JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A04224, doi:10.1029/2010JA016045, 2011
Outer magnetospheric structure: Jupiter and Saturn compared
D. R. Went,1 M. G. Kivelson,2,3 N. Achilleos,4,5 C. S. Arridge,5,6 and M. K. Dougherty1
Received 27 August 2010; revised 11 January 2011; accepted 10 February 2011; published 20 April 2011.
[1] The Jovian dayside magnetosphere is traditionally divided into three different regions
with the outermost region, colloquially referred to as the cushion region, existing between
the outer edge of the magnetodisk and the magnetopause. Magnetometer and plasma data
from 6 different spacecraft are used to determine the average properties of this region,
including its characteristic thickness at the subsolar point, and these observations are
compared with data from the Saturnian magnetosphere obtained using the Pioneer, Voyager,
and Cassini spacecraft. Significant differences are found in the structure of the two
rotationally driven magnetospheres with the Saturnian system showing little evidence for the
cushion region seen at Jupiter. These differences are discussed in terms of the parameter
regimes pertinent to each planet, and the potential effect of magnetodisk warping at Saturn
is discussed. It is tentatively suggested that while the Jovian magnetodisk typically breaks
down several tens of planetary radii inside the magnetopause, thus allowing plasma‐
depleted flux tubes beyond it to relax into the cushion region configuration, the Saturnian
magnetodisk may persist until much closer to the magnetospheric boundary. A number of
observational tests of this hypothesis are proposed, and the need for improved observations
at both planets is stressed.
Citation: Went, D. R., M. G. Kivelson, N. Achilleos, C. S. Arridge, and M. K. Dougherty (2011), Outer magnetospheric
structure: Jupiter and Saturn compared, J. Geophys. Res., 116, A04224, doi:10.1029/2010JA016045.
1. Introduction
[2] The structure and dynamics of the Saturnian magnetosphere are often described as being intermediate between
those of Jupiter and the Earth [Gombosi et al., 2009]. Plasma
in the terrestrial magnetosphere is forced into large‐scale
motion by the solar wind–driven Dungey cycle [Dungey,
1961] beginning with dayside reconnection between magnetospheric and interplanetary magnetic fields. The open flux
tubes thus produced are then convected over the polar regions
of the planet by the fast‐flowing solar wind, during which
time they lose much of their mass content, before meeting
again in the magnetotail and reconnecting to form closed flux
tubes. These closed flux tubes can then return to the dayside
and complete the circulation.
[3] In contrast, despite some evidence for Dungey cycle
operation [Cowley et al., 2003], the overall dynamics of the
Jovian magnetosphere appear to be dominated by internal
sources of angular momentum. Plasma released by the vol1
Blackett Laboratory, Imperial College London, London, UK.
Department of Earth and Space Sciences, University of California,
Los Angeles, California, USA.
3
Institute of Geophysics and Planetary Physics, University of
California, Los Angeles, California, USA.
4
Department of Physics and Astronomy, University College London,
London, UK.
5
Centre for Planetary Sciences at UCL/Birkbeck, London, UK.
6
Mullard Space Science Laboratory, Department of Space and Climate
Physics, University College London, Dorking, UK.
2
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JA016045
canically active moon Io is continually picked up by the Jovian
magnetic field and, through field‐aligned currents linked to the
ionosphere, rapidly accelerated to near‐corotational velocities [Bagenal and Sullivan, 1981]. Vasyliũnas [1983] then
described the resulting circulation as plasma is driven radially
outward by the combined action of the centrifugally driven
interchange (b < 1) and ballooning (b < 1) instabilities
[Kivelson and Southwood, 2005] where b (beta) is the ratio
of plasma to magnetic pressure. Despite distorting the Jovian
magnetic field [Smith et al., 1974] this outward transportation
and ballooning is thought to be restricted on the dayside by
the dynamic pressure of the solar wind acting on the magnetopause. Only on the nightside (where confinement by the
magnetopause becomes negligible) can plasma‐loaded flux
tubes expand without restriction. Eventually a critical point
is reached beyond which magnetic curvature can no longer
support the outward acting forces associated with expansion
[Goertz, 1983] and a plasmoid is released downtail in a burst
of reconnection [Woch et al., 2002]. The resulting plasma‐
depleted flux tubes, still anchored to the planet, are then
able to dipolarize [Kivelson, 2005] and return to the inner
magnetosphere [Kivelson and Southwood, 2005] where they
become reloaded with iogenic plasma so that the so‐called
Vasyliũnas cycle can repeat.
[4] Saturn, like Jupiter, is also a fast rotator with significant sources of plasma in its magnetosphere. Moreover, the
inner moon Enceladus is known to have a highly dynamic
atmosphere with active geysers that play a similar role at
Saturn to the volcanism seen on Io [Dougherty et al., 2006;
Porco et al., 2006; Pontius and Hill, 2006]. Consequently,
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contribution from those involved in the Dungey cycle [Cowley
et al., 2003] being significant only during periods of enhanced
solar wind activity [Badman and Cowley, 2007]. The resulting circulation pattern is described in detail by Kivelson and
Southwood [2005] and well illustrated by Figure 7 of that
paper.
[6] Saturnian equivalents of the inner magnetosphere and
magnetodisk have recently been reported by Arridge et al.
[2008a] but are observed only when the magnetosphere is
in an expanded configuration. In this paper we complete the
comparison with Jupiter by considering evidence for an outer
magnetospheric cushion region at Saturn, paying particular
attention to those expanded passes where the magnetodisk,
which forms a vital component of the cushion region’s definition at Jupiter, is expected to be present. We begin with a
review of the extensive observations made at Jupiter.
2. The Cushion Region at Jupiter
Figure 1. Schematic representation of the Jovian magnetosphere with (top) the noon meridian viewed from dusk and
(bottom) the equatorial plane viewed from above. In both
cases the Sun is to the left. The inner (blue), middle (yellow),
and outer (green) magnetospheres are not shown to scale. Figure adapted from Smith et al. [1976].
there is now growing evidence for a Saturnian circulation
pattern consistent with the Vasyliũnas cycle [André et al.,
2007; McAndrews et al., 2009] although, in contrast to the
Jovian dynamics discussed above, the solar wind appears to
retain considerable influence on the outer magnetosphere
[McAndrews et al., 2008], magnetotail [Bunce et al., 2005]
and aurora [Bunce et al., 2008]. The complex interaction
between solar wind and internally driven transport processes
at Saturn is thus a topic of much research.
[5] Returning our attention to Jupiter, the dayside magnetosphere is traditionally divided into three qualitatively distinct spatial regions (Figure 1) as discussed by Smith et al.
[1976]. The inner magnetosphere (R ] 10–20 RJ) is dominated by Jupiter’s strong internal dipole with a smoothly
varying, southward directed field close to the equator. Further
out, the increasing significance of centrifugal forces leads to a
radially stretched magnetodisk (20 ] R ] 60 RJ) where the
ballooning instability dominates the magnetic field geometry.
