VOL. 7, NO. 1
GEOPHYSICALRESEARCHLETTERS
SPATIAL
DISTRIBUTION
OF PLASMA
JANUARY 1980
IN THE IO TORUS
Fran Bagenal and James D. Sullivan
Center for SpaceResearch,M.I.T., Cambridge, 02139
George L. Siscoe
Departmentof AtmosphericSciences,Universityof California, Los Angeles,90024
Abstract. In situ measurementsof ion densitiesand temperatures
have been analyzed to produce profiles of these plasma parameters
However, at the sametemperaturethe oxygenions have a larger thermal speed.Therefore, in regionswhere all the ionic speciesappear to
have the same temperature, the proportions of O* and S:* that are
combined in the A/Z* = 16 peak can be determined using a fitting
procedurewith appropriate thermal widths for the different species.
Although there are no resolved peaks of S:* amd Na* in the cold
region these ions could be presentin the hot torus. Nevertheless,in
the analysispresentedhere, S*, O *, S:* and O:* are taken to be the only speciesof significance. It has been shown that SO:* (A/Z* = 64)
and severalheavier ions are also present(Sullivan and Bagenal, 1979)
but they are minor contributors to the total ion densityand are found
outside the energy per charge range of this analysis.
The functional form of a sum of isotropic convectedMaxwellian
distributions is used to make a multidimensional parameter search
for a least squaresfit to the observedspectra. This producesin situ
values of ion densities,the common temperature, and the common
component of the bulk motion of the plasma into the detector for
each high resolution spectrum, measured at 192 second intervals
along the spacecrafttrajectory. A few of the spectrahave not been included in the presentanalysisbecauseof missingpoints in the spectra
alongthe Voyager1 inboundtrajectorybetween7 and 5 Rj. The
temperature profile shows a sharp decreaseby a factor of •,,50 be-
tween5.8 and 5.2Rj corresponding
to a temperature
gradientof
•,7 x l0sperRr Theelectrondensity
profile,inferredfromtheion
densitymeasurements,
hastwo maximaat 5.7 and 5.3 Rr A twodimensionalmodel of the spatial distribution of various ionic species
in the Io plasma torus has been constructed.Using this model a contour map of electron densityin a meridional plane has been made, it
exhibitsa well-defined
inneredgeto thetorusat 5.6 Rj. Thecontour
map of S* ion densityindicates
that mostof the S* ionsare concentrated closeto the centrifugal symmetrysurfaceand radially inward
of thelargerelectrondensitymaximumnear5.7 Rr
Introduction
In situ measurementsof ions in the plasma torus of Juipter's
satellite
Io weremadewiththeplasmascience
instrument
on 6øard
Voyager 1 (for initial Voyager resultsregardingthe Io plasma torus
seeBridge et al. 1979; Broadfoot et al., 1979; Scarf et al., 1979; and
Warwick et al., 1979). Warwick et al. (1979) have constructeda twodimensional model by extrapolating their smoothed electron measurementsalong the trajectory under assumptionsof axial and mirror
symmetry. Although suchelectrondensitymodelsof the torus are invaluable, they provide little information about its ion properties,
e.g., composition, densitiesand temperaturesof the various ionic
species.This paper presentsprofiles of positive ion temperature and
densities along the inbound trajectory. These profiles are used,
without smoothing, to constructcontour maps of the plasma density
in a meridional plane. The symmetry assumptionsof Warwick et al.
(1979) have beenmaintained with the additional assumptionsthat the
ions are isothermal along a given magnetic field line and that their
distribution along each magnetic field line is governed by a scale
height.
(e.g.,around5.7 Rj). It ishopedthat furtherworkwill includesome
of thesemissingspectraas well as data obtained at radial distances
greaterthan•,7 Rj.
