Polar confinement of Saturn's magnetosphere revealed by in situ Cassini observations

Abstract

Plasma rotation plays a large role in determining the size and shape of Saturn's disk-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 characterized 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 approximately 19% in this direction). This figure is insensitive to variations in D P .

A Self‐Regulating Equilibrium Magnetopause Model With Applications to Saturn

“…This is consistent with Pilkington et al, 2014Pilkington et al, 's (2014 treatment of in situ data obtained by the Cassini spacecraft between 2007 and 2009, which indicates that the magnetopause boundary at Saturn is best described by a flattened surface along the north-south direction. This is consistent with Pilkington et al, 2014Pilkington et al, 's (2014 treatment of in situ data obtained by the Cassini spacecraft between 2007 and 2009, which indicates that the magnetopause boundary at Saturn is best described by a flattened surface along the north-south direction.…”

“…This is consistent with Pilkington et al, 2014Pilkington et al, 's (2014 treatment of in situ data obtained by the Cassini spacecraft between 2007 and 2009, which indicates that the magnetopause boundary at Saturn is best described by a flattened surface along the north-south direction. Just upstream from the cusp, for example, the flattening parameter takes a value of ≈ 0.81: This corresponds to a flattening of the boundary of ≈ 19% and is consistent with Pilkington et al, 2014Pilkington et al, 's (2014 treatment of Cassini data. This value corresponds to a simple dilation to an axisymmetric magnetopause boundary along the north-south direction: Due to the high concentration of low-latitude crossings in the data set, and since a compressed boundary cannot describe the "flaring" of the surface downstream from the cusp, it seems reasonable that we obtain a larger value for the flattening parameter .…”

“…In the absence of an upstream solar-wind monitor, observed magnetopause crossings were used to model the shape of the boundary including variable solar wind pressure Kanani et al, 2010) and the behavior of the surface in response to variations in the internal plasma pressure distributions (Achilleos et al, 2010;Pilkington et al, 2014;Sorba et al, 2017). In the absence of an upstream solar-wind monitor, observed magnetopause crossings were used to model the shape of the boundary including variable solar wind pressure Kanani et al, 2010) and the behavior of the surface in response to variations in the internal plasma pressure distributions (Achilleos et al, 2010;Pilkington et al, 2014;Sorba et al, 2017).…”

The Meridional Magnetic Field Lines of Saturn

“…Beyond the magnetopause on the dayside, δ usually exceeds 1.0 because the magnetic fields fluctuate from varying solar wind conditions. In fact, this variability serves as a good marker for the magnetopause, and Figure indicates the statistical validity of the model magnetopause (Pilkington et al, ).…”

Solar Wind and Internally Driven Dynamics: Influences on Magnetodiscs and Auroral Responses

“…Because of the presence of a disk-like current sheet at Jupiter, Jupiter's magnetospheric boundaries are expected to exhibit strong polar flattening (e.g., Slavin et al 1985;Huddleston et al 1998;Pilkington et al 2014), meaning that the magnetosphere extends further from the planet in the dawn-dusk direction than in the north-south direction. have specifically discussed the issue of polar flattening by use of their Jupiter simulations considering a variety of external conditions.…”