Multifluid Magnetohydrodynamic Investigation of the Global Dynamics of Saturn's Magnetosphere
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The dynamics of Saturn's magnetosphere are driven internally by the planet's strong magnetic field, rapid rotation rate, and inner-magnetosphere plasma source, and externally by the solar wind. We use a multifluid magnetohydrodynamic simulation of Saturn's magnetosphere to investigate the production and transport of new plasma, the dynamics of Saturn's magnetotail, and response to seasonal variation. Saturn's closely aligned magnetic dipole and rotational axes are inclined 26.7° relative to the plane of its orbit. As a result, the magnetospheric morphology is strongly influenced by the solar wind; the plasma sheet is deformed into a "basin" at solstice, with decreasing curvature as the planet approaches equinox. Internally, new water group plasma is produced by ionization of Saturn's distributed neutral cloud, while charge-exchange collisions between magnetospheric ions and neutrals result in a loss of momentum from the plasma. New plasma is accelerated towards corotation by the magnetic field, while centrifugal stresses cause it to move radially outward. In order to prevent runaway inflation of the magnetosphere, this plasma must eventually escape, either through the flanks or down the magnetotail. The Saturn multifluid model features three ion species (protons, water group ions, and a heavy tracer), allowing us to track the dynamics of each ion species, as well as the evolution of the electron pressure. We have modified the simulation to include neutral cloud interactions, specifically photoionization, electron impact ionization, and symmetric charge exchange, enabling simulation of mass-loading as a function of local plasma variables. Our 3D multifluid global simulation provides global context for in-situ observations, which, while valuable, only provide data from a single spatial location at a given time. We use this model to study the production and outflow of plasma in the inner magnetosphere, as well as the characteristics of inward-moving outer-magnetosphere injection fingers. We also investigate the impact of seasonal changes on the global magnetospheric configuration and dynamics, as well as the plasma production and transport processes in the inner and middle magnetosphere. Finally, we investigate the evolution of plasmoids and their possible role in removing inner magnetosphere plasma. We validate our results using data from the Cassini Plasma Spectrometer and Magnetometer instruments (CAPS and MAG).