NASA Technical Reports Server (NTRS) 20150008406: The Plasmaspheric Plume and Magnetopause Reconnection

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GeophysicalResearchLetters
RESEARCHLETTER The plasmaspheric plume and magnetopause reconnection
10.1002/2013GL058802
B.M.Walsh1,T.D.Phan2,D.G.Sibeck1,andV.M.Souza1,3
KeyPoints: 1NASAGoddardSpaceFlightCenter,Greenbelt,Maryland,USA,2UniversityofCalifornia,SpaceScienceLab,Berkeley,
•Theplasmasphericplumegreatly California,USA,3NationalInstituteforSpaceResearch/INPE,SaoJosedosCampos,Brazil
impactsmagnetopausereconnection
•Theplumereducessolar
wind-magnetospherecouplingina
AbstractWepresentnear-simultaneousmeasurementsfromtwoTHEMISspacecraftatthedayside
localizedregion
magnetopausewitha1.5hseparationinlocaltime.Onespacecraftobservesahigh-densityplasmaspheric
plumewhiletheotherdoesnot.Bothspacecraftobservesignaturesofmagneticreconnection,providing
Correspondenceto:
atestforthechangestoreconnectioninlocaltimealongthemagnetopauseaswellastheimpactofhigh
B.M.Walsh,
[email protected] densitiesonthereconnectionprocess.Whentheplumeispresentandthemagnetosphericdensityexceeds
thatinthemagnetosheath,thereconnectionjetvelocitydecreases,thedensitywithinthejetincreases,and
thelocationofthefasterjetisprimarilyonfieldlineswithmagnetosheathorientation.Slowerjetvelocities
Citation:
Walsh,B.M.,T.D.Phan,D.G.Sibeck, indicatethatreconnectionisoccurringlessefficiently.Inthelocalizedregionwheretheplumecontacts
andV.M.Souza(2014),Theplasma- themagnetopause,thehigh-densityplumemayimpedethesolarwind-magnetospherecouplingbymass
sphericplumeandmagnetopause
loadingthereconnectionsite.
reconnection,Geophys.Res.Lett.,41,
223–228,doi:10.1002/2013GL058802.
Received21NOV2013 1.Introduction
Accepted31DEC2013
Acceptedarticleonline4JAN2014 Ithasbeenpredictedthatsolarwind-magnetospherecouplingislimitedduringtimeswhendenseplasma-
Publishedonline21JAN2014 sphericplumescontactthedaysidemagnetopause.Thesepredictionsarebasedontheoryandmodeling
whichshowthatthereconnectionratedecreaseswhentheplasmadensityincreases[e.g.,CassakandShay,
2007;Birnetal.,2008].Astheplumecontactsthemagnetopause,itshouldmassloadtheregionandslow
magnetopausereconnection.Thisinturnshoulddecreasethelevelofsolarwind-magnetospherecoupling.
Observationally,thecontrolofsolarwind-magnetospherecouplingbytheplumehasbeeninferredinsta-
tisticalworkbyBorovskyandDenton[2006].BorovskyandDenton[2006]examinedmanyyearsofsolarwind
measurementsandgeosychronousplumeobservationstoshowreducedsolarwind-magnetospherecou-
plingasmeasuredthroughgeomagneticindices.Globalmagnetohydrodynamic(MHD)modelingshows
thatmassloadingofthemagnetopausereconnectionwillgreatlyslowthereconnectionrateinalocalized
region,whichisconsistentwithpreviousresults[Borovskyetal.,2008].
