Outer magnetospheric structure: Jupiter and Saturn compared

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. Dougherty 1 

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 plasmadepleted 

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 Blackett Laboratory, Imperial College London, London, UK. 

2 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. 

Copyright 2011 by the American Geophysical Union. 

0148‐0227/11/2010JA016045 

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 volcanically 

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 plasmadepleted 

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. 

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 R J ) 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 R J ) 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. The Cushion Region at Jupiter 

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 ( ∣B R ∣ while 

radially stretched, nondipolar field lines are associated with 

∣B R ∣ > ∣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, ∣B R ∣ > ∣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 R J Ulysses explores a radially stretched region, 

shaded yellow, where ∣B R ∣ > ∣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 (R C

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Figure 2. Ulysses observations in the Jovian magnetosphere. (a) Jovian System III magnetic field components 

(B R ,red;B , blue or white; B ,green)and±∣B∣ (black). (b) Normalized poloidal field components 

(∣B ∣/∣B∣, blue or white; ∣B R ∣/∣B∣, red). (c) Angle INT between the observed magnetic field, B OBS ,and 

the internal magnetic field, B INT . 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, ∣l M ∣,which beyond 50 R J corresponds to l M = 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 R J , 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 R J 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 R J ) 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 B R 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 R J . 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 (dn e /n e ≈ 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 B R 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 

¼ r P þ B2 

þ N i ðm i þ m e ÞW 2 r: ð1Þ 

R C 2 0 

[13] Equation (1) describes the first‐order balance between 

the magnetic curvature force (left), pressure gradient force 

(right) and centrifugal force (far right). Here R C is the local 

radius of curvature of the field, B 2 /2m 0 is the magneticpressure, 

P is the plasma pressure (assumed to be isotropic), 

N i is the number density of ions, m e and m i 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 

unit vector ^n points in the direction of the outward normal 

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 R J ) 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 R J . 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 R J 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. 

[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 B R and B components to be more comparable 

than in the cushion region and, while the B R 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 (∣B R ∣ > ∣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 R J which suggests 

a mean cushion region “inertial subsolar thickness” of order 

L CR ∼ 20 R J . The 16 R J 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 R J thick cushion 

region with a center 65 R J 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, L CR , and the mean 

rotational voltage across the cushion region at the subsolar 

point, V CR , must be appropriately scaled to the smaller Saturnian 

magnetosphere. Here we perform this scaling using the 

mean subsolar standoff distance of the magnetopause, R SS , 

and the total rotational voltage across the magnetosphere, 

V ROT , as shown below using values from Table 1: 

 

L CR ðSÞ L CR ðJÞ 

R 

SSðSÞ 

6R S 

ð3Þ 

R SS ðJÞ 

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 X JSMAG 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 R J and 2 R J 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, E CR = −v CR × B CR where 

v CR and B CR are the cushion region’s plasma velocity and 

magnetic field, respectively, across the subsolar cushion 

region as shown below: 

jV CR j¼jE CR L CR jjv CR jjB CR jjL CR j: 

Here we have assumed that v CR , B CR and L CR 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 v CR ≈ 0.5WR CR where R CR 

is the mean radial distance to the center of the cushion region. 

With an outer magnetospheric field strength of order 10 nT, 

ð2Þ 

 

V CR ðSÞ V CR ðJÞ 

V 

ROT ðSÞ 

200kV: 

ðJÞ 

V ROT 

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 R S . 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 R S 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 

ð4Þ 

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Table 1. Comparison of Physical and Magnetospheric Parameters for Jupiter and Saturn 

Parameter Jupiter Saturn 

Definite Value 

Equatorial radius, R P [Cox, 2001] 71492 km 60268 km 

Magnetic moment [Cox, 2001] 1.55 × 10 20 Tm 3 4.6 × 10 18 Tm 3 

Rotation period, t 9.925h [Cox, 2001] 10.66h [Cox, 2001] 