Finally, between the magnetodisk and magnetopause, the
more dipolar yet disordered outer magnetosphere or cushion
region (described by Kivelson [1976] as “a layer of magnetic
turbulence”) is suggestive of reduced centrifugal stresses and
plasma‐depleted flux tubes. Kivelson and Southwood [2005]
interpret these flux tubes as those involved in the final (mass
release) stages of the Vasyliũnas cycle with a secondary
2.1. Typical Characteristics: Ulysses
[7] A typical inbound pass through the Jovian magnetosphere was made by the Ulysses spacecraft [Wenzel et al.,
1992] along a low‐latitude (<15°) prenoon meridian [Haynes,
1995]. To illustrate the qualitative structure of the observed
magnetic field we consider changes in two parameters
derived from the magnetic field data. The first of these is
the ratio of the poloidal field components to the total field
magnitude (∣B∣/∣B∣,∣BR∣/∣B∣) and the second is the angle,
INT, between the observed magnetic field, BOBS, and the
magnetic field associated with the Jovian internal dipole,
BINT [Randall, 1998]. These observations are presented in
Figures 2b and 2c, respectively, between distances of 20–
90 RJ with the magnetopause located at 88 RJ. The data is
plotted with increasing radial distance (decreasing UT time
for an inbound pass) on the x axis.
[8] Close to the magnetic equator, field lines with a dipolar
configuration are southward directed with ∣B∣ > ∣BR∣ while
radially stretched, nondipolar field lines are associated with
∣BR∣ > ∣B∣. Away from the equator the changing direction
of dipolar magnetic field lines invalidates this interpretation and the angular parameter INT must be used to characterize the field instead. A critical angle (INT = 50°) is defined
above which the magnetic field will be considered significantly nondipolar and if, in addition to this, ∣BR∣ > ∣B∣,
we describe the resulting field configuration as a radially
stretched magnetodisk.
[9] With these points in mind the Ulysses data of Figure 2
can be seen to show three distinctly different magnetospheric
regions. A quasi‐dipolar inner magnetosphere, shaded blue,
exists close to the planet where INT < 50°. Between distances
of 30–70 RJ Ulysses explores a radially stretched region,
shaded yellow, where ∣BR∣ > ∣B∣ and INT reaches values
close to 90°. This is the Jovian middle magnetosphere or
magnetodisk which, on closer inspection, is found to consist of a relatively thin current sheet (RC < 2 RJ [Mauk and
Krimigis, 1987], where RC is the radius of curvature of the
field lines on the magnetic equator) surrounded by two
radially stretched regions of opposite polarity. Crossings of
this current sheet, observed as reversals in BR, occur whenever the magnetic equator passes over the spacecraft. In
the spacecraft rest frame the magnetic equator “flaps” up
and down, with approximately the planetary rotation period,
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Figure 2. Ulysses observations in the Jovian magnetosphere. (a) Jovian System III magnetic field components (BR, red; B, blue or white; B, green) and ±∣B∣ (black). (b) Normalized poloidal field components
(∣B∣/∣B∣, blue or white; ∣BR∣/∣B∣, red). (c) Angle INT between the observed magnetic field, BOBS, and
the internal magnetic field, BINT. Horizontal dashed lines denote the critical magnetodisk angles of 50°
and 180 − 50 = 130°. (d) Thirty‐minute normalized magnetic field RMS fluctuation. (e) SWOOPS thermal
electron density (blue or white) and temperature (red). Vertical dashed lines denote local minima in absolute
magnetic latitude, ∣lM∣,which beyond 50 RJ corresponds to lM = 0°. The inner magnetosphere (blue), magnetodisk (yellow), transition region (white), cushion region (green), boundary layers (cyan), magnetopause
crossings (red), and magnetosheath (grey) are shaded. The radial distance, planetocentric latitude, and local
time of the spacecraft are shown along the x axis.
due to the rocking motion of the rotating planetary dipole
which is inclined by ≈10° to the rotation axis. Beyond the
magnetodisk the field direction rotates slowly and, for R >
83 RJ, Ulysses explores a third magnetospheric region which
is once again dipolar. This is the outer magnetosphere or
cushion region, shaded green, separated from the magnetodisk by a roughly 14 RJ transition region of intermediate
properties, shaded white. In the transition region the disturbed
magnetic field rotates through ∼90° and the two poloidal field
components are comparable in magnitude.
[10] The cushion region electron density (measured by
the SWOOPS instrument [Bame et al., 1992]) is ∼10−2 cm−3
while, in contrast, the first crossing of the magnetodisk
plasma sheet in the middle magnetosphere is associated with
an electron density almost an order of magnitude higher.
At the radial distance of this crossing (≈68 RJ) the electron
density observed in the magnetodisk lobes is comparable to
that seen previously in the overlaying cushion region. Both
the plasma sheet and magnetodisk lobe density decrease upon
approaching the planet, a counterintuitive observation interpreted by Phillips et al. [1993] as an effect associated with
Ulysses unusual trajectory. In order to launch Ulysses into
a polar orbit around the sun, the planetocentric latitude of
the spacecraft increased significantly on approaching Jupiter.
This is clearly evident from the top right panel of Figure 3
where the meridional projection of Ulysses’s Jupiter flyby
trajectory is shown in red. The high planetocentric latitude
also explains the BR dominated field seen in the quasi‐dipolar
( < 50°) inner magnetosphere. As can be seen from Figure 2e,
where density is plotted in blue/white and temperature in red, a
transient density (temperature) increase (decrease) is observed
at roughly 24 RJ. This feature is interpreted by Phillips et al.
[1993] as a spacecraft traversal of high‐latitude open field
lines.
[11] The highly disturbed nature of the transition region
field is clear when the RMS fluctuation (calculated from the
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Figure 3. In situ exploration of (top) Jovian and (bottom) Saturnian magnetospheres with spacecraft trajectories projected onto the (left) equatorial and (right) instantaneous meridional planes. The inner and outer
dashed lines represent the nominal locations of the magnetopause and bow shock, respectively. Pioneer 10,
blue; Pioneer 11, gold; Voyager 1, pink; Voyager 2, black; Ulysses, red; Galileo, cyan; Cassini, grey; Cassini Rev 20 inbound, dashed purple.
Pythagorean sum of the standard deviations of each field
component computed over a 30 min time interval; Figure 2d)
is examined for this region at 1 min resolution. The thermal
electron density appears to correlate with some of these
fluctuations (dne /ne ≈ 2–10) and was studied in detail by
Southwood et al. [1993]. Some of the density enhancements
were found to be associated with magnetic field rotations
reminiscent of the current sheet crossings seen in the magnetodisk, however their occurrence in the transition region
appears to be unrelated to the spacecraft’s magnetic latitude
and they are often accompanied by simultaneous reversals in
both BR and B. These are interpreted as spacecraft encounters
with a highly warped magnetodisk, strongly tilted in the
meridional plane. Other density enhancements are not associated with a clear rotation of the magnetic field but are,
instead, associated with a large decrease in the magnitude
of the B dominated background magnetic field. These phenomena are termed “magnetic nulls” and are discussed in
detail by Haynes et al. [1994] and Southwood et al. [1995].
