JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 106, NO. All, PAGES 25,541-25,556, NOVEMBER
1, 2001
Ring current oxygen ions escaping into the magnetosheath Q.-G. Zong•, B. Wilken2, S. Y. Fu3, T. A. Fritz4, A. Korth2, N. Hasebe 5 D 9
ß
J. Williams 6 and Z.-Y. Pu a
Abstract. Storm-related magnetosphericoxygen bursts were observedin the dayside magnetosheathduring the coronal mass ejection on January 10, 1997. These singly chargedoxygen ion events exhibited a clear antisunward flow. The oxygen ions are associatedwith a strong negative interplanetary magnetosheath field (IMF). The averageB• was almost - 50 nT, and the field projection in the x-y plane (GSE) was nearly constantin the Sun/dawn sectorforming an
angleof 450relativeto the Earth-Sunaxis. The magnetopause wasidentified as a rotational discontinuityby usingthe principal axis analysis(PAA) method. The three-dimensionalpolar versus azimuthal angle distribution of the oxygen ions showedthat the oxygen flow has a north to south velocity component. The observationssuggestthat the daysidereconnectionprocessis generally steady. The energydispersioncan be explainedwith the time-of-flight (TOF) effect assuming oxygenions are escapingfrom the magnetospherealong the reconnectedfield lines. The lack of hydrogenand helium ions during the observedoxygen bursts can be explained, as only oxygen ions are resupplied by the gradient drift in the inner magnetospherebecauseof their larger bounce periods with respect to hydrogen and helium ions. Therefore only oxygen ions are observedcontinuouslyin the
m•gnetoshe•th. The estimated oxygenescape r•te •mountsto 0.61x 1023ions/s, about 33% of the input rate of the ring current. The observationsimply that the stormtime ring current is asymmetric. A large amount of ring current oxygen ions escapefrom the magnetosphereinto the magnetosheath. 1. Introduction
Fermi mechanism,operating at the quasi-parallel bow shock, can accelerate incident solar wind ions to ener-
Both the solar wind and the Earth's magnetosphere giesup to about 200 keV with an isotropicangular discan contribute to the energeticion populations of the tribution[Lee,1982].Solarwind 0 6+ ionswhichhave magnetosheath.The strength of the two sourcesvaries undergoneFermiacceleration(quasi-parallelshock)can accordingto geomagneticactivity and interplanetary
be further acceleratedto reachMeV energiesthrough multiplereflectionsby the quasi-perpendicular shockif quasi-perpendicularshock drift acceleration can only interplanetarymagneticfield (IMF) linesare beingconacceleratea seedsolarwind populationto peak energies nectedto the shockin a quasi-parallelconfigurationat of about 10 keV because of the limited drift distance one time and within a few minuteschanged•toa quasiat the Earth's bow shock[Schwartzet al., 1983]. The perpendicular configuration, or viceversa[Freemanand Parks, 2000]. •Max-Planck-Institutf/Jr Aeronomie,Katlenburg-Lindau, Although the observationsin the past provideoverGermany. Now at Center for SpacePhysics,Boston University, whelmingevidencethat the magnetosphere generally Massachusetts. provides the main contribution to the energetic particle 2Max-Planck-Institut fiir Aeronomie,Katlenburg-Lindau, populationfoundin the magnetosheath [Sibecket al., Germany. particleses3Department ofGeophysics, PekingUniversity, Beijing,China. 1999],the questionof howmagnetospheric 4Centerfor SpacePhysics, BostonUniversity, Massachusettss.caperemainsopen. Theseparticleshavebeensuggested 5Advanced Research Centerfor Scienceand Engineering,to migrate from the dayside into the magnetosheath Waseda University, Tokyo, Japan. via differentmechanisms:(1) magnetospheric particles 6AppliedPhysics Laboratory, JohnsHopkinsUniversity, Lau- are acceleratedin the daysidereconnection regionand rel, Maryland. crossthe magnetopauseat a rotational discontinuity along reconnected magneticfield lines [Speiseret al., Copyright 2001 by the American GeophysicalUnion. 1981;Cowley,1982];(2) energeticmagnetospheric parPaper number 2000JA000127. ticlesare free to streamoutwardalonginterconnected 0148-0227/01/ 2001JA000127509.00 magnetosheathand magnetospheric magneticfield lines conditions[Sibecket al., 1987]. The mechanismof the
25,541
25,542
ZONG ET AL.: RING
CURRENT
OXYGEN
IONS IN THE MAGNETOSHEATH
with no accelerationprocessinvolved[Scholeret al., vation showedthat ring current ions (oxygen,helium, 1981];(3) they can crossthe tangentialdiscontinuityof and hydrogen)can leak out of the inner magnetosphere the magnetopause and enterthe magnetosheath [Sibeck during intensestorm activities, building an ion layer in et al., 1987; Sibeck and McEntire, 1988; Paschalidis the magnetosheathnear the magnetopause.Energetic et al., 1994],or leak into the magnetosheath causedby oxygenion enhancementsare repeatedly detected in the particle scattering in the magnetopausecurrent sheet
magnetosheathwith a total duration of about 150 min
duringenhancedsolarwind dynamicpressure[Zon9and [Zongand Wilken,1998;Zonget al., 1999]. Wilken, 1999]. The initial resultsonthe coronalmassejection(CME) During the dayside reconnection,it is expected that magnetosheathplasma not only is transported into the magnetospherewhich forms a boundary layer on open magnetosphericfield lines, but can alsobe reflecteddue