Althoughmanymeasurementsoftheplumeexistatgeosynchronousorbit[e.g.,Moldwinetal.,1994;
Elphicetal.,1996;BorovskyandDenton,2008],fewspacecraftmeasurementshavebeenreportedofthe
plumeactuallycontactingthemagnetopause[e.g.,Suetal.,2000;McFaddenetal.,2008a;Walshetal.,
2013].Thecurrentstudyprovidessimultaneousspacecraftobservationsatthemagnetopausewitha
separationinlocaltime.Onespacecraftobservesahigh-densityplumecontactingthemagnetopause
nearlocalnoonwhileanotherspacecraftencountersthemagnetopauseprenoonanddoesnotobserve
theplume.Severalstudieshavepresentedcaseswithmultiplespacecraftobservingreconnectionat
themagnetopausewithlocaltimeseparation[Phanetal.,2000;Dunlopetal.,2011],butthisisthefirst
involvingtheplume.Sincethemagnetosheathpropertiesareroughlythesameatthetwospacecraft,
theobservationsserveasatestfortheimpactoftheplumeonmagnetopausereconnectionandsolar
wind-magnetospherecoupling.
2.Instrumentation
InsituobservationsfromtheTimeHistoryofEventsandMacroscaleInteractionsduringSubstorms(THEMIS)
spacecraftareused.Magneticfieldmeasurementsaremadewiththefluxgatemagnetometer(FGM)[Auster
etal.,2008].OnboardplasmamomentsfromtheESAinstrument[McFaddenetal.,2008b]areusedfor
bulkflowvelocities,andenergeticparticlemeasurementsareobtainedfromtheSolidStateTelescope.
TheelectrondensitymeasurementscomefromthespacecraftpotentialmeasuredbyEFI[Bonnelletal.,
2008],sincemuchofthecoldplasmasphericpopulationisbelowtheenergythresholdofESA(∼8eV).
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15 3.Observations
Duringa45minintervalfrom13:10to13:55UTon15
September2008,bothThAandThDcrossthemag-
netopauseanumberoftimes.Thespacecraftare
10
primarilyequatorialandareseparatedby1.5hin
magneticlocaltimeat11.6hand10.0h,respectively.
]
RE ThelocationsofthespacecraftareshowninFigure1.
[
X 5 Figure2presentssolarwindobservationsfromThB
whichislocatedjustupstreamofthebowshockat
GSM(X,Y,Z)=(26,−16,3)R .Theinterplanetarymag-
E
0 neticfield(IMF)Bzcomponentremainssouthwardfor
theentireintervalwiththeexceptionofseveralshort
periodsnear13:41UT.Formuchoftheinterval,the
5 0 -5 IMFclockangle(CA=tan−1(By))isgreaterthan120◦.
Bz
Y [R ] Themagneticfieldstrengthandelectrondensity
E
remainnearlyconstantduringthisinterval,indicating
Figure1.ThelocationsofThA(blue)andThD(red)on steadyinputstothemagnetopause.Propagationtime
15September2008ingeocentricsolarmagnetospheric
ofthemeasurementsfromtheupstreamspacecraft
(GSM).Thegreyregionisanominallocationofthe
(ThB)tothesubsolarmagnetopauseisestimatedat
plasmasphereandplume.
lessthan13minbasedonthebulkflowvelocity.
Figure3showsthemeasurementsfromThAandThDduringthesametimeperiodwhileeachencounters
themagnetopause.Bothspacecraftmeasuretheelectrondensityinthemagnetosheathtobe∼5–10cm−3;
however,thedensitymeasuredinsidethemagnetospherebythetwospacecraftdiffersby2ordersof
magnitude.InsidethemagnetosphereThDobservesadensityof0.4–0.7cm−3,typicalforthedayside
outermagnetosphere.ThAmeasuresthedensityoftheplumeplasmacontactingthemagnetopausetobe
18–72cm−3.Themagneticfieldandvelocitymeasurementsarepresentedintheboundarynormalcoordi-
natesystem(LMN)whereLisalongtheoutflowdirection,MisalongtheXline,andNisthecurrentsheet
normal.Thecoordinatesystemwasidentifiedthroughminimumvarianceofthemagneticfield(MVAB)
[SonnerupandCahill,1967].Thespacecraftobservejetsorenhancementsinthev componentinboththe
L
positiveandnegativedirectionswhileencounteringtheboundarylayer.Thisindicatesthatthespacecraft
weremakingobservationsbothaboveandbelowthereconnectionsiteasseenbyTrenchietal.[2008].
