Anti-planetward auroral electron beams at Saturn.

Abstract

Strong discrete aurorae on Earth are excited by electrons, which are accelerated along magnetic field lines towards the planet. Surprisingly, electrons accelerated in the opposite direction have been recently observed. The mechanisms and significance of this anti-earthward acceleration are highly uncertain because only earthward acceleration was traditionally considered, and observations remain limited. It is also unclear whether upward acceleration of the electrons is a necessary part of the auroral process or simply a special feature of Earth's complex space environment. Here we report anti-planetward acceleration of electron beams in Saturn's magnetosphere along field lines that statistically map into regions of aurora. The energy spectrum of these beams is qualitatively similar to the ones observed at Earth, and the energy fluxes in the observed beams are comparable with the energies required to excite Saturn's aurora. These beams, along with the observations at Earth and the barely understood electron beams in Jupiter's magnetosphere, demonstrate that anti-planetward acceleration is a universal feature of aurorae. The energy contained in the beams shows that upward acceleration is an essential part of the overall auroral process.

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Anti-planetward auroral electron beams at Saturn
J. Saur1†, B. H. Mauk1, D. G. Mitchell1, N. Krupp2, K. K. Khurana3, S. Livi1, S. M. Krimigis1, P. T. Newell1,
D. J. Williams1, P. C. Brandt1, A. Lagg2, E. Roussos2& M. K. Dougherty4
Strong discrete aurorae on Earth are excited by electrons, which
are accelerated along magnetic field lines towards the planet1.
Surprisingly, electrons accelerated in the opposite direction have
beenrecentlyobserved2–6.Themechanismsandsignificanceofthis
anti-earthward acceleration are highly uncertain because only
earthward acceleration was traditionally considered, and obser-
vations remain limited. It is also unclear whether upward accel-
erationoftheelectronsisanecessarypartoftheauroralprocessor
simply a special feature of Earth’s complex space environment.
Here we report anti-planetward acceleration of electron beams in
Saturn’smagnetospherealongfieldlinesthatstatistically mapinto
regions of aurora. The energy spectrum of these beams is quali-
tativelysimilartotheonesobservedatEarth,andtheenergyfluxes
in the observed beams are comparable with the energies required
to excite Saturn’s aurora. These beams, along with the obser-
vations at Earth2–6and the barely understood electron beams in
Jupiter’s magnetosphere7,8, demonstrate that anti-planetward
acceleration is a universal feature of aurorae. The energy con-
tained in the beams shows that upward acceleration is an essential
part of the overall auroral process.
Measurements by the Cassini spacecraft9,10in Saturn’s magneto-
sphere reveal electron distributions on near equatorial segments that
contain electron beams (Fig. 1). These electron beams are strongly
aligned with respect to the local magnetic field and have great
similarity to those observed at Earth2–6.
In order to constrain the spatial occurrence of the electron beams,
weexamineallavailabledataofthefirstfourCassinispacecraftorbits
aroundSaturn(identifiedasSOI(Saturnorbitinsertion),0a,0b,and
0c).Thedataarebinnedintodifferentcategories,whichwedisplayas
a function of their position in Saturn’s magnetosphere (Fig. 2).
Segments of Cassini’s orbit marked red display regions where we
find magnetic-field-aligned electron beams. On the inbound pass—
that is, the dawn–noon sector of the magnetosphere—we see on two
orbits (0a, 0b) electron beams at radial distances between 11RSand
23RS(RSindicates Saturn’s radius). Measurements during the other
twoorbitsdonotsatisfyourobservationalconstraintsandrenderthe
electron distribution undetermined. On the outbound pass—that
is, in the post-midnight to dawn sector—we find electron beams
during orbits SOI, 0a and 0c at radial distances between 19RSand 33
RS. The spacecraft attitude on other parts of the outbound orbits
does not seriously constrain the electron distribution. On orbital
segments with electron beams we do not simultaneously observe ion
beams.
The electron beaming is observed over all available energies from
20keVto800keV,andnotjustoveranarrowrangeofenergy(Fig.3).
Theenergydistributionsofthebeamsfollowroughlyapowerlawasa
function of energy, with spectral index between 21.6 and 22.6. The
broad nature of the energy spectrum suggests a stochastic accelera-
tion process and not a simple potential drop along the magnetic field
lines, consistent with the analogous Earth beams2–6. While the
spacecraft is moving radially inward during orbit 0a, the energy
flux of the beams increases by about a factor of 10 at the lowest
energies.