related oxygen event on January 10, 1997 were de-
scribedby Zong and Wilken [1998]. In this paper,
on the basisof observationsobtained by both the high energyparticle- lower-energydetector(HEP-LD) and to interactionwith the currentlayer. Cowley[1982]has energeticparticlesand ion composition(EPIC) instruproposedthat cold magnetosphericions or ring current ments onboard Geotail, we will demonstrate that these ions can be accelerated in the current layer, can be re- energetic oxygen ions probably escape from the ring flectedback into the magnetosphere,or can leak into the current region into the dayside magnetosheathvia a magnetosheath. Southward streaming energetic ions quasi-steady reconnection process. Furthermore, we are consistentwith a picture of an outflow of magne- will suggestthat oxygen ions can be continuouslysuptospheric particles along reconnectedfield lines with plied by the gradient drift in the inner magnetosphere, a reconnectionneutral line south of the magnetopause and therefore,hydrogenand helium ionsseemto be contraversal.In a casestudy,Scholev[1983]foundthat in- tinually depleted and only oxygenions can be observed in the magnetosheath. sidethe magnetopausethe phasespacedensitiesof •30 to •65 keV protons were larger for particles streaming away from the magnetopausethan toward the magne- 2. Geotail Observations topause. They also found that phase space densities The data presentedin this paper are obtainedby the parallel to the field in the magnetosheathwere larger HEP-LD instrument[ Wilken et al., 1998;Doke et al., than the density within the magnetosphere. This has been interpreted in terms of reflection and acceleration 1994]. The HEP-LD is an advancedion spectrometer of some of the energetic ring current particles at the with a position-sensitive time-of-flight(T) and energy magnetopausecurrent layer under quasi-steadyrecon- (E) detectionsystemwhich determinesthe massof innectionconditions.Moreover,Daly [1982]pointedout cidentenergeticions. Combinedwith the sectoredspin that particleswith 90o pitch anglemovingacrossthe plane of the spacecraft,it allowsthe imaging of flux magnetopausevia a reconnection processwould keep distributionsover the completeunit spherein phase space. The energy ranges for hydrogen,helium, and their pitch angles unchanged. Geomagneticstorms(definedby the ring current de- oxygenare approximately75 - 4000 keV, 100- 4000 keV, velopment)are causedby enhancedmagnetospheric con- and 140- 4000 keV, respectively.Althoughthe HEPvection. During the main phase, particles are injected LD spectrometeris not sensitiveto the chargestate of into the ring current region by an enhanceddawn-dusk the incident particles, the EPIC instrument on Geoelectricfield, with ions drifting to the dusksideand elec- tail can providechargestate information for ionsin the trons to the dawnside. Not all particles can complete energyrange30 to 230 keV/e [Williamset al., 1994]. drift orbits through the dayside magnetosphere.The EPIC measurements documentthe dominantpresence ions may be expected to leak from the dusksidemag- of singly ionized oxygenin the selectedeventsof this netosphereinto the magnetosheath, and electronsmay paper. leak from the dawnsidemagnetosphere. On January10 and 11, 1997,a coronalmassejection
A global simulationstudy by Win91eeet al. [1996] (CME) encounteredthe Earth's magnetosphere and showsthat the leakageof particles(H+ and O+ ions) produceda moderate geomagneticstorm. The CME from the ring current into the magnetosheathshould is manifestedby a largenegative/positive excursionin occur frequently. During changesof the IMF orienta- the IMF Bz [Zongand Wilken,1998].On January10, tion, the leaking particles can produce sporadic ener- the satellite crossedthe magnetopauseat about 0800 getic particle events. The most intense events appear UT and was traveling in the dusksidemagnetosheath to be associatedwith southward turnings of the IMF adjacentto the magnetopause.Figure I showsthe trarather than with northward turnings. This model also jectory of GEOTAIL on January 10 and 11, 1997. The
predictsthat ring currentO+ ionsmaybe an important nominal positionsof the bow shockand the magnecomponent to these energetic particle events. topausehave beenscaledaccordingto the modelgiven Typical valuesof the O+ energydensityin the quiet by Fairfield [1971]. time ring current are about 6% with respectto the proPlate I gives an overview of the HEP-LD and the ton energy density and more than 100% during large magneticfield measurements from the magneticfield storms[Da91iset al., 1999]. Recently,Geotail obser- (MGF) instrumentbetween1000 and 1200UT on Jan-
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Plate 1. An overviewof high energyparticle- lowerenergydetector(HEP-LD) and magnetic field (MGF) data for January10 and 11, 1997. 'Fromthe top the graphshow:integralcounting rates"all ions"and "O"; countingratesfor hydrogenand helium;color-codedazimuthalintensity distributionsof the all ion rates; GSE componentsand magnitudeof the magneticfield (in nanoteslas).The circlesin the third graphmark the magneticfield directionprojectedon the equatorial plane.