[12] A consideration of the radial force balance condition
for an isotropic (sub)corotating plasma [Southwood and
Kivelson, 2001] allows us to relate the observed magnetodisk and cushion region field configurations to the ambient
population of magnetospheric plasma:
^
n
B2 =0
B2
þ Ni ðmi þ me ÞW2 r:
¼ r P þ
RC
20
ð1Þ
[13] Equation (1) describes the first‐order balance between
the magnetic curvature force (left), pressure gradient force
(right) and centrifugal force (far right). Here RC is the local
radius of curvature of the field, B2/2m0 is the magneticpressure, P is the plasma pressure (assumed to be isotropic),
Ni is the number density of ions, me and mi are the electron and
mean ion masses, respectively, W is the angular frequency of
plasma rotation and r is the perpendicular distance from the
spin axis of the planet about which the plasma rotates. The
^ points in the direction of the outward normal
unit vector n
to the field line. According to this equation, higher‐density
plasmas will tend to “stretch out” the magnetic field
(decreasing the radius of curvature in order to increase the
stabilizing tension force) whereas lower‐density plasmas,
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at a given r and w, can be successfully constrained by a less
stretched configuration.
[14] The effect described above is negligible in the low‐b
inner magnetosphere (dominated by Jupiter’s strong internal
dipole) however it becomes increasingly important at larger
radial distances where the plasma and magnetic pressures
become comparable. Since the cushion region itself is found
at large (>50 RJ) radial distances, the quasi‐dipolar nature of
the cushion region field suggests, in light of equation (1), that
the Jovian outer magnetosphere is depleted of magnetospheric plasma relative to the radially stretched magnetodisk.
In this picture the cushion region consists of flux tubes which
have recently lost much of their mass content as a result of
Vasyliũnas and Dungey cycle reconnection while the transition region corresponds to the distorted outer edge of the
magnetodisk. Dense clumps of plasma occasionally break off
the outer edge of the magnetodisk and move through the
overlaying cushion region where they are observed in magnetic field data as sharp decreases in the total magnitude of the
magnetic field. These are the “magnetic nulls” of Haynes
et al. [1994].
2.2. Spatial and Temporal Variability
[15] A total of six magnetometer‐carrying spacecraft have
explored the Jovian magnetosphere to date (Pioneer 10,
Pioneer 11, Voyager 1, Voyager 2, Ulysses and Galileo; see
Figure 3) covering all local times in the equatorial plane out
to distances of ∼100 RJ. New Horizons was not equipped with
a magnetometer and, while Cassini skimmed the dusk magnetosphere en route to Saturn, it did not penetrate far enough
for the full magnetospheric structure to be determined. Considered together, the available observations reveal significant temporal and spatial variability in the properties of the
cushion region described above.
[16] Considering the temporal variability first, the expansion and contraction of the Jovian magnetosphere (usually
an equilibrating response to changes in solar wind dynamic
pressure) is thought to result in magnetopause and, by
extension, cushion region motion, relative to the planet, at
velocities comparable to or greater than those of an exploring
spacecraft [Sonnerup et al., 1981; Cowley and Bunce, 2003].
In the rest frame of the planet this motion results in the
cushion region “sweeping” back and forth over the exploring
spacecraft at the same time as the spacecraft itself moves
relative to Jupiter. The inbound leg of the Pioneer 10 flyby
illustrates this point well with the spacecraft crossing the
cushion region‐to‐magnetodisk boundary 3 times in the
space of just 3 days, covering a radial distance of over
40 RJ while doing so. Such dynamical considerations act
to modulate the time a spacecraft spends inside the cushion
region and makes the true “inertial” thickness of the region
impossible to determine from single spacecraft data. The
same unpredictable boundary motion also has consequences
for the stability and thickness of the underlying plasma sheet
and is likely to control the probability of plasma blobs
breaking off from the magnetodisk [Southwood and Kivelson,
2001]. This may, in turn, introduce a temporal variability
to the nature and extent of cushion and transition region
field fluctuations. Other potential sources of variability, such
as the bursty nature of magnetotail reconnection [Woch et al.,
2002] and the variable activity levels seen at Io [Bagenal
et al., 2004] are probably of secondary importance.
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[17] From a spatial perspective, Kivelson and Southwood
[2005] identified a local time asymmetry in the cushion
region with the region of quasi‐dipolar field being more
evident in the morningside magnetosphere as opposed to
afternoon. In the predusk sector Kivelson and Southwood
[2005] found the BR and B components to be more comparable than in the cushion region and, while the BR component
reversed sign rather irregularly, clear spectral peaks were
found at the rotation period of Jupiter. These observations
were interpreted as a result of the plasma sheet thickening as
it rotates toward dusk, reducing the contrast between centrifugally stressed and plasma‐depleted flux tubes, combined
with a gradual refilling of the cushion region by plasma that
has broken off the outer edge of the dynamically unstable
plasma sheet. Such complex variability is difficult to quantify in the absence of multispacecraft observations and, as a
result, it is beyond the remit of this paper to consider such
variability in detail.
2.3. Average Properties
[18] Previous studies of the cushion region [Smith et al.,
1976; Balogh et al., 1992; Kivelson et al., 1997] considered
spacecraft data on an individual basis only and, as we have
seen above, such a study tells us little about the average properties of the region. We address this problem for the
first time by considering observations made by all Jupiter
exploring spacecraft to carry a magnetometer to date. We do
this by defining the average thickness of the cushion region
at Jupiter to be the average separation between the cushion
region’s inner boundary (projected to the subsolar point using
the Joy et al. [2002] magnetopause model) and the mean
subsolar location of the Joy et al. [2002] magnetopause. Here
we use the Joy et al. [2002] magnetopause location determined from a single‐gaussian fit to the spacecraft observations. Determining the instantaneous location of the cushion
region’s inner boundary does not require knowledge of the
speed at which the boundary itself is moving though considerable ambiguity is often involved in its determination due
to the gradual nature of the transition between the cushion
region and magnetodisk. To reduce this ambiguity, passes on
which a stable magnetodisk configuration (∣BR∣ > ∣B∣, INT >
50°) could not be identified were excluded from the analysis due to the resulting ambiguity in distinguishing adjacent
magnetospheric regions. A total of 13 transition points could
be identified in this way (between 1973 and 2003) and their
distribution in the equatorial plane is shown in Figure 4.
[19] The mean location of the cushion region’s inner
boundary maps to a subsolar location of 54 RJ which suggests
a mean cushion region “inertial subsolar thickness” of order
LCR ∼ 20 RJ. The 16 RJ standard deviation in the location of
the inner boundary is similar to that seen in the location of
the Joy et al. [2002] magnetopause and is consistent with the
effects of variable solar wind dynamic pressure modulating
the size of the magnetospheric cavity. The range of observations is, of course, larger than the standard deviation quoted
above and, once again, the Pioneer 10 inbound pass provides
a good illustration of the variability.
[20] Uncertainties in both the mean and standard deviation
of the cushion region’s inner boundary were estimated using a
Monte Carlo method similar to that of Achilleos et al. [2008]:
500 random subsamples, each comprising 75% of the total
number of observations used in this investigation, were used
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the absolute rotational voltage across a 20 RJ thick cushion
region with a center 65 RJ from the planet is then approximately 6 MV. A comparison between this value and the
mean Dungey cycle reconnection voltage of 0.25 MV
[Badman and Cowley, 2007] led Badman and Cowley [2007]
and Kivelson and Southwood [2005] to conclude that the
Dungey cycle contribution to the cushion region flux content
is negligible under typical solar wind conditions. The outer
magnetosphere of Jupiter is thus, predominately, a rotational
phenomenon.
