Computer Science
Scientific paper
Jan 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt.........1m&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0017, Part 0606 231 pages; [Ph.D. dissertation].United States - - Massachusetts
Computer Science
Saturn, Ionosphere, Plasmasphere
Scientific paper
A number of puzzling phenomena were revealed when the Voyager spacecraft flew past Saturn in 1981 to measure the ionized portions (ionosphere) of its upper atmosphere (thermosphere). Most of these issues have remained unexplained in the intervening 25 years due to a lack of conclusive observational data. With the arrival of Cassini at Saturn in July 2004, however, a new era of observations began, providing the promise of fresh evidence and demanding the development of a contemporary theoretical framework in order to re-examine old mysteries and understand new discoveries.
This dissertation presents studies of Saturn's ionosphere and inner plasmasphere based on new time-dependent photochemical and diffusive transport models that solve the ion equations of continuity in one dimension. Calculations are conducted within the overall framework of a self-consistent, three-dimensional general circulation model (GCM) of Saturn's thermosphere, and the results of these studies are combined with GCM results to provide the building blocks of a new comprehensive model, the Saturn-Thermosphere- Ionosphere-Model (STIM).
The one-dimensional model calculations are used to constrain and investigate a number of unresolved issues and to make testable predictions based on those investigations. Five primary topics are addressed: (1) the additional loss processes required to bring predicted electron densities into agreement with observations, (2) the discrepancy between theory and observations regarding the diurnal variation of peak electron density, (3) the effects of shadowing by Saturn's rings on its ionosphere, (4) the yet unknown electron and ion temperatures at Saturn, and (5) the ionospheric contribution to Saturn's plasmasphere.
The models show that a steady influx of water into Saturn's atmosphere--from
its rings or icy satellites--is required to explain observed electron densities. Additionally, the time-variability of the water source may be the cause of frequently observed ionospheric plasma depletions. The first calculations of the time-dependent effect of attenuation of sunlight by Saturn's rings indicate that they cause large latitudinal gradients within the ionosphere, and may provide radio frequency windows through which atmospheric lightning is observed. Warm plasma temperatures are predicted in Saturn's upper atmosphere, with a strong dawn/dusk asymmetry and a large diurnal variation.
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