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GSE
25,544
ZONG ET AL.: RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH
X=6.0,Y=9.4,Z=0.4Re
10.01.1997 11:00-11:10UT
- Oxygen cts
10.5
Legend Tail In B Out B Dusk Dawn Plate 2. Three-dimensional polarversusazimuthalangledistributions of oxygenionsin the time intervalbetween1100and 1110UT. Circledplussign(in B) indicatesthe magneticfield vectorB at this point into the sphere;solidcircle(out B) marksthe magneticfieldvectorB at this point out the sphere. "N" marks the north direction.
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ZONG ET AL.- RING CURRENT
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Plate 3. Energy spectrogramversusuniversaltime for suprathermalH+, He++, and 0 + ions and electronsobtainby the energeticparticlesand ion composition(EPIC-ICS) instrument The time
interval
is identical
to Plate
1.
25,546
ZONG ET AL.' RING CURRENT
OXYGEN
Geotail
IONS IN THE MAGNETOSHEATH
Orbit
January 10 - 10, 1997 (BS/MP: Fairfield JGR7._•6,1971,6700)
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Figure 1. Geotail trajectory for January 10 a.nd 11, 1997. Nominal positionsof the magnetopauseand bow shockin GSE coordinatesystem(in RE) are shownas dashedcurves.
uary 10, 1997. The different graphs show energetic oxygen, helium, and proton ion fluxes, and the particles'flow direction,togetherwith the magneticfield in GSE coordinates. The spacecraftwas initially traveling in the magnetosheathand then crossedthe magnetopausefrom the magnetosheathinto the magnetosphereat about 1113 UT. This region identificationis
the spacecraftmoved into the magnetosphere,and the all ions counting rate jumped to a higher level, but the oxygenion counting rate returns to a lower level similar to that observedbefore the two oxygen peaks.
The angular distributions(the third graph in Plate 1) in the magnetosheath (before1113UT) showstrong anisotropy,in contrast to the isotropic distributions in
basedon the Geotail plasma[Mukai et al., 1994]and the magnetosphere.The magnetic field direction is almagneticfieldmeasurements [Kokubunet al., 1994]. most at the center of the azimuthal angular distribuThe top graphin plate I showstwo energy-integrated tions of the all ions rate in the magnetosheath. This ion counting rates versus time profiles. The solid line labeled "all ions representsa rate summed over all ion speciesand energies,usually dominatedby protons;the
indicates that the projected anisotropy of these ions coincideswith the magnetic field direction in the X- Y plane. The projected anisotrol•y of the oxygen ions dashedline labeled "O refers to oxygenions summed clearly coincideswith the magneticfield directionin the over all energies.The secondpanel showsenergyinte- X- Y plane, being parallel to the magnetic field. The grated rates for hydrogenand helium. flow was tailward with a component in the dusk-ward From 1000 to 1113 UT the "all Ions countingrate direction. A representationof oxygenions detectedby profileindicatesthat when GEOTAIL wastravelingin the HEP-LD instrumentin three dimensions(including the magnetosheath,the oxygencount rate is at a lower field direction and the orientation of the unit spherein level until two major oxygen peaks are detected from GSE coordinates)is displayedin Plate 2. The magni1055 to 1103 UT and 1107 to 1113 UT. After 1113 UT tude of the magneticfield in the magnetosheathis about
ZONG ET AL.' RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH
Geotail
HEP-LD
10.01.1997
11'01-11:09
25,547
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Figure 2. Differential energydistributions forsuprathermal H+, He++, andO+ ionsintegrated
over the time of the oxygenevent in Plate 1.
55 nT, and only 40 nT in the magnetosphere(seethe bottom graph of Plate 1). As alreadymentioned,Plate 2 showsa high-resolution three-dimensionalangular distribution of oxygenionsin the interval 1100 to 1110 UT. The highly inclined local
As mentioned earlier, the HEP-LD spectrometer is not sensitive to the charge state of the incident ions. However,the EPIC instrumentation on Geotail can provide charge state information for ions in the energy
ative to the north direction. The oxygen ions occupy a
of the heads are essentially identical, and measure ion
range30 to 230 keV/e. The EPIC ion compositionsenmagnetosheath fieldlinesforma polarangleof 450rel- sor (ICS) is composedof three separateheads. Two
300x 1120(polarX azimuth)angularsegment on the spectra and composition,which providesflux, compofor two 300 unit spherenearly perpendicularto the magnetic field. strion,spectra,and angulardistributions The beam-like oxygenions have an averagepitch angle polar angleranges(-380to- 80; +80 to +380). The of • 840 relative to B.
