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LETTER doi:10.1038/nature09928The auroral footprint of Enceladus on SaturnWayne R. Pryor1,2*, Abigail M. Rymer3*, Donald G. Mitchell3, Thomas W. Hill4, David T. Young5, Joachim Saur6,Geraint H. Jones7,8, Sven Jacobsen6, Stan W. H. Cowley9, Barry H. Mauk3, Andrew J. Coates7, Jacques Gustin10, Denis Grodent10,Jean-Claude Gerard10, Laurent Lamy11, Jonathan D. Nichols9, Stamatios M. Krimigis3,12, Larry W. Esposito13, ´Michele K. Dougherty14, Alain J. Jouchoux13, A. Ian F. Stewart13, William E. McClintock13, Gregory M. Holsclaw13,Joseph M. Ajello15, Joshua E. Colwell16, Amanda R. Hendrix15, Frank J. Crary5, John T. Clarke17 & Xiaoyan Zhou15Although there are substantial differences between the magneto- associated with changes in the magnetic field perturbation (Fig. 1c),spheres of Jupiter and Saturn, it has been suggested that cryovolcanic suggesting an actual change in the total field-aligned current density. Atactivity at Enceladus1–9 could lead to electrodynamic coupling Jupiter, variations in auroral radio emission14 and a ‘string-of-pearls’between Enceladus and Saturn like that which links Jupiter with ultraviolet aurora associated with the Io footprint15 have been inter-Io, Europa and Ganymede. Powerful field-aligned electron beams ´ preted as being due to multiple reflections of a standing Alfven waveassociated with the Io–Jupiter coupling, for example, create an current system driven by Io. It is possible that the flickering in energy ofauroral footprint in Jupiter’s ionosphere10,11. Auroral ultraviolet the beams observed downstream of Enceladus is the equatorial sig-emission associated with Enceladus–Saturn coupling is anticipated nature of a standing wave pattern like that observed at the Io footprint.to be just a few tenths of a kilorayleigh (ref. 12), about an order of It has been suggested that the locations of beams observed near Io aremagnitude dimmer than Io’s footprint and below the observable ´ controlled by the product of the Alfven wave travel time towards Jupiterthreshold, consistent with its non-detection13. Here we report the and the plasma convection speed past the moon16. If this value, in unitsdetection of magnetic-field-aligned ion and electron beams (offset of the moon’s radius, is larger at Saturn, then the beams are expected toseveral moon radii downstream from Enceladus) with sufficient be further downstream than at Io.power to stimulate detectable aurora, and the subsequent discovery We estimate the wave travel time to be of the order of 150 s (using theof Enceladus-associated aurora in a few per cent of the scans of the electron density derived from Cassini data17 and assuming a dipolemoon’s footprint. The footprint varies in emission magnitude field). Assuming an average plasma velocity of ,20% of full co-rotationmore than can plausibly be explained by changes in magneto- between Enceladus and the onset of the beams, we find a downstreamspheric parameters—and as such is probably indicative of variable shift of the beams of 3.6 RE. This is consistent with the observed distanceplume activity. downstream from the moon where the beams begin. However, given There have been 12 close Cassini encounters with Enceladus in the six the spatial1 and (likely) temporal18,19 variability of the Enceladus vents,years since the spacecraft arrived at Saturn. During a fly-by on 11 August filamentary current structures associated with local variable mass load-2008, the spacecraft passed within 55 km of the moon at 21:06 UT. ing might contribute to the variability of the observations. Assuming theCassini approached Enceladus from downstream (with respect to the field-aligned electrons are incident on Saturn’s ionosphere, thebackground plasma flow) while moving north–south (Supplemen- observed flux excites hydrogen molecules at Enceladus’ footpoint, pro-tary Fig. 1). Just before closest approach, a spacecraft roll brought two ducing ultraviolet emission between 3 6 0.2 and 12 6 3.0 kR. That isplasma sensors into the optimum orientation for measuring along above the measurement threshold of the Cassini UltraViolet ImagingSaturn’s (approximately dipolar) magnetic field lines. At this time, Spectrograph (UVIS)20.powerful ion and electron beams were observed propagating