1
Tethys and Dione: Sources of outward flowing plasma in
Saturn’s magnetosphere
J. L. Burch,1 J. Goldstein,1 W. S. Lewis,1 D. T. Young,1 A. J. Coates,2 M. K. Dougherty,3 & N.
André4
1
Southwest Research Institute, P. O. Drawer 28510, San Antonio, Texas, 78228-0510 U.S.A.
2
Mullard Space Science Laboratory, Hombury St. Mary, Dorking, Sur RH5 6NT United Kingdom
3
Imperial College, Blackett Lab, London, SW7 2BZ United Kingdom
4
Research and Scientific Support Department, ESA, 2200 AG Noordwijk, The Netherlands
Saturn’s magnetosphere has proved to be one of the most dynamic and interesting in the
solar system. While plasma transport at Saturn is clearly dominated by rapid corotation,1,2
numerous strong inward plasma injections and subsequent dispersive drifts around the
planet have been observed and reported.3-5 The character of the plasma injections in the
inner region of the Saturn magnetosphere (between radial distances of roughly 6 to 9
Saturn radii) is observed to be consistent with several of the predictions of centrifugal
interchange theory. One of the major outstanding questions, up to now, has been how
plasma flows outward from the inner magnetosphere as is predicted from interchange
theory in order to prevent unlimited accumulation of the injected plasma. Does the outflow
occur in localized phenomena similar to the inward injection events and as predicted by
several theoretical models, or is there a general outwelling of plasma with no discrete
events? The research reported in this paper demonstrates that general outwelling is what
in fact occurs. This conclusion is based on electron pitch-angle distributions of the so-called
butterfly shape, which imply sources at localized radial distances well inside the point of
observation. These distributions fill the spaces between the discrete injection events in the
events reported here. The shapes of the butterfly distributions are consistent with sources
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near the orbits of the satellites Dione (for electrons with energies of several keV) and
Tethys (for energies below 1 keV).
Magnetospheres are the plasma-filled regions surrounding planets whose physical phenomena
are dominated by planetary magnetic fields. Unlike at the Earth, where planetary rotation (and
the resulting corotation of plasma) produces observable but globally minor effects on the
transport of plasma in the magnetosphere, the dynamics of the giant magnetospheres of Jupiter
and Saturn are dominated by corotation.6 For example, at the Earth any significant effects of
planetary rotation on the plasma extend only out to about half the distance to the sunward
boundary of the magnetosphere, while at Saturn and Jupiter corotation effects remain strong all
the way out to this boundary.
In rapidly rotating magnetospheres it has long been predicted that the centrifugal
interchange instability should be important for plasma transport.7-9 Centrifugal interchange is the
plasma analog of the Rayleigh-Taylor instability in stratified fluids, but instead of a gravitydriven turnover of fluids there is an exchange of plasmas of different characters that is driven by
centrifugal force.10 The theory of centrifugal interchange at Jupiter was developed
comprehensively over many years leading up to the observations made by the Galileo
spacecraft.11-13 At Jupiter several clear events were observed near the orbit of Io, which provides
a strong volcanic source of plasma that is easily distinguished from the surrounding ambient
plasma. For the observed cases, only inward transport of plasma was seen. However, to prevent
unlimited accumulation of plasma in either the outer or inner regions of the magnetosphere, both
inward and outward transport must occur.
Since its arrival in the Saturn system in summer 2004, Cassini has revealed a very dynamic
magnetospheric environment, with strong plasma injection events occurring frequently and at all
local times in the inner magnetosphere at radial distances between about 6 and 9 Saturn radii
(RS).3-5 These events can be observed both locally (where and when they occur) and remotely
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(through observation of the drifts of injected ions and electrons around the planet). Although
dramatic in character—because of the clear difference in density and temperature of the injected
plasma from those of the surrounding plasma—the radial flow velocities of the events are
predicted to be very small compared to the rotation velocity and so are difficult to observe
directly. For Saturn, as for Jupiter, all of the published events are characteristic of inward
injection.