Rotation frequency (W =2p/t) 1.76 × 10 −4 s −1 1.64 × 10 −4 s −1 

Axial tilt [Cox, 2001] 3.12° 26.73° 

Dipole tilt [Cox, 2001] 10° 2R J [Mauk and Krimigis, 1987] < 2 R S [Achilleos et al., 2010a] 

Magnetopause subsolar 75 R J [Joy et al., 2002] 24 R S [Achilleos et al., 2008] 

Standoff distance, R SS 

Magnetospheric flux tube ∼10 −3 KgWb ‐1 [Pontius and Hill, 1989] ∼10 −3 KgWb ‐1 [McAndrews et al., 2009] 

Mass content, s 

Dungey Cycle reconnection 0.25 MV [Badman and Cowley, 2007] 0.045 MV [Badman and Cowley, 2007] 

Voltage, V D 

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 

(B R ,red;B , blue or white; B ,green)and±∣B∣ (black). (b) Normalized poloidal field components 

(∣B ∣/∣B∣, blue or white; ∣B R ∣/∣B∣, red). (c) Angle between the observed magnetic field, B OBS , and the magnetic 

field of the Burton et al. [2009] internal dipole, B DIP . 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 R S (R SS = 28.5 R S ) 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 R S 

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∣, 

∣B R ∣/∣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°, ∣B R ∣ > ∣B ∣) region, shaded yellow, from 25 R S 

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 R S 

“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 quasidipolar 

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 ( ∣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°, ∣B R ∣ > ∣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 R S ) 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 (R SS ≥ 23 R S ) Dayside Passes at Saturn as Identified Using the Arridge et al. 

[2006] Magnetopause Model and the Innermost Magnetopause Crossing for Each Pass 

Pass Direction R SS /R S LT/Decimal Hours l/Degree Magnetodisk? Cushion? 

Voyager 2 OUT 23 6 0 YES None observed 

Cassini Rev 00B OUT 27 6 −5 YES None observed 

Cassini Rev 020 IN 29 6 0 YES None observed 

Cassini Rev 015 OUT 26 7 0 YES None observed 

Cassini Rev 009 OUT 27 7 −9 YES None observed 





Cassini Rev 000 IN 25 8 −15 NO N/A 







Cassini Rev 057 OUT 24 12 7 NO N/A 






Cassini Rev 052 IN 29 16 4 NO N/A 




magnetopause prevents a magnetodisk from forming for all 

but the most expanded magnetospheric conditions (Cassini 

Rev 49 outbound, R SS >29R S ) and plasma‐depleted flux 

tubes, if present, are difficult to distinguish from their plasmaloaded 

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 quasidipolar, 

B ‐dominated field between the dayside magnetopause 

and magnetodisk at Saturn is in stark contrast to the 

∼20 R J 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. 

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 timeaveraged 

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 R S 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 plasmadepleted 

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 

(R SS >23R S ) in order to ensure that the interior 

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 R S 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 plasmadepleted 

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 plasmadepleted 

flux tubes will be able to relax into a more dipolar 

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Figure 6. The critical ion density, N C , 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 m i m e 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, R C , under these assumptions we obtain 

B 2 

R C 

ðk þ 1Þ 0 N i m i W 2 ; 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 E KE ∼ (3/2) 

k B T i , where T i is the mean ion temperature, the mean ion 

gyroradius, R g , for equatorially mirroring particles may be 

expressed as 

R g ¼ m 

i 2k B T 1=2 

i 

; ð6Þ 

jq i jB m i 

where q i is the mean electromagnetic charge on an ion. 

Setting R C = R g in equation (5), substituting for equation (6) 

and rearranging for N i gives 

jq i jB 3 

m 1=2 

i 

N C 

; ð7Þ 

ðk þ 1Þ 0 m 2 i W2 2k B T i 

where N C 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 N C 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 q i = 1 is assumed for 

both magnetospheres and the mean ion mass, m i , 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 R J , well inside the 88 R J 

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 R J . 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 

subset of passes is actually greater than the 75 R J 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 R J 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 R J . 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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