from Two weeks later, on 26 August 2008, the UVIS recorded threeSaturn’s northern hemisphere (Fig. 1). Neither sensor was accessible successive polar views (two of which are shown here as Fig. 2) thatto particles originating from Saturn’s southern ionosphere. Beams were show an unambiguous auroral footprint (boxed area at top left ofobserved from 3.6 to at least 23.3 RE (radius of Enceladus RE 5 252 km) Fig. 2a and b). UVIS spectra of the footprint look similar to the simul-downstream (positive X in Fig. 1) from Enceladus. At 21:05 UT, ,1 min taneously measured emissions from the brighter main auroral ovalbefore closest approach, with Cassini still 3.6 RE downstream of the seen near 75u latitude in Fig. 2. Both compare well with an H2 elec-moon, the flow of magnetic-field-aligned ions and electrons abruptly tron-impact laboratory spectrum and are thus consistent withceased. (The final burst of low energy electron flux observed after closest emissions due to electrons precipitating on atomic and molecularapproach at ,21:07 in Fig. 1b is actually the tail of a non-field-aligned hydrogen at an emission altitude ,1,100 km above the 1 bar level indistribution and is produced by a different process to that which pro- the atmosphere21. Using this altitude and a quantitative Saturn mag-duces the beams.) netic field model22, we calculate that the northern Enceladus footprint At approximately 20:59 and 21:02 UT, the magnetic-field-aligned should occur at a latitude of 64.5u N for nominal magnetosphericelectrons flicker in energy between peaks near 10 eV and 1 keV; bi- conditions. (The southern footprint would occur at 61.7u S becausemodal electron populations are observed for about 1 min either side of Saturn’s magnetic dipole, although spin-aligned within observationalthese transitions (Fig. 1b). These changes in the characteristic energy of uncertainties, is displaced about 0.04 RS (Saturn radii) northward fromthe field-aligned electron flux, while not currently well understood, are Saturn’s geometric centre22.) The modelled footprint latitude is not1 Science Department, Central Arizona College, Coolidge, Arizona 85128 USA. 2Space Environment Technologies, Pacific Palisades, California 90272, USA. 3Applied Physics Laboratory, Johns HopkinsUniversity, Laurel, Maryland 20723, USA. 4Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA. 5Space Science and Engineering Division, Southwest Research Institute, SanAntonio, Texas 78238, USA. 6Institut fur Geophysik und Meteorologie, Universitat zu Koln, Cologne, D-50923, Germany. 7Mullard Space Science Laboratory, Department of Space and Climate Physics, ¨ ¨ ¨University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK. 8The Centre for Planetary Sciences at University College London/Birkbeck, London WC1E 6BT, UK. 9Department of Physics and 10 ´Astronomy, University of Leicester, Leicester LE1 7RH, UK. Laboratoire de Physique Atmospherique et Plane ´taire, Departement d’Astrophysique, Geophysique et Oceanographie, Universite de Lie ´ ´ ´ ´ `ge,Lie B-4000, Belgium. 11Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique, Observatoire de Paris, Centre National de la Recherche Scientifique, Universite Pierre et Marie Curie, `ge, ´Universite Paris Diderot, 92195 Meudon, France. 12Academy of Athens, Soranou Efesiou 4, 115 27, Athens, Greece. 13Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, ´Colorado 80303, USA. 14Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London SW7 2AZ, UK. 15Jet Propulsion Laboratory, Pasadena, California 91109, USA. 16Department ofPhysics, University of Central Florida, Orlando, Florida 32816, USA. 17Astronomy Department, Boston University, Boston 02215, USA.*These authors contributed equally to this work. 2 1 A P R I L 2 0 1 1 | VO L 4 7 2 | N AT U R E | 3 3 1 ©2011 Macmillan Publishers Limited. 