GEOPHYSICAL
RF.
SEARCH
LETFERS,
VOL.19,NO.2, PAGES
79-82,
3ANUARY
24,1992
THE ABUNDANCE OF O++ IN THE JOVIAN MAGNETOSPHERE
F. Bagenall,
D. E. Shemansky
2,R. L. McNutt
Jr.3,R. Schreier
4 andA. Eviatar4
Abstract. From a synthesisof data from the PlasmaScience
andUltraviolet Scienceinstrumentson the Voyager 1
spacecraftwe presenta radial profile of O++ abundance
between4.9 and 42 Rj. We observea sharprise in O++
mixingratio near 7.5 RI, coincidentwith a sharprise in
effectiveelectrontemperature
at the outerboundary
of theIo •
plasmatoms.Beyond8.5 Ri the O++ mixingratiois foundto
beroughlyconstant
whichindicatesfreezingof the ionization
prevailing
attheouteredgeof thehottoms.
Rj. Ouraimis to testthehypothesis
thattheplasmain the
sheethasdiffusedoutfromIo andrepresents
a frozen-in
pictureof thecomposition
andionization
stateof thehottoms.
The PLS Observations
The Voyager PLS instrumentconsistsof four modulatedgridFaradaycupswhichmeasureion andelectroncurrentsin
anenergy-per-unit-charge
rangeof 10 to 5950V (Bridgeet al.
[1977]).Whentheplasma
flowisof highsonicMuchnumber
anddirectedinto the sensors,the spectralpeaksfor the
different ion speciesare well-resolved and the detector
Thequestion
of theionization
partitioning
stateof theplasma responseis quite straight forward, allowing accurate
in the magnetosphere of Jupiter has not been resolved
determination
of thedensity,
temperature
andflowvelocityof
satisfactorily
despitethe PioneerandVoyagerflybysand eachspeciesin the cold regionof the torusinside5.6 Rj
intensiveandextensiveremoteobservationsby meansof
(Bagenal
andSullivan
1981;Bagenal
1985)andin regions
of
telescopes
at groundobservatories,on board rocketsand from
theplasmasheetin themiddlemagnetosphere
(McNuttet al.
Earth orbit. Peculiarities in the distribution of ions have raised
1981).Determination
of thecomplexresponse
of thedetector
basic issues for the mechanics of sourceprocesses, to trans-or sub-sonicplasmaflow has allowed analysisof
subsequent
physicalchemistryand masstransport. Of
suprathermal
components
of the ion spectraand spectra
particular interest is the apparent discrepancyin the
obtained
in between
theseregionsof coldplasma(Sandsand
Introduction
determination
of the abundance
of O++. The presence
of O++
ionsin the cold torus with a massto chargeratio of 8 was
originallynotedin the VoyagerPlasmaScience[PLS]dataat
thetimeof the Voyager1 encounter
by Bridgeet al. [1979]
(andfurther discussedby Bagenal and Sullivan [1981];
McNuttet al. [1981]; Bagenal [1985]; Bagenal[1989]).
AlthoughVoyager observationsindicate the existenceof
McNutt, 1988).
Figure1a showsa spectrum
obtainedat a distanceof about
5.1 Rj fromJupiterexhibitingthe3 majorspectralpeaksfor
corotating
ionswith mass/charge
ratios(A/Z) of 8, 16 and32.
The A/Z=8
ion is considered to be O++ rather than S4+
becauseof the absenceof S3+ at A/Z=32/3. There is also a
minorpeakat A/Z=64 whichis presumably
dueto eitherSO2+
significant
quantities
of O++ions,ground-based
spectroscopicor S2+. Whenthe two outermajorpeaksof the spectrum,
observations
by Brownet al. [1983]of the5007]i lineof OIII
obtained
negative
results.
Theyplaceafirmupperlimitof less
than
4 cm-3ontheO++number
density.