4 Bz
2
BT] 0 By
[n --42 Bx
-6
e
gl 150
Ang] 100
k de
c[ 50
o
Cl 0
10
8
B|T] 6
|[n 4
2
0
10
ne-3m]
c
[
1
X [R ] 26.4 26.4 26.4 26.4 26.4 26.4
E
Y [R ] -16.1 -16.1 -16.0 -16.0 -16.0 -16.0
E
Z [R ] 3.3 3.2 3.1 3.1 3.0 2.9
E
hhmm 1300 1310 1320 1330 1340 1350
2008 Sep 15
Figure2.UpstreamsolarwindmeasurementsfromThB.Thespacecraftisjustupstreamofthebowshockat(X,Y,Z)=
(26,−16,3)R .Fromtoptobottom,thepanelsaremagneticfieldvector,IMFclockangle,magneticfieldmagnitude,and
E
electrondensity.
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10.1002/2013GL058802
ThD - No Plume ThA - Plume
gy 10000 a 108 eV) gy 10000 e 108 eV)
ctron Ener[eV] 1100000 111000765-2cm-s-sr- ctron Ener[eV] 1010000 111000765-2cm-s-sr-
Ele 10 104 ev/( Ele 10 104 ev/(
100.0 100.0
] ]
n-3m 10.0 n-3m 10.0
c c
[ 1.0 b [ 1.0 f
0.1 0.1
400 400 g
c
200 200
s] s]
Vm/ 0 Vm/ 0
k k
[ -200 [ -200
-400 -400
60 60
40 40
BT] 200 BT] 200
n n
[ -20 [ -20
-40 d -40 h
-60 -60
hhmm 1310 1320 1330 1340 1350 hhmm 1310 1320 1330 1340 1350
2008 Sep 15 2008 Sep 15
Figure3.THEMISmeasurementsforthesametimeperiod(13:10-13:55UT)areshownforThDandThA.(a,e)Electronenergyspectra,(b,f)electrondensity
derivedfromspacecraftpotential,and(d,h)magneticfieldcomponents.Bothspacecraftcrossthemagnetopauseanumberoftimesduringtheinterval.Theblack
arrowsindicatefullboundarycrossingswithreconnection.
Weattributethedifferenceindensitybetweenthetwospacecrafttobefromalocalizedplasmaspheric
plumegiventhemagnitudeofthedensityandthelocationoftheobservations.Anumberofstud-
ieshavedemonstratedaconnectionbetweentheplasmasphericstructureandgeomagneticactivity
[e.g.,Chappelletal.,1970;HigelandLei,1984;Carpenteretal.,1993].Duringgeomagneticallydisturbedperi-
ods,theplasmapauseerodesradiallyinwardandadrainageplumecanforminthedusksectorextending
sunwardtowardthemagnetopause[Spasojevic´etal.,2003].Thecurrenteventisconsistentwiththispicture
asitoccursduringamoderategeomagneticstormwithaSym-Hindexof−40nT.Duringthistimeperiod,
theenhancedmagnetosphericconvectionbringstheplasmasphericplumetothemagnetopausewhereit
isobservedbyoneofthetwospacecraftattheboundary.
4.Reconnection
Duringthetimeperiodfrom13:10to13:55UT,bothspacecraftexperienceanumberoffullandpartial
magnetopausecrossings.Forcloseranalysisofthestructureofreconnection,weselectfullmagnetopause
crossingswithreconnectionoccurring.Weidentifyfullcrossingsbyarotationofthemagneticfieldanda
changeindensity.Forarotationaldiscontinuityatthemagnetopause,MHDpredictsthattheoutflowwill
beAlfvènicinthereferenceframeoftheXline.InthecaseofasymmetricreconnectiontheAlfvénspeedis
ahybridoftheAlfvénspeedoneachsideofthecurrentsheet.A15stimeperiodjustinsideandoutsidethe
magnetopauseisaveragedtoobtaintheplasmaparametersinthemagnetosphereandmagnetosheath.