Radial profilesof plasma
Figure 1 shows the radial profile of the temperature of the
plasma derived from fits to the data assuminga common temperature
for all species,i.e., the plasma is locally isothermal. There appear to
be two very distinct regions;the hot torus where T --4 x 10•K and a
cold region closer to Jupiter where T •,8 x 10:K. This drop in
temperature by a factor of *50 occurswithin a very small radial dis-
tance(5.8 to 5.2 Rj), corresponding
to a temperature
gradientof
•,7 x 10•Kper Rj. In the coldregionwherethe peaksin theenergy
per charge spectracorrespondingto different ionic speciesare well
resolved, independent fits for the temperaturesof each speciesindi-
In situ Measurement
catethatinwardof •,,5.4Rj, theassumption
of anisothermal
plasma
is valid (for examplesof spectra,seeBridge et al., 1979, and Sullivan
and Bagenal, 1979). Within the torus, wherethe plasmaappearsto be
much hotter, the individual ion peaks stronglyoverlap. By specifying
which ionic speciesare presentand by assumingthat they all have the
same temperature, individual ion densitiescan be determined. The
value of common temperature derived from the fit is sensitiveto the
ratio of O* to S:* for the A/Z* = 16 peak (located near the center of
the broad distribution). Uncertainties in the density and temperature
determinations can be found by investigatingthe two extreme cases
by taking either S:* or O* to be totally absent (see the discussion
below). In the hot region closeto Io, the apparent sourceof the ions,
it seemsunlikely that the plasma has had time to thermalize. In that
case, a more realistic model for the hot region would assumea common gyrospeedat the point of ionization, i.e., temperaturesproportional to the massesof the ions. Fits to the data under this assumption
are not yet complete.
The valuesof the individual ion densitieshave beenmultiplied by
their respectivechargestateand summedto producethe electrondensity that would balancethe chargeof all the ions detected.This measurementof electron density is independentof the identification of
the ion speciesas the instrument measuresthe positive ion current.
Figure 2a shows in the in situ electron densitiesas a function of
The plasma instrument measuresions and electrons within an
energyper chargerange of 10 to 5950 volts (for detailed propertiesof
the instrument, see Bridge et al., 1977). During the Voyager 1 inbound traversal of the torus, the main sensorwas oriented toward the
direction from which the torus plasma, corotating in Jupiter's
magnetic field, appearsto be flowing. On the outbound traversal of
the torus the instrument orientation was lessfavourable and analysis
of the data is more complex and as yet incomplete.
Near the orbit of Io corotating ions with mass to charge ratios,
A/Z*, greater than •6 can be detected.At this distancethe energy
per chargerangeincludesthe ionic specieswhichdominatethe ultraviolet emissions(Broadfoot et al., 1979). Radially inward from the
torus, there are well resolvedpeaks at A/Z* valuesof 8, 16, and 32
(Bridgeet al., 1979). There is someambiguityin the identificationof
the correspondingionic speciesfor each of thesepeaks. The peaks at
A/Z* values of 8 and 32 are presumably dominated by O :* and
S+thoughthere could be contributions from S4* and O:*. Unfortunately, the common ratio of A/Z* = 16 for S:* and O* resultsin
the superpositionof the spectralpeaks of thesetwo abundant ions.
Copyright
1980 by the American Geophysical Union.
Paper 9L1664
0094-8276/80/009L-
41
1664501.00
42
Bagenal et al.'
, ,,
, i r,
,,
Spatial
Distribution
of Plasma
havior of both the S* and electron densitymay be usedto investigate
i , , , , I , , ,,
the two-dimensional
Two-dimensional
z
105
•
0q
q
structure of the torus.
Structure
The distribution of isotropicplasma along a magneticfield line
can be found by balancingthe forces acting on the plasma in the
directionof the field. The centrifugal force tendsto confinethe ions
to the centrifugalsymmetrysurface(equator) and the magneticmirror force tendsto confine them to the magneticequator. The angular
separationbetweenthesetwo surfacesfor Jupiter is about 3 o (Hill,
Desslerand Michel, 1974). The ratio of the centrifugalto the mirror
forcesfor equatorially
confinedparticlesis 2Kc/3Kñ whereKcis the
kineticenergyof the ion movingat corotationalspeedand Kñ is the
5
6
7
8
L-SHELL
Fig. 1. The profile of ion temperature(versusmagneticL-shell) determined from multiple-ion energyper chargespectra,assumingthe ions
to be isothermal.