We show here evidence for widespread outward plasma transport in Saturn’s
magnetosphere using electron data from the Cassini Plasma Spectrometer (CAPS).14 These
observations involve analysis of particle pitch angle, which is defined as the angle between a
charged particle’s velocity vector and the magnetic field vector (B) about which it gyrates. The
inward injections are observed as very localized (several thousand to perhaps 20,000 kilometers
in width) events containing plasmas with higher temperatures and lower densities than their
surroundings.4 Within the injection events the higher-energy electrons at several keV have
trapped distributions (with fluxes peaked near 90° pitch angle) while the low-energy electrons
are often field-aligned, indicating the presence of field-aligned currents. We focus here on the
electrons in regions outside the injection events. These electrons have very unique angular
distributions, which have been previously described as “butterfly” (fluxes peaked at magnetic
pitch angles intermediate between 0° and 90° or between 90° and 180°). We conclude that the
butterfly distributions result from a general outflow of plasma that fills the regions outside the
inward injection events. Such angular distributions have been observed in the Earth’s
magnetosphere where they result from preferential loss of particles with pitch angles near 90°
because their drift paths intersect the dayside boundary of the magnetosphere, and this effect is
strongly energy dependent.15,16 When this effect occurs at all energies, as we have observed, then
it becomes a unique identifier of either an encounter with a localized reduced magnetic field, as
has been observed at Jupiter in the Io wake,17 or outward transport from a radially localized
source in a magnetic field that drops off with radial distance as occurs around magnetized
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planets. In such instances, the change from a pitch-angle distribution peaked at 90° to a butterfly
distribution is required to conserve the first adiabatic invariant (perpendicular energy divided by
magnetic field strength) as the magnetic field strength decreases. The fact that this effect is
observed by CAPS over a large range of radial distances eliminates the possibility of an
encounter with a localized weak magnetic field. Because the butterfly distributions will only
result from outward transport when the source region is localized in radial distance, it is possible
to use the measured pitch angle distributions to determine the planetary radial distance of the
source region. The results presented here show that the source region for outward transport is
near the orbit of the satellite Dione for electrons in the energy range of a few keV and near the
orbit of Tethys for electrons with energies from a few hundred eV to 1 keV, suggesting the
possibility of plasma tori associated with both of these satellites.18
Data from CAPS and the Cassini Magnetometer19 are shown in Figure 1 from an outbound
orbit segment through the nightside magnetosphere very near the equatorial plane. Two major
plasma injections can clearly be seen (near 18:57 and 19:05 UT) along with other very narrow
injections. In each case a clear density depression, a very large electron temperature increase,
and a significant increase in magnetic field magnitude are observed. These variations are
typically seen for injections close to the equatorial plane outside about 6 Rs, while the injections
observed at higher latitude typically show a diamagnetic effect (a decrease in magnetic field
strength) and less pronounced density depressions while still showing the temperature increase.
Since these injections involve lower densities and far higher energies than are seen in the
intervening regions, they are presumed to have source regions at larger distances where plasmas
are typically hotter and less dense. In addition, analysis of the vector components of the magnetic
field in these regions show a pronounced dipolarization as compared to the adjacent regions,
which should result from an inward plasma injection.
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The data shown in Figure 1 are for electrons with pitch angles near 90°, and the regions
between the injections seem quiescent with lower-energy and much weaker fluxes. However, at
pitch angles away from 90° the fluxes, while remaining at lower energies, reach values
comparable to those inside the injection events. Plots of count rate (proportional to energy flux)
versus pitch angle for eight different energies for the time period 1910-1915 UT are shown in
Figure 2. The double-peaked traces are clear examples of butterfly distributions. We note that at
the highest four energies (generally >a few keV) the peaks all occur at similar pitch angles (near
both 45° and 135°) while at the lower four energies the peaks occur at pitch angles significantly
closer to the field line (near 30° and 150°). As noted above, the butterfly distributions should
result from outward transport with a source region limited in radial distance. If so, then there will
be a critical pitch angle at which the magnetic field strength at the electron’s mirror point is
equal to the equatorial field strength at the source. Assuming a trapped distribution at the source
(peaked at 90°) and adiabatic transport, the peaks in the butterfly distributions will occur at the
critical pitch angles. As shown by the vertical lines in Figure 2, the distributions in the left-hand
panel are consistent with a source near the radial distance of Dione (6.26 RS) while those in the
right-hand panel are consistent with a source near the radial distance of Tethys (4.88 RS).
To confirm this mapping, it is helpful to analyze data obtained at the Dione radial distance;
the corresponding pitch-angle distributions for a different orbit on October 30, 2005 are shown in
Figure 3. The solid vertical line shows the critical pitch angle for Tethys (the critical angle for
Dione would be 90°). In addition to showing the expected trapped distribution at the higher
energy in the left-hand panel, butterfly distributions peaked near the Tethys critical pitch angles
are seen at lower energy in the right-hand panel. There is an occasional dip in the 90° fluxes at
the higher energies near Dione indicating the beginning of a weak butterfly distribution. Our
modeling shows that this observation is consistent with the finite width of a possible Dione
plasma torus.