2.4. Scaling Jupiter to Saturn
[23] For comparative purposes both the mean inertial subsolar thickness of the cushion region, LCR, and the mean
rotational voltage across the cushion region at the subsolar
point, VCR, must be appropriately scaled to the smaller Saturnian magnetosphere. Here we perform this scaling using the
mean subsolar standoff distance of the magnetopause, RSS,
and the total rotational voltage across the magnetosphere,
VROT, as shown below using values from Table 1:
LCR ðS Þ LCR ð J Þ
Figure 4. (top) Nominal location of the Joy et al. [2002]
magnetopause (dashed line) with the 1 s variability shaded.
Circles represent individual spacecraft observations of the
cushion region inner boundary: Pioneer 10, blue; Pioneer 11,
gold; Voyager 1, pink; Voyager 2, black; Ulysses, red;
Galileo, cyan. (bottom) The single‐fit gaussian distribution
of magnetopause locations (dashed line) and cushion region
inner boundary observations (solid line) projected onto the
+ve XJSMAG axis using the Joy et al. [2002] magnetopause
model.
to calculate the mean and standard deviation of the subsolar
location of the cushion region’s inner boundary. The standard
deviation of the resulting distributions was then used as a
measure of the uncertainty in each parameter. The size of
these uncertainties (3 RJ and 2 RJ for the mean and standard
deviation, respectively) is probably indicative of the small
number of observations available for analysis.
[21] The mean rotational voltage across the cushion region
at the subsolar point is a useful parameter for quantifying the
relative contribution of different magnetospheric processes
to its formation and evolution. It can be estimated by integrating the motional electric field, ECR = −vCR × BCR where
vCR and BCR are the cushion region’s plasma velocity and
magnetic field, respectively, across the subsolar cushion
region as shown below:
jVCR j ¼ jECR LCR j jvCR jjBCR jjLCR j:
ð2Þ
Here we have assumed that vCR, BCR and LCR are mutually
orthogonal. Assuming that cushion region plasma rotates
at roughly 50% of the rigid corotation speed, the mean azimuthal velocity may be written as vCR ≈ 0.5WRCR where RCR
is the mean radial distance to the center of the cushion region.
With an outer magnetospheric field strength of order 10 nT,
RSS ðS Þ
RSS ð J Þ
VROT ðS Þ
VCR ðS Þ VCR ð J Þ
VROT ð J Þ
6RS
ð3Þ
200kV :
ð4Þ
If the thickness of the cushion region scaled linearly with the
typical subsolar distance to the magnetopause, one would
expect a typical subsolar width of order 6 RS. Such a thickness
should be readily apparent in Pioneer, Voyager and Cassini
magnetometer observations. However, it is important to note
that there is no a priori reason to believe that cushion region
properties will scale linearly with these parameters. A more
sophisticated scaling would take into account differences in
the upstream solar wind dynamic pressure, dayside reconnection voltage, internal mass loading rate, planetary rotation
rate and magnetospheric flux content at each planet. However
our poor understanding of how these parameters interact to
form the cushion region prevents us from constructing a
scaling constant with greater physical significance at this
time. The value of the above scaling comes instead from the
simple and intuitive comparison between the Jovian and
Saturnian magnetospheres that the scaling results permit.
A Saturnian cushion region with a mean subsolar inertial
thickness of roughly 6 RS will occupy approximately the same
fraction of its parent magnetosphere as the cushion region
seen at Jupiter. Similarly, a Saturnian cushion region associated with a mean subsolar rotational voltage of roughly
200kV would contain the same fraction of the magnetosphere’s total rotational voltage as the cushion region seen
at Jupiter.
3. The Cushion Region at Saturn
3.1. Typical Characteristics: Cassini Rev 20
[25] A typical pass through the (expanded) Saturnian
magnetosphere is represented by the inbound leg of Cassini’s
Rev 20 orbit (9 January to 17 January 2006, dashed purple
in Figure 3) along a low‐latitude dawn meridian. Spacecraft
data are presented in Figure 5 with increasing radial distance
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Table 1. Comparison of Physical and Magnetospheric Parameters for Jupiter and Saturn
Parameter
Jupiter
Saturn
Equatorial radius, RP [Cox, 2001]
Magnetic moment [Cox, 2001]
Rotation period, t
Rotation frequency (W = 2p/t)
Axial tilt [Cox, 2001]
Dipole tilt [Cox, 2001]
Total rotational voltage, VROT
Definite Value
71492 km
1.55 × 1020 Tm3
9.925h [Cox, 2001]
1.76 × 10−4 s−1
3.12°
10°
400 MV [Badman and Cowley, 2007]
60268 km
4.6 × 1018 Tm3
10.66h [Cox, 2001]
1.64 × 10−4 s−1
26.73°
<1°
12 MV [Badman and Cowley, 2007]
Long‐Term Average
∼8 nT
24 amu [Thomas et al., 2004]
∼103 kgs‐1 [Vasyliũnas, 2008]
> 2 RJ [Mauk and Krimigis, 1987]
75 RJ [Joy et al., 2002]
∼3 nT
18 amu [Khurana et al., 2007]
∼102 kgs‐1 [Vasyliũnas, 2008]
< 2 RS [Achilleos et al., 2010a]
24 RS [Achilleos et al., 2008]
∼10−3 KgWb‐1 [Pontius and Hill, 1989]
∼10−3 KgWb‐1 [McAndrews et al., 2009]
0.25 MV [Badman and Cowley, 2007]
0.045 MV [Badman and Cowley, 2007]
Outer magnetosphere ∣B∣a
Plasma ion mass, mi
Active moon source rate, S
Equatorial radius of curvature, RC
Magnetopause subsolar
Standoff distance, RSS
Magnetospheric flux tube
Mass content, s
Dungey Cycle reconnection
Voltage, VD
a
Average outer magnetospheric field strengths were obtained from this study.
Figure 5. Cassini Rev 20 observations in the Saturnian magnetosphere. (a) KRTP magnetic field components (BR, red; B, blue or white; B, green) and ±∣B∣ (black). (b) Normalized poloidal field components
(∣B∣/∣B∣, blue or white; ∣BR∣/∣B∣, red). (c) Angle between the observed magnetic field, BOBS, and the magnetic field of the Burton et al. [2009] internal dipole, BDIP. Horizontal dashed lines denote the critical
magnetodisk angles of 50° and 180 − 50 = 130°. (d) Thirty‐minute normalized magnetic field RMS variance. (e) CAPS/ELS thermal electron density (blue or white) and temperature (red). Vertical dashed lines
denote points of K = 100° Kurth longitude [Kurth et al., 2008], separated by roughly one planetary rotation.
The inner magnetosphere (blue), transition region (white), magnetodisk (yellow), magnetopause (red),
and magnetosheath (grey) are shaded. The radial distance, planetocentric latitude, and local time of the
spacecraft are shown along the x axis.
