Southwood: Presidential address
Saturn’s
mysterious magnetism
In his 2013 RAS Presidential
Address, David Southwood looks
at the surprises that have come
from the exploration of Saturn’s
deceptively simple magnetic field.
W
riting a proposal to NASA from Imperial College to put a magnetometer on
the Cassini spacecraft, 25 years ago,
I never dreamt that the basic result promised
would remain unresolved today. The key goal
of the instrument was to determine the internal
magnetic asymmetry of Saturn. We had no idea
of the surprises to come.
So far, Saturn remains an aligned rotator,
with planetary rotation and field symmetry axes
aligned within our present capacity to measure.
However, magnetic axial symmetry is broken
globally outside the planet’s surface. A complex
web of exterior fields associated with the planet’s magnetosphere and ionosphere obscures the
apparently simple aligned rotator within; how
this happens is the primary topic of this paper.
Saturn’s abiding axisymmetry remains a troubling problem for planetary dynamo theorists.
As Cao et al. (2011, 2012) point out, Cowling’s
anti-dynamo theorem (Cowling 1933), which
states that an axisymmetric magnetic field cannot be maintained by an axisymmetric motion,
made an aligned rotator with a dynamo field
A&G • February 2014 • Vol. 55 1: This false-colour image, made from Cassini near-infrared data, shows green aurorae streaking
out about 1000 km above the cloud tops of Saturn’s south polar region. Note the alignment of the
aurora with the planet’s banded clouds and rotation axis. Blue indicates reflected sunlight at
2–3 µm, green, light from hydrogen ions at 3–4 µm, and red, thermal emission at 5 µm. The dark
spots and banded features in the image are silhouettes of clouds and small storms that outline the
deeper weather systems and circulation patterns. (NASA/JPL/ASI/Univ. Arizona/Univ. Leicester)
unlikely. Following the first reports of Saturn’s
axisymmetric field, Stevenson (1982) suggested that there was a conducting layer above
the dynamo region in which a skin effect in the
stratified rotation suppressed the non-axisymmetric terms. However, no dynamo model has
yet successfully produced the effect envisaged.
Alternatives are being sought; Saturn’s field is
substantially weaker than the highly asymmetric field of Jupiter, which may be a clue.
Another clue is the observed polar intensification of the field; recently, Cao et al. (2011, 2012)
have introduced differential rotation in the
dynamo region to produce this and interest is
centring on Couette flow models where the flow
that makes the field occurs between inner and
outer shells and where the field becomes very
substantially wound up. Models currently produce energy in the poloidal field that is perhaps
0.05 of that in the toroidal field; in a convection
driven dynamo, the ratio is close to unity.
Seven years of Cassini data (from orbit insertion in 2004 to 2011) have shown no firm deviation from axisymmetry. Nonetheless, Cassini
has not flown in close orbits; it was only during
Saturn orbit insertion (SOI) in June 2004 that
Cassini braved the rings and passed only 0.3 RS
(RS = Saturn radius, 60 278 km) above the cloud
tops. The field detected during the SOI pass does
appear to show a deviation from the standard
model. However, the single pass had very limited planetary longitude coverage and the data
preclude any definitive statement. However, it
does whet one’s appetite for the later stages of
the Cassini mission when Cassini will repeatedly pass close to the planet in what are called
the “proximal” orbits. Near periapsis the orbit
will be taken inside the innermost ring (the D
ring) passing only a few thousand kilometres
above the planet’s cloud tops. Before that time,
we have to complete the challenge of calibrating
and removing efficiently, the rotating external
fields that the Saturn system creates.
Saturn’s rotation
During the Voyager spacecraft flybys of Saturn
in 1980 and 1981, the planetary radio astronomy instrument gave the first indications of
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Southwood: Presidential address
2: Data from inbound
passes of Pioneer 11
and Voyager 2 plotted
against time but
also radial distance
from Saturn (in
units of Saturn radii,
RS = 60 278 km).
CA marks closest
approach. Both
approaches were in
the morning sector
of local time. The
component shown
is the azimuthal
component Bf. The
continuous lines show
sinusoids adjusted
for the Doppler shift
associated with
spacecraft angular
motion on the
assumption that the
signals are rotating
with the planet. In
other words, the
signals appear to be
stationary in what was
believed at the time
to be the planetary
frame.