L-shell (assuminga dipole magnetic field). The profile shows two
localdensitymaximaat 5.7 and5.3 Rj witha sharpdropin densityby
a factor .of 3 to 5 between them. Radially inward of the maximum at
5.3 Rj, the densityagaindropsrapidly,thistimeby morethanan
order of magnitude.Comparisonof the electrondensityprofile with
the temperatureprofile (Figure 1) revealsthat the secondsmallerelectron density peak occurs inward of the sharp drop in temperature.
The two-peak structureof the electrondensityprofile was also shown
in earlier papers of Bridge et al. (1979) and Warwick et al. (1979).
Comparisonof the electrondensityprofile with the measurements
of
Warwick et al. (1979) suggeststhat in the cold region there are very
gyro-kineticenergy(Siscoe,1977). For the ions observedin the Io
torus, this ratio is everywheregreaterthan 3, and in the cold inner
region, greater than 60. Thus the magnetic mirror force may be
neglectedin derivingthe off-equatorial densitydistribution,and the
plane of mirror symmetryliesmuch closerto the centrifugalthan the
magnetic equatorial surface.
Coulomb collisionsbetween heavy ions will tend to make them
isotropic and isothermal along an individual magnetic field line.
Similarly,the temperature
of the electrons(Te)will becomethe same
as the ion temperature
(Ti). Observations
suggest
the two temperaturesare comparable(Scudder,private communication;Broadfoot et
al., 1979). Balancingthe centrifugaland pressuregradientforcesfor
eachionic speciesseparatelyproducesa simpleexponentialvariation
of density,ni, withdistance
fromthecentrifugal
equator,z, namely
n = ni exp[-(z/Hi)2]
whereni is the equatorialdensityand the scaleheightHi =
[2kTi/3mi%22]
'/2(kisBoltzmann's
constant.
S2istherotation
rateof
fewionswithA/Z* < 6. In thetorus(6 - 7 Rj) therearelessthan200
electronscm-3 unaccountedfor by the measurementsof ions with 8 •<
A/Z* •< 32. In the hot region simultaneousfits for S*, S2', O* and 02*
usingthe procedureoutlined above, producesa ratio of total number
of oxygenions to sulphur ions
[n(O'*) + n(O*)l/[n(S'*)
(a)
+ n(S*)]
of around unity. This result is not consistentwith a ratio of two
which would be expectedif the sourceof ionsis completedissociation
and ionizationof SO2(SO2
-} S* + 20*) from an atmosphere
of Io
z
(Pearl et al., 1979; Kumar, 1979). Therefore, if the four speciesused
in this model are in fact the major contributors in this region of the
spectrumthen a sulphursourcein addition to SO2may be requiredto
explainthe ion measurements.On the other hand, the oxygento sulphur ratio couldbe very differentif thereare substantialquantitiesof
ionic specieswhichhave so far beenassumedabsent,e.g., O,*, Na*,
S3' and S'* or if there is significant loss of neutral atoms.
The S* densityprofile (Figure 2b) has a similar structureto that
of the electronprofile exceptthat the inner S* densitymaximum has a
value much larger than its densityin the hot torus. Moving towards
I
I , , I J I I LI J I I I f J , , , ,
103
(b)
Jupiterfrom the locationof this peak at m5.3 Rj the spacecraft
measureda drop in the densityof S* ions of nearly three ordersof
v
magnitude
withina radialdistance
of 0.4 Rj. In thehottorusregion
the calculatedS* densityvaries accordingto the assumedidentification of the A/Z* = 16 peak. The sensitivityof the S* densityhasbeen
investigated
by takingthe extremecasesof A/Z* = 16corresponding
to a singlespecies
of eitherS2' or O*, shownby verticalbarson Figure
2b. Similarly,if thereis considerable02* or Na* presentthe S* density
in the hot torus will have been over estimated in this analysis. However, the majority of the S* ionsare found wherethe A/Z* = 32 peak
is well resolved.
The spacecrafttook a little under half a rotation period of
Jupiter (i.e., m5 hours)to traversethe regioncorresponding
to the
tOt _
7.