Because of the similarity of the observed beams to the beaming
observed at Earth, both in terms of the narrowness of the beaming
LETTERS
Figure 1 | Segments of Cassini’s orbit in Saturn’s magnetosphere show
electron distributions that contain magnetic-field-aligned electron
beams. This distribution was taken during a 10min segment of Cassini’s
inbound orbit 0a at 11:27 on 26 October 2004 at a radial distance of 21.3RS
withaprioritychannelinanenergyrangefrom28.1to49keV.Zeroand1808
are along the locally measured magnetic field28, which is where we find
electron beams. Electron beam intensities f are displayed normalized to the
maximum value max(f) of the segment. The electron data are measured by
the low energy telescope of the Low Energy Magnetospheric Measurement
System (LEMMS)9,10on board the Cassini spacecraft. It measures the
electron distribution relative to the magnetic field direction in different
energychannelswithinatotalenergy rangefrom20.2keVto 829keVandan
instrumentfull-widthconeof158.Adetailedanalysisoftheangularwidthof
the equatorial beams, including the detector response function, provides an
upper limit of 108 for the angular half width, but permits any smaller angle.
A source distribution close to Saturn that is narrow in angular width at its
sourcewouldbeconsistentwithourobservedmaximum108width.Thisstill
holds if pitch angle scattering is included. With a pitch angle diffusion
coefficient29D0¼ 5 £ 1023s21taken from Jupiter’s magnetosphere at a
comparable radial distance, we estimate that beams with an energy of
,30keV widen by 88 while travelling from Saturn out to the location of the
Cassini spacecraft. If we assume alternatively that the source electron
distribution is not only broad in energy but also in its angular distribution,
that is, isotropic, then the action of the magnetic mirror force requires the
electrons to be accelerated close to Saturn in order to be field-aligned at the
equator. Under the assumption of isotropy, a maximum 108 width at the
spacecraft places the electron acceleration region somewhere below 5RS
from Saturn’s atmosphere. Additional pitch angle scattering requires the
origin of the beams to be much closer to Saturn.
1Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA.2Max-Planck-Institut fu ¨r Sonnensystemforschung,
37191 Katlenburg-Lindau, Germany.3Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, California 90095, USA.4Blackett Laboratory, Imperial College, London
SW7 2AZ, UK. †Present address: Institut fu ¨r Geophysik und Meteorologie, Universita ¨t zu Ko ¨ln, Albertus-Magnus-Platz, 50923 Ko ¨ln, Germany.
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and the breadth of the energies involved, our most reasonable
inference is that the origin of the electron beams is close to Saturn,
in analogy with the demonstrated origin of such beams at Earth2–6
(seealsoFig.1legend).Similarconclusionshavebeenreachedforthe
origin of electron beams in Jupiter’s magnetosphere7and in
the wake of Jupiter’s satellite Io11. Independent support for a near
Saturn source comes from Cassini radio observations12,13. These
place the source region of Saturn’s kilometric radiation, which is
associated with Saturn’s auroral electron acceleration12, over its
auroral zone.
In order to investigate the longitudes and latitudes where the
beams originate near Saturn, we use the latest magnetic field model14
developed with Cassini and Voyager data. In Fig. 4, we show the
electron beams, measured near the equator, mapped with the
magnetic field model into Saturn’s atmosphere as red data points.
We overlay the projected locations of the beams with two auroral
images15, obtained before the Cassini period when the beams where
observed. No auroral images are available for the same time period
when the Cassini spacecraft made the electron measurements.
Although Saturn’s aurorae vary in time16,17, our auroral images
(Fig. 4) as well as 68recent further images18provide agood statistical
framework for the average location of Saturn’s auroral features.