Figure 2 showsproton, helium, and oxygen energy spectra obtained during the oxygen burst event. It shouldbe noted that the low-energyhydrogen(30 to
75 keV) and helium (70 to 100 keV) are identifiedby the time-of-flight signalonly. The oxygenspectrumhas two well-developed peaks at about 250 and 550 keV, and the oxygen flux beyond 150 keV exceedsthat of hydrogen and helium ions.
energyrange is 50 keV to 3 MeV. The third head primarily measureselectron fluxes > 32 keV and >110
keV (integral)for polar angleranges(-300to +30ø) [Williamset al., 1994]. The EPIC measurements document the dominant presenceof singly ionized oxygen
during the aboveoxygenevents: (1) between1000 and 1030 UT: strong north to south flux for all ion species, anti-sunward flux is • 10 times stronger than sunward
flux, O+• flux is •0 100 times strongerthan the high
25,548
ZONG ET AL.' RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH
Principle ofTheOxygen Event OnJan. 10,1997 ObsetdByGEOTAIL-HEP-LD and IC$Immzmeat ZGSE
Oxyg• •vent•
ICS1 XGSE
, ICS2
Figure 3. Principleof the oxygen eventson January10, 1997,observed by bothGeotailHEP-
LD and EPIC-ICS instrument.Openinganglesand orientations of HEP-LD and EPIC-ICS are
given.
chargedstatesolarwind particleflux (probablysome EPIC-ICS observationsare in closeagreementwith the HEP-LD measurements throughoutthe time intervals. 0+6); (2) between 1055and1103UT: predominantly For the time interval 1000 to 1030 UT the intensiO+l (stilla strongnorthto southdifference) withsome
and the moderately protons(fluxpeaksequatorially, no north-south differ- tiesof ion species,the composition, anisotropic angular distributions indicate the presence ence),antisunward fluxis muchstronger thansunward of magnetospheric particles in the magnetosheath which flux, O+l is muchgreaterthan high chargestate oxycould leak from the local or from the somewhat distant gen(0 +6 < 1%); and (3) between1107and1113UT: virtuallyno O+• and no O+• measured in the 10-210 dawnward magnetopause. For the time interval 1030 to 1113 UT the ion compokeV/e energyrange. One "canpresume that the oxy-
is differentfromthe nearby genmeasured at higherenergies is O+•, butthereare sitionin the magnetosheath
in the previousinterval.The energetic no measurements to supportthis presumption.Given magnetosphere all the O+• aroundbeforeand after this interval,it is population appearsto be oxygen-rich. The fluxof protons and helium decrease to a very low level. The poshighlylikelythat the oxygenmeasured hereis O+• (S. itive and reverse oxygen ions' energy dispersion events P. Christon,private communication,2000). A summary plot of the EPIC-ICS instrumentfor the are observedwith muchhigherintensitiesthan in the magnetosphere (theICS northdetector only). oxygen event on Januauy 10, 1997 is given in Figure adjacent of twooxygenpeakshavebeen 3. Plate 3 representsthe ion flux for different composi- The temporalsignatures tions (H, He, O) in two directions,south(EPIC-ICS 1 explainedas the resultof a Geotail out-and-inpassage spatiallayerof energy-dispersed sensor)and north (EPIC-ICS 2 sensor),in the first six througha quasi-steady images. The bottom plot showsthe electron integral oxygen ions[Zongand Wilken,1998]. For the time interval 1113to 1120UT (Geotail safielflux for two energybands (38 keV to infinity and 110 at about1113UT), hykeV to infinity). A simplifiedrepresentationof angles lite enteredthe magnetosphere and orientation
of the Geotail HEP-LD
and EPIC-ICS
sensorsfor the oxygen event is given in Figure 3. The
drogenandheliumionsareisotropic, theoxygen fluxreturns to the normal flux level observed in the magneto-
ZONG ET AL.- RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH
sphere,and the energeticelectronsstart to be detected, which is consistentwith the regionidentification.
3. Interpretation 3.1. Magnetopause Discontinuity
25,549 i
100.0
and Discussion
50.0
as a Rotational
From Plate 1, we know that the Geotail satellite crossedthe magnetopause into the magnetosphere at about 1113 UT. After the satellite entered the magne-
tosphere,the oxygenflux intermediatelydroppedto a
N
o.o
-50.0
low level.
In order to decidewhether the magnetopauseis closed
(tangentialdiscontinuity)or open (rotationaldiscontinuity), a principal axis analysis(PAA) was performed to study the characteristicsof the magnetopause,fol-
-100
.