.
5
6
L-StqF, LL
7
8
data shownhere, so it is unlikely that theselarge densitychangesover
Fig. 2. In situ density measurementsversusL-shell. Panel (a) electrons, (.). The measurementsof Warwick et al., (1979) are shownby
a few minutes time scale could be due to azimuthal variations. Simi-
x. In panel(b) ionswithA/Z* = 32, assumed
to beS*(o). Theverti-
larly, the latitude of the spacecraftremainedsmallso that the struc-
cal bars show uncertainties in the density determination due to model
dependence.
ture shown must be in the radial direction. Therefore, the radial be-
Bagenal et al.'
Spatial
Distribution
Jupiter).Theeffective
ionicmass,m?,(Siscoe,
1977)takesintoac-
of Plasma
S•'DENSITY IN MERIDIONRL PLRNE
count the consequence
of includingthe electronsin the force balance
'
byreducing
theionmass,in effect,bya factorof (Z•'+ 1) if Te = Ti.
This factor will be proportionallysmallerif the electronsare colder.
The distributionof ionsalonga field line hasbeencalculatedincluding the electrostatic interactions between the different ionic
speciesand their accompanyingelectrons.Thesecalculationssuggest
that an independentscaleheightapproximationis valid for the ionic
speciesand limited spatial region consideredin this analysis.However, there might be a significanteffect on minor ionic specieswith
low massto chargeratios. Thesespecieswould be drawn off the centrifugal equator by the electrostaticpotential set up by the more
dominant heavy ions.
The equation above has been used to determine for each ionic
species
off-equatorialdensities
whicharethenweightedby the correspondingchargestates,linearlyinterpolatedand summedup to give
theelectrondensitymapshownin Figure3. The cross-sectional
shape
of the torus shown is similar to that derived from the remote ultra-
violet observations
of Broadfootet al. (1979). The electrondensities
shownhereare underestimates
becauseall of the ionicspecies
are not
includedin this analysis.
The moststrikingfeatureof this densitymap is the sharpinner
!
'
SPACECRAFTLOCATION
q
5
6
7
8
D I $TI::INCE (R J)
Fig. 4. Contours of equal densityof ions with A/Z* = 32 assumedto
beS*in a meridional
plane(centered
onthecentrifugal
symmetry
surface). The location of the spacecraft at the time of each spectral
measurementusedis shown by +. Contour levelsare in stepsof 200
cm-3(1 Rs = 71 398km).
edgeof the torus at the L-shell of L = 5.6. This is the location of the
sharpdrop in temperature(Figure2) and the regionin whichScarf
et al. (1979) observedauroral hiss. Radially inwards from the torus
the plasmaappearsto be closelyconfinedto the centrifugalequator.
Althoughtheequatorial
electron
density
hastwomaxima(1700cm-3
at 5.3Rs and2100cm-3at 5.7Rs),thetotalioncontent
perunit
L-shell (NL') increasesproportionallywith L. In an accompany
paper, Richardsonet al. (1980) discussthe significanceof this observation and showthat the two densitymaximado not necessarily
imply an episodicsource.
Figure4 showsa contourmapfor S*ionsalonemadein the same
way as Figure 3. The sharpinner edgeof the torus remainsbut the
majorityof the S* ionsforma triangular-shaped
featurepointing
towardsJupiter, situatedentirelywithin the regionwherethe A/Z*
= 32 spectralpeak is well resolved.This shapeis similar to the torus
cross-section
inferredfrom ground-based
observations
of S* optical
emission(Nash, 1979;Pilcher,1979;Traugeret al., 1979).Theseoptical observationsshowagreementwith the assumptionthat the A/Z*
= 32peakisindeeddominated
byS*ions.In thehottorusa comparisonof theS*numberdensities
derived
fromtheground-based
andin
situmeasurements
is not yet possiblebecauseof the modeldependent
uncertaintiesassociatedwith composition.
