JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. E9, PAGES 20,125-20,148,AUGUST 30, 1998 Galileo orbiter ultraviolet observations of Jupiter aurora JosephAjello, • DonaldShemansky, 2 Wayne Pryor, 3 Kent Tobiska, 4 Charles Hord, 3 StuartStephens, 4 Ian Stewart, 3 John Clarke, • Karen Simmons, 3 William McClintock, 3 CharlesBarth, 3Jeremy Gebben, 3Deborah Miller, 3 and Bill Sandel6 Abstract. In 1996 duringthe first four orbitsof the satellitetour the Galileo ultraviolet spectrometer(UVS) (11304320/•) and extreme ultraviolet spectrometer (EUVS) (540-1280/•) performednear-simultaneous observations of theJupiter aurora in boththe northand south polar regions.These observations are modeledto providethe absolute surface brightness of the aurora from the H2 RydbergSystems (B, B', B", C, D, D' -->X bandsystems). The spectraldistribution andbrightness of theEUV auroraare sensitiveto H2 abundance,H2 temperature,and CI-I• abundance.Analysis of theemission spectra indicates that the EUV aurora (800-1200/l 0 are producedover a rangeof altitudescorresponding to slant column abundances of H2 from 10 •6 to 1020cm '•' or greater. The UVS spectra of thefar ultraviolet (FUV) from 1130 to 1700]i are opticallythin in H2,but highlysensitive to theCI-h columnabundance and to the secondary electronenergy distribution. The slantcolumn abundance of CI-I• absorbersfound from modelsof theFUV spectravaried in therange 0 - 10x 10•6 cm '2, indicating the presence of bothhigh altitude aurora,at or abovethe homopause,and deep aurora. The FUV spectrashow C2H•absorption bands near1520/•. The surface brightness ofthe aurora from the H2 Rydberg Systems ranged from 100 to 600 kR and of H Lyman c• was60 to 130 kR for a 2000 km wide oval. The totalpower input to theatmosphere from particle deposition is estimatedto be ~ 1 x 10•4 W. I. Introduction energy output of the aurora and through self-absorption is Galileo began its planned l 1-orbit tour of the Jupiter sensitive to H a abundanceand temperature,as well as CHn systemin December1995. The I.JV subsystemon the Galileo abundance.The energeticsand CH4 abundancesought to be orbiter consists of two separatespectrometers. The extreme self-consistent between the two sets of data. ultravioletspectrometer (EUVS) measuresradiation from 540 These unique observations consisting of combined EUV to 1280• andis on thespinning portion of the spacecraft.and UVS spectraare an important step in understandingthe The ultraviolet spectrometer(UVS)is mounted on the scan Jupiter aurora. Voyager auroralEUV spectraobtained inside platformand operates over the rangefrom 1130 to 4320 • the magnetospherewere partially compromisedby a high [Hord et al., 1992]. In the early orbits in 1996 at the time radiation environment.'A single preliminary model is shown the boresight of the EUVS crossedthe dawn terminator of by Broadfoot et al. [1981]. Morphological studies by Jupiter, simultaneousEUVS and UVS observations of the Herbert et al. [1987] of the northern aurora, from the aurora from the same source location were performed. The outbound leg of Voyager 2, pinpointed the System 1I[ spectra measured by each instrument provide important longitude,)•- 210øasa regionof maximumbrightness of H information about the energetics and structureof the aurora. Lytx and Ha bandsand the southernmostextent of the aurora The UVS far ultraviolet (FUV) observations furnish the CHn as the footprint of the Io toms. In addition, the Voyager column abundanceabove the aurora and energy output of the UVS spectral resolution was not sufficient to observe the Rydbergband systemsof H a, but provide no information on CHn absorptionstructure. Recently, Morrissey et al. [1997] the H a foregroundabundance. The EUVS also measuresthe obtainedsimultaneous EUV/FUV spectra (900-1650 ]•) and imaging of the Jovian aurorawith the Hopkins Ultraviolet Telescope (HUT)and Hubble Space Telescope (HST). The HUT model analysis was limited to the wavelengthsfrom •JetPropulsion Laboratory, California Institute of Technology, 1000to 1650]k usingmolecular parameters of the H 2 Lyman Pasadena. and Werner system.Significant residuals existed in the 900- 2Department of Aerospace Engineering, University of Southern California, Los Angeles. 1000 ]i wavelengthregion from the unmodeledhigher 3Laboratory for Atmospheric and Space Physics, University of Rydberg states. The Galileo EUVS observations provide Colorado, Boulder. excellent signal-to-noise (S/N) measurements inside the nFederal Data Corporation, Jet Propulsion Laboratory, Pasadena, Jovian magnetosphere. The emergent spectrum from the Califomia. •SpacePhysics Research Laboratory, University of Michigan,Ann Jupiter atmosphereis sensitive to gas temperatureand I-Ia Arbor. abundancealong the optical path. The resonancebands (v' 6 Lunarand Planetary Laboratory, University of Arizona,Tucson. 