0
'
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-100.0
'
I
'
100.0
•
I
'
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]
0.0
lowing the procedureproposedby $onnerup and Cabill
[1967]and Russelland Elphic [1979]. In this analysis the B• axisis definedby the directionof maximumvari-
I
-50.0 I
'
•
I
I
50.0
100.0
,
i
=
ance for the magnetic field during the time interval of ,
*
the crossing. The othertwoaxes,By andBz, areoriented along the directions with intermediate and minimum field variations, respectively. Figure 4 displays hodogramsfor the magnetic vectorsin the PAA coordinate system. The hodogramswere computed from 17 magneticfield vectorsobservedwhen the spacecraft crossedthe magnetopausebetween 1113:48and 1114:36 UT. The bottom hodogram in Figure 4 demonstrates the large variation in amplitude and the rotation of
50.0 =
=
.
=
_
=
.,
0,0-
-50.0
-
the vectorin the B•-B; plane. The directions of the principal axes in the GSE system are describedby the
followingeigenvectors:B•'(-0.8330,-0.5523, 0.0335); ß
*
By'(-0.4680, 0.6709,-0.5753) andBz'(-0.2952 , 0.4949, 0.87a). The ratio of the intermediate to minimum eigenvalue is 11.0, and I B• I/B is 0.16; hencethe normalcomponents of the magnetopauseboundary are very well established[Sonnerupand Cahill, 1967;Leppingand Be-
=
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Figure 4. Principalaxisanalysis(PAA)hodograms of the magneticvectorfrom 11!3:48to 1114:36UT in the ,
*
minimumvariance coordinate system(B•, By, andBz
in nanoteslas)with the axescorresponding to the maxharmon,1980]. The normalfield componentB• is 5.9 imum, intermediate, and minimum variances, respecnT, which is substantially different from zero. There- tively. B and E denotethe beginand end of the traces. fore the magnetopauseis a rotational discontinuity at this time, and energetic oxygen ions may flow out of the magnetosphereinto the magnetosheath along reconnected field lines.
3.2. Difference Between IMF
and the Magnetic
nents which were obtained from the Wind spacecraft. At that time the spacecraft was located at XGSE "" 88 RE upstream. This CME is manifested by a large
negative/positiveexcursionin IMF Bz [Burlagaet al., 1998]. Only duringthe negativeBz phasedo both planAs mentioned before, on January 10 and 11, 1997, etary Dst and ASY-H indicesshowsignificantactivity.
Field in the Magnet0sheath
a CME encounteredthe Earth's magnetosphere. The top two plots of Figure 5 display the ring current in-
The shaded area marks the interval in which the oxygen events were observedby the Geotail satellite when dexD st andthe asymmetry indexASY - H [Kawasaki it was located in the magnetosheath. and Akasofu,1971]. The asymmetricring currentinComparing Figure 5 with Plate 1, we can see that dex AS¾- H is definedas the range betweenthe max- the IMF Bx componentdetected by WIND is negative, imum and the minimum deviation in selectedground about -5 nT, however,in the dusksidemagnetosheath, magnetographtraces as a function of local time for the the B• is positive at about 20 nT as obtained by GEOhorizontal component in a dipole polar field. The bot- TAIL. The By componentin the magnetosheath also
tom three plots are the IMF Bx, By and Bz compo- showsa substantialdifferencecomparedto the IMF By
25,550
ZONG ET AL.' RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH 5O
-5(
lOO
5o
28 lO
lO
-lO
2o
lO
-lO -2O
00:00
12:00
10 Jan.
24:00
-
12:00
24:00
11 Jan., 1997 [UT]
Figure5. Interplanetary magnetic fieldcomponents (Bx,By,andB•) andplanetary index during a coronal mass ejection (CME)event onJanuary 10and11,1997.Thefirsttwoplotss
aregeomagnetic activityindices DstandA$¾- H. Thebottomthreeplotsshow theIMF Bx,
By,Bzcomponents. Theshaded arearepresents thetimeinterval inwhich energetic oxygen ions
weredetected whenthespacecraft waslocated in themagnetosheath.
component. The positiveB• component in the magne- A schematic of the magneticreconnection in the subtosheathmay implythat a large-scale magnetic field solarregionof the magnetosphere is givenin Figure6. reconfiguration is occurring nearthe dayside magne- The positiveB• componentis detectedon the southside topause, which led to the difference between Geotail and Wind observations.
of the neutral line in the magnetosheath.The southwardstreamingof the observed energetic oxygenionsis
ZONGET AL.. RINGCURRENTOXYGENIONSIN THE MAGNETOSHEATH
25,551
Magneto,phtic Oxygen IonLeak into the Magnetosheath aReconnect/on Ptoce z
Bx
-
Oxygen Ions :,,,Bx +
'•'
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200 key
1•
500 keV
ß
1000 keV
ß
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TO T1 T2 T3
Figure 6. Schematic of the reconnection process onthe dayside magnetosphere for southward interplanetary magnetic field(IMF). Ringcurrentoxygenionsleakintothe magnetosheath via a reconnection process. The motionsof magnetic fieldlinesandthepositions of the oxygenions released fromthe neutrallineat pointmarkedwith crossandtheirprogressive pointsareshown to scale.The relativemotionof Geotailthroughthe oxygenlayercaused by a solardynamic pressure pulse[Zongand Wilken,1998]is alsodisplayed.