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(decreasing UT time for an inbound pass) on the x axis. The
final magnetopause crossing was made at a radial distance of
45.6 RS (RSS = 28.5 RS) on the equatorial dawn meridian. The
spacecraft then proceeded inward, moving through the dayside magnetosphere over a period of 10 days, to a 5.6 RS
periapsis near the dusk terminator. As in section 2 for Jupiter,
the qualitative structure of the observed magnetic field is most
readily apparent when we consider the ratio of the poloidal
field components to the total field magnitude (∣B∣/∣B∣,
∣BR∣/∣B∣) and the angle, INT, between the observed magnetic
field and the field associated with the Burton et al. [2009]
internal dipole.
[26] Two magnetospheric regions are apparent on this pass
(c.f. the three observed at Jupiter) with a quasi‐dipolar region
close to the planet, shaded blue, and a radially stretched
(INT > 50°, ∣BR∣ > ∣B∣) region, shaded yellow, from 25 RS
out to the magnetopause. These two regions are interpreted as
Saturnian equivalents to the Jovian inner magnetosphere
and magnetodisk, respectively, separated by a roughly 7 RS
“transition region,” shaded white in Figure 5, where the mean
field rotates through ∼30° and has properties intermediate
between the two adjacent regions. It should be noted, however, that this is a very different type of transition region to the
one described at Jupiter as it involves two qualitatively different magnetospheric regions. The absence of an equivalent
transition region for the Ulysses pass at Jupiter is most likely
due to differences in the spacecraft trajectories and the speed
at which each spacecraft made transition into the quasi‐
dipolar region. Unlike the Ulysses observations made at
Jupiter there is no evidence for a third, quasi‐dipolar region
between the magnetodisk and magnetopause which might be
interpreted as a Saturnian equivalent of the cushion region.
[27] Both the inner magnetosphere and magnetodisk show
variability with a period close to the approximately 10h30m
planetary rotation period, however, unlike the periodic phenomena seen in the Jovian magnetosphere, these variations
cannot be interpreted as a rotational flapping of the magnetosphere due to the small (<1°) tilt of the Burton et al. [2009]
internal dipole. They instead constitute the famous yet
enigmatic rotational modulation (or “camshaft signal”) first
identified in magnetic field data by Espinosa and Dougherty
[2000]. The superposition of the associated “camshaft field”
onto Saturn’s internal dipole creates a signature, near the
equatorial plane beyond 15 RS, very similar to that of a
rotating, tilted current sheet [Southwood and Kivelson, 2007].
However, for many of the low‐latitude orbits considered in
this study the mean location of the current sheet is well above
the equatorial plane such that crossings of the magnetodisk
center (with BR reversing sign) are rare. The vertical displacement of the magnetodisk is primarily a result of the
ambient solar wind which, due to Saturn’s significant (26°)
axial tilt, impacts the magnetosphere from above (southern
winter) or below (southern summer) the equatorial plane. The
resulting asymmetry in magnetic pressure north and south of
the equator then acts to warp the magnetodisk into a bowl
shape [Arridge et al., 2008b].
[28] Observations of the thermal electron density and temperature are made by the CAPS/ELS instrument [Young et al.,
2004; Lewis et al., 2008] throughout the Rev 20 orbit. In
the quasi‐dipolar inner magnetosphere the thermal electron
number density reaches values in excess of 1 cm−3 and the
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temperature is very low with a minimum, close to the planet,
of just a few kiloelectron volts. The plasma density (temperature) decreases (increases) rapidly with distance and,
in the radially stretched magnetodisk, the density often
approaches values comparable with the noise level of the ELS
instrument [Arridge et al., 2009]. At some points along
the trajectory the count rate of thermal electrons drops below
the threshold above which reliable plasma moments can be
generated and thus the moments presented in Figure 5 are
assumed to come from the denser regions of the Saturnian
plasma sheet. Here the number density of thermal electrons
is found to be of order 10−3 cm−3 while the corresponding
temperature is of order 100 eV.
3.2. Saturnian Survey
[29] During the period 1 July 2004 to 1 March 2009 Cassini
made a total of 212 passes (106 orbits) through the Saturnian
magnetosphere; six additional passes (3 flybys) were made
collectively by Pioneer 11, Voyager 1 and Voyager 2 in
the preceding interval of 1979–1981. Of these 218 passes,
26 were associated with a low‐latitude, dayside trajectory and
an innermost magnetopause crossing satisfying RSS ≥ 23 RS.
The majority (16) of these were made through the postdawn
(0600 < LT <1000) sector of the magnetosphere where,
in analogy with Jupiter, the cushion region should be most
evident, with five more made in the noon (1000 < LT < 1400)
and predusk (1400 < LT < 1800) sectors, respectively. The
general properties of these passes are shown in Table 2 and,
as at Jupiter, only those passes associated with a stable, well
defined magnetodisk (INT > 50°, ∣BR∣ > ∣B∣) were inspected
for evidence of a cushion region.
[30] For the 12 passes on which an unambiguous magnetodisk was identified, spacecraft observations reveal an
extremely dynamic magnetosphere varying over multiple
time scales of order minutes to days. The position and/or
orientation of the magnetodisk appear to vary both during and
between spacecraft passes at Saturn, as does the mean angle
by which the magnetic field deviates from that of the Burton
et al. [2009] dipole. In general, however, the magnetosphere
is characterized by a two‐layered geometry (as observed
on Cassini Rev 20; Figure 5) with a highly dipolar, B‐
dominated region close to the planet, analogous to the Jovian
inner magnetosphere, and a radially stretched magnetodisk
( > 50°, ∣BR∣ > ∣B∣) at large radial distances out to the
magnetopause. There is no evidence for a third magnetospheric region, between the magnetodisk and magnetopause,
which might be interpreted as a Saturnian equivalent to the
cushion region seen at Jupiter. Rotational periodicities are
often present at Saturn, both in the inner magnetosphere and
magnetodisk, but the physical origin of these periodicities is
different to those seen at Jupiter and crossings of the magnetodisk are rare. Finally, the field in the Saturnian magnetodisk is also “less stretched” than that at Jupiter, as evidenced
by the two poloidal field components lying closer together
in value. These observations are consistent with the magnetodisk modeling work of Achilleos et al. [2010a] which
shows that field lines in the Saturnian magnetodisk have a
much larger radius of curvature (>2 RS) than their Jovian
equivalents.
[31] The remaining passes can be divided into two
categories. In the noon sector the reduced distance to the
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Table 2. Summary of Low‐Latitude (l < 20°), Expanded (RSS ≥ 23 RS) Dayside Passes at Saturn as Identified Using the Arridge et al.
[2006] Magnetopause Model and the Innermost Magnetopause Crossing for Each Pass
Pass
Voyager 2
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
Cassini Rev
00B
020
015
009
016
019
003
018
000
005
017
013
008
003
012
057
048
051
052
049
044
052
051
050
048
Direction
RSS /RS
LT/Decimal Hours
l/Degree
Magnetodisk?
