GEOPHYSICAL RESEARCH LETTERS, VOL.26,NO.22,PAGES 3349-3352, NOVEMBER 15,1999
Bursty Energetic Oxygen Events in the Dayside Magnetosheath: GEOTAIL Observations Q.-G. Zong and B. Wilken Max-Planck-Institut fiir Aeronomie,Katlenburg-Lindau,Germany
Abstract. with
Two energetic oxygen ion enhancements drift westward but electronseastward, most of the ions
a clear anti-sunward
flow were observed in the may be expectedto leak from the dusksidemagnetodaysidemagnetosheathwhen GEOTAIL waslocatedat sphere,and electronsleak from the dawnsidemagneto(4.3, 16.3, 0.7) Rz on 26 January 1994. The oxygen sphere. ions peaked at energiesof about 300 keV. The comOxygen ion beams in the energyrange 0.1 to 17.1 mencementof the two oxygen enhancementsat 1405 keV have been observedin the low latitude boundary and 1440 UT were in coincidence with two substorm layer by AMPTE/CCE during someperiodsof northinjectionsat 1349 and 1422UT. The oxygeneventsare ward IMF on Oct. 6, 1984. This may suggestthat characterizedby strongimpulsiveincreasesin the flux the oxygen population is injected into the dayside low whereasenergeticprotonsand helium ions were essen- latitude boundary layer directly from the high latitude tially absent. The magnetosheathfield projectionin the ionosphere [Fuseliefet al., 1995]. On rare occassions x-y plane (GSE) wasalmostconstantand pointedinto energetic O+ ionshavealsobeenobserved upstreamof
thedusk/tailsector forming anangleof135oto theSun-
the Earth's bow shock. A brief burst of these O + ions
Earth axis. The field magnitudewas 35 nT with rather little fluctuations and the Bz component was negative. These observationsshowthat energeticoxygenions can leak out of the magnetosphereinto the magnetosheath duringisolatedsubstormactivity (stormconditionsare not required). Enhancedsolar wind dynamic pressure compressedthe magnetopauseinward and may have helped to observeenergeticoxygenions to leave from the magnetopause current sheetthus escapinginto the magnetosheath.
with a duration
of about 20 minutes had been detected
on Sept. 5, 1984,by the AMPTE/IRM spacecraft.This shortO+ burst clearlyshowsthe characteristics which should be expectedfor ions leaking out of the magnetospherewithout strong interaction in the foreshock region[M6biuset al., 1986].This is in agreementwith the resultsby Krimigis et al. [1986]. Recently, GEOTAIL observationsshowedthat energeticions (oxygen,helium,and hydrogen)leak out of the inner magnetosphereand can form layers in the magnetosheath (in the vicinity of the magnetopause) during intensestorm activities. Energeticoxygenion 1. Introduction enhancementsare frequently detected in the magnetosheath and show occassionallya total duration of It is now well known that both the solar wind and about 150 min [Zong and Wilken, 1998; Zong et al., the Earth's magnetospherecontribute to the energetic 1999]. ion populationsof the magnetosheath.The strength of In this paper, we report two energeticoxygenion enthe two sourcesvaries accordingto geomagneticactivhancementsdetectedin the daysidemagnetosheathon ity and interplanetaryconditions[Sibecket al., 1987]. January 26, 1994. The ion enhancementswere observed The magnetosphere is a su•cient reservoirof energetic at 1405 and 1440 UT in near coincidence with substorm particles that are energized mainly in the tail during substorms anddrift to the dayside.Roederer[Roedeter, injectionsat 1349and 1422 UT. The Dst index wasonly 1970]showedthat the drift pathsof energeticparticles around-20 nT on this day. These observationssuggest that oxygen ions can escapefrom the inner magnetoin the outer magnetosphereintersectthe magnetopause. Not all of the particleswhich follow thesedrift paths can sphereinto the magnetosheathduring isolatedsubstorm activity.
successfullydrift through the daysidemagnetosphere.
Magnetospheric energetic particleswith large(• 90ø) pitch angles provide the dominant sourceof particles 2. Observations
availablefor escape.Greenspan et al. [1985]notedthat the 90o pitchangleparticlesin substorm injections fail
The data to be presentedare obtainedby the HEP-
to traversethe noon meridian, indicating that they have LD instrument onboardtheGEOTAILspacecraft [Doke been lost at the magnetopause.Becauseenergeticions et al., 1994]. The HEP-LD instrumentenergyranges Copyright 1999bytheAmerican Geophysical Union. Papernumber1999GL003634. 0094-8276/99/1999GL003634505.00
for hydrogen,heliumand oxygenaxeapproximately75 - 4000keV, 100- 4000keV, and 200- 1500keV, respectively. It shouldbe notedthat a lowerenergyband 30 to 75 keV for hydrogenand 70 to 100 keV for helium 3349
3350
ZONG AND WILKEN: ENERGETIC OXYGEN BURSTS IN THE DAYSIDE MAGNETOSHEATH
are identifiedby the time-of-flight T signalonly because the energyis outsidethe nominal detector range. Figure I givesan overviewof the HEP-LD and MGF measurements from 1300 to 1600 UT on January 26, 1994, which showsenergetic oxygen, helium, and proton ion fluxes, the particle flow directions in the equatorial plane, together with the magnetic field in GSE coordinates.