3: Magnetic field data
from the Imperial
College magnetometer
on the Cassini spacecraft
for the period 2–11 April
2010 inclusive. 1 min
resolution data averaged
over 1 hr are shown. The
coordinate system is
based on the planetary
radial (r), co-latitudinal
(q) and azimuthal (f)
directions. The upper
panel shows the radial
(Br – black trace) and
southward (Bq – red
trace) components of the
field. Near periapsis, the
Bq (red) component goes
off scale peaking at 457 nT. The lower panel shows the residual radial and southward components
after subtracting the standard offset planetary dipole field model along with the azimuthal
component Bf (green trace).
rotational asymmetry (Desch and Kaiser 1981).
They discovered the Saturn kilometric radiation (SKR) pulsing with a period near 10.7 hr
(639 m 24±7 s). The period of the radio pulses
was immediately interpreted as the deep interior rotation period of the planet. Puzzlingly,
the direct measurements of the planetary magnetic field during Pioneer and Voyager flybys
of Saturn by separate teams known for their
competitive streak revealed an internal planetary magnetic field, axisymmetric to octupole
order (represented often as a dipole offset along
1.14
the rotation axis to the north by 0.04 RS). It was
assumed that there must be a high order and
probably high latitude magnetic anomaly ordering the radio data that the flyby orbits near the
ecliptic had not detected.
Saturn’s kilometric radio signals could not
be detected at Earth. The Ulysses solar polar
orbiting spacecraft was, however, equipped
with a very sensitive radio receiver and had its
perihelion near Jupiter’s orbit, roughly half way
between the Sun and the orbit of Saturn. Where
Ulysses and Saturn were on the same side of the
Sun, the SKR could be detected. Using measurements from the Ulysses spacecraft from 1994 to
1996, Galopeau and Lecacheux (2000), found a
small variation, ~1% per year, in the period of
the SKR radio emission. This seems too rapid
to arise from secular changes in the rotation
period of so massive a body as Saturn and the
first doubts about the use of pulsed radio emissions for determining an object’s rotation rate
were sown. Doubts even arose over whether
the pulsed signals were generated by a rotating
source – although this assumption, unlike the
former, has survived.
At Imperial we found ourselves facing a more
complicated task at Saturn than we’d envisaged
a decade before. In preparation, we looked at
Voyager and Pioneer magnetic data from two
decades earlier. Re-analysis of the Pioneer and
Voyager spacecraft magnetic data by Espinosa
and Dougherty (2000) and Espinosa et al.
(2003a, b) indeed showed features pulsing with
a period of 10.7 hr. However, the polarization of
the magnetic pulsations showed that they came
from a source outside the planet. Figure 2 shows
the sinusoidal transverse fields detected – it was
as if there was a rotating pressure enhancement
or “cam” rotating far out in the planet’s magnetosphere. Moreover, although it took years to
be sure that the radio signal sources were rotating, it was proved right away that the magnetic
signals had an azimuthal wave number of 1
and they were rotating in the same sense as the
planet. The term “cam” has stuck, at least for
the field in the dipolar region of the magnetosphere. A cam removes rotation symmetry and
that is what Saturn’s external fields do.
Cassini arrives
The Imperial College group’s discovery was
vindicated when the Cassini spacecraft arrived
in orbit in mid-2004 and found that the 10.7 hr
oscillations were more or less ubiquitous in
Saturn’s magnetosphere. If Espinosa et al.’s
(2003a) polarization argument for the external force was subtle to some, what made its
external nature abundantly clear was that the
amplitude did not fall off as the cube or higher
order power of radial distance from the planet.
Indeed, the signal amplitude was remarkably
constant over radial distances of tens of R S ,
as shown in figure 3. In 11 days, the spacecraft comes from a distance of 36.6 R s from
the planet, passes through periapsis at 3.5 R S
and then returns to 34.3 R S. The lower panel
shows the field after the model (axisymmetric
offset dipole) field has been subtracted, eliminating the background field. A large depression
in the Bq component remains around closest
approach. This is the effect of the ring current,
the energetic charged particles trapped in the
field and drifting about the planet.