ELECTRON DENSITY IN MERIDIONRL PLRNE
1
Discussion
Plasma measurements made in the torus and therefore close to
the apparent source, Io, indicate plasma temperaturescloseto 4 x
10•K(35eV).Thecorresponding
thermal(gyro)speeds
of ,x,15km s-'
for sulphur
and•20 km s-' for oxygen
areonlya fractionof thecorotationvalueof 57kms-' presumably
because
of theshielding
of the
corotational electric field by currents near Io (Eviatar, Siscoeand
Mekler, 1979; Goertz 1979). With the plasmasourceat Io, the density
gradient is unstable at L-values greater than •6 which results in
enhancedoutward diffusion (Richardsonet al., 1980); thus the slowly
decreasingdensity producesan indistinct outer edge to the torus.
Radially inward of Io's orbit the plasmadiffusesslowly, remaining sufficientlydensefor electroncollisionsto excitethe ionsand generate the considerableultraviolet and optical emissionsthat first indicatedthe presenceof the Io plasmatorus (Kupo, Mekler and Eviatar,
1976). The ions in this region are rapidly cooledby the radiation. The
excitation mechanismis very dependenton the electron densityand
temperatureso that as the plasmacoolsand collapsestoward the centrifugal equator, the density is enhancedwhich further increasesthe
cooling by radiation. Hence, there are cold isothermal ions inside a
very sharp temperature gradient which forms a well-defined inner
edgeof the torus.
Recombination has a weaker dependenceon temperature and
densitythan the collisionalexcitation mechanism,but, eventually, as
the ions diffuse towards Jupiter, they recombine to form neutrals
which are slung off through the outer magnetosphereto be recycled
as energeticheavy ions (Eviatar et al., 1976; Krirnigis et al., 1979;
Vogt et al., 1979); or to provide a source in the magnetosheath
(Broadfoot et al., 1979); or to escapethe Jovian systementirely.
Summary
1) In situ ion measurements
showa sharpdrop in temperature
bya factorof 50at about5.6Rs.corresponding
to a temperature
gradientof •7 x 10•KperRs.Thetemperature
dropoccurs
between
two
local maximain the equatorialplasmaelectrondensityof at least
2100
and1700
cm
-•atradial
distances
of5.7and5.3Rs,respectively.
2) The inner boundaryof the Io plasmatorus is very well de-
+ SPACECRAFT LOCATION
finedbya steep
electron
density
gradient
at L •5.6 Rs.Themajority
q
5
6
?
8
DISTFINCE(RJ)
Fig. 3. Contours of equal densityof electronsin a meridional plane
(centeredon the centrifugalsymmetrysurface).The location of the
spacecraftat the time of eachspectralmeasurementusedis shownby
+. Contourlevelsarein stepsof 200cm-• (1 Rs = 71 398km).
oftheS*ions
arefound
inatriangular-shaped
region
near5.3Rsand
are confinedto low centrifugallatitudes.It is proposedthat as the
plasmadiffusesinwardit is rapidlycooledby radiationand collapses
toward the centrifugalequator.
3) The outerboundaryof the torusis lessdistinctand is compatible with enhancedoutwarddiffusionfrom a plasmasourceat Io.
Acknowledgements.The authors would like to thank H. S.
44
Bagenal et al.'
Spatial
Bridge, J. W. Belcher and C. K. Goertz for helpful discussionsand
valuable assistancein preparing this paper. We would also like to
thank D. Shemanskyfor useful commentsas a referee. This work was
funded in part by JPL Contract 953733 and NASA Grant NGL
22-009-015 (Wash).
References
Bridge, H. S., J. W. Belcher, R. J. Butler, A. J. Lazarus, A.M.
Mavretic, J. D. Sullivan, G. L. Siscoe, V. M. Vasyliunas, "The
Plasma Experiment on the 1977 Voyager Mission", Space Sci.
Rev. 21. 259 (1977).
Bridge, H. S., J. W. Belcher, A. J. Lazarus, J. D. Sullivan, R. L.
McNutt, Jr., F. Bagenal, J. D. Scudder,E. C. Sittler, G. L. Siscoe,
V. M. Vasyliunas, C. K. Goertz, C. M. Yeates, "Plasma Observations Near Jupiter: Initial Results from Voyager 1". Science204.