Reconnectionduringthesecrossingsisidentifiedwhenthejetspeediswithin25%ofthepredictedhybrid
AlfvénvelocityfromCassakandShay[2007].Thejetvelocitiesareobtainedbysubtractingthereconnecting
componentoftheexhaustvelocity(v )fromthebackgroundflowintheLdirection.Theexhaustvelocityis
L
selectedasthemaximumvaluewithinthecurrentsheet.
Thebackgroundflowistakenfromthesidewiththelargermassinflowwhichisdeterminedfrom𝜌∕B
followingCassakandShay[2007].TheratioofmassinflowforeachcrossingisgiveninTable1withthe
assumptionthattheeffectivemassissimilarfortheplasmaonbothsidesofthemagnetopause.AtThD,the
magnetosheathmassinflowclearlydominatesandsubtractingthebackgroundv fromthemagnetosheath
L
isaclearchoice.AtThA,themagnetosphericmassinflowislargerbutdoesnotdominate.Inthisscenarioit
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Table1.ParametersforReconnectionMeasurementsa
Time vjet vC+S njet ns nm Bs Bm MassInflow
hh:mm SC (km/s) (km/s) (cm−3) (cm−3) (cm−3) (nT) (nT) Ratio JetField
13:29 D 363 441 1.7 4.2 0.5 21 63 0.04 ’sphere
13:34 D 362 364 2.9 8.4 0.4 24 63 0.05 ’sphere
13:44 D 255 283 7.2 11.9 0.7 23 54 0.04 ’sphere
13:26 A 190 213 15.2 9.8 20.3 51 29 2.35 ’sheath
13:28 A 126 163 14.9 11.6 18.3 30 28 1.69 ’sheath
13:31 A 221 176 15.9 9.4 55.0 38 56 3.97 ’sheath
a“SC”istheTHEMISspacecraft.vC+SisthecalculatedCassakandShayhybridAlfvénvelocity.Thesubscript
mindicatesmagnetosphericsidewhilesisthemagnetosheathside.Theratioofmagnetospheretomagne-
tosheathmassinflowis nmBs.“JetField”istheorientationofthemagneticfieldvectoratthetimeofthemaxi-
mumjetvelocity.AfieldnosBrmientationwith+B ismagnetosphericwhile−B ismagnetosheathforthisevent.
L L
islikelythatbothsidesarecontributingplasma,sothedecisionislessclear.Sinceweareonlyselectingone
sidetosubtractabackground,wechoosethesidewiththelargermassinflow.Inthesecrossingsv from
L
themagnetosheathissubtractedwhennoplumeispresent(ThD),whilev fromthemagnetosphereissub-
L
tractedwhentheplumeispresent(ThA).Wenote,however,thatourcriteriaforreconnectionaremetusing
v fromeithersideoftheboundary.
L
UsingthecrossingbyThDat13:34UTasanexample(showninFigure4),themaximumv component
L
withintheexhaustis370km/swhilethebackgroundcomponent(magnetosheathinthiscase)is8km/s.
Thisgivesajetvelocityofv =v -v =362km/s.ThehybridAlfvénspeedis364km/s,sothe
jet exhaust background
jetmeasurementis99%ofthepredictedvalueandisthereforeselectedasareconnectionevent.Withthese
criteria,wehaveidentifiedthreeperiodsforeachspacecraftwhenreconnectionisobservedduringafull
magnetopausecrossing.ThetimesoftheseperiodsandreconnectionparametersareprovidedinTable1.
ThetimesarealsoshowninFigure3withblackarrows.WenotethatthederivationofthehybridAlfvén
velocityfromCassakandShay[2007]assumesv =0km/sonbothsidesofthecurrentsheetwhichdoesnot
L
occurinthesemeasurements.Thebackgroundflowissubtractedfromtheexhaustmeasurementtoobtain
anappropriatereferenceframeforcomparisonwiththeprediction.