Electron
, o?
Flux
(1994
Jan.
26)
oe
•
, o•
•
104
•
....
1
•
:
!o--•i-
•__ ..................
:LT;UT--
I I 0
'los r
.io•'
,, .....
,,z., lO -s
The top panel in Figure I showstwo energy-integrated ion countingrates versustime profiles. The dashedline labelled "All Ions" representsa rate summedover all ion speciesand energies,usuallydominatedby protons;the solid line labelled CNO refers to oxygen ions summed over all energies. The secondpanel showsenergy integrated rates for selectedparticle species(solid and dashedlines for hydrogenand helium, respectively). Two oxygen bursts in the interval 1400 to 1500 UT havebeenobserved,the first oxygenburst beganat 1404
! 8g
_
.................
LT--UT*0.6
-: •0 2
............. 13
14
15
T;rI'Ie
[UT
16
]
Figure 2. Energeticelectron fluxes in five or six differential energy channels,respectively,versustime obtained in the geostationary orbit by four Los Alamos
NationalLaboratory(LANL) spacecrafton January2.6,
1994. The two substorm onsets are marked by dashed lines and the beginningsof the two oxygenburstsin the UT and lasted about 10 min, the secondburst started at magnetosheathis indicated by solid lines. The space1440 UT and lasted 20 min. The intensity of the second craft designationsare shown on the left side. burst was much stronger than that of the first one, the countingrate wasalmostoneorder of magnitudehigher.
At this time, the countingrate of H+ and He ionsare
jected anisotropiesof theseions coincideswith the magnetic field directionin the X-Y plane (note: the angular distributionsshowlook directions!).The flow was tailward with a component in the duskward direction. In the time interval from 1300 to 1600 UT, the magimuthal angular distribution of oxygenions in the magnetic field in the magnetosheath(the region identifinetosheathare about 1800, indicatingthat the procation is based on the plasma data, no shown here) was almost stable at 35 nT, although the magnetic field componentsvaried significantly. The azimuthal direcMPA½/UoB Geotail HEP-LD 26.01.1994 WU Mode: s/•+T, ed_B, •t----I rain tion of the magnetic field is also quite stable as shown by the small circles in the third panel of Figure 1. It shouldbe noted that the two oxygenburstsoccurredin intervals with negative Bz component. The commencementof the two oxygenbursts at 1405 and 1440 UT appears to be in close coincidencewith substorm onsets at 1349 and 1422 UT, as documented by the geostationaryelectron flux variations in Figure very low. The projected anisotropiesof the oxygen ions was shown in panel 3 in Figure 1. The angles between the magnetic field direction and the center of the az-
2 which shows the differential
electron
flux versus time
from the SOPA instrumentson four Los Alamosgeosynchronoussatellites on January 26, 1994. The energy channelscoverthe range between30 and 315 keV (for
detailsconsult[Belianet al., 1992]). Figure 2 showsfor the satellite nearest to local midnight (1987-097), between 1300 and 1349 UT, the familiar UT X
Y Z
300 4.3
15.1 0.8
1330 3.9
15,7 0.7
1400 3.4
16.3 0.7
1430 3.0
16.9 0.7
1500 2.6
17,4 0.7
1530 2.2
17.9 0.7
1600 1.8
18.4 RE O.7 OSi5
feature
of a slow electron
flux
decline
over 30
min usually associatedwith a substorm growth phase. Then, at 1349 UT and 1422 UT, two particle injection events were observed.
After
1450 UT
the fluxes de-
Figure 1. An overviewof HEP-LD and MGF data creasedto normal level during the substorm recovery from 1300 to 1600 UT, January 26, 1994. From the phase. top the panelsshow: Integral countingrates 'All Ions' Figure 3 showsproton, helium, and oxygen energy and 'CNO Ions'; counting rates for hydrogen and hespectra obtained for the second strong oxygen burst lium; colour-codedazimuthal intensity distributions of the all ionsrate (look direction);GSE components and event. In contrast to the flux profiles in 1, the enmagnitudeof the magneticfield (in nT). The circlesin ergy distributions extend to somewhat lo•er speciesthe third panel mark the magnetic field direction pro- dependent energies. As can be seen from this figure, jected on the equatorial plane. hydrogen and helium show a falling spectrum whereas
ZONG AND WILKEN: Geotai
I
DE.