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RESEARCH LETTER 20:52:30 20:55 20:57:30 21:00 21:02:30 21:05 21:07:30 Proton diff. flux a 20 (cm–2 sr–1 s–1 keV–1) 55–90 keV ion flux INCA 10 0 b 1 1012 DEF (s–1 m–2 sr–1) log(energy (keV)) Closest ELS electrons Pitch angle (°) 0 approach 30 Datagap 1011 –1 20 1010 –2 10 109 c 20 2.0 Upward electron Azimuthal B-field perturbation (nT) flux (mW m–2) 10 1.0 0 0.0 Anti-draped ΔB –10 Draped ΔB –1.0 Upward FAE flux +Z North Time (UT) 20:55 21:00 21:05 21:10 X (REnc) 23.3 13.4 3.6 –6.2 +Y +X Y (REnc) –2.0 –1.4 –0.4 0.8 Toward Along Z (REnc) 41.7 23.0 4.3 –14.3 Saturn co-rotationFigure 1 | Cassini particle and field observations on 11 August 2008. (MAG)28,29. Positive DB (red) is in the direction of co-rotation; the signaturea, Protons (55–90 keV) observed by the Cassini Ion and Neutral Camera expected from simple field line draping around an obstacle is characterized by(INCA)26. Colours indicate proton differential flux. Contours 30u and 60u from negative DB (blue) above the equator. Positive DB (red) indicates an anti-the magnetic field direction are overplotted in white. b, Electrons (1 eV to draped perturbation in the super-co-rotational sense. Overplotted in green is22 keV) measured by the most field-aligned detectors of the CAPS Electron the total upward field-aligned electron (FAE) energy flux derived by numericalSpectrometer (ELS)27. Electron differential energy flux (DEF) is indicated by the integration of the electron data in b (see Supplementary Information forcolour bar. Field-aligned electrons are observed when the instrument measured additional discussion and error analysis). Calculations of electron energy loss inwithin ,20u of the magnetic field line, as indicated by the white line (pitch H2 atmospheres indicate that 1 mW m22 of particle energy input producesangle). The blacked out regions are data gaps. c, Azimuthal perturbation (DB) ,10 kR of auroral ultraviolet emission30. The observed electron energy flux isin the magnetic field during this interval, calculated by subtracting a model therefore expected to produce a ultraviolet emission brightness betweenbackground field from the total field measured by the Cassini magnetometer 2.8 6 0.2 and 11.9 6 3.0 kR.very sensitive to auroral altitude; it would shift only 0.04u equatorward The location of the observed northern footprint is consistent withif the assumed auroral altitude were increased to 1,200 km. It is also not the expected location. The brightness centroid of the first spot (Fig. 2a)very sensitive to the size of the magnetospheric cavity, varying by only was at latitude 64.1u 6 0.4u and longitude 286.0u 6 0.5u, thus about,0.1u over the whole range of sizes observed during the 6-year Cassini 1.7u downstream of the sub-Enceladus longitude of 287.7u. Here wemission. have set errors equal to the pixel size. The brightness centroid of the a b 1 2 5 10 20 50 100 200 500 1,000 1 2 5 10 20 50 100 200 500 1,000 EUV counts per pixel EUV counts per pixelFigure 2 | Cassini images of Saturn’s northern aurora, including the image represents two spacecraft slews across the planet. The colour bar showsEnceladus auroral footprint. a, b, Successive UVIS EUV polar-projected EUV emission per pixel. The white boxes are centred on 64.5u N and the sub-images of Saturn’s north polar region from 26 August 2008 (day of year 239); Enceladus longitude, cover 4u in latitude and 10u in longitude, and enclose the02:16–03:28 UT (a) and 03:38–04:50 UT (b). Images were formed by slowly predicted magnetic mapping of the satellite Enceladus to Saturn’s daysideslewing the spacecraft and its long-slit ultraviolet spectrometer. During this atmosphere. Satellite footprint emission is visible in both boxes. The north poleinterval, Cassini moved from sub-spacecraft latitudes of 74u N to 65u N, and is at the centre; the latitude circles are 5u apart, and the hashed white linefrom 8.1 to 6.0 RS (Saturn radius RS < 60,300 km) from Saturn’s centre. Each indicates the day/night terminator. The Sun is to the left.3 3 2 | N AT U R E | VO L 4 7 2 | 2 1 A P R I L 2 0 1 1 ©2011 Macmillan Publishers Limited. All rights reserved
