Haythornthwaite, Richard P; (2023) Ionospheric studies at Saturnian satellites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An ionosphere refers to the region of charged particles contained in the upper regions of a body's atmosphere. They are typically fed from the neutral atmosphere, which ionises due to sources such as solar photons or precipitating particles. Except Mercury, all planets in the solar system have ionospheres, in addition to this, ionospheres have been observed at several moons including: Europa, Triton and Titan. This thesis covers several studies regarding ionospheric composition and dynamics at the Saturnian moons of Enceladus and Titan, utilising data from the Cassini-Huygens mission which studied these moons between 2004 and 2017. The first study focuses on ion velocities in Enceladus's plume as measured by sensors from the Cassini Plasma Spectrometer instrument. One of the key findings from the Cassini mission was a large water ice plume emanating from the south polar region of the moon Enceladus. Cassini made several passes through the plume and the Cassini Plasma Spectrometer sensors made insitu measurements of positive and negative ions. The recorded energies of the ions were used to infer ion velocities along Cassini's trajectory. Two populations were found, associated with faster and slower moving water ion populations. These were linked with velocity characteristics found in the neutral water population. Comparing the velocities of positive and negative ions also indicated the presence of an electrostatic field in the plume. The second study covers heavy positive ion measurements in Titan's ionosphere. Previous studies of Titan's ionosphere have revealed a plethora of hydrocarbon and nitrile cations and anions. Using data from the Cassini Ion Beam Spectrometer sensor taken across five Titan flybys, positive ions were studied with masses ranging from 170 amu up to 350 amu. Examining the possible molecular structure of the ions, the most abundant ions masses were found to be consistent with molecular ions of polycyclic aromatic compounds, including polycyclic aromatic hydrocarbons and nitrogen-bearing polycyclic aromatic hydrocarbons. This was further supported by examining the mass difference between the most abundant ions. A difference of 12 or 13 amu was found to be the most common, indicating the addition of a carbon or carbon-hydrogen molecule, consistent with polycyclic aromatic hydrocarbon growth pathways. The third and fourth studies were performed in tandem and cover ion velocities within Titan's ionosphere. Previous studies have examined ion velocities parallel to Cassini's trajectory over a selection of Titan flybys from early in its mission. Here, ion velocities both parallel and perpendicular to Cassini's trajectory were derived, known as alongtrack and crosstrack velocities respectively, across a large series of flybys, allowing for a statistical approach. For both crosstrack and alongtrack directions, the positive and negative ions velocities were found to be proportional to each other, agreeing with the expectation from ion-neutral collisional coupling. Alongtrack velocities were further investigated through a comparison to the expected superrotation in Titan's ionosphere. Some measurements did not agree with expectations from neutral wind measurements and provide evidence for complex ion wind structures within Titan's ionosphere.

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