There was a surprise in that the ring current extends in to periapsis. Initially, charged
A&G • February 2014 • Vol. 55
Southwood: Presidential address
mat­erial from the rings was suspected as the
primary source. However, in a separate shock
delivered by the Saturn system, geysers were
discovered on the tiny moon Enceladus, first
through the magnetic perturbation (Dougherty
et al. 2006) and then subsequently by imaging.
Enceladus, whose orbit is at ~4 RS , just outside
the main rings, is the primary source of positively charged material, water group ions, in the
inner magnetosphere.
The data in figure 4 are from the same time as
the data in figure 3, but the ring current depression has been removed from the Bq component.
Moreover, the ordinate is no longer time but a
rotating phase with period matching the SKR
radio period. Although the signals are not
entirely sinusoidal, they are periodic and also
well fit by the rotating phase at all local times
and over an enormous range of radial distance.
Within R ~ 10–12 RS , the transverse DBr and Bf
components are in quadrature with DBr leading
Bf . The rotation of the fields is in the same sense
as the planetary rotation. Beyond R ~ 10–12 RS,
the background field switches fairly rapidly to
being radially in or outwards (as can be seen in
figure 3). DBq and Bf are then in quadrature with
DBq leading and DBr and Bf in antiphase.
Figures 3 and 4 have been chosen when the
spacecraft orbit is both in the equatorial plane
of the magnetosphere and also near equinox.
At equinox, solar illumination of the northern
and southern polar caps is about equal and
the ionosphere and magnetosphere are at their
most symmetric. But the signals are not symmetrical. During cycles 390–395, in figure 4,
the spacecraft is on closed dipolar field lines,
i.e. field lines that extend from one hemisphere
to another in the inner magnetosphere. It is
surprising that all three components are present in a plane of symmetry – the equatorial
plane – of the system. Were the signals also to
be north–south symmetric one would expect
at least one field component to have a node,
i.e. equal zero. The lack of symmetry implies
transport of momentum and energy across the
equator. The explanation is that the signals are
made up of two separate northern and southern components, oscillating at slightly different
frequencies, another one of Saturn’s surprises.
A further surprise
The discovery of north–south asymmetry in
the 10.7 hr signals came very early in 2009.
Gurnett et al. (2009) showed that radio signals
originating in the northern and southern hemisphere were pulsing at slightly different rates:
the southern hemisphere signals were pulsing at
648 minutes period, whereas the northern signals pulsed at 636 minutes. I wondered whether
the magnetic signal had a similar asymmetry
and fate allowed me to find out. In late 2008, the
Cassini orbit had been adjusted to adopt a very
high inclination with a short orbital period of
A&G • February 2014 • Vol. 55 4: The data from the
same time (2–11 April
2010) as in figure
3. The ring current
signature has been
removed from the
Bq component. The
abscissa is no longer
time but the phase
of a signal rotating
about the planet at the
SKR pulse period (the
phase used is that of
the mean frequency).
The annotation is in
cycles from 1 January
2010.
5: The three panels show data from two successive orbits of Cassini. The red trace is shifted by
281 hr (roughly one orbit period) with respect to black trace. Data have been smoothed to remove
variations on timescale of less than one hour and detrended. Black trace data are from days 33–59
2009 (2 February 20:00 to 28 February 13:00) and red trace data are from days 45­–71 2009 (14
February 13:00 to 12 Mar 11:00 UT). The labelling and shading indicate the hemisphere in which
the spacecraft is. The phase difference between signals an orbit apart is zero when the spacecraft
is in the south for all three field components. In the north, the phase difference between orbits is
close to p, again for all three components.
around 10 days. Moreover, the orbit was more
or less symmetric north–south, ideally suited
for the discovery that northern and southern
magnetic signals were indeed also oscillating
at slightly different periods (Southwood 2011).
Figure 5 shows the three field components for
three identical orbits in total, shown by dint of
the red trace being shifted 281 hours back in
time, very close to one orbital period. Shading
indicates when the spacecraft is in the southern
hemisphere, where it is evident that the red and
black components are in phase. In the north,
the red and black components – one orbit apart
– are in antiphase. There must be a slight difference in period between north and south, just
as in the radio data. The 1.7–2% difference in
radio period meant that the expected beat cycle
between northern and southern periods would
be about 50–60 cycles, giving a half beat period
of about 12 days. The happy and remarkable
coincidence of the relation between the orbital
period in 2008–2009 and the half beat period
provided the most unambiguous phase differences possible.