Morerecently,
Moos
etal. [1991]reporta low leveldetection
of emission
fromonI
at1661,1666/•bytheHopkins
Ultraviolet
Telescope
in
December
1990.TheyinferthatO++comprised
about2% of
thetomsiondensity.
which correspond
to O++ andS+ respectively,were fitted
independently,
the two ionicspecieswere foundto havethe
sametemperature
(to within 1%). This is notsurprising
asthe
time scalefor ion-ionthermalequilibrationis only aboutan
hour for these plasma conditions. The middle peak
corresponds
to ionswith A/Z=I 6, presumablya combination
of O + and S++ ions. If these ions with A/Z=I
In thisLetter,we shallpresent
a radialprofileof the O++
abundance
thatis theresultof combiningVoyager
1 PLSdata
fromtheinnertoms(4.9-6Rj) andtheplasmasheet
(11.7-42
Ri)witha recent
analysis
ofspectra
obtained
bytheVoyager
1
Ultraviolet
Science[UVS] instmment
between5.75and8.25
6 are also in
equilibriumandsharethe sametemperature,then the S++
energydistributionwouldappearto havehalf the width of the
O+ distribution.If oneassumes
thermalequilibriumone can
then estimate the relative contributions of S ++ and O + ions to
the A/Z=I 6 peak.The spectrumin Figure la illustratesthat
underthisassumption,
the spectrum
canbe fittedvery closely
with the A/Z=I 6 peak dominated by O+ ions. At higher
energiesthereappears
to be a smallsuprathermal
tail to the ion
1Astrophysical,
Planetary
& Atmospheric
Sciences
Dept.,
spectrum.Usingthe sub-sonicresponseof the detector,we fit
Universityof Colorado,Boulder,Colorado
this tail with a Maxwellinn distribution of hot O+ ions which
2Lunar
andPlanetary
Lab.,University
ofArizona,
nowat
havea temperature
of about100eV but compriseonly2% of
the total plasmadensity.The compositionof the cold toms
reportedby Bagenal(1985) can be summarizedas equal
proportionsof S+ and O+ with S++ and O++ eachhaving
Aerospace
Engineering,
University
of Southern
California
3Visidyne
Inc.,Burlington,
Mass
4Geophysics
& PlanetarySciencesDept., Tel Aviv
Papernumber92GL00070
mixing ratiosof 5%.
Outside5.6 Rj, wherethe plasmaflow becomestrans-sonic,
the spectralpeaksof different ion speciesare generallynot
resolvedby the PLS instrument.Thusthe PLS insturmentwas
0094-8534/92/92GL-00070503.00
not ableto determinethe compositionin the warmregionof
Copyright
1992bytheAmerican
Geophysical
Union.
79
80
Bagenalet al.: The Abundance
of O++ in theJovianMagnetosphere
106
.
S3+
o"
lO
•0 +
" '
"-----•_SO•
lO
•o•
•
•
•
soo
,, ,,
• o•
........
•
•o
•o
•oo
s•
•Q (Volts)
• •o
•ooo
•Q (Volts)
Figure1. Voyager1 PLSionspectraat (left) 5.1 and(right)27 Rj. Thedata(histogram)
havebeenfittedwith a convected
Maxwellianfunction(smooth
curve)for eachionspecies
to produce
a bestfit thedata(crosses).
the toms.Beyond11.7 Ri, however,the Voyagerspacecraft
traversedregionsof supersonicplasmawherespectralpeaks
rathercoarse.The spectracorresponding
to 5.75 and 8.25 RI
are shownin Figure 2a.
