Outer magnetospheric structure: Jupiter and Saturn compared

A04224
WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE
A04224
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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