "ontinuation of tis igor and e"itement for many years to "ome.


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&'RTHER REA$ING

AC2I&:&F&;!S 4 !2& I;!&';A!I4;A? FA;&!4S#2&'IC S!(Y %IFS*,&dited by ). )attri"k and &. 'o$fe, &uropean Spa"e Agen"y, ;oord/i5k, !e

;eter$ands, 1+.

S4?A' !&''&S!'IA? #2YSICS, &dited by '. ?. Caroi$$ano and D. F. orbes, .'eide$ #ub$ising Co., 1+3.

#2YSICS 4 !2& D4:IA; FA;&!4S#2&'&, A. D. ess$er, Cambridge (niersity#ress, 1+3.

FA;&!IC '&C4;;&C!I4; I; S#AC& A; ?A)4'A!4'Y #?ASFAS, &dited by &.@. 2ones, Dr., eopysi"a$ Fonograp 30, Ameri"an eopysi"a$ (nion, @asington,.C., 1+.

:&;(S, &dited by .F. 2unten, ?. Co$in, !.F. onaue and :.?. Foro, !e (niersityof Ariona #ress, 1+3.

H(A;!I!A!I:& AS#&C!S 4 FA;&!4S#2&'IC #2YSICS, ?.'. ?yons and .D.@i$$iams . 'eide$ #ub$ising Co., 1+

!2& &A'!2S FA;&!IC I&? I!S 2IS!4'Y, 4'II; A; #?A;&!A'Y#&'S#&C!I:&, '. !. Ferri$$ and F. @. F"&$inny, A"ademi" #ress, 1+3.

FA;&!4S#2&'IC C(''&;!S. &dited by !. A. #otemra, eopysi"a$ Fonograp

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C.T. Russell - The Magnetosphere Q3