LETTER RESEARCHsecond spot was at latitude 64.3u 6 0.3u and longitude 316.8u 6 0.4u, 10. Connerney, J. E. P., Baron, R., Satoh, T. & Owen, T. Images of excited H31 at the foot of the Io flux tube in Jupiter’s atmosphere. Science 262, 1035–1038 (1993).about 0.8u downstream of the sub-Enceladus longitude of 317.6u. 11. Clarke, J. T. et al. Ultraviolet emissions from the magnetic footprints of Io,These values are very close to the anticipated Enceladus footprint Ganymede, and Europa on Jupiter. Nature 415, 997–1000 (2002).latitude of 64.5u. This agreement confirms both the identification with 12. Pontius, D. H. Jr & Hill, T. W. Enceladus: a significant plasma source for Saturn’sEnceladus and the accuracy of the magnetic field model, a ‘ground- magnetosphere. J. Geophys. Res. 111, A09214, doi:10.1029/2006JA011674 (2006).truth’ observation which proved vital in confirming the detailed mag- 13. Wannawichian, S., Clarke, J. T. & Pontius, D. H. Jr. Interaction evidence betweennetic field configuration of Jupiter23 and is equally valuable at Saturn. Enceladus’ atmosphere and Saturn’s magnetosphere. J. Geophys. Res. 113,The predicted southern footprint has not yet been detected (28 of the A07217, doi:10.1029/2007JA012899 (2008). 14. Gurnett, D. A. & Goertz, C. K. Multiple Alfven wave reflections excited by Io, origin of310 non-detections were of the southern ionosphere); the southern the Jovian decametric arcs. J. Geophys. Res. 86 (A2), 717–722 (1981).footprint may be dimmer than its northern counterpart, as is the case 15. Bonfond, B. et al. UV Io footprint leading spot: a key feature for understanding thefor the main aurora24. UV Io footprint multiplicity? Geophys. Res. Lett. 35, L05107, doi:10.1029/ 2007GL032418 (2008). The 504 km diameter of Enceladus maps along Saturn’s magnetic 16. Jacobsen, S. J. et al. Location and spatial shape of electron beams in Io’s wake. J.field lines to a quasi-elliptical spot ,52 3 29 km in Saturn’s atmo- Geophys. Res. 115, A04205, doi:10.1029/2009JA014753 (2010).sphere, for average magnetospheric conditions. This spot would be 17. Persoon, A. M., Gurnett, D. A., Kurth, W. S. & Groene, J. B. A simple scale heightspatially unresolved by UVIS. For a steady fixed source, the UVIS model of the electron density in Saturn’s plasma disk. Geophys. Res. Lett. 33, L18106, doi:10.1029/2006GL027090 (2006).observations suggest emission connected to an Enceladus interaction 18. Saur, J. et al. Evidence for temporal variability of Enceladus’ gas jets: modeling ofregion at the equator extending as far as 20 RE downstream with a Cassini observations. Geophys. Res. Lett. 35, L20105, doi:10.1029/radial extent between 0 and 20 RE, consistent with the location of the 2008GL035811 (2008). 19. Smith, H. T. et al. Enceladus plume variability and the neutral gas densities inbeams observed at the equator. Saturn’s magnetosphere. J. Geophys. Res. 115, A10252, doi:10.1029/ The slewing UVIS slit passed over and recorded the Enceladus- 2009JA015184 (2010).related spot on 26 August 2008 at 3:00, at 4:20 and at 8:02 UT. The 20. Esposito, L. W. et al. the Cassini ultraviolet imaging spectrograph investigation.spot dims as it moves from near dawn towards noon (8:20 to 9:12 to Space Sci. Rev. 115, 299–361 (2004). ´ 21. Gerard, J.-C. et al. Altitude of Saturn’s aurora and its implications for the11:54 local time). The total combined (extreme ultraviolet (EUV) plus characteristic energy of precipitated electrons. Geophys. Res. Lett. 36, L02202,far ultraviolet (FUV)) spot brightnesses in the three UVIS images were doi:10.1029/2008GL036554 (2009).1,550 6 340 R, 1,130 6 200 R and 450 6 290 R. These should be con- 22. Burton, M. E., Dougherty, M. K. & Russell, C. T. Model of Saturn’s internal planetary magnetic field based on Cassini observations. Planet. Space Sci. 57, 1706–1713sidered lower limits, assuming the spatial pixel is uniformly filled by (2009).signal, because the true emission region size is not known (see Sup- ˜ 23. Connerney, J. E. P., Acunna, M. H., Ness, N. F. & Satoh, T. New models of Jupiter’splementary Information for more details). Thus (even when visible) magnetic field constrained by the Io flux tube footprint. J. Geophys. Res. 103,the Enceladus auroral footprint varies in brightness by a factor of about 11929–11939 (1998). 24. Nichols, J. D. et al. Saturn’s equinoctial auroras. Geophys. Res. Lett. 36, L24102,3. At Jupiter, footprint emission variability is principally caused by the doi:10.1029/2009GL041491 (2009).rocking of the magnetospheric plasma sheet, as the magnetic dipole 25. Kanani, S. J. et al. A new form of Saturn’s magnetopause using a dynamic pressuremoment is inclined with respect to the spin axis. At Saturn, there is no balance model, based on in situ, multi-instrument Cassini measurements. J. Geophys. Res. 115, A06207, doi:10.1029/2009JA014262 (2010).substantial rocking of the plasma sheet at the location of Enceladus, 26. Krimigis, S. M. et al. Magnetospheric imaging instrument (MIMI) on the Cassinibut we still see these large brightness variations. This variation could, mission to Saturn/Titan. Space Sci. Rev. 114, 233–329 (2004).in principle, reflect variations of plume activity18,19, of ionization rates 27. Young, D. T. et al. Cassini plasma spectrometer investigation. Space Sci. Rev. 114,(owing to varying background plasma conditions), or of magneto- 1–112 (2004). 