Desai, RT; (2018) Negative Ions in Outer Solar System Plasmas: TItan, Europa & Rhea. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Negative ions exist in many astrophysical environments. They are the major source of opacity in stars such as our sun and played a role in forming the first stars in the early-universe. In recent years, significant populations of negatively charged ions have been discovered in plasma environments within the outer solar system, discoveries which in many cases were serendipitously made by instruments not designed for this purpose. This thesis uses a combination of spacecraft data analysis and numerical simulations to study negatively charged ions observed by Cassini at the Saturnian moons, Titan and Rhea, and by Galileo at the Jovian moon Europa. The first study focuses on detections by Cassini's Electron Spectrometer of negatively charged ions and aerosols in Titan's ionosphere. An advanced model of the instrument response function is developed and used to statistically identify the presence of carbon chain anions, the first such detections within a planetary atmosphere. Observations of slightly larger anions are interpreted as a hitherto unknown chemical regime involving chemical structures incompatible with previously observed, or even known, anion species. Moreover, a multi-flyby analysis shows the smaller species to deplete with decreasing altitude at a rate proportional to the growth of the larger aerosols, a correlation which suggest these anions are coupled to this growth process. These results demonstrate the importance of tracing a route from small to large species to understand how complex organics can be produced within a planetary atmosphere. The second study derives from Galileo magnetometer observations near Europa which, surprisingly, showed both left-hand and right-hand wave power at the Chlorine ion gyrofrequency. These waves are generated by the outflow of newly ionised material and, in this case, the mix of polarisations was inferred to result from the presence of both positive and negative Chlorine ions. To test this hypothesis, a hybrid particle-in-cell simulation technique is used to carry out the first study focussing on negative ions generating the Alfv\'en-cyclotron instability in order to constrain the non-linear relation between sources densities, wave amplitudes and polarisation, for Chlorine pickup ions in conditions representative of the Europa plasma environment. Through relating simulated and observed wave properties, Chlorine pickup ion densities are constrained to within 0.1-1.5 cm$^{-3}$ in localised regions in Europa's plasma wake, with not less than 5\% resulting from a negatively charged component. These results suggest the moon is a net source of these species with implications for the salinity of the global sub-surface ocean. The third study focuses on both positive and negative pickup ions detected by Cassini's Plasma Spectrometer at Rhea. Using velocity space analysis and expressions derived for partially filled velocity-ring distributions, the positive pickup ions observed during two Rhea flybys are identified as compatible with CO$_2^+$. The negative pickup ions, observed during the first targeted Rhea flyby, are identified as incompatible with the previous identification of O$^-$ and instead are shown as compatible with heavier species in the range of 26$\pm$3 u. These are consequently attributed to carbon-based compounds, such as CN$^-$, C$_2$H$^-$, C$_2^-$, or HCO$^-$, and are suggested to derive from carbonaceous material on the moon's, likely negatively charged, surface. Unidentified, dark, possibly carbon-bearing material, is apparent in near-infrared observations of the surfaces of Rhea, Dione, Pheobe, Iapetus, Hyperion, Epimetheus and throughout Saturn's F-ring. These identifications thus provide context for understanding what minority species could be present within the surfaces of the icy moons of the giant planets.

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