Mt'•de:
Ed__B.
26.01.
HEP-LD •../E-.*-T.
D•'f=
ENERGETIC 199.4
i .,:• :.,•l. oX--2.8.
i kV
OXYGEN 1 _-•;: oo
Y--17.2.
•----•).7
BURSTS IN THE DAYSIDE
kit RE
Legende
lO
.•l
o
2
Oxygen
I 0ø
10-2
•
......
•'6o '
..... i
F•nergy
+
i b•o (keY)
MAGNETOSHEATH
3351
and the occurranceof oxygenions in the sheathis consistentwith the time taken (about 16 min) for oxygen ions with 200 keV to drift from the mid-night to the dayside. 2. On Januaray 26, the magnetopausewas strongly compressedby the enhancedsolar wind dynamic pressure. This can causenormally trapped magnetospheric ion drift pathsto temporarilyopento the magnetopause
[Wilken et al., 1986]. In fact, on January 26, 1994, the solar wind speedincreasedcontinuallyfrom 350 to 560 km/s, the solarwind ion densityenhancedfrom 5
to a maximumof 25 cm-a (thesedata aretakenfrom
/omniweb/form/dx.html). Figure 3. Differential energy distributionsfor supra- http: //nssdc.gsfc.nasa.gov thermalH+, He++ and0 + ionsintegratedoverthe time Therefore, the radial position of the magnetopauseat of the secondoxygenevent in Figure 1. the subsolarpoint wascompressed to 7.2 Re (the averagepositionis 10.8 Re) for the peak dynamicpressure value. This displacementis in good agreement with the oxygenspectrumhasa well developedpeak at about GEOTAIL observationsat the magnetopause. Under 300 keV. In case all particle speciestravelled with the this compressed condition,energeticparticlesfrom the same velocity as oxygen, then the proton peak would inner magnetospheremay be captured by the magnebe expected at 20 keV and the helium peak at 80 keV. topausecurrent sheet which tends to be thickenedby Both valuesare below the energywindow of the instruthe contribution of magnetosphericenergeticions. ment. This notion is consistentwith correspondingobTypical valuesof O+ contributionto the ring curservations of the EPIC instrumenton GEOTAIL (privat communicationwith D. J. Williams, 1999). The oxy- rent energydensityare • 6% during quiet time and gen flux beyond 150 keV exceedsthat of hydrogenand morethan 50%duringgreatstorms[Dagliset al., 1999]. Cowley[1982]has pointedout that the magnetopause helium ions. may actully be a two-scalelayer, one scale is about ..• 100km (a gyroradius of typicalsheathions)for the cur3. Interpretation and Discussion rent provided by the magnetosheathions, the other is The solar wind is most likely not the source which • 500 km by the ring current ions (if the ring current providedthe observedoxygenionsin the dusksidemag- speciesis dynamicallysignificant). An ion moving in netosheath,becauseof the followingreasons:(1) The the magnetopausecurrent sheetwill followa doublespiof 2 Larmor radii (Speiser mechanism of a quasi-perpendicularbow shock drift ral orbit with a displacement because would accelerate a seed solar wind population to peak orbit[Speiser,1965]) into the magnetosheath energiesof only about 10 keV, the pitch angleof these of the northward magnetic field in the magnetosphere
particlespeaksnear90ø. (5•) The Fermimechanism, operatingat the quasi-parallelbow shock,would accelerate incident solar wind ions to energiesup to about
200keV [Lee,1982].The accelerated ionsconvectdownstream by the solar wind flow, but mostly in the dawnsidemagnetosheath.In this case,the azimuthal angular distributions should be isotropic rather than strongly
anisotropicas observedby the GEOTAIL HEP/LD instrument. Furthermore, the solar wind source will not supplyoxygenionsin sufficientnumbers,sincethe oxygen abundancein the solarwind is lessthan 0.05%. A reasonableconclusionis that the observedoxygenions originated in the magnetosphere. The followingis an intuitive mechanismfor energetic particle escapefrom the magnetosphereinto the magnetosheathwhen the magnetopauseis compressedby the increaseddynamic pressureof the solar wind. A simplifiedschematicis shownin Figure 4. 1. Injected magnetosphericions drift to the duskside whereaselectronsdrift to the dawnsideduring a substorm activity (Note: the presentcaseis not a stormtime substorm, the Dst index was only around-20 nT, Kp was3) (Figure4a). The timeelapsedfromthe observation of the electroninjection by the LANL spacecraft
(c)
Figure 4. A schemefor ion leakagecausedby enhanced solarwind dynamicpressure.(A) the normal magnetopause with the thicknessof a gyroradius of sheath
ions; (B) the magnetopause is thickenedby captured ring current ions if the magnetopauseis compressed; (C) the oxygenionsleavefrom the magnetopause when the magnetopauseis further compressed.