ThD - No Plume ThA - Plume
Energy [eV] 111111000000123456 a 111111100000002345678V/(cm^2-sec-sr-eV) Energy [eV] 111111000000123456 e 111111100000002345678V/(cm^2-sec-sr-eV)
100.0 b e 100.0 f e
3] 10.0 3] 10.0
nem- nem-
c 1.0 c 1.0
[ [
Sphere Sheath Sheath Sphere
0.1 0.1
100 100
0
V m/s] -100 V m/s] -100
k -200 c k -200 g
[ -300 [ -300
-400 -400
80 80
60 60
40 40
BnT] 200 BnT] 20
[ -20 d [ -20 h
-40 -40
-60 -60
Seconds 30 00 30 00 30 Seconds 00 30 00
2008 Sep 15 1333 1334 1335 2008 Sep 15 1331 1332
Figure4.MagnetopausecrossingfromThDandThA.(a,e)Ionenergyspectragram,(b,f)electrondensityderivedfrom
spacecraftpotential,(c,g)bulkflowcomponents,and(d,h)magneticfieldcomponents.Themagneticfieldandbulk
flowareinboundarynormalcoordinatesdeterminedthroughMVAB.Theverticaldashedbarsindicatethetimeofthe
peakjetvelocity.
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Twonear-simultaneouscrossingsfromThAandThDareshowninFigure4forcloseranalysis.These
arerepresentativeoftheothercrossingsbyeachspacecraft.InthisexampleThAcrossesthemagne-
topausefromthemagnetosheathtothemagnetosphereat13:31:37UT.Threeminuteslater,ThDpasses
fromthemagnetospheretothemagnetosheathat13:34:45UT.ThDmeasuresadensityof0.2cm−3
insidethewhileThAmeasuresamagnetosphericdensityof55cm−3(Figures4band4f).Partofthecold
plumeplasmacanbeseenasafluxenhancementintheionenergyspectragramwithanenergynear
10eV(Figure4e).Thiscoldpopulationcoexistswiththehottermagnetosphericpopulationobservedby
bothspacecraft.
Thedensityasymmetrybetweenthespacecraftimpactsseveralaspectsofthereconnection.Thefirstaspect
isthelocationofthereconnectionjetinrespecttotheXline.InthecaseofThD,thedensityishigherin
themagnetosheath,resultinginasymmetricreconnectionwherethejetliesprimarilyonmagneticfield
lineswithmagnetosphericorientation(+B )(Figures4cand4d).Thisistypicalforterrestrialmagnetopause
L
reconnection.ThAobservesahigherdensityinthemagnetosphere,causingthereverseasymmetry.Inthis
scenariothejetliesprimarilyonmagneticfieldlinesofmagnetosheathorientation(−B )orlowerdensity
L
(Figures4gand4h).Ineachcasethejetisdisplacedtowardthesidewithlowerdensityandlower𝜌∕B,con-
sistentwithpredictionsbyMHDtheory[CassakandShay,2007].Themagneticfieldorientationduringthe
peakjetvelocityfortheothercrossingsisconsistentwithwhateachspacecraftobservesinthisexample
andisincludedinTable1.
Themagnitudeofthejetvelocitiesarealsodifferentbetweenthespacecraft(Figures4cand4g).ThD
observesasignificantlylargerjetvelocity(v =362km/s)thanThA(v =162km/s).ThDalsoobserveslower
jet jet
densityinsidethemagnetopausethanThA.Thejetiscausedbythemagneticcurvatureforcethatacceler-
atestheexhaustplasmatothehybridAlfvénspeed.TheAlfvénspeedisinverselyproportionalton1∕2,soa
slowerjetvelocityinthecrossingswithhigherdensityisconsistentwiththeobservations.Inadditiontothe
velocity,thedensitywithineachexhaustjetisalsodifferentinthetwocases.Thedensityshowsmixingof
theplasmapopulationsfrombothsides.InthecaseofThA,whenreconnectionishighlyasymmetric,the
bulkofthecontributioncomesfromthemagnetosheathsideorthesidewithlargerdensity.Thevelocity
anddensitywithineachjetaregiveninTable1.