987 (1979).
Broadfoot, A. L., M. J. S. Belton, P. J. Takacs, B. R. Sandel, D. E.
Shemansy,J. B. Holbert, J. M. Ajello, S. K. Atreya, T. M. Donahue, H. W. Moos, J. L. Bertaux, J. E. Blamont, D. F. Strobel, J.
C. McConnell, A. Dalgarno, R. Goody, and M. B. McElroy, "Extreme Ultraviolet observations from Voyager 1 Encounter with
Jupiter", Science204. 979 (1979).
Eviatar, A., G. L. Siscoeand Yu Mekler, "Temperature Anistropy
of the Jovian Sulphur Nebula", Icarus, 139, 450 (1979).
Eviatar, A., Yu Mekler, F. V. Coroniti, "Jovian Sodium Plasma",
Astrophys. J. 205. 622 (1976).
Goertz, C. K. "Io's interaction with the plasma torus". submitted to
J. Geophys. Res. (1979)
Hill, T. H., A. J. Dessler and F. C. Michel, "Configuration of
the Jovian Magnetosphere," G.R.L., 1, 3. (1974).
Krimigis, S. M., R. P. Armstrong, W. I. Axford, C. O. Bostrom, C.
Y. Fan, G. Gloeckler, L. J. Lanzerotti, E. P. Keath, R. D. Zwickl,
J. F. Carbary, and D.C. Hamilton, "Low-Energy Charged Particle Environment at Jupiter: A First Look", Science204.998 (1979).
Kumar, S., "The stabilityof an SO2atmosphereon Io", Nature, 280,
758, (1979).
Distribution
of Plasma
Kupo, I., Yu. Mekler, and A. Eviator, Detection of ionized sulphur
in the Jovian magnetosphere",Astrophys. J., 205. L51, 1976.
Nash, D. B., "Jupiter Sulphur-PlasmaRing", EOS. 60, 307 (1979).
Pearl, J., R. Hanel, V. Kunde, W. Maguire, K. Fox, S. Gupta, C.
Ponnamperuma, and F. Rantin, "Identification and gaseousSO2
and new upper limits for other gaseson Io", Nature, 280, 755,
1979.
Pilcher, C. B., "Images of Jupiter's Sulfur Ring", submittedto Science, August, 1979.
Richardson, J., G. L. Siscoe,J. D. Sullivan, and F. Bagenal, "TimeDependentRadial Diffusion in the Io Plasma Disk", G.R.L., this
issue, 1980.
Scarf, F. L., D. A. Gurnett, and W. S. Kurth, "Jupiter Plasma Wave
Observations: An Initial Voyager 1 Overview," Science,204. 991.
1979.
Siscoe,G. L., "On the equatorial confinementand velocityspacedistribution of satelliteions in Jupiter's magnetosphere",J. Geophys.
Res. 92. 1641. 1977.
Sullivan, J. D., and F.Bagenal, "In situ identification of various
ionic speciesin Jupiter's magnetosphere", Nature, 280. 798, 1979.
Trauger, J. T., C. Miinch, F. L. Roesler, "A Study of the Jovian
(SII) Nebula at High Spectral Resolution", AstrophysicalJ., in
press, 1980.
Vogt, R. E., W. R. Cook, A. C. Cummings, T. L. Garrard, N.
Gehrels, E. C. Stone, J. H. Trainor, A. W. Schardt, T. Conlon, N.
Lal, and G. B. McDonald, "Voyager 1: Energetic Ions and Electrons in the Jovian Magnetosphere", Science,204. 1003, 1979.
Warwick, J. W., J. B. Pearce, A. C. Riddle, J. K. Alexander, M.D.
Desch, M. L. Kaiser, J. R. Thieman, T. D. Carr, S. Gulkis, A.
Boischot, C. C. Harvey, B. M. Pedersen, "Voyager 1 Planetary
Radio Astronomy ObservationsNear Jupiter", Science,204, 995,
1979.
(Received November 1, 1979;
accepted November 26, 1979)