The VoyagerUVS spectraareblendsof denseline spectraof
sulfur and oxygen. The reduction of the data requires
can be resolved. Since we cannot assume that conditions of
thermalequilibriumprevail in the plasmasheet,we cannot
determinetherelativecontributions
of O+ andS++to thepeak
in the spectrumat mass/charge
= 16 (McNutt et al., 1981). In
Figurelb, we showa spectrumof thermalplasmaobtainedat
27 Rj in whichthe presenceof an ionsof massto chargeratios
of 8, 32/3, 16 and32 are apparent.Again,thepeakat ADZ=8
couldbe dueto a combination
of O++ andS4+. However,the
comparisonto model calculationsof the electron excitation
process.
The synthesis
of thespectrato determinepartitioning
of speciesdependson electrontemperature. Between 5.75
and 7.5 RI the electron temperature is determined from
analysisof the UVS spectraand is consistentwith the in situ
measurements [Sittier and Strobel,1987]. From 7.5 RI
outwardthetemperatureprofilederivedfrom the PLS electron
uniqueidentificationof the peakat 32/3 with smallamountsof
S3+,indicates
thatS4+musthavemuchlowerabundance
and
databy Sittier and Strobel[!987] was applied,becauseof
lossof accuracyin the UVS temperaturemeasurements
at the
highertemperatures
(20 eV) of the outertoms.
The higherelectrontemperatureof the outerregionsof the
tomsis demonstrated
by differencesbetweenthe two spectra
thatO++ is thedominantcomponent
at ADZ=8.
Ultraviolet SpectralEvidence
The Voyager ! UVS instrumentscannedthe Joviansystem
for 30 daysbeginningMarch 15, 1979. These systemscans
haverecentlybeenreanalyzedwith the aid of updatedatomic
shown in Figure 2a. The emission lines toward shorter
wavelengthshave greaterstrengthat 8.25 Rj, forced by the
higherelectrontemperatures,
producing
significantdifferences
in spectralshape.The actual differencesin ion partitioning
betweenthesetwo locationsis in fact greaterthan apparent
from the two spectrabecausedifferencesin the variationof
data. In order to detect the weaker emission from the low
densityregionsin the outer toms, the 30 days of data were
binnedin stepsof 0.5 RI, and the radial scaleis, therefore,
0.50
•
•
3
•
o
I
-
==
"' i
i
i '!
I
! '!
i
i
i
'
i
i
i
I
I
i ' ',
i'"' i
i
!
i
,
i
i
,
•
1
-
--
'---'
5.75
Rj
• O.lO
-
•3+ .
•
•
1
0
550
•
65O
750
850
950
O.Ol
-•050
t150
5.!
i
i
i
•
I
6.0
i
i
i
i
i
6.5
i
i
i
i
I
7.0
i
i
i
i
I
7.5
i
i
I
i
i •
8.0
i
i
i ,,
8,5
X(A)
Figure2. (Left)Spectra
fromVoyager
1 UVS at5.75and8.25 Rj (normalized
at 6853,).Modeled
transitions
aremarked
onthe
figure.(Right)Theionpartitioning
fromfitsto UVS spectra
(exceptO++ opencircleswhicharefrommodelcalculations).
81
Bagenal
et al.:TheAbundance
of O++ intheJovian
Magnetosphere
collisionstrengthwith electrontemperatureaffectthe relative
sulfurand oxygenexcitation efficiencies. The featurenear
sonicresponseof the PLS detector)with a plasmahaving the
UVS ion compositionandmovingwithin a few percentof
rigid corotation.The spectrumrequiredthat in addition to a
thermalpopulation
at 60 eV, theions showa suprathermal
tail
(comprisingup to 30% of the total charge density) with
600/• in the 8.25 Rj spectrumis dominatedby O++ andis
measurable
only in the spectraat 7.75 and 8.25 Rj, because
ofa combinationof sufficientlyhigh electrontemperatureand
increased
mixingratio.In contrast
thefeaturenear900/!,is
mainlydueto S+ andvirtually absentat 8.25 Rj principally
becauseof reduced mixing ratio. The total oxygen ion
abundance
is accurately
measured
fromthefeature
near8343,.