0)of the Rydberg band systems [Ajello etal., 1984, 1988i appearin the 850-1100]i wavelengthrange. If the gas temperatureis near 1000 K and the Ha columndensity above Copyright1998 by the AmericanGeophysical Union. 10•g cm a, thenthe atmosphereis thick for both the (v', 0) Papernumber 98JE00832. and (v', 1)progressions,which extendto 1160 ]k, a 0148-0227/98/98JE-00832509.00. spectral region that begins to include the UVS. Multiple 20,125 20,126 AJELLO ET AL.: EUV AND FUV OBSERVATIONS OF JUPITER AURORA I SUN 50 Rj 00 Rj . <15'Rj ß .:.-: RADIATION • CONSTRAINT' ENCOUNTER .......... PERIOD -•-Z-AX!S APPROACH EUV + I o TORUS S/C PERIOD MAGNETOSPHERE .i.:::.?.::!:::.::!?.::.i.ii?.::!:::.i:.:::.i:.i...........P..E. RIOD i::i::i Figure 1. North ecliptic view of the Galileo trajectory during the orbital tour. The spacecraftz axis is nominally pointed in the Earth direction,and the UVS/EUVS can be boresightedto look at the toms and aurora.The orbit numbersare indicated.The white area is the time during which the bulk of the UVS/EUVS observationswere planned.Jupiter and the Io toms are available in the region called Io toms period. The 15 Rj radiation constraint is a predictedclosest approachdistance to operate the instrument. The dark stippledarea is the time when the cone angleof Jupiter system is less than '90ø and unobservableby the UVS becauseof spacecraftobstructions. scatteringin these bands is unimportant,since emissions in days per orbit for the toms and 1/4 day (6 hours) for the opticallythick bandsare fluorescedat longerwavelengths in auroralobservations by the EUVS. UVS observationsof the optically thin bands of the same progression. aurorawere planned for each orbit to accompanythe EUVS. In this paper we report the EUVS auroralobservations The total Observationtime per pole per orbit, considering obtained on the "Big Four" set of orbits. The three sets of the small duty cycle of the spinning EUVS, was 12 s. observations begin just prior to the satellite/Jupiter Excellent EUV spectra were obtained from these brief encounters:G1, C3, and E4. There were no EUVS/UVS aurora exposures.The EUVS acquiredhigh-quality spectra of the or torus spectra on G2. These first four orbits have large aurora because of the smaller than expected radiation distances from Jupiter during the toms and aurora environment and an increased thickness of radiation observationperiods, roughly definedby 90 ø Jupiter phase shieldingrelative to the Voyager UVS. Modeling techniques angle. These orbits provided the least interferencefrom developedby Shemansky[1985] wereused as a basis for the radiation and the longest and most sensitiveobservations of EUV and FUV models. the Jupiter auroraand Io toms. The geometry is shown in Previous spectral information about the U V aurora is Figure 1 with the Io toms period. The period included4 3/4 known from two decades of FUV measurements by AJELLOET AL.: EUV AND FUV OBSERVATIONSOF JUPITERAURORA 20,127 InternationalUltraviolet Explorer(IUE), HUT, and HST thedata into two wavelengthregions: the blue 1230-1300/• Goddardhigh-resolution spectrometer (GHRS). The past andthe red 1557-1619 /• asproposed by Yunget al. [1982]. historyof FUV observationsis extensive,consisting of Theratio of red/bluehas been used as a measureof the CH4 both high-resolutionspatial imaging (HST) in the Lyman (principalhydrocarbon) optical depth in front of the auroral bandsand H Lymantx andspectral measurements of varying layerwith SystemIll longitude.The two principalresults of spectral coverage and resolution (GHRS high resolution this studyshowed that the auroralbrightness and the color echellespectra of 0.07 /• full widthat half maximumratio peakedat 180ø longitude,indicating a maximum (FWHM)to IUElow resolution of 11 /• FWHM)[Clarke et columndensity of CH4 at 180ø longitude with a variationof al., 1994; Feldmanet al., 1993; Durranceet al., 1982; a factorof 2-3. This resultcould be interpretedas arising Livengoodet al., 1990;Trafton et al., 1994;Prange et al., froman atmospherethat is tmiformin longitudebut has a 1995,1997a,b; Harris et al., 1996]. longitudedependent energy in primary precipitating The mostrecent spectral model of the FUV aurorawas particles.A reconciliationof the long-termIUE resultsof publishedby Liu andDalgarno [1996]. Using a primeset of Livengood(e.g., maximumbrightness and color ratio at HSTGHRS observations from past observer cycles, Liu and 180ø longitude) with the WFPC 2 andFOC imaging is given Dalgarnoderive Ha auroral rotational temperatures of 400- byPrange et al. [1997a]and Ballester et al. [1996].Prange 600 K. The modelincludes the line transitionprobabilities et al. [1997a]give a summaryof the southernaurora from of Abgrall et al. [1993a,b],