28. Dougherty, M. K. et al. The Cassini magnetic field investigation. Space Sci. Rev. 114,spheric size (when the magnetosphere is compressed, auroral emis- 331–383 (2004).sions are generally enhanced). The last two factors do not typically 29. Jia, Y.-D. et al. Time varying magnetospheric environment near Enceladus as seenexhibit order-of-magnitude variations17,25. The most likely cause for by the Cassini magnetometer. Geophys. Res. Lett. 37, L09203, doi:10.1029/ 2010GL042948 (2010).the observed large-scale variability, therefore, is time-variable cryo- 30. Waite, J. Jr et al. Electron precipitation and related aeronomy of the Jovianvolcanism from Enceladus’ south polar vents, suggesting that plume thermosphere and ionosphere. J. Geophys. Res. 88, 6143–6163 (1983).activity was particularly high during August 2008. Thus, systematic Supplementary Information is linked to the online version of the paper atmonitoring of Enceladus’ ultraviolet auroral footprint might provide www.nature.com/nature.evidence of plume variability, which is an important open issue. Acknowledgements We acknowledge support from the NASA/ESA Cassini Project and NASA’s Cassini Data Analysis Program.Received 13 July 2010; accepted 10 February 2011. Author Contributions A.M.R. and W.R.P. discovered the electron beams and the auroral1. Porco, C. C. et al. Cassini observes the active south pole of Enceladus. Science 311, footprint, respectively, and wrote most of the paper. D.G.M. discovered the ion beams 1393–1401 (2006). and contributed to the text and interpretation. T.W.H. contributed extensively to the text2. Spencer, J. R. et al. Cassini encounters Enceladus: background and the discovery and interpretation. D.T.Y. is CAPS PI and contributed extensively to the text and of a south polar hot spot. Science 311, 1401–1405 (2006). interpretation. J.S., G.H.J., S.J., B.H.M. and A.J.C. advised on the interpretation of the in3. Dougherty, M. K. et al. Identification of a dynamic atmosphere at Enceladus with situ data. S.W.H.C. performed the field line mapping and provided advice on the paper. the Cassini magnetometer. Science 311, 1406–1409 (2006). J.G., D.G., J.-C.G., L.L. and J.D.N. advised on the interpretation of the UVIS data. S.M.K. is4. Tokar, R. L. et al. The interaction of the atmosphere of Enceladus with Saturn’s the MIMI PI and oversaw the ion data. M.K.D. is the MAG PI and oversaw the plasma. Science 311, 1409–1412 (2006). magnetometer data. L.W.E. is the UVIS PI and oversaw the UVIS data. A.J.J. and F.J.C.5. Jones, G. H. et al. Enceladus’ varying imprint on the magnetosphere of Saturn. designed the auroral observation campaign. A.I.F.S., W.E.M., J.M.A., J.E.C. and A.R.H. Science 311, 1412–1415 (2006). helped to process the UVIS data. J.T.C. provided advice on the HST observations. X.Z.6. Spahn, F. et al. Cassini dust measurements at Enceladus and implications for the contributed to auroral discussions related to comparisons with terrestrial auroral origin of the E ring. Science 311, 1416–1418 (2006). processes.7. Waite, J. H. et al. Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure. Science 311, 1419–1422 (2006). Author Information Reprints and permissions information is available at8. Hansen, C. J. et al. Enceladus’ water vapor plume. Science 311, 1422–1425 www.nature.com/reprints. The authors declare no competing financial interests. (2006). Readers are welcome to comment on the online version of this article at9. Brown, R. H. et al. Composition and physical properties of Enceladus’ surface. www.nature.com/nature. Correspondence and requests for materials should be Science 311, 1425–1428 (2006). addressed to A.M.R. (firstname.lastname@example.org). 2 1 A P R I L 2 0 1 1 | VO L 4 7 2 | N AT U R E | 3 3 3 ©2011 Macmillan Publishers Limited. All rights reserved