characteristic
energiesof 200 eV. When we tried to fit PLS ion
spectrafartheroutin the toruswith the sameion composition
we found that a large (20%) reduction in flow speed from
corotationwasnecessary
to matchthe spectrum.On the other
The separationof O++ and O+ in the spectra,with the
exception
oftheextreme
conditions
noted
above,
is•lifficult
to
determinefrom the UVS spectra, as discussedby Brown et
al. [1983] andShemansky[1987]. The actualamountof O++
between 5.75-6.75
Rj at Voyager 1 encounter therefore
remainsuncertain because of the lack of definition in both of
theVoyagerinstruments.The valuesindicatedby the open
circlesin Figure2b in this regionarein fact modelpartitioning
calculationsrather than from reductionof the spectra.From
7.25RI outwardthe higherelectrontemperatures
andchanges
inspectral
shaperequiretheinsertionof measurable
O++with
a sharprisein themixingratio[O++] / [e] fromvalues< 0.02
inside7.0 Rj to 0.2 at 7.75 Rj.
Radial Distribution of O ++
hand, if we assumedrigid corotation, we found that we
requiredwhat seemedat the time to be an unreasonablylarge
abundance
of low mass-to-charge
ionssuchasO++. With the
recentanalysisof UVS spectrain the outer toruswe returned
to the PLS spectrum at 8 Rj which we fit assumingrigid
corotation.The resultingcontributionof O++ to the spectrum
(labeledPMODEL in Figure3) is consistentwith the current
analysisof the UVS spectrain thisregion.
The radial profile of O++ abundance shown in Figure 3
raisesthree main issues:(a) the enhancementof O++ in the
cold toms, which remainsunresolved(Bagenal, 1989); (b)
the abruptincreasein ionization at the outer boundaryof the
torus,presumablydueto the presenceof hot electrons;(c) the
enhancementof the oxygenabundancebetween7 and 8 RI.
The observedspatial distributionof O++ ions is consistent
with electronimpactionizationof O+ beingthemainsourceof
By combiningthesenewUVS resultswith the O++density
determinations
of McNutt et al. [1981] and Bagenal[1989] we
areableto produceradialprofilesof the O++ mixingratioand
densitybetween 4.9 and 42 Ri (Figure 3). At the outer
boundary
ofthetoms,
where
thetotalplasma
•ensity
drops
by
anorderof magnitude,the O++ mixing ratio increasesby an
orderof magnitude.Fartherout, in the plasmasheet,the O++
mixingratio is fairly constantat about 10-20%.
Whileion composition
cannotbe determined
fromthePLS
ionspectrain the warmtorus,fittingthe spectrawith a given
composition puts constraints on the flow speed and
temperature
of the ions.For example,Bagenal[!989] found
thattheion spectraat 6 Rj couldbe modelled(usingthesub-
O++ratherthancharge
exchange
•therateof whichdecreases
sharplyoutsidethe densetomsregion). The numberof targets
available for impact ionization also decreaseswith radial
distance,but this canbe compensated
for by local increasesin
electrontemperatureand in energeticelectronfluxes. It is in
the same region of the outer boundary of the torus that not
only do Sittier and Strobel [1987] report an increase in the
temperatureof the cold electronpopulationby aboutan order
of magnitude, but McNutt et a1.[1990] also note the very
localized intensification of suprathermalelectron fluxes.
Whetherthesechangesare linked to heavy ion precipitation
and auroral activity as suggestedby McNutt et al. [1990]
remains an open question. Turning to the issue of the
!o
oo
.Ol
-
_
I
I
!o
Zx= Ba,
geno! [ 1909]
'-•
.
0 = UVS
. =PMODEL
• = McNtztt
et cd.[1981]
I
5
......,
I
7
,,.... ,
-
A
*4•
•
•
I
!0
20
Radial Distance (R•)
•I
30
,*
{1
,,
Figure
3.Profiles
of(above)
mixing
ratios
and(below)
absolute
values
ofO++obtained
frombothPLSandUVSobservations.