consistent with an outflowof magnetospheric particles namicpressurepulseon January10, 1997 [Zongand alongreconnected fieldlineswith a largepitchangle Wilken,1998].The observed time-of-flight (TOF) ef(Plate2 andFigure3) anda reconnection neutralline fectsof oxygenionsare givenin Figure7. The upper existingnorthwardof the Geotailspacecraft. 3.3. Oxygen Energy Dispersion in the
plot in Figure 7 showsa reversedispersionfrom 1055 to 1102 UT, and the bottom plot showsa normal dispersion from 1107 to 1115 UT.
Magnetosheath
The spectraof magnetospheric oxygenarrivingat the magnetosheath on a given field line dependupon the An inverse energydispersion hasbeenobserved by
Geotailoutbound, namely, thelower-energy oxygen ions time elapsed after the field line was reconnected. The are detectedcloserand the higher-energy oxygenions relationbetweendistance(S) and the particle'sflight are detected furtherawaywith respectto the magne- time (At) is: topause;a normalenergydispersionhas beenseenby an inboundcrossing (seePlate3). The Geotailinbound
andoutboundmotionrelativeto the magnetopause in 30 min(1050to 1120UT) wascaused by thesolardy-
$- vat - •m Ecos OAt, where $ is the distance between the source and the
25,552
ZONG
ET AL.'
RING
CURRENT
OXYGEN
1102 UT:o.
,
-
IONS IN THE
MAGNETOSHEATH
, 1055 -- 1102 UT
-100
•
-.
-200
-300
-400
I
1000
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½:::: 600 •
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Figure 7. Therelationof thedelaytimeand1/vZ-•measured in different energychannels by both HEP-LD (solidcircles)and EPIC-ICS (crosses)instrumentson board Geotail satelliteon January 10, 1997. The best fits of those points are given by solid lines.
observedpoint, E is the particle's energy and 0 is the pitch angle. For oxygenions the relation betweenthe particle's energy and the distanceto the sourcecan be simplified as 58.2
cos0
•At,
signatures,the position of the sourceregion can be estimated at a distance of S = 12.8RE away from the Geotail satellite, assumingthat the reconnectionposition is not changedduring this oxygenevent. This is consistentwith both the magnetopauseand bow shock as the source location
if we consider that the resolution
of the pitchangleis +7.50. wherethe distance(S) is in RE, andthe energy(E) in
It can be arguably ruled out that the solar wind is the sourceof the presently describedenergetic oxygen The time delay At of the reversedispersionfrom 1055 ions in the dusksidemagnetosheath: The mechanism to 1102 UT is relative to 1102 UT, when the particles of the quasi-perpendicularshock drift accelerationcan with the highestenergy are observed;the time delay only acceleratea seedsolarwind populationto peak enAt of the normal dispersionfrom 1107 to 1115 UT is ergiesof about 10 keV becauseof the limited drift diskeV.
relativeto 1107 UT, when the particleswith the high- tanceat the Earth'sbowshock[Schwartzet al., 1983]. est energy are observed. From Plate 2 we can see the The Fermi mechanism,operating at the quasi-parallel oxygenions having fine beam-like structure with an av- bow shock,can accelerateincident solarwind ionsto en-
eragepitchangleof -• 84ø. Tracingbackthe dispersedergiesup to about 200 keV with an isotropicangulardis-
ZONG ET AL.' RING
CURRENT
OXYGEN
IONS IN THE MAGNETOSHEATH
Asymmetc Storm
25,553
RingCurrent
x Escaping Oxygen ions (About 33%)
6.1X1• IonsJs Dusk
Dawn
Y i ß
lccons
Ions
CRRES
Observation:
D,t - 100nT
NENL
1.6 x 1028 Oxygen ions Input rate: 1.Sx102•/S
Figure 8. A view of the magnetosphereand magnetosheathin the ecliptic plane. Drift paths of energetic magnetosphericions intersect the dusksidemagnetopause. During the conditions of a southward magnetic field in the magnetosheathand a rotational discontinuity in the magnetopause, energetic ions will be lost into the magnetosheathand the ring current becomes asymmetric. NENL denotes near Earth neutral line.
tribution[Lee,1982].Suprathermal 0 6+ ionscouldbe because the particledistributions arestronglyanisotropi reflectedefficientlyfrom the quasi-parallelshock,where rather than isotropic. However,daysidereconnection they gain enoughenergyto be subsequentlyreflectedat may opena path for magnetospheric ionsescapinginto the quasi-perpendicularshock. After five to ten reflec- the magnetosheath.