Auroral activity occurs over a range of latitudes that correspond
well to the spatial range where we find electron beams, that is, from
the magnetopause to as far inside as 11RS. In these images it appears
thatmainfeaturesofSaturn’sauroraearelocatedatlatitudesbetween
708 and 758, which correspond to Saturn’ s middle magnetosphere,
according to our field model. For example, the red data points near
808latitudeonthenoonside(Fig.4)characterizeelectronbeamsjust
inside the magnetopause boundary. Saturn’s aurorae lie in general
well equatorward of this boundary. From the recent 68 images18, an
extendedreferenceoval forquiet solar wind conditions19at728could
be extracted, which corresponds to 12RSin the noon sector of the
magnetosphere, where we observe electron beams. This shows that
main features ofSaturn’sauroraeoriginate inside the magnetosphere
significantly planetward of its magnetopause. Since the mapping of
the beams depends on the magnetic field model, we independently
also use a simple dipole magnetic field. The results are shown with
yellow data points in Fig. 4. With this simple model, some regions
giving rise to Saturn’s aurorae need to lie even further inside 11RS.
Beamsataradialdistanceof11RS,however,appeartobetoofaraway
fromtheopen/closedfieldlineboundariesorthetailofthemagneto-
sphere to be created there, the long-standing expectation for the
source regions of Saturn’s aurorae20–22. Our findings suggest that all
three planets—Saturn, Jupiter23and Earth24,25—have (to a varying
degree)contributionsto theirauroral sourceregionswellinside their
magnetospheric boundaries.
For the observed near-equatorial energy spectra of Fig. 3, we
estimate the corresponding energy flux at the low altitude source
region, that is, close to Saturn (Fig. 3 legend). We do not measure
electrons with energies less than 20keV, but owing to the beams’
power law behaviour, we expect a significant flux below 20keV. If we
therefore extrapolate the spectra to 2keV, we find an energy flux of
0.13mWm22close to Saturn, based on measurements at 21.4RS
(0.1mWm22not extrapolated). The projected fluxes at Saturn
increase strongly for measurements taken closer to Saturn. For
example, we find 1.5mWm22at 13.7RSalso extrapolated to 2keV
(0.3mWm22not extrapolated). If we assume that the electron beam
intensity correlates with auroral intensity, then the higher energy
fluxes further inside of Saturn’s magnetosphere additionally support
the idea that main features of Saturn’s aurorae originate deep inside
its magnetosphere.
Figure 2 | Electron beams in Saturn’s magnetosphere. Locations of
electron beams in Saturn’s magnetosphere for the first four orbits of
Cassini’s tour are shown in red. Segments in solid black indicate locations
that we positively identify not to be field-aligned; segments in thin black,
undetermined distribution (mostly owing to insufficient data coverage);
dark blue cross, innermost magnetopause crossing; turquoise,
magnetosheath; dotted circles are separated by 10RS(RS, Saturn’s radius)
and are for orientation purposes only. The x-axis is perpendicular to
Saturn’s spin axis and points in direction to the Sun. The y-axis is chosen so
that the x-axis, the y-axis and Saturn’s spin axis display a right-handed
coordinate system. Cassini travels in an anticlockwise fashion in these
panels.
Figure3|FourelectronbeamenergyspectraduringCassini’sinboundorbit
0a in 2004. Each spectrum is calculated from the electron distributions
(similar to Fig. 1) of eight different energy channels within the instrument
energy range of 20–800keV; four parameters for each spectrum are given in
the key, namely, the day of 2004, the time in the day, the magnetic field and
the radial distance from Saturn. In each distribution we only calculate the
energy within the beam, that is, we only count the electron flux that lies
within an 11.258 cone with respect to the local magnetic field. We can use
these spectra to estimate the total electron energy fluxes at the low altitude
source region (close to Saturn) by taking into account the characteristics of
theLEMMSdetectorandthewidthofthelosscone.Weassumethatthetotal
energy flux within a magnetic flux tube is equal at the equator and at the
originoftheacceleration.Thetotallowaltitudeenergyflux(20–800keV)for
thespectrumat13.7,19.9,20.6and21.4RSisrespectively0.34,0.16,0.1and
0.1mWm22( ¼ 1023Wm22), with a spectral power law slope of
respectively 22.6, 22.5, 22.1 and 21.6. We expect the energy spectrum to
extend (as a similar power law) to lower energies. When we extrapolate the
spectra to 2keV, we find a total electron energy flux of 1.5, 0.6, 0.18 and
0.13mWm22, respectively.