Bagenal
et al.:TheAbundance
of O++ in theJovian
Magnetosphere
82
coupling,in Time-Variable
enhancement
of oxygen
between7 and8 RI, wenowfacethe Bagenal,F., Toms-magnetosphere
Phenomenain theJovianSystemed. Belton et al., pp. 196possibility
thattheremaybeanadditional,
ratherweaksource
210, NASA, SP-494, !989.
of oxygen
ionsbeyond
7.25Rj. A candidate
source
is theicy
BagenalF. and J.D. Sullivan, Direct plasmameasurements
in
satelliteEuropaassuggested
by Bagenal[! 989]and,onthe
basisof Pioneer10 data,by IntriligatorandMiller [1982].
Thecoldplasma
in themid-magnetosphere
plasma
sheetmay
be regardedas a 'frozen'imageof the hot torusif, in the
regionoutside
thetorus,therelevantreaction
timesarelonger
than thetime of diffusivetransportto regionsof still lower
density.Shemanskyet al. [1985] have derivedanalytic
expressions
for thecoefficients
of excitationandionization.
Evaluation of the O ++ creation rate coefficient as a function of
electronenergyshowsthatthereis a broadmaximumaround
400eVof about3x10-8cm3s
-1. Partitioning
calculations
with
thediffusivetranportoccuringon time scalesof theorderof
60-100 days and with the small fraction of supratherma!
electronsin thedensepartof thetomsproducea mixingratio
of just a few percentfor O++. The existenceof enhanced
suprathermal
electronfluxesoutsidethedensetomsmakesthe
creationof O++ ionsby electronimpactfeasibleoutto about8
RI. At greaterradialdistance,in the tenuousplasmasheet,the
mixingratioof O++ canbe expectedto remainconstant
asthe
total densitydecreases.
In conclusion,it is apparentfrom the combinationof the
resultsdescribedabovethat.O++ waspresentthroughout
the
Jovianmagnetosphere
at the time of the Voyager1 encounter
in early 1979. The obviousquestionwhich arisesfrom the
aboveconcernsthe non-observationof O++ by Brown et al.
[1983] and the weak emissions reported by Moos et al.
[1991]. Sincethe abundanceof O++ in the denseregionof the
tomsis extremelysensitiveto the electronenergydistribution
[Shemansky,1987] it hasbeensuggested
thatthe absenceof
O++ in the observationsby Brown et al. [1983] could be
explainedby a depletionof hot electrons.It is alsopossible
that the Earth-basedobservations
were dominatedby the high
densityregionof the toms wherethe O++ abundance
is very
small. In fact, the 2% O++ mixing ratio reportedby Moos et
al. [1991] is consistentwith partitioningcalculationsfor the
densepart of the toms. Analysisof the corresponding
UVS
data from the Voyager 2 encountershowsa lower effective
the Io torus and inner magnetosphere of Jupiter,
J. Geophys.Res., 86, 8447-8466, 1981.
Bridge, H.S., J.W. Belcher, R.J. Butler, A.J. Lazarus, A.M.
Mavretic,J.D. Sullivan, G.L. SiscoeandV.M. Vasyliunas,
The plasma experiment on the !977 Voyager mission,
SpaceSci. Rev., 21, 259-287, 1977.
Bridge, H.S., J.W. Belcher, A.J. Lazarus, J.D. Sullivan,
R.L. McNutt, F. Bagenal, J.D. Scudder,E.C. Sittler, G.L.
Siscoe, V.M. Vasyliunas, C.K. Goertz and C.M. Yeates,
Plasma observationsnear Jupiter: Initial results from
Voyager !, Science, 204, 987-991, 1979.
Brown, R.A., Shemansky, D.E. and R.E. Iohnson, A
deficiency of O++ in the Io plasmatorus,Astrophys.Y.,
264, 309-323, 1983.