tions at the quasi-perpendicular shock,0 6+ ions can reachMeV energies,whereasproton and alpha particles only reach several hundred keV through this process. However, this model strongly relies upon the fact that a singlefield line can be connectedthe shockin a quasiperpendicular configuration at one time and within a few minuteschangesto a quasi-parallelconfiguration,or
As we know,driftingionsin the magnetosphere with pitchangle900wouldpreferentially leakfurtherout in radial distance and further toward the dusk meridian
[Bakeret al., 1988;Wilkenet al., 1986]. The higherenergyparticles,becauseof their larger gyroradius,extend further into the magnetosheaththan the lower-
energyparticles[Scholeret al., 1981].The oxygenpar-
viceversa[Freemanand Parks, 2000]. The accelerated ticles stream away from the magnetopause alongthe ions may be convecteddownstream by the solar wind reconnected field lineswith pitch anglesup to 90ø. An flow in the magnetosheath under nominal solar wind energetic magnetospheric particlewith 90o pitchangle conditions. As meant in section 2, the magnetosheath whichcrosses the magnetopausevia a reconnectionconfield is rather steady and the IMF magneticfield configuration is not changedduring the oxygen events;therefore it is highly unlikely that the very anisotropicand energeticoxygenions originate at the bow shock. Leakageof trapped magnetosphericoxygen into the magnetosheathby a diffusiveprocessis highly unlikely,
figurationwill almostkeepits pitchangle[Daly,1982], sincea very limited energycan be gainedfrom the reconnectionprocess.The magnetictensionat the magnetopauseappliedby reconnectioncanonly add a velocity of the order of the Alfv•n velocity,associatedwith the changein B acrossthe magnetopause.For a 50 nT
25,554
ZONG ET AL.: RING
CURRENT
OXYGEN
IONS IN THE MAGNETOSHEATH
field rotatedby 180ø, AB is 100 nT. Assuminga typ- the negativeBz phaseof the CME, thesering current ical hydrogenplasmadensityof 20 cm-3, the Alfv•n ions can be leaked along reconnectedfield lines in a velocityis m 490 km/s. For oxygenionsthis amounts processwhich doesnot necessarilyproduceextra accelto an energy gain of about 20 keV. However, the ob- eration[Scholeret al., 1981]. The observations in this servedoxygenions have energiesabove 140 keV, which means that an acceleration from reconnectionis quite negligible. If magnetosphericparticles escapefrom the reconnectionregion as suggestedin Figure 6, Geotail will observeenergy-dispersed particlesmovingalongthe field lines. In this framework an energy dispersioncan
paper suggestthat part of the oxygenions leak from the ring current into the magnetosheath.This is con-
sistentwith a strongasymmetricring current [Korth et al., 2000]. The secondplot of Figure 5 showsthe asymmetric ring current index ASY- H for January 10 and 11,
be explainedasa time-of-flight(TOF) effect[Sarrisand 1997. The maximum values of the ASY- H index are detectedduring the main phaseof the magneticstorm. Axford, 1979]as follows. The reconnectedfield lines carrying energeticoxygen This strongasymmetricring current may be causedby ionsmove northward and southwardaway from the neu- the lossof the ring current oxygenions. In this storm tral line. Oxygen ions on a given field line are acceler- the oxygenescapefrom the daysidemagnetopause into ated more or lesssimultaneously,so the faster particles will be at any time further away from the reconnection region. The particles are releasedfrom the neutral line at To, and at subsequenttimes T1, T2, T3 the reconnected magnetic field lines move away from the neutral line. The energetic particles are energy dispersed along these field lines becauseof the different veloci-
themagnetosheath isoftheorderof 0.61x 1023ions/s, assumingthat the bulk velocity of the oxygen beam
is v • 500 km/s and the densityp - 0.3 x 103m-3. The crosssection of the oxygen beam is assumedas
A = 5 RE2 (RE = 6371km (Earthradius)).Thisis about 33% of the total oxygeninput rate of the ring current as preliminarily estimated from measurement
ties of the oxygenions (seeFigure 6). In this casean on the CRRES satellite during a moderatemagnetic inverse energy dispersionwill be observedby Geotail storm[Fu, 2000]. outbound, and a normal energy dispersionwill be seen by an inbound crossing.It should be pointed out that 3.5. Why Only Oxygen Ions and No Hydrogen
the IMF at that time has a significantdawn-duskBy
and Helium
Ions
Are
Observed
component, so that the magnetic tension will causean The bounceperiodof a 250 keV oxygenion in a dipole east-westmotion on the most recently reconnectedfield field at L=8 is about 88 s, which meansno particles
lines [Gosling,1990]. This meansthat the energydis- shouldbe observedon openfield lines(reconnected field persionmay extend in the GSE Y- Z plane rather than lines)if thereis no further supply.However,the partithe X- Z plane. clescan be suppliedby longitudinal drift, if the extent The two temporal dispersion signatures of oxygen of the reconnectionregionis not muchlarger than the
ions from
1055 to 1102 UT
and from
1107 to 1115 UT
