Plasma Interactions in Titan's Ionosphere

Abstract

The Cassini mission has collected vast amounts of in situ data within the ionosphere of Saturn's moon Titan and has shown the complexity of the interaction of Saturn's magnetospheric plasma with Titan. Models of the interactions have been created; however, none have been able to completely describe the observed phenomena. Most notably, modeled electron densities are much larger than the electron densities observed by instruments aboard the Cassini spacecraft. This thesis will explore the possible causes of this discrepancy between measured and modeled electron densities using models calculating the production of ions due to solar photons and magnetospheric electrons precipitating down magnetic field lines and into the ionosphere, temperature calculations of the thermal electron population (electrons with energies less than 2 eV), and chemical reactions in the ionosphere. The results of these models will be compared to data collected by instruments aboard Cassini. Modeled ion production rates and thermal electron temperature profiles will be shown to be in good agreement with ion production rates derived from data collected by the Ion – Neutral Mass Spectrometer (INMS) and electron temperatures measured by the Radio and Plasma Wave Science – Langmuir Probe above 1000 km. Modeled ion mass spectra will be generated near the ionospheric peak and will be compared with the INMS measured mass spectra to examine the effects of chemical loss processes on the ion densities. From this analysis it will be shown that the overabundance of modeled electrons is not caused by over production of ions and that chemical loss processes, predominantly the electron dissociative recombination coefficient of HCNH+, need to be reexamined. After the model has been proven to reproduce accurate profiles of ion production and temperature, ion production profiles will be generated using solar photons and magnetospheric electron fluxes for four canonical cases detailed in the work of Rymer et al. [2009] and a globally averaged model of the neutral densities based on INMS neutral measurements from more than 30 flybys of Titan. These generic profiles can be combined to predict ionospheric observations made by the Cassini spacecraft for a variety of solar zenith angles and magnetospheric conditions.