Wellbrock, A; (2013) Cassini Plasma Spectrometer observations of Titan's ionospheric electrons and ions. Doctoral thesis , UCL (University College London).

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Abstract

The work in this thesis uses data from the Cassini spacecraft in the Saturnian system to study the ionosphere and magnetic environment of Saturn’s largest moon Titan. The main instrument utilised is the Cassini Plasma Spectrometer – Electron Spectrometer (CAPS-ELS). Following the presentation of relevant background information, the first part of the thesis is concerned with the analysis of electrons in Titan’s ionosphere and exosphere, whereas in the second part we investigate observations of organic negative ions that can reach masses as high as 13,800 amu/q. The first of two electron topics is the study of photoelectrons in Titan’s ionosphere. We report on observations of photoelectrons (Coates et al., 2007a) in Titan’s exosphere and ionospheric tail that were created in Titan’s lower sunlit ionosphere. We compare observations to hybrid model results (Sillanpää et al., 2011) to confirm that photoelectrons can travel to these locations via magnetic field lines and discuss the implications for Titan’s ionospheric environment and magnetic tail. In the second electron topic we explore regions in Titan’s topside ionosphere where electrons undergo a change in energy of up to 100 eV. These events are observed predominantly on the hemisphere of Titan where the electric field of Saturn’s corotating magnetospheric plasma points away from the moon. A number of events also appear to be associated with pick up ions. The negative ion part of this thesis describes the investigation of factors that control the masses and number densities of negative ions (Coates et al., 2007b, 2009). These have been observed in the altitude range 950 – 1400 km which makes up the main part of Titan’s ionosphere. We find that the highest masses and densities are observed at the lower altitudes within this range. In addition, we reveal solar zenith angle trends for different mass groups, including a region of predominantly low densities near the day-night terminator.

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