longitudinaldrift per bounceperiod[Scholer,1983].If
in Plate I are interpreted to be the result of an out- the reconnectionregion is large in longitudinalextent, and-in passagethrough a quasi-steadyspatial layer of again no particles should be seen on field lines in which energy-dispersedoxygen ions by a reconnectionprocess the distancefrom the boundaryof the reconnectionre-
persistingfor at least 20 min (after crossingthe magnetopause,Geotail lost contactwith the oxygenlayer). Together with the rather steady magnetosheathfield observedby Geotail satellite throughout all the events,
gionis largerthan the longitudinaldrift path within one bounceperiod. The drift of a 250 keV oxygenin a dipole fieldduringan oxygenbounceperiodleadsto a longitudinal extentof 2.77 RE, or about 20ø. The bouncetime the resultsmay imply that reconnectioncan be a steady of a 250 keV proton in a dipole field at L-8 is about process. 24.8 s. The longitudinaldrift during the abovebounce periodleadsto an extent of 0.8 RE, whichis about 50 3.4. Asymmetric Ring Current in longitude. The particleswith pitch anglescloseto to the (longitudinal) boundary A schematic modelof the asymmetric stormtimering 900wouldbe lostclose region,whereasparticlesmirroring current is given in Figure 8. Drift paths of ener- of the reconnection getic magnetosphericparticles with large pitch angles at lower altitudes could be observed well within the re-
(• 90ø)willintersect theduskside magnetopause [Roed-connectionregion. This effect could also lead to a fieldeter, 1967]. During a suddenmagnetospheric com- aligned anisotropyon open field lines. pression,as might happen during a magnetic storm, If the longitudinalextent of the reconnection region the magnetopause movesinward,and normallytrapped is larger than 0.8 RE (proton gradientdrift during a magnetosphericion drift paths temporarily opento the bounceperiod) and lessthan 2.77 RE (oxygengradimagnetopause [Wilkenet al., 1986;Sibecket al., 1988]. ent drift during a bounceperiod), then all protonson Because ions drift westward but electrons drift eastthe reconnected field lines will be lost into the magward, most of the ions may be expectedto leak from netosheathin one proton bounce period becausethey the dusksidemagnetosphere,and electronsmay be ex- cannot be suppliedby the gradient drift. Oxygen ions, prected leak from the dawnsidemagnetosphere.During however,will be observedcontinuallybecausethey are
ZONG ET AL.' RING CURRENT OXYGEN IONS IN THE MAGNETOSHEATH
resuppliedby the oxygenion gradient drift. Therefore only oxygenions and almost no hydrogenand helium ions can be observedin the daysidereconnectionregion. In general, the region in which magnetosphericring
25,555
the neutral line. The escapedmagnetosphericions can be resuppliedby the gradient drift in the inner magnetosphere. However, this longitudinal extent may lead to
only oxygenions (no hydrogenand helium ions) being
currentparticles mightescape alongmerged fieldlines observedby Geotail HEP-LD. The escapedoxygen flow was clearly tailward with a dawn to dusk component; these oxygen ions may also the stagnationpoint [Russell,1995]or subsolarpoint reach upstream of the bowshockbut that depends on [NishidaandMaezawa,1971]. Cowley[1982,1995]has the magnetic field configuration in the magnetosheath. pointed out that the reconnectioncannot continuously Furthermore, the estimated oxygenescaperate is 0.61 x take place acrossall longitudes;reconnectionsignatures 1023ions/s,whichis about33%of the total oxygen inoccur in only 25% of all magnetopausecrossingsun- put rate of the ring current. If the O+ ionscontribute der southwardmagnetic field conditionsin the magne- 60% of the energydensityto the ring current [Daglis tosheath. This implies the longitudinal extent of the et al., 1999], the escapedions lead to at least a 20% neutral line is about 6.28 RE if the subsolar position asymmetry of the stormtime ring current. In order to is located at 8 RE as estimated by Zong and Wilken get a whole picture of the escapingring current oxygen [1998].In the reconnection region,nearly 12.7%of the ions,for example, the planetary and interplanetary consize of the neutral line can be resupplied by the proton ditions, escapingposition, etc., a statistical study will drift, whereas44% of this size can be resuppliedby the be made in future work. oxygendrift. Thus the appearanceof ring current particles in the reconnectionregionwill dependstronglyon Acknowledgements. This work was supportedin part by the portion of the reconnectionregion encounteredby ChineseNSFC (Grant No. 49984002 and49834040) andthe ChineseKey ResearchProjectof G20000784. the spacecraft.
is limited by the longitudinal extent of the neutral line. As we know, the reconnection may take place near
Janet Luhmann thanks the referees for their assistance in
evaluatingthis paper.
4. Summary and Conclusion
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(Received April 3, 2000;revisedDecember 6, 2000;
December6, 2000.) Potemra, Energetic magnetosphericions at the dayside accepted