Leicester leaps in
The Leicester group, primarily Gabby Provan
and David Andrews working with Stan Cowley,
had done a lot of work surveying and codifying
the 10.7 hr signals since 2004. They set to work
to identify northern and southern signals and
their time evolution (Andrews et al. 2010, Provan et al. 2011) throughout the mission. Importantly it was established that the northern and
southern compressional signals (i.e. the component along the background field) had different
phase relationships with the transverse components. In the closed field regions, the northern/
southern Bq component is in phase/antiphase
with the transverse component in the meridian.
This can then be fed into the decoding of the signals. An excellent analysis resulted (Andrews et
al. 2012) which traced separately the northern
and southern signals almost continuously from
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Southwood: Presidential address
6: The figure shows the traces of the peak normalized amplitudes (R) of south (A) and north (B)
signals resulting from using a demodulation process that then homes in on the signal using
requirements of phase coherence, level of agreement between components and continuity.
The ordinate is period in hours. The horizontal dashed line shows the guide periods for initial
demodulation in each case (10.75 and 10.62 hr respectively). Full details of the signal processing
methods used are given in Andrews et al. (2012). The abscissa is marked in days since 1 January
2004. The start of each year is marked along the top. (Provided by D Andrews, adapted from fig. 7
of Andrews et al. 2012)
7: Schematic illustrating
the Andrews et al. (2012)
determination of the
distribution of northern
and southern signals.
Cassini’s arrival at Saturn. Data from Andrews
et al. (2012) is displayed in figure 6.
Cassini orbit insertion took place in southern
Saturnian mid-summer. By 2011, it was past
equinox (August 2009). Andrews et al. (2012)
found that the ratio of southern to northern
amplitude reduced from around 3 in mid-summer to about 1 around the time of equinox. Tantalizingly, the northern and southern periods do
approach a mean period of ~642 min around
equinox. Could this mean that solar illumination of the polar cap was at the root of the seasonal variation? Probably not. The ionospheric
seasonal illumination hypothesis has a basic
flaw. If a seasonal north–south period difference were associated with changes in the solar
illumination of the polar caps, the ionization
(and therefore the ionospheric conductivity) of
the summer cap would be the greater in summer. Prima facie, one expects the summer cap
then to be more tightly bound to the neutral
atmosphere and so to rotate faster not slower
1.16
in mid-summer. For the moment, the seasonal
variation remains a mystery to be resolved.
Work continues on tracking the evolution of
northern and southern periods (Provan et al.
2013). Post-equinox, the system seems to have
adopted a different behaviour with abrupt
(~1 Cassini orbital period) switches between
southern and northern period. The periods
themselves are reported to be relatively stable,
637.8 min (north) and 641.4 (south), but one
can note that the southern rotation remains the
slower although we are now well past equinox.
Theories, speculation…
The magnetic oscillations modulate nearly all
Saturn magnetospheric processes and it is thus
likely that they arise from a global process.
Gurnett et al. (2007) and Goldreich and Farmer
(2007) proposed a two-cell magnetospheric circulation rotating with the planet transporting
material from the Enceladus orbit out to interplanetary space as the cause. The idea had first
been put forward by Hill et al. (1981). As an
author on the Gurnett paper, I keep the idea in
my mind (and suspect that a variant of it will be
in the final answer), but, as presented, it doesn’t
work for a global circulation. Notably, in the
dipolar regions of the magnetosphere, the heavy
Enceladus material is observed to rotate at only
about half the planetary rotation speed.
In parallel, Southwood and Kivelson (2007)
picked on the north–south asymmetry of the
signals and proposed a rotating system of fieldaligned “cam” currents connecting northern
and southern ionospheres, roughly at the limit
of the dipole field’s dominance. The strongest
currents in the Saturn system have been detected
in the vicinity of the magnetic shells in question,
(see, for example, Talboys et al. 2009). Southwood and Kivelson (2009) developed the idea
further, suggesting that the rotating currents
and their shear fields interact with a field pattern
fixed with respect to the Sun due to the solar
wind stress beyond the dipole region. The peak
solar-wind-induced stress is in the mid-morning
hours, consistent with the radio observations.
