  Plasma rotation plays a large role in determining the size and shape of
Saturn's disc-like magnetosphere. A magnetosphere more confined to the equator
in the polar regions is expected as a result of the interaction between this
type of obstacle and the solar wind. In addition, at times away from equinox, a
north-south asymmetry is expected where the magnetopause will be further
confined in one hemisphere but less confined in the opposite hemisphere.
Examining the extent of this confinement has been limited by a lack of
high-latitude spacecraft observations. Here, for the first time, direct
evidence for polar confinement of Saturn's magnetopause has been observed using
in-situ data obtained by the Cassini spacecraft during a series of
high-inclination orbits between 2007 and 2009. Following techniques established
by previous authors, we assume an equilibrium between the solar wind dynamic
pressure (which Cassini is generally unable to measure directly), and the
magnetic plus plasma pressure inside the magnetosphere. This assumption thus
allows us to estimate the upstream solar wind dynamic pressure ($D_P$) for a
series of magnetopause crossings, and hence to determine the expected location
and global shape of the magnetopause as a function of $D_P$. A clear divergence
from the familiar axisymmetric models of the magnetosphere is observed which
may be characterised by an 'apparent flattening parameter' of 0.81+0.03/-0.06
(representing a simple dilation of the nominal axisymmetric boundary along the
$Z_{KSM}$ axis such that the extent is reduced by 19% in this direction). This
figure is insensitive to variations in $D_P$.

Earth and Planetary Astrophysics