JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. A12, PAGES 21,231-21,243, DECEMBER 1, 1993 Modeling the Europa Plasma Torus RON SCHREIER AND AHARON EVIATAR 1 Department of Geophysics and Planetary Sciences, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Israel VYTENIS M. VASYLIONAS Max-Planck-Institut fgr Aeronomie, Katlenburg-Lindau, Germany JOHN D. RICHARDSON Center for Space Research, Massachusetts Institute of Technology, Cambridge The existenceof a torus of plasma generated by sputtering from Jupiter's satellite Europa has long been suspectedbut never yet convincingly demonstrated. Temperature profiles from Voyager plasma observations indicate the presence of hot, possibly freshly picked-up ions in the general vicinity of the orbit of Europa, which may be interpreted as evidence for a local plasma torus. Studies of ion partitioning in the outer regions of the Io torus reveal that the oxygen to sulfur mixing ratio varies with radial distance; this may indicate that oxygen-rich matter is injected from a non-Io source, most probably Europa. We have constructed a quantitative model of a plasma torus near the orbit of Europa which takes into account plasma input from the Io torus, sputtering from the surfaceof Europa, a great number of ionization and charge exchangeprocesses, and plasma loss by diffusive transport. When the transport time is chosen so that the model's total number density in consistent with the observed total plasma density, the contribution from Europa is found to be significant although not dominant. The model predicts in detail the ion composition,charge states, and the relative fractionsof hot Europa-generatedand (presumed) cold Io-generated ions. The results are generally consistentwith observationsfrom Voyager and can in principle (subjectto [imitationsof data coverage)be confirmedin more detail by Ulysses. INTRODUCTION incorporated.Bagenal et. al. [1992],in an investigationof It has long been accepted that Io is the dominant source the paucityof O++ in the Io torusreported by Brownet al. of plasma in the magnetosphereof Jupiter and that con- [1983],found that O++ pervadesthe entiremagnetosphere tributions from the icy satellites are insignificant. A re- and that while the sulfur density drops off with increas- ported detection of Europa-associated plasma in the Pio- ing radial distance, as expected, the oxygen does not. This neer 10 data set [Intriligator and Miller, 1982]did not win again led to the suggestionthat Europa may be playing a broad acceptance. Early evaluationsof the sourcestrength role as a magnetosphericplasma source. Renewed interest of Europaranged from 8 x 1026s -• fromsputtering by ra- in the possibility of a Europa torus was stimulated by the diation belt particles[Johnson et al., 1981]down to as low Ulyssesencounter with Jupiter. Observationsmade during as 3 x 1024s -• from sputteringby corotatingiogenic ions the Ulyssesflyby of Jupiter confirmedthe existenceof signif- [Eviatar et al., 1981]. The latter estimatewas made be- icantamounts of O++ in regionsof the magnetospherewell fore the results of the low-energysputtering experimentsof off the zenomagneticequatorial plane [Geisset al., 1992]. Bar-Nun et al. [1985] becameavailable and significantly The Ulyssesinstruments were turned off, however, during undervaluesthe source rate. Later analysesby Sieveka and the close approach of the spacecraft to the planet, and no Johnson[1982] and Squyreset al. [1983]led to a deeperun- data were acquired in the equatorial plane. Our results com- derstandingof the interaction of the surfaceof Europa with plement the Ulysses ion mass spectrometer results as we the magnetosphereplasma [Eviatar et al., 1985]. strive to create a continuous latitudinal profile of plasma Baganal[1989] reported a significantenhancement of a density and composition. This, of course, implies basic time hot heavy-ion component outside the hot torus, as shown independence, which is indeed a limitation of the vMidity in Figure l a, and suggestedthat this oxygen component of the conclusions. Confirmation of the Europa source hy- might be associatedwith Europa. pothesisawaits more detailed analysisof the Ulyssesplasma These are preliminary results as the correctionsfor elec- data set. tron and ion feedthrough to the measured currents in the We present the results from a numerical model of the Eu- PlasmaScience (PLS) instrumenthave not yet been fully ropa torus. Sputtering from Europa's icy surface by mag- netospheric particles is a source of molecular and atomic water fragments, and ionization and other atomic processes • Also at Department of AtmosphericSciences, University of California, Los Angeles. produce plasma in the vicinity of the satellite's orbit. We first summarize what is known today about the plas- Copyright 1993 by the American Geophysical Union ma conditionsnear Europa. We then present the model and Paper number 93JA02585. examine the effect of various parameters on the results, and 0148-0227/93/93JA-02585505.00 compare the calculated density and composition with the 21,231 21,232 SCHREIERET AL.: MODELING THE EUROPAPLASMA TORUS I0000 _ _ _ _ 1000=-_ • •/•• -= ioo_-- • •. -- • _ _ I0 - ,, -: ß T•(average) : _ _ I --- / • T•(cold) _ • T•(hot) 0,1 ,,,,I,,,,1•,1,,,,I,•1,,,,I,,,,I,,,,I .... 