3352
ZONG AND WILKEN: ENERGETIC OXYGEN BURSTS IN THE DAYSIDE MAGNETOSHEATH
and the southwardmagneticfield in the magnetosheath (Figure 4b). 3. Oxygen ions may have a better chanceto leak to the sheath from the magnetopausecurrent sheet if the magnetopauseis sufficientlycompressed.The gyroradiusof an oxygenion with an energyof 300 keV is about to= 5312 km in a 60 nT magneticfield, which is about ten times of the thicknessof the magnetopause. The large gyroradiusof oxygenionsallowsthem to accessthe magnetosheathand to escapeinto the sheath. This may explain the absenceof protonsin the oxygen bursts(seeFigure 4c). The above concept offers an explanation for the occurrenceand compositionof so called oxygenbursts in the dayside magnetosheathunder certain conditions. On the other hand, field line reconnection, as an alternative, would provide particle accessto the magnetosheath as well. The outflow speedin the order of the
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O+ ionsduringan upstreamparticleevent,Geophys. Res. with a clear anti-sunwardflow and energiespeaked at Lett., 13, 1372-1375, 1986. about 300 keV wereobservedin the magnetosheath.At Roederer,J. G., Dynamicsof geomagnetically Trappedradithis time on 26 January,1994,the GEOTAIL spacecraft ation, Springer-Verlag,New York, 1970. was locatedin the magnetosheathat 17:00LT and at a Sibeck,D. G., R. W. McEntire, A. T. Y. Lui, R. E. Lopez, radial distanceof 16 Re. The oxygeneventsare charS. M. Krimigis, R. B. Decker,L. J. Zanetti, and T. A. Potemra, Energeticmagnetospheric ions at the dayside acterizedby strongimpulsiveincreasesin the flux. magnetopause: Leakage or merging?, J. Geophys.Res., (2) The commencement of the two oxygenbursts were observed at 1405 and 1440 UT in near coincidence
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Speiser, T. W., Particle trajectories in model current with substormonsetsat 13:49and 14:22UT (measured sheet;1. Analytic solution, J. Geophys.Res., 70, 4219,
by geostationaryelectroninjections),approximately16
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and 18 min delayedrelative to the respectivesubstorm Wilken, B., D. N. Baker, P. R. Higbie, T. A. Fritz, W. P. Olson, and K. A. Pfitzer, Magnetosphericconfiguration onset. These observationssuggestthat energeticoxyand energeticparticleeffectsassociated with a ssc:a case genionscan alsoleak out of the magnetosphere into the studyof the CDAW eventon March 22, 1979,J. Geophys.
magnetosheathduring an isolatedsubstorm(a storm
Res., 91, 1459-1473, 1986.
conditions are not necessarilyrequired as suggestby
Zong,Q.-G.,andB. Wilken,Layeredstructure of energetic [Zongand Wilken,1998;Zonget al., 1999]). oxygenionsin the magnetosheath, Geophys. Res.Lett., 25, 4121-4124, 1998. (3) Duringthe time intervalof the oxygenburststhe T. Doke,and magnetosheath fieldprojectionin the x-y plane(GSE) Zong,Q.-G.,B. Wilken,J. Woch,G. Reeves, T. Yamamoto, Energetic particle bursts in the near-Earth wasalmostconstantly pointinginto the dusk/tailsector magnetosheath duringa storm,Physics andChemistry of formingan angleof 135o to the Sun-Earthaxis. The the Earth, ϥ, 293-298, 1999. field magnitude in the sheath was 35 nT with rather little fluctuations.
B. Wilken and Q.-G. Zong, Max-Planck-Institut (4) Enhancedsolarwind dynamicpressuremay have flit D-37191, Katlenburg-Lindau, Germany. (e-mail: helped to causeenergeticoxygen ions of polar iono-
[email protected];
[email protected]) sphericorigin to escapefrom the inner magnetosphere
into the magnetosheath. Acknowledgments.
We acknowledge the effort of teams
who made important data sets available on web sites such as
http:// cdaweb.gsfc.nasa.gov/cdaweb (Dst, IMF Bz, and solar wind density)and http://leadbell.lanl.gov/ (geostationaryelec- (ReceivedJuly, 7, 1999; revisedAugust 27, 1999; acceptedSeptember1, 1999.) tron data).
