Taylor, Sam A.; (2019) Photoionisation and Spacecraft-Shadow Interactions in Saturn's Inner Magnetosphere. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, data analysis and modelling techniques are used to study the impact of solar illumination on the plasma and neutral molecules in Saturns inner magnetosphere. The data used are from the Cassini spacecraft, a joint NASA-ESA-ASI flagship mission to explore the Saturn system. The first study focuses on photoelectron observations by the CAPS-ELS instrument at Saturns icy moon Enceladus. The geometry of each flyby is analysed and encounters are selected for suitability for observing photoelectrons originating in Enceladus neutral plume material. Fitting techniques are used to subtract the background thermal electron population, revealing characteristic photoelectron spectra and several previously unidentified peaks. A photoelectron model is adapted to create a synthetic production spectra of photoelectrons in the plume. Comparison between the subtracted electron spectra and the model verifies that the observed peak structure can be entirely explained by the photoionisation of plume neutrals. The second study investigates the effects of eclipsing by Saturn on the photoelectron population in the neutral torus. Despite the photoionisation source being obscured, during several shadowing events characteristic photoelectron signatures are observed throughout. This unexpected observation leads us to conclude that there must be a transport process delivering photoelectrons into the shadow. A detailed model is developed to show the geometry of each event including: shadow geometry, spacecraft trajectory, simple magnetic field model and a neutral density model. Analysis of this model suggests that photoelectrons are being produced in the neutral torus of Saturn and transported along field lines deep into the shadow of Saturn. Finally, the effects of shadowing on the spacecraft are investigated. The sudden loss of illumination appears to affect both the local plasma as photoionisation of neutrals is switched off and the spacecraft itself in the form of rapid spacecraft potential changes. Along with spacecraft effects, there is also evidence of electron density drops inside the shadow, likely due to the removal of photoionisation as a plasma source in darkness.

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