4 5 6 7 8 9 10 II 12 13 4 L-shell (offset) Fig, ! a, Flacl.ia].profiles of the temperatures of various components of the ion p]a.smaa.s observed by the P I.,S instrumenton Voyager.1, The plot is taken from Bage•l ['1989], O.SO The main cluster consistsof the A, B, and C cups, which are mounted about a cone whosecentral axis points toward Earth. The D cup points perpendicularto the spacecraft- Sun direction. When the spacecraft was inbound toward Jupiter, the D cup pointed approximatelyinto the corota- tion direction during both the Voyager 1 and Voyager2 en- counters. A detailed description of the instrument and its modesof operationis givenby Bridg• •t al. [1977].This is an electrostaticinstrument which has the capability to pro- 0.01 .... ' .... ' .... ' .... ' .... ,, ,A,, vide well-resolvedparticle spectra wheneverthe flow is cold 5.5 6.0 6.5 7.0 7.5 8.0 8.5 and directed into the detectors. Such conditions held in the ;j mid-magnetosphereplasma sheet [McArutt •t aL, 1981]. Fig. 1b. Ion partitioningfrom fits to UVS spectra(except O ++ opencircles which are frommodel calculations), taken from Figure 2 showsspectra from the Voyager2 P LS instru- •l et •l. [1992]. ment obtainednear closest approach; the correspondingbest fits (giventhe input assumptions)to thesespectra are shown observations.We alsoinvestigate the relative importanceof in the panelsof Figure 3. The spectrumin Figure 2a is ob- iogenicand eurogenicsources in order to determinewhether tained 1.5 Rj south of the magnetic equator at a radial Europa providesa significantcontribution to its plasma en- distance of 10.16 /{j. Well-defined peaks appear in the A, vironment, that is, whether it is meaningful to discussa B, and C cups. Figure 3a showsa fit to this spectrumusing Europa plasma torus. a MaxwellJanplasma comprised of O+,S+, O++,SS+,Na +, andK ++ [McArutt,this issue]. Another example of a spec- EUROPA'S PLASMA ENVIRONMENT trum in which well-definedpeaks appear is shownin Figure The plasma conditionsin the vicinity of Europa can be 25 and its associatedfit is shown by Figure 3b. Figure 2c summarized as follows: The ion number density lies in the presentsa spectrum obtained at closestapproach, outside range30 to 50 cm-s [Belcher,1983]. The temperatureof the orbit of Europa at a similar distance(1.5 Rj) off the the thermal or cold component rises from 50-60 eV at L magnetic equator. Since the plasma is warm, the currents ~ 6 to 200-400 eV atL ~ 9-11, and there is an additional from individual speciesoverlap. Nevertheless,this spectrum (30%) hot componentat ~ 2 keV [Bagenal,1989]. The hot showsthe presenceof a hot locally picked-up background componentcan be attributed to pickup ions created outside plasmaas found by Bagenal[1989]. The crowdingof six of Europa's orbit and heated adiabatically. The electron ion componentsinto 50 energychannels (Figures 3a and distribution function is also describedby two components: prevents a unique determination of plasma parameters. a coldcomponent (26 eV) witha numberdensity of 36 cm -3 An agreementbetween a numerical model and the results anda hot (1.2 keV) andtenuous component (3.1 cm-3) of obtained from the analysis of the PLS spectra is an im- suprathermal electrons. Any successfulmodel of a steady portant test for the model. Though the processof fitting state torus must reproduce these observationsof Europa's ion distribution functions to the PLS spectra is not unique environment. (i.e., the A/Z-•16 componentmight be attributedeither to O+ andS++.), general characteristics of the model param- PLS OBSERVATIONS eters (e.g., the total numberdensity and the partitioning The Voyager PLS instrument consistsof four modulated- between(cold)iogenic and (hot) eurogenicplasma) should grid Faradaycups (A, B, C andD) whichmeasure ions and match those of the fits. The poor quality of the fits to the D electronsin the energy-per-chargerange 10 to 5950 eV/q. cup currentsmay imply that significantelectron feedthrough SCHRI•II•RI•T AL.: MODI•LINGTHI• EUROPAPLASMA TORUS 21,233 VOYRGER SPECTRRL DRTR 10• R CUP 106 B CUP Illl II III III III! . •0 5 _ •o s z • - • . •o • - - 103 I I I I I I I I I I I I I I I lO3 0 •2 6q 96 128 0 32 6q 96 128 CHRNNELc•NBER 106 lO6 I""'"'""'"l •o 5 ,JOS z z w 8o • •o • lOs 0 32 6q 96 128 o 32 60, 96 128 CHANNEL NUMBER CHANNEL NUMBER JUPITER MRCNETOSPNERE B - (-36.388 , qq.816 , -71.952 ) CANNA VOYAGER 2 CHARGE DENSITY ESTIMFITE = (11.9367 ) 1979 190 2000:10.0q5 VCUP - (96.509 , 102.57q , 103.225 , 16.127 ) KM/S R = 10.290 RHO(HAG) = 10.180 Z(MRG) = -1.506 RIGID COROTATION SPEED = 127.763 VOYAGER SPECTRAL DRTR R CUP 108 lO6 B cup .