Intriligator,D.S. andW.D. Miller, Firstevidencefor a Europa
plasmatorus,J. Geophys.Res., 87, 8081-8090, ! 982.
McNutt, R.L., J.W. Belcher and H.S. Bridge, Positive ion
observationsin the middle magnetosphere of Jupiter,
J.Geophys.Res., 86, 8319-8342, 1981.
McNutt, R.L., F. Bagenal and R.M. Thorne, Observationsof
auroral secondaryelectronsin the Jovian magnetosphere,
Geophys.Res. Lett., 17, 291-294, 1990.
Mekler, Yu. and A. Eviatar, Time analysisof volcanicactivity
on Io by means of plasma observations,J. Geophys.
Res.,85, 1307-13!0, 1980.
Moos, H. W., Feldman, P. D., Durrance, S. T., Blair W. P.,
Bowers, C. W., Davidsen, A. F., Dixon, W. V., Ferguson,
H. C., Henry, R. C., Kimble, R. A.,Kriss, G. A., Kruk, J.
W., Long, K. S. and O. Vancura, Determinationof ionic
abundances in the Io torus using Hopkins Ultraviolet
Telescope,Astrophys.J., 382, L105-L108, !991.
Sands,M.R., and R.L. McNutt, Jr., Plasmaflow in Jupiter's
dayside middle magnetosphere, J. Geophys. Res.,93,
8502-8518, ! 988.
Shemansky,D.E., Ratio of oxygento sulfurin the Io plasma
torus,J. Geophys.Res.,92, 6141-6146, 1987.
temperature
for theelectrons
anda significantly
lowernumber Shemansky,D.E., J.M. Ajello and D.T. Hall, Electronimpact
densityof O++ ionsin thetoms[Shemansky,
1987]consistent
excitationof H2: Rydbergbandsystems
andthebenchmark
with time-variabilitiesreportedby Mekler andEviatar [1980]
dissociativecrosssectionfor H Lyman-(•, Astrophys.J.,
andWalker and Kivelson [1981]. The forthcomingencounter
296, 765-773, 1985.
of the Ulysses spacecraft with Jupiter should provide
Sittier, E.C., Jr. and D.F. Strobel, io plasmatoruselectrons,
additionalinformationonthe partitioningof the Io toms. We
J. Geophys.Res.,92, 574!-5762, 1987.
predict that if the level of activity is sufficiently high to
Walker, R., and M.G. Kivelson, Multiply reflectedstanding
Alfven waves in the Io torus:Pioneer 10 observations,
maintaina tomsof hotplasmacontainingenoughsuprathermal
electrons,the plasma instumentation
on Ulyssesmay observe
Geophys.Res. Lett., 8, 1281-1284,1981.
a significantabundance
of O++.
Acknowledgements.The work at Tel Aviv Universitywas
supportedby the United States-Israel Binational Science
Foundationunder grant 88-00120. The authorsacknowledge
support by NASA grants NAGW-87, NAGW-! 06 and
NAGW-1622. Work at VisidyneInc. waspartially supported
via subcontract
fromNASA supported
programsat MIT.
F. Bagenal,A.P.A.S.Dept.,U. of Colorado,BoulderCO
80309-0391.
R. L. McNutt Jr.,Visidyneinc., Burlington,MA 01803.
R. Schreierand A. Eviatar, Dept. of Geophysicsand
Planetary
Sciences,
Tel Aviv University,69 978 RamatAviv,
Israel.
D. E. Shemansky,
Aerospace
Eng.Dept., U. of Southern
California,Los AngelesCA 90089.
References
(ReceivedSeptember11, 1991;
Bagenal,F., Plasmaconditionsinside Io's orbit,J. Geophys.
Res.,90, 311-324, !985.
Revised: November 12, 1991;
Accepted:December19, 199!)