Pilkington, NM; (2016) The Impact of Interior Plasma Dynamics on the Shape and Size of Saturn's Magnetosphere. Doctoral thesis , UCL (University College London).

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Abstract

This thesis explores the impact of rotational and plasma dynamics on the geometry and variability of the location of Saturn's magnetopause, using data obtained by the Cassini spacecraft. A significant departure was identified between the observed location of the high-latitude magnetopause and that predicted by the axisymmetric Kanani et al. model. This model was derived from equatorial measurements made earlier in the mission. The departure demonstrated for the first time that Saturn's magnetosphere is subject to significant 'polar flattening'. It was found that variability in the suprathermal plasma pressure causes dramatic changes in the location of the magnetopause. Such variability had previously been attributed to changes in the solar wind dynamic pressure, but these observations showed that internally driven plasma dynamics imparts a similar degree of variability in the location of the standoff point (up to ~10-15 planetary radii). This effect was incorporated into an empirical magnetopause model and significantly improved the agreement between the observations and the model predictions. In the final part of this study, the structure of the magnetopause was examined in greater detail. Firstly, it was found that the magnetosphere extends further at dawn than dusk. This may be caused by the flow of plasma around the planet, which opposes the solar wind on the morning side of the magnetosphere, but flows in the same direction as the solar wind at dusk. The large dataset collected throughout this study also spans approximately nine years, a significant fraction of a Saturnian season. Additional analysis revealed that the geometry of the magnetopause changes with season, in agreement with theoretical studies. Specifically, a north-south asymmetry is introduced away from equinox whereby the magnetopause is further flattened in the winter hemisphere relative to the north-south symmetric equinoctial magnetopause. This is in addition to the contribution from the centrifugally confined thermal plasma.

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