Nonetheless, ideas developed before 2009,
however suggestive, had not come to terms with
the dual-period nature of the signals. Why does
Saturn’s external magnetic field seem to break
its symmetry separately, north and south? The
most important clue lies in a further result from
Andrews et al.’s (2012) survey. When the Cassini spacecraft was on flux tubes in either polar
cap, the signal detected was, within the ~10%
uncertainty, pristine, i.e. pure southern or pure
northern. On the other hand, when the spacecraft was in the closed field regions the signal
was a mixture. The result, summed up in the
schematic in figure 7 suggests that the sources
are indeed directly linked to the motion of the
caps (Southwood and Cowley, in prep.).
Unfortunately, the speed at which the spacecraft passes through the polar cap boundaries
(at between 70–75° invariant latitude) is so high
that the location of the change from pristine to
mixed signals is not precise. In the central polar
caps the flux tubes extend far out into interplanetary space; the plasma there is likely to be
tenuous. Very little torque is needed from the
ionosphere and these tubes will probably rotate
at close to 10.7 hr. On the flanks of the cap,
however, there is likely to be solar wind mat­
erial which has entered either directly through
the dayside cusp or through magnetic reconnection on the dayside as part of the Dungey cycle
(Cowley et al. 2004). Details of the high-latitude
circulation of this material may well yield the
answer to our problem.
Can a rotating circulation be set up in the
regime between the permanently open field of
the polar cap proper and the permanently closed
field lines which contain the heavy ionized mat­
erial from Enceladus and the rings in the deep
interior of the magnetosphere? Jia et al. (2012)
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Southwood: Presidential address
8: Aurorae on Saturn can last for days – very
different from those on Earth, which last
only about 10 minutes. And on Saturn it is
the pressure of the solar wind that appears
to drive auroral storms, whereas on Earth
the main driver is the Sun’s magnetic field,
carried in the solar wind. (NASA, ESA, J
Clarke [Boston Univ.], Z Levay [STScI])
and Jia and Kivelson (2012) have provided a
simple modification of a University of Michigan global magnetospheric computer simulation that was calibrated to fit the magnetic fields
seen, but has turned out to then explain many
other observed features of the magnetosphere.
Their model imposes twin vortices (azimuthal
wave number m = 1, Southwood and Kivelson
2007) in the neutral atmosphere in each hemisphere centred near the expected open–closed
field lines boundary. The neutral motion then
drives plasma vortices and results in a global
magnetospheric response.
The proposal of a neutral atmosphere source
(originally made by Smith 2006) is not unique,
but the success of the Michigan work in modelling many observed aspects of the system is a
big step forward. It shows what magnetospheric
consequences can follow from a simple twinvortex rotating ionospheric flow pattern. However, because the Michigan model treats the
ionosphere–magnetosphere coupling along the
field direction in a quasi-static limit, very similar results arise for vortices set up by motions
driven in the ionized medium. The location of
the northern and southern vortices not only
at magnetically conjugate sites but also on the
magnetic shells where the dipole field is ceasing
to be dominant make this seem an appealing
line of enquiry.
An example of the 10.7 hr magnetic oscillaA&G • February 2014 • Vol. 55 tions deep in the northern polar cap and tail
lobe of Saturn is shown in figure 9, from a distant high-latitude Cassini pass in late 2006.
Barely perceptible in the top panel is a compressional oscillation in field strength which shows
up in the radial component shown as second
(dotted) trace oscillation plotted in the middle panel. The oscillations look unremarkable
until one recalls not only that the spacecraft is
1.8 × 106 km from the planet, but also that the
signals imply a very large-scale length. An order
of magnitude estimate of the spatial displacement of the plasma (from frozen-in field theorem) yields
x ~ l|| b⁄|B|
where l|| is a parallel scale length. A minimum value for l|| would be approximately the
spacecraft radial distance, 30 RS , although it is
likely to be larger (e.g. the parallel wavelength
of an Alfvén wave, ~600 RS). Taking the latter
value is a reductio ad absurdum – the scale of
motion would then be ~100 RS , exceeding the
transverse dimensions of the system. Magneto­
hydro­dynamics deals with this by introducing a
compression magnetic component to limit transverse motion, seen here as the small compressional Br oscillation. A reasonable deduction
for the plasma displacement amplitude might be
x ~ 5–10 RS. In fact, as the transverse field components can be seen to be nearly in quadrature
and of similar magnitude, the local plasma is
not moving just back and forth but is executing
a circular motion, transverse to the field direction with radius of order x. Most, if not all, of
the polar cap flux tubes are rotating similarly.