OUT
OUT
IN
OUT
OUT
OUT
IN
OUT
IN
IN
OUT
IN
IN
IN
IN
IN
OUT
OUT
OUT
OUT
OUT
OUT
IN
IN
IN
IN
23
27
29
26
27
29
32
30
32
25
27
24
25
26
27
23
24
23
27
28
29
24
29
25
33
25
6
6
6
7
7
7
7
7
7
8
8
8
9
9
9
9
12
12
12
12
13
14
16
16
16
17
0
−5
0
0
−9
0
0
0
0
−15
−4
0
−19
−18
−1
−19
7
0
1
2
−2
5
4
4
−6
0
YES
YES
YES
YES
YES
YES
YES
YES
YES
NO
YES
YES
NO
NO
NO
NO
NO
NO
NO
NO
YES
NO
NO
NO
NO
NO
magnetopause prevents a magnetodisk from forming for all
but the most expanded magnetospheric conditions (Cassini
Rev 49 outbound, RSS > 29 RS) and plasma‐depleted flux
tubes, if present, are difficult to distinguish from their plasma‐
loaded counterparts. This fundamental ambiguity could not
be resolved using CAPS/ELS plasma moments as the lack of
clear magnetodisk crossings (discussed above for Rev 20)
prevented us from obtaining the central plasma sheet density
necessary for comparison. Here it is important to remember
that, at Jupiter, the cushion region and magnetodisk lobes
have essentially the same density; it is only the density at the
magnetic equator (synonymous with the center of the plasma
sheet in the magnetodisk) that changes upon entering the
cushion region. The dusk sector field, in contrast, is typically
disturbed such that a stable magnetospheric configuration,
either magnetodisk‐like or dipolar, is difficult to define.
Similar observations were made in the dusk sector of the
Jovian magnetosphere where the plasma sheet becomes
so thick [Kivelson and Southwood, 2005] that exploring
spacecraft rarely leave the disturbed central region. In such
cases a “disk‐like” interpretation of the magnetic field
geometry is no longer applicable.
4. Discussion
[32] The lack of a persistent, unambiguous region of quasi‐
dipolar, B‐dominated field between the dayside magnetopause and magnetodisk at Saturn is in stark contrast to the
∼20 RJ thick, local time dependent cushion region typically
observed at Jupiter. The apparent implication of this discovery is that the Saturnian dayside magnetosphere typically
lacks an outermost layer of plasma‐depleted flux tubes and
that dynamical processes in the Saturnian outer magnetosphere manifest themselves in a very different way to those
observed at Jupiter.
Cushion?
None
None
None
None
None
None
None
None
None
observed
observed
observed
observed
observed
observed
observed
observed
observed
N/A
None observed
None observed
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
None observed
N/A
N/A
N/A
N/A
N/A
4.1. The Importance of Plasma Return Flows
[33] Both the Jovian and Saturnian magnetospheres are
associated with a constant time‐averaged magnetospheric
mass content and, as a result, the time‐averaged rate at which
plasma is generated in the inner magnetosphere (primarily by
processes related to Io and Enceladus) must equal the time‐
averaged rate at which plasma is lost through reconnective processes occurring in the magnetotail. The difference
between the Jovian and Saturnian inner magnetosphere
source rates is typically assumed to be around an order of
magnitude [Vasyliũnas, 2008]; however, caution must be
applied when using this figure for the following important reason. Inner magnetosphere source rates are typically
derived from a measurement of the “momentum loading” of
magnetospheric field lines and, as a result, involve a significant contribution from charge exchange processes which
change the momentum of magnetospheric plasma while not
significantly altering its total mass. The inclusion of such
charge exchange processes in the momentum loading calculation thus results in an overestimation of the true magnetospheric source rate by an unknown factor that is difficult to
determine through experimental means.
[34] The modeling work of Delamere et al. [2007] suggests
that charge exchange processes may be an order of magnitude
more important at Saturn than at Jupiter such that the true
difference between the Jovian and Saturnian inner magnetosphere source rates may then approach a factor of 100. From
an order of magnitude perspective, the average mass content
of flux tubes in the Jovian and Saturnian magnetospheres
(Table 1) is comparable. This fact, combined with the idea
that average mass input equals average mass output, then
implies that flux tubes in the Jovian magnetotail lose mass
roughly 100 times faster than their Saturnian equivalents.
This is similar to the factor of 30 difference in the total
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rotational voltage across each magnetosphere (Table 1) and
suggests that Vasyliũnas cycle reconnection is of a similar
dynamical importance in each magnetosphere. Both systems
should therefore contain a similar number of plasma‐depleted
flux tubes (relative to their total flux content) and, all other
things being equal, a Saturnian equivalent to the cushion
region seen at Jupiter should be evident in the dayside outer
magnetosphere.
[35] Badman and Cowley [2007] estimate the mean
reconnection voltage associated with the Dungey cycle to be
0.25 MV for Jupiter and 0.045 MV for Saturn. The difference
between these two values is smaller than for the Vasyliũnas
reconnection voltage however theoretical calculations [Badman
and Cowley, 2007] suggest a layer of plasma‐depleted flux
tubes, adjacent to the dawn magnetopause at Saturn, with
typical thickness of 0.5–2 RS if the Vasyliũnas cycle is
neglected. No evidence for such a layer was observed in this
study.
[36] Considering the Dungey and Vasyliũnas cycles as
separate and distinct processes may be an oversimplification
of reality as the microphysics of magnetotail reconnection
at Jupiter and Saturn are poorly understood. Kivelson and
Southwood [2005] suggest that the Dungey and Vasyliũnas
cycles may close during the same reconnective episode in the
magnetotail and, in this case, the total number of plasma‐
depleted flux tubes moving back around to the dayside will be
altered from that expected when the two cycles are considered
separately. Regardless of these details the above discussion
makes it clear that plasma cycle return flows are likely to play
an important role in the dynamics of Saturn’s outer magnetosphere and that plasma‐depleted flux tubes returning to the
dayside should be ubiquitous in spacecraft observations.
4.2. Depleted Flux Tube Configuration
[37] If there are plasma‐depleted flux tubes in the Saturnian
outer magnetosphere, why are these flux tubes not associated
with the quasi‐dipolar magnetic field configuration seen
in the Jovian cushion region? The warped nature of the
Saturnian magnetodisk will change the magnetic field configuration expected near the equator (particularly at large
distances from the planet) in a way that is sensitive to the solar
wind dynamic pressure and the instantaneous location of the
spacecraft. However, the fact that the Jovian cushion region
is observed even when the magnetodisk is tilted more than
10° away from the equator suggests that such warping is
unlikely to account for the observed lack of a cushion region
at Saturn.
[38] Of more importance may be the dynamical motion of
the magnetodisk at each planet. At Jupiter the magnetodisk
flaps up and down (in the rest frame of the spacecraft) due
to the 10° tilt between the planets dipole and rotation axes.
The regular transition into different magnetospheric regions
that results from this motion may make differences in magnetic field topology easier to detect. At Saturn the equivalent
motion is far more complicated and, at present, poorly
understood. However, should the magnetodisk at Saturn
move in a more restricted fashion than at Jupiter, differences in topological regions may be less obvious in spacecraft
observations.