5.Discussion
Apredictedeffectoftheplasmasphericplumeatthemagnetopauseisthatthedenseplasmainthe
magnetospherewillslowreconnectionanddecreasesolarwind-magnetospherecoupling[Borovsky
andDenton,2006].Theoryofsymmetric[Sweet,1958],aswellasasymmetricreconnection[Cassakand
Shay,2007],showsthereconnectionratetoscalewiththeAlfvénspeedorinverselywithn1∕2.Asthe
densityincreases,thereconnectionratedecreases.Themultispacecraftobservationspresentedhere
showreconnectionoccurringoverarangeoflocaltimewithsimilarmagnetosheathbutgreatlydifferent
magnetosphericdensities.
Withcurrentspacecraftinstrumentation,observationalmeasurementsofadimensionlessreconnection
ratecanvarybyafactorof2ormoreforasingleeventdependingonwhattechniquesareused[Phan
etal.,2001].Thesetechniquesoftenrequireidentifyingaboundarynormalofthecurrentsheetwhich
canbedonethroughseveralmethods,aswellasidentifyingthevelocityoftheXlinestructureunderan
assumptionofconstantmotion.Bothofthesecanintroducesignificanterrorsintothemeasurement.An
alternativewaytomeasuretheefficiencyofreconnectionistomonitorthevelocityoftheexhaustjet.
Althoughitdoesnotproducearatequantitativelysimilartoothermethods(i.e.,B /|B|orv /v ),theeffi-
n n A
ciencyofreconnectionattwospacecraftcanbecomparedbythemagnitudeofthereconnectionjet.Using
thejetvelocityassumesthatthedimensionlessreconnectionratestaysroughlyconstantoverthetime
periodofthespacecraftpass.Ifthereconnectionratedoesnotchangesignificantly,thevelocityofthejetis
afunctionoftheinflowspeedwhichisinturnameasureofthereconnectionrate.
ThDobservesfasterjetsattendingcrossingswithoutaplumethanThAobserveswithaplume.Thisiscon-
sistentwiththeideathatthereconnectionratedecreaseswithhigherdensitiesandthattheplasmaspheric
plumecanslowreconnectionwhenitcontactsthemagnetopause.Onalargerscale,theseobservations
showthatreconnectionwillbeimpactedinalocalizedregionwheretheplumecontactsthemagne-
topause.Thesizeoftheplumeatthemagnetopausemustalsodeterminetheimpactontheoverallsolar
wind-magnetospherecoupling,andameasureofthissizeisneededtoquantifythiseffect.Inadditionto
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affectingreconnection,thenewlyopenedfieldlineswilltransportplasmasphericplasmaastheyconvect
overthepolestothenightsidewheretheplasmaisdepositedintothetailandlobes[Elphicetal.,1997].
Tounderstandtheglobalimpactoftheplumeonsolarwind-magnetospherecoupling,onemustfollow
theentirecirculationpatternbeginningwiththeentrainmentofdenseplasmasphericmaterialonnewly
reconnectedmagneticfieldlines.
6.Conclusion
Near-simultaneousspacecraftobservationsatthemagnetopauseshowthepresenceofalocalizedregion
withhighdensitiesfromtheplasmasphericplume.Separatedby1.5hinmagneticlocaltime,bothspace-
craftmeasuremagneticreconnection;however,onlyonemeasuresthehigh-densityplume.Whenthe
plumeispresent,thereconnectionjetshavelowervelocitiesandlargerdensities.Adecreasedjetvelocity
indicatesalocalizedreductioninsolarwind-magnetospherecouplingduetothepresenceoftheplasmas-
phericplume.Theseobservationsalsoshowthatthepropertiesofreconnectionareafunctionofthelocal
plasmadensityandvaryalongthesurfaceofthemagnetopause.
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