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Our findings suggest that plasma originating near the orbits of Tethys and Dione are
important sources of outflowing plasma with energies from a few hundred eV to several keV in
the inner Saturn magnetosphere. As a result, the large amount of plasma data now being obtained
by Cassini can be used to remote sense plasma sources and transport throughout the inner
magnetosphere of Saturn. Figure 4 is a sketch that illustrates our results. The orbit of Cassini is
shown as it proceeds outward beyond the orbit of Dione. The observed inward plasma injections
are shown by the red lines along the orbit. Plasma in the suggested Dione and Tethys tori flow
outward, presumably as an effect of the interchange instability as evidenced by the appearance of
inward injections. Electron distributions with pitch angles peaked near 90° at the source evolve
to peaks toward the field line at specific angles determined by the ratio of the magnetic field
strength at the source and at the observation point. Although an electron source at the orbit of
Enceladus (at 3.95 RS) cannot be ruled out, we have not seen clear evidence of critical pitch
angles consistent with a source there (21° and 159° for Figure 2 and 30° and 150° for Figure 3).
It is therefore possible that the inward injections and the resulting turbulent mixing produced by
interchange do not reach inward to that radius.
While our suggestion of plasma sources at the orbits of Dione and Tethys is unique, our
observation of a general plasma outflow except for discrete injection events is supported by other
recently reported Cassini observations using different techniques.20-22
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Acknowledgements This research was supported by JPL Contract 959930 with SwRI. Helpful comments by Drs.
Frank Crary, Barry Mauk, Ed Sittler, Michelle Thomsen, and Hunter Waite are greatly appreciated.
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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 W.S.L. ([email protected]).
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Figure 1 Plasma and magnetic field data obtained by Cassini on October 28, 2004. a,
Energy-time spectrogram of electron counts from the CAPS-ELS instrument. The
counts are color-coded and are proportional to the electron energy flux. The pitch
angles of the particles for Anode 3 (shown) is near 90°, indicating locally trapped
particles. Eight anodes are used to cover nearly the full range of pitch angles from ~0°
to ~180°. b, Electron density integrated over the full ELS energy range from 1 eV up to
26 keV after subtraction of spacecraft photoelectrons. c, Electron temperature
calculated from ELS energy-angle distributions. d, Deviation of the magnetic field
magnitude from the ambient values, which varied from 30.1 to 24.5 nT across the plot.
At the bottom of the plot are noted the universal time (UT), the radial distance of Cassini
in units of the Saturn radius (Rs), the latitude (Lat) and local time (LT) of Cassini.
Figure 2 Scatter plots of the logarithm of counts (per 2 seconds) versus pitch angle for
eight energies (noted to the right of each plot) for 1910-1915 UT on October 28, 2004.
Vertical lines in the plots in the left-hand column (the four highest energies) denote the
critical pitch angle for a source region of trapped electrons at the orbit of Dione. The
corresponding lines in the right-hand column (the four lowest energies) denote the
critical pitch angle for a source region at Tethys’ orbit. The rapid falloffs in count rate
that appear across each plot are an artifact resulting from partial shadowing of the field
of view by other Cassini instruments.
Figure 3 Scatter plots of the logarithm of counts (per 2 seconds) versus pitch angle for
two energies (noted to the right of each plot) for 0545-0550 UT on October 30, 2005
when Cassini was at the orbital radius of Dione. Vertical lines in the right-hand plot
denote the critical pitch angle for a source region of trapped electrons at the orbit of
Tethys. At the higher energy (left-hand panel) the distribution is peaked near 90°
indicating a source within the Dione plasma torus.
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Figure 4 Sketch of Saturn and its satellites Tethys and Dione. The plasma tori of the
two satellites are approximated in blue for Tethys (indicating lower energy plasma) and
green for Dione (indicating higher energy plasma). The orbit of Cassini is shown in black
with red lines showing the positions of observed inward plasma injections. Butterfly
electron pitch-angle distributions consistent with outward plasma flow are observed
throughout the regions in between the inward injections. Shown along the gold magnetic
flux tubes are trajectories for electrons with 90° equatorial pitch angle within the satellite
tori and for electrons with equatorial pitch angles of 43.5° and 136.5° at the Dione
distance (indicating a source at the Tethys radius) and 45° and 135° at Cassini
(indicating a source at the Dione radius). Electrons observed at Cassini to originate at
the Tethys radius, with equatorial pitch angles near 30°and 150°, are not shown.