[39] Our analysis of Saturnian spacecraft data was limited
to passes associated with a relatively expanded magnetosphere (RSS > 23 RS) in order to ensure that the interior
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magnetodisk configuration, so essential for defining entry
into the cushion region, was present. When the magnetosphere is in a compressed state the magnetodisk configuration
vanishes and the entire dayside magnetosphere is associated
with a quasi‐dipolar geometry very similar to that of the
extensively studied terrestrial magnetosphere. Under such
conditions it is very difficult to identify plasma‐depleted
flux tubes based on their magnetic field configuration alone
and a detailed study of the associated plasma data becomes
important. However, because of the very low densities
observed in the outer magnetosphere, the collection and
interpretation of such measurements is fraught with difficulties and beyond the scope of this investigation.
[40] The Dungey cycle contribution to the cushion region’s
flux content is expected to increase during compressed
magnetospheric conditions [Badman and Cowley, 2007]
however the response of the dominant Vasyliũnas cycle is
less certain. Although the steady state contribution of the
Vasyliũnas cycle to the cushion region’s flux content must
depend only on the internal mass loading rate, the physical
mechanism by which this flux is added to the region may be
controlled by the upstream solar dynamic pressure [Zieger
et al., 2010] in a complicated fashion. However, even if
the Vasyliũnas cycle is neglected, Badman and Cowley
[2007] still predict a cushion region thickness of order
0.5 RS under expanded magnetospheric conditions. This
suggests that restricting our analysis to the expanded magnetosphere will not significantly bias our conclusions in
this study.
[41] One potential explanation for the observations
described above lies in the physics of the magnetodisk itself.
The disk is created as corotating magnetospheric plasma,
strongly confined to the centrifugal equator, balloons outward under the influence of centrifugal forces. The frozen
in magnetic field gets dragged out with the expanding plasma
and eventually adopts the classic magnetodisk configuration.
This expansion is generally thought to be restricted on the
dayside by the dynamic pressure of the solar wind acting on
the magnetopause such that only on the nightside of the planet
can the expansion continue to the point of reconnection. Such
reconnection typically removes mass from the system and
keeps the total mass content of the magnetosphere in a state
of long‐term quasi‐equilibrium.
[42] Consider, however, a situation in which the dynamic
pressure of the solar wind is particularly low such that the
magnetopause retreats to large distances on the dayside. Is
it possible, under these conditions, for the stretching of the
magnetodisk to continue to the point of reconnection on the
dayside too? And if so, what are the likely implications of this
reconnection for the large‐scale structure and dynamics of
the system? In the compressed state, where the magnetodisk
extends all the way to the magnetopause, any plasma‐
depleted flux tubes present on the dayside will be draped over
the magnetodisk and, as a result, retain a significant radial
component. The north–south thickening of the current sheet
due to this “draped magnetic flux” is likely to be negligible
and probably undetectable. In the expanded state the radial
distance at which magnetodisk reconnection occurs also
represents the maximum distance at which a stable magnetodisk‐like configuration can be maintained. Beyond this
distance corotation can no longer be enforced and plasma‐
depleted flux tubes will be able to relax into a more dipolar
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Figure 6. The critical ion density, NC, required for magnetodisk breakdown in the (top) Jovian and (bottom) Saturnian magnetospheres is shown in red. The measured thermal electron density is shown in blue,
and the 1 s variability in the mean location of the Joy et al. [2002] (Jupiter) and Arridge et al. [2006] (Saturn)
magnetopause is shaded in grey, mapped to the local time of the magnetopause crossing. Vertical black lines
denote the actual location of the magnetopause observed by each spacecraft.
configuration reminiscent of the cushion region. Small
clumps of plasma containing closed loops of magnetic
field may detach from the outer edge of the magnetodisk
but, for the most part, this plasma will remain inside the
magnetopause and eventually diffuse onto adjacent magnetospheric field lines. Eventually, this plasma will be lost in
the magnetotail.
[43] Can this effect explain the formation of the cushion
region at Jupiter and, similarly, the absence of a cushion
region at Saturn? The radial distance at which the magnetodisk breaks down can be estimated from the magnetospheric
force balance condition (equation (1)) if we make the simplifying assumptions that mi me and that the poorly constrained outward acting pressure gradient forces are equal
to some multiple, k, of the centrifugal force.
[44] Rearranging equation (1) for the field line radius of
curvature, RC, under these assumptions we obtain
B2
RC ;
ðk þ 1Þ0 Ni mi W2 ð5Þ
from which it is immediately apparent that, for increasing outward forces, the field line radius of curvature will
decrease. This decrease cannot continue indefinitely, however, as the radius of curvature will eventually approach
values comparable with the ion gyroradius. At this point
the MHD assumptions upon which equation (1) is based will
begin to break down and, for a low‐energy plasma, the
magnetic field is likely to develop an x line. Reconnection at
this x line may explain many of the magnetic nulls observed
in the transition region at Jupiter by Haynes [1995].
[45] Assuming that ions have a kinetic energy EKE ∼ (3/2)
kBT i, where T i is the mean ion temperature, the mean ion
gyroradius, Rg, for equatorially mirroring particles may be
expressed as
Rg ¼
mi 2kB Ti 1=2
;
jqi jB
mi
ð6Þ
where qi is the mean electromagnetic charge on an ion.
Setting RC = Rg in equation (5), substituting for equation (6)
and rearranging for Ni gives
NC jqi jB3
mi 1=2
;
ðk þ 1Þ0 m2i W2 2kB Ti
ð7Þ
where NC is now the critical number density required for
the ion gyroradius and field line radius of curvature to be
equal. To estimate the distance at which this critical density
is reached in each magnetosphere the value of NC must be
compared with the observed number density measured by in
situ spacecraft. This comparison is presented in Figure 6 for
the Ulysses (inbound) pass at Jupiter and Cassini Rev 20
(inbound) pass at Saturn where it has been assumed that ion
and electron number densities and temperatures are equal.
This assumption is qualitatively consistent with the results
of Thomsen et al. [2010].
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[46] For the purposes of this study a constant electron
temperature of 150 eV and 120 eV has been assumed for
Jupiter and Saturn, respectively, based on in situ plasma
measurements by the Ulysses and Cassini spacecraft. The
magnetic field profile was determined from a power law fit
to the observed magnetodisk lobes with the assumption
(based on Ulysses observations) that the field magnitude in
the center of the magnetodisk is roughly 10% of its value in
the lobes. The azimuthal velocity of the plasma was allowed
to vary with radial distance according to the Achilleos et al.
[2010a] model for Saturn and the Hill [1980] model for
Jupiter. Finally, a mean ion charge of qi = 1 is assumed for
both magnetospheres and the mean ion mass, mi, is set at
24 amu for Jupiter [Thomas et al., 2004] and 18 amu for
Saturn [Khurana et al., 2007]. We further assume that pressure gradient forces are comparable to centrifugal forces at
Saturn (k = 1) and an order of magnitude larger than centrifugal forces (k = 10) at Jupiter. This is the largest possible
pressure gradient contribution allowed at each planet by
Achilleos et al. [2010a, 2010b].