At the author’s request, a simple computational test was recently undertaken by Xianzhe
Jia of the University of Michigan. Jia (personal
communication, 2013) looked at an axisymmetric cylindrical column of collision-free plasma
threaded by a magnetic field with the field line
feet embedded at one end in a perfect conductor. The plasma was forced into rotation about
the symmetry axis by setting the conductor
rotating. The plasma not only started to rotate
but slewed back and forth transverse to the
background field at the rotation rate. The latter
motion, introduced by the plasma, is called a
kink motion in solar physics (Edwin and Roberts 1983). The slewing removes the original
rotational symmetry and the kink motion sets
up compressional oscillations in the rotating
tube. If this simple simulation can be extended
to fit the more complex environment of the open
field lines of the Saturn polar cap mag­neto­
sphere, one has a solution for the origin of the
global 10.7 hr Saturn magnetic field oscillations.
In fact, the ionosphere is not a perfect conductor. Nonetheless, at ionospheric levels the
travel time for MHD signals around the planet
is certainly much less than the 10.7 hr period.
The signal will be quasi-static and be described
1.17
Southwood: Presidential address
Planets and pulsars
9: 10.7 hr magnetic oscillations deep in the polar cap/tail lobe of Saturn. The data is taken from the
Imperial College magnetometer on the Cassini spacecraft. 10 rotation cycles and around 110 hr of
data are shown. The top panel shows the strength of the mainly radial background field, the two
lower panels show at larger scale the meridional (q) and azimuthal (f) components. Also shown in
the middle panel is the oscillation in radial component (filtered to zero mean).
by an electrostatic potential: equipotentials
would be instantaneous streamlines for horizontal motion. The observed m = 1 (longitudinal wave number) nature of the Saturn signals
(Southwood and Kivelson 2007) means the
potential varies around the polar cap boundary and the electric field cannot be zero there;
in consequence, the rotating disturbance moves
the polar cap boundary north and south. At any
instant, there should be poleward motion in one
(planetary) longitude sector and equatorward
in the other, i.e. an equatorward bulge on one
side of the polar cap boundary and a depression or poleward displacement on the other. The
auroral zone does in fact rock in just this way
(Nichols et al. 2008, 2010).
Why are the signals present? The computation
by Jia shows that they are associated with the
transmission of torque along the field to maintain the polar cap in rotation. Hitherto it had
been assumed that a torque would be uniformly
exerted on the open polar cap, putting a local
time-independent twist into the field (Isbell et
al. 1984, Milan et al. 2005). There is no observational evidence of that field.
tion. However, in time-honoured fashion, Saturn’s external field has displayed a new mode of
behaviour post-equinox and has started switching between northern and southern dominance
in an almost chaotic manner (Provan et al.
2013). We’re not out of the woods yet. ●
Concluding remarks
D J Southwood, Blackett Laboratory, Imperial
College, London, UK.
Acknowledgments. The author expresses
particular thanks to Xianzhe Jia (University
of Michigan) for his computational work; to
David Andrews, particularly for provision
of figure 6 to the author’s specifications; and
Michele Dougherty for her superb stewardship
of the Cassini magnetometer since Cassini SOI.
He also acknowledges useful discussions with
many colleagues. Stan Cowley, Emma Bunce,
Margaret Kivelson, Gabby Provan and Vytenis
Vasyliunas deserve particular mention. Support
is also acknowledged from the University of
Michigan Dept of Ocean Atmospheres and Space
Science general fund as well as a Senior Research
Investigator position at Imperial College. Cassini
magnetometer data reduction at Imperial College
has been supported by UK Space Agency through a
contract from the European Space Agency.
This report is one of work in progress. The
magnetic field of Saturn has provided surprise
after surprise. We’ve come a long way in sorting out the external field but the story is not
over. The Imperial College Cassini magnet­
ometer was expected to identify the manner in
which the internal planetary field departs from
axisymmetry; the final multiple orbits of Cassini
should provide a dénouement. In the meantime,
we hope to wrap up our understanding of the
external field prior to this new phase of explora-
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1.18
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