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Saturn’s observed auroral ultraviolet intensities are typically in the
range of 1kilorayleigh (1kR) to up to 70kR (ref. 16), which
corresponds to an energy flux of 0.1–7mWm22. The energy fluxes
that we infer from the observed electron beams of 0.1–1.5mWm22
are surprisingly large, and comparable to the energies needed to
excite Saturn’s aurorae.
Weconcludethatelectronbeamsareacceleratedaway fromSaturn
on field lines that map into the auroral regions of Saturn. At Earth,
anti-planetward accelerated electrons have also been observed2–6.
Their significance and their mechanisms remain however, uncertain,
both owing to a lack of comprehensive studies and owing to the fact
that currently no generally accepted model for the upward accelera-
tion exists, although a few ideas have been proposed26,27. With the
observations at Earth only, it is not clear whether anti-planetward
accelerated auroral electrons are unique to the special properties of
the complex Earth environment or if they are a general property of
aurorae.Recently,electronbeamshavealsobeenobservedinJupiter’s
magnetosphere7,8. Their origin and their physical relation to Jupiter’s
aurorae are, however, unclear, and the huge qualitative differences
between Jupiter’s and Earth’s magnetosphere bring into question the
extent to which the beams at Earth and Jupiter are analogous.
Observations of beams at Saturn, that is, in a magnetospheric
configuration that lies between that of Earth and Jupiter, suggest
that anti-planetward acceleration of electrons is a universal property
of aurorae. The energy fluxes in the anti-planetward accelerated
beams compete with the fluxes contained in the beams that are
directed towards the planet, and therefore generate aurorae. Thus in
addition to the universality of anti-planetward acceleration, Saturn’s
aurorae suggest that anti-planetward acceleration is also a key player
in the energetics of the auroral process.
Received 13 June; accepted 2 November 2005.
1.Paschmann, G., Haaland, S. & Treumann, R. Auroral Plasma Physics (Kluwer
Academic, Dordrecht, 2003).
Klumpar, D. M. in Physics of Space Plasmas (1989) (eds Chang, T., Crew, G. B. &
Jasperse, J. R.) 265– -276 (SPI Conference Proceedings and Reprint Series No. 9,
Scientific Publishers, Cambridge, Massachusetts, 1990).
Klumpar, D., Quinn, J. & Shelley, E. Counter-streaming electrons at the
geomagnetic equator near 9 earth radii. Geophys. Res. Lett. 15, 1295– -1298
(1988).
Carlson, C. et al. FAST observations in the downward auroral current region:
Energetic upgoing electron beams, parallel potential drops and ion heating.
Geophys. Res. Lett. 25, 2017– -2020 (1998).
Ergun, R. et al. FAST satellite observations of electric field structures in the
auroral zone. Geophys. Res. Lett. 25, 2025– -2028 (1998).
Marklund, M. et al. Temporal evolution of the electric field accelerating
electrons away from the auroral ionosphere. Nature 414, 724– -727 (2001).
Frank, L. A. & Paterson, W. R. Galileo observations of electron beams and
thermal ions in Jupiter’s magnetosphere and their relationship to the auroras.
J. Geophys. Res. 107, A1478, doi:10.1029/2001JA009150 (2002).
Toma ´s, A. et al. Energetic electrons in the inner part of the Jovian
magnetosphere and their relation to auroral emissions. J. Geophys. Res. 109,
A203, doi:10.1029/2004JA010405 (2004).
Krimigis, S. et al. Magnetosphere Imaging Instrument (MIMI) on the Cassini
Mission to Saturn/Titan. Space Sci. Rev. 114, 223– -329 (2004).
10. Krimigis, S. et al. Dynamics of Saturn’s magnetosphere from MIMI during
Cassini’s orbital insertion. Science 307, 1270– -1273 (2005).
11. Williams, D. J., Thorne, R. M. & Mauk, B. H. Energetic electron beams and
trapped electrons at Io. J. Geophys. Res. 104, 14739– -14753 (1999).
12. Kurth, W. S. An Earth-like correspondence between Saturn’s auroral features
and radio emission. Nature 443, 722– -725 (2005).
13. Gurnett, D. A. et al. Radio and plasma wave observations at Saturn from
Cassini’s approach and first orbit. Science 307, 1255– -1259 (2005).
14. Khurana, K. K., Arridge, C. S. & Dougherty, M. K. A versatile model of
Saturn’s magnetospheric field. Geophys. Res. Abstr. 7, 05970 (2005);
khttp://www.cosis.net/abstracts/EGU05/05970/EGU05-J-05970.pdfl.