[47] Using these numbers the critical number density at
both planets (Figure 6, red lines) is seen to decrease with
radial distance, primarily due to the decreasing magnitude of
the magnetic field. The electron number density measured by
in situ spacecraft (Figure 6, blue lines) also decreases with
radial distance and, for the Ulysses inbound pass at Jupiter,
the two curves cross at roughly 50 RJ, well inside the 88 RJ
distance to the magnetopause. Beyond this distance the
number density at the center of the plasma sheet is almost an
order of magnitude higher than the critical value suggesting
that magnetodisk breakdown in this region is likely. Such
breakdown is supported by the observation of tearing islands
inside the dayside magnetodisk by Russell et al. [1999] as
well as the magnetic “nulls” of Haynes [1995].
[48] Interpreting the Cassini data is more difficult. The low
electron density in Saturn’s outer magnetosphere is near the
ELS detection threshold and this, combined with the lack
of clear current sheet crossings, makes it hard to tell how the
in situ number density at the center of the plasma sheet
compares with the critical value. Because of this, definite
statements about the structure and dynamics of Saturn’s
magnetodisk will have to await more extensive observations
and higher‐sensitivity plasma measurements. Qualitatively,
however, it appears likely that the measured number density
is comparable to the critical value throughout much of
Saturn’s magnetodisk but that it rarely exceeds this critical
value to the extent observed at Jupiter. This implies that
magnetodisk breakdown is less likely at Saturn (although
the current observations cannot rule it out entirely) which
is certainly consistent with the observational absence of a
cushion region in the Saturnian magnetosphere.
[49] In light of this discussion an important point must
be raised with regards to the mean subsolar thickness of the
cushion region seen at Jupiter. In estimating this value we
have assumed that the mean subsolar standoff distance of the
magnetopause is 75 RJ. However, in section 4 it has been
suggested that the Jovian cushion region is more likely to
form when the magnetosphere is in an expanded configuration. It is thus possible that many of the Jovian cushion region
observations used in this analysis were made when the
magnetosphere was in just such an expanded configuration
and that the mean magnetopause location for this particular
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subset of passes is actually greater than the 75 RJ mean
obtained by Joy et al. [2002]. Such biasing would result in
the cushion region having a mean inertial subsolar thickness larger than the 20 RJ figure obtained in section 2.3. A
more accurate estimation of the cushion region’s thickness
should become possible once future missions to Jupiter begin
returning data.
4.3. Future Studies and Extensions
[50] The ideas presented in this paper can be tested in
two ways. A detailed study of the ion‐electron plasma in the
outer regions of both planets magnetospheres should reveal
whether plasma‐depleted flux tubes (of any magnetic configuration) exist in these systems. The above statement will
hold true even when the magnetodisk is seen to persist all the
way out to the magnetopause or, in the case of Saturn, when
the magnetosphere is so compressed that no magnetodisk
is observed at all. Such observations will, additionally, allow
plasma in the outer magnetosphere to be better characterized
in general and, as a result, allow the radial distance (and
critical density) at which the magnetodisk breaks down to be
better constrained. Extensive, high‐quality plasma measurements of this nature are currently unavailable (both at Jupiter
and at Saturn) but this situation may change as new and
improved spacecraft reach the outer planets. Meanwhile,
in the more immediate future, qualitative statements may
become possible as existing instrumental data sets are studied
in more detail. We expect plasma‐depleted flux tubes to be
common in the outer magnetospheres of both these planets,
regardless of the associated magnetic field topology.
[51] We have also suggested that the thickness of the
cushion region is inversely (though not necessarily linearly)
proportional to solar wind dynamic pressure and that, for
extremely high dynamic pressures, it may not be seen at
all. This is difficult to examine at present owing, primarily,
to the difficulties involved in calculating the thickness of
the cushion region (and the upstream solar wind dynamic
pressure) for individual passes. The aforementioned lack of
plasma data is also an important issue. The proposed EJSM
and Juno missions (Juno is scheduled for launch in August
2011 while EJSM is currently under review) may improve
the statistics in such a way that qualitative statements can
be made on this possibility but the real answer must await
a multispacecraft investigation of the cushion region. The
EJSM mission, potentially consisting of two concurrent spacecraft, may yet provide such an opportunity.
[52] The Juno mission will, for the first time, allow the full
three dimensional structure of the Jovian magnetosphere to
be explored and this, combined with high‐latitude Cassini
orbits, will allow the effects of magnetodisk warping to be
quantified at both planets. Finally, a better understanding of
the microphysics of magnetotail reconnection (for example,
do the Dungey and Vasyliũnas cycles share a common x
line?) will allow many of our theoretical expectations, at
both planets, to be better constrained and compared with
observations.
5. Summary
[53] This paper has characterized the Jovian outer magnetosphere (or cushion region) using, for the first time, observations made by multiple exploring spacecraft. We find that,
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while the instantaneous location of both the inner and outer
boundaries are highly variable, the cushion region has a
mean subsolar thickness of order 20 RJ. The cushion region,
which is more evident in the morningside magnetosphere
as opposed to afternoon, is interpreted by Kivelson and
Southwood [2005] as a layer of plasma‐depleted flux tubes
which have recently lost mass in the magnetotail as part of
the Vasyliũnas and Dungey cycles. Using magnetometer
and plasma data from Cassini and other spacecraft, we have
shown that the Saturnian magnetosphere typically lacks this
outer layer of quasi‐dipolar flux tubes with the Saturnian
magnetodisk instead persisting right out to the magnetopause.
[54] In spite of this observation, arguments are presented
suggesting that Saturn’s outer magnetosphere must contain a
large number of plasma‐depleted flux tubes. The nondipolar
geometry of these flux tubes is discussed from a number
of perspectives, emphasizing the complicating factors of
magnetodisk warping and variations in the size of the magnetospheric cavity. We show that the Jovian magnetodisk
typically breaks down well inside the planetary magnetopause while, at Saturn, evidence for this breakdown is
weaker although it cannot, at present, be ruled out entirely.
While conclusive statements must await higher‐quality plasma
data and statistics, we tentatively propose that Saturn’s
magnetodisk typically persists right out to the magnetopause,
robbing any plasma‐depleted flux tubes that lay beyond it
of the essential space that is required for them to relax into a
more dipolar configuration reminiscent of the cushion region
seen at Jupiter. Observational tests of this theory have been
proposed and potential developments and extensions of this
work are discussed.
[55] Acknowledgments. The authors would like to acknowledge
useful discussions with Krishan Khurana with regards to the content of
this paper. D. R. Went was funded by an STFC postgraduate studentship
at Imperial College London.
[56] Masaki Fujimoto thanks the reviewers for their assistance in evaluating this paper.
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N. Achilleos, Centre for Planetary Sciences at UCL/Birkbeck, Gower
Street, London WC1E 6BT, UK.
C. S. Arridge, Mullard Space Science Laboratory, Department of Space
and Climate Physics, University College London, Holmbury St. Mary,
Dorking RH5 6NT, UK.
M. K. Dougherty and D. R. Went, Blackett Laboratory, Imperial College
London, London SW7 2AZ, UK. ([email protected])
M. G. Kivelson, Institute of Geophysics and Planetary Physics,
University of California, Los Angeles, CA 90095, USA.
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