1. LETTER doi:10.1038/nature09928
The auroral footprint of Enceladus on Saturn
Wayne 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 Zhou15
Although 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. At
activity 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 wave
associated with the Io–Jupiter coupling, for example, create an current system driven by Io. It is possible that the flickering in energy of
auroral 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 are
magnitude dimmer than Io’s footprint and below the observable ´
controlled by the product of the Alfven wave travel time towards Jupiter
threshold, consistent with its non-detection13. Here we report the and the plasma convection speed past the moon16. If this value, in units
detection of magnetic-field-aligned ion and electron beams (offset of the moon’s radius, is larger at Saturn, then the beams are expected to
several 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 the
of Enceladus-associated aurora in a few per cent of the scans of the electron density derived from Cassini data17 and assuming a dipole
moon’s footprint. The footprint varies in emission magnitude field). Assuming an average plasma velocity of ,20% of full co-rotation
more than can plausibly be explained by changes in magneto- between Enceladus and the onset of the beams, we find a downstream
spheric parameters—and as such is probably indicative of variable shift of the beams of 3.6 RE. This is consistent with the observed distance
plume 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 the
Cassini approached Enceladus from downstream (with respect to the field-aligned electrons are incident on Saturn’s ionosphere, the
background 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 is
plasma sensors into the optimum orientation for measuring along above the measurement threshold of the Cassini UltraViolet Imaging
Saturn’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 three
Saturn’s northern hemisphere (Fig. 1). Neither sensor was accessible successive polar views (two of which are shown here as Fig. 2) that
to particles originating from Saturn’s southern ionosphere. Beams were show an unambiguous auroral footprint (boxed area at top left of
observed 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 oval
before 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 with
ceased. (The final burst of low energy electron flux observed after closest emissions due to electrons precipitating on atomic and molecular
approach 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 in
distribution 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 magnetospheric
electrons flicker in energy between peaks near 10 eV and 1 keV; bi- conditions. (The southern footprint would occur at 61.7u S because
modal electron populations are observed for about 1 min either side of Saturn’s magnetic dipole, although spin-aligned within observational
these transitions (Fig. 1b). These changes in the characteristic energy of uncertainties, is displaced about 0.04 RS (Saturn radii) northward from
the field-aligned electron flux, while not currently well understood, are Saturn’s geometric centre22.) The modelled footprint latitude is not
1
Science Department, Central Arizona College, Coolidge, Arizona 85128 USA. 2Space Environment Technologies, Pacific Palisades, California 90272, USA. 3Applied Physics Laboratory, Johns Hopkins
University, Laurel, Maryland 20723, USA. 4Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA. 5Space Science and Engineering Division, Southwest Research Institute, San
Antonio, 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 of
Physics, University of Central Florida, Orlando, Florida 32816, USA. 17Astronomy Department, Boston University, Boston 02215, USA.
*These authors contributed equally to this work.
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2. 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-rotation
Figure 1 | Cassini particle and field observations on 11 August 2008. (MAG)28,29. Positive DB (red) is in the direction of co-rotation; the signature
a, 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 is
22 keV) measured by the most field-aligned detectors of the CAPS Electron the total upward field-aligned electron (FAE) energy flux derived by numerical
Spectrometer (ELS)27. Electron differential energy flux (DEF) is indicated by the integration of the electron data in b (see Supplementary Information for
colour bar. Field-aligned electrons are observed when the instrument measured additional discussion and error analysis). Calculations of electron energy loss in
within ,20u of the magnetic field line, as indicated by the white line (pitch H2 atmospheres indicate that 1 mW m22 of particle energy input produces
angle). The blacked out regions are data gaps. c, Azimuthal perturbation (DB) ,10 kR of auroral ultraviolet emission30. The observed electron energy flux is
in the magnetic field during this interval, calculated by subtracting a model therefore expected to produce a ultraviolet emission brightness between
background 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 with
if 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 we
mission. 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 pixel
Figure 2 | Cassini images of Saturn’s northern aurora, including the image represents two spacecraft slews across the planet. The colour bar shows
Enceladus 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 the
02:16–03:28 UT (a) and 03:38–04:50 UT (b). Images were formed by slowly predicted magnetic mapping of the satellite Enceladus to Saturn’s dayside
slewing the spacecraft and its long-slit ultraviolet spectrometer. During this atmosphere. Satellite footprint emission is visible in both boxes. The north pole
interval, 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 line
from 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.
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3. LETTER RESEARCH
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Supplementary Information is linked to the online version of the paper at
monitoring 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 auroral
1. 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 text
2. 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 in
3. 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. is
4. 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 at
8. 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 at
9. 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. (abigail.rymer@jhuapl.edu).
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