15. Ge ´rard, J.-C. et al. Characteristics of Saturn’s FUV aurora observed with the
Space Telescope Imaging Spectrograph. J. Geophys. Res. 109(A9), A09207,
doi:10.1029/2004JA010513 (2004).
16. Clarke, J. T. et al. Morphological differences between Saturn’s ultraviolet
aurorae and those of Earth and Jupiter. Nature 433, 717– -719 (2005).
17. Prange ´, R. et al. An interplanetary shock traced by planetary auroral storms
from the Sun to Saturn. Nature 432, 78– -81 (2004).
18. Grodent, D., Gerard, J.-C., Cowley, S. W. T., Bunce, E. J. & Clarke, J. T. Variable
morphology of Saturn’s southern ultraviolet aurora. J. Geophys. Res. 110,
A07215, doi:10.1029/2004JA010983 (2005).
19. Crary, F. J. et al. Solar wind dynamic pressure and electric field as the main
factors controlling Saturn’s aurorae. Nature 433, 720– -722 (2005).
20. Bhardwaj, A. & Gladstone, G. R. Auroral emissions of the giant planets. Rev.
Geophys. 38, 295– -353 (2000).
21. Cowley, S. T. W., Bunce, E. & Prange ´, R. Saturn’s polar ionospheric flows and
their relation to the main auroral oval. Ann. Geophys. 22, 1379– -1394 (2004).
22. Cowley, S. T. W. et al. Reconnection in a rotation-dominated magnetosphere
and its relation to Saturn’s auroral dynamics. J. Geophys. Res. 110, A00201,
doi:10.1029/2004JA010796 (2005).
2.
3.
4.
5.
6.
7.
8.
9.
Figure4 |Saturn’selectronbeamsandaurorae. Shown are the locationsof
all observed electron beams that mapped into Saturn’s polar atmosphere
(red and yellow data points) over-plotted in comparison with two
snapshots15of Saturn’s aurora, a and b. Data points in red are calculated
with the latest magnetic field model14, data points in yellow are calculated
with a simple dipole magnetic field. To the left is the morning side of Saturn
and bottom is towards noon; circles display 708 and 808 planetary latitude.
Saturn’s aurora varies in time, but electron beams map in a statistical sense
well into regions of Saturn’s aurora. As a technical note, in the Khurana
magnetic field model14the dayside magnetopause is located at 20RS. For
orbits 0a and 0b, the magnetopause is located at 21.6RSand 22.9RS.
Therefore a few locations of electron beams cannot be mapped with this
magnetic field model into Saturn’s atmosphere, and are neglected in this
figure. (This figure has been reproduced from figure 3 of ref. 15. Copyright
2004 American Geophysical Union. Reproduced/modified by permission of
the AGU.)
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23. Clarke, J. T. et al. Ultraviolet emissions from the magnetic footprints of Io,
Ganymede and Europa on Jupiter. Nature 415, 997– -1000 (2002).
24. Feldstein, Y. I. & Galperin, Y. I. The auroral luminosity in the high-latitude
upper atmosphere: its dynamics and relationship to the large-scale structure of
the Earth’s magnetosphere. Rev. Geophys. 23, 217– -275 (1985).
25. Mauk, B. H. & Meng, C.-I. in Auroral Physics (eds Meng, C. I., Rycroft, M. J. &
Frank, L. A.) 223– -239 (Cambridge Univ. Press, Cambridge, 1991).
26. Temerin, M. & Carlson, C. W. Current-voltage relationship in the downward
auroral current region. Geophys. Res. Lett. 25, 2365– -2368 (1998).
27. Anderson, L. et al. Characteristics of parallel electric fields in the downward
current region of the aurora. Phys. Plasmas 9, 3600– -3609 (2002).
28. Dougherty, M. K. et al. The Cassini Magnetic Field Investigation. Space Sci. Rev.
114, 331– -383 (2004).
29. Williams, D. J. & Mauk, B. H. Pitch angle diffusion at Jupiter’s moon
Ganymede. J. Geophys. Res. 102, 24283– -24287 (1997).
Acknowledgements We thank D. Grodent for helpful discussions.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to J.S. (saur@geo.uni-koeln.de).
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