Hardy, Flavien; (2021) Morphology and Dynamics of Saturn's Magnetopause. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes how the structure and dynamics of Saturn’s magnetopause can be explored by modelling the interactions between the planetary environment and the solar wind. Saturn’s magnetosphere has significant internal sources of plasma: a cold, dense equatorial population originating from the moon Enceladus, and hotter, more tenuous population in the outer magnetosphere. As solar wind particles approach this system, they experience the influence of different internal drivers, including Saturn’s magnetodisk magnetic field and the internal hot plasma. The magnetopause is, to first order, the region where the overall pressure from these internal drivers balances the solar wind dynamic pressure. We model the boundary in three dimensions by describing these interactions from considerations of pressure balance. We find that Saturn’s magnetopause becomes increasingly flattened at the poles as the system size increases, with sharp indents, or ‘cusps’, at high latitudes. The boundary responds to sudden changes in solar and/or internal conditions by moving closer to or further from the planet. By using magnetopause crossings from the Cassini spacecraft, we derive a novel method which unites external and internal drivers in our study of magnetopause compressibility. We find that Saturn’s magnetopause responds to changes in the pressure budget in ways that depend on its size: it behaves similarly to that of Earth when it is compressed, and closer to that of Jupiter when it is expanded. More precisely, we generalise the concept of magnetopause compressibility to describe it as a function of system size at Saturn. In doing so, we also explain previous observational estimates of compressibility as distinct ‘sub-regions’ within this compressibility-size relation. Finally, the interactions between Saturn’s magnetosphere and the solar wind flow are modulated by planetary seasons. We introduce the planet’s obliquity in our model and build a framework which describes the magnetopause under any seasonal configuration. We find that, throughout a Kronian year, the nose of the boundary traces a ‘figure eight’-shaped locus centred on the Sun-planet line. The amplitude of the locus is such that the nose is periodically displaced from the rotational equator, thus creating a clear North-South asymmetry. In particular, the polar cusps are found to move complementarily closer to the nose or terminator. We compute the magnetic field generated by the currents flowing along the magnetopause and find that its inclusion in the internal field causes the current sheet to hinge seasonally, as was evidenced by previous observational studies.

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