The Ionosphere of Saturn.

A one-dimensional theoretical model of the Saturn upper atmosphere and ionosphere was developed and used to study the effects of various free parameters on the structure and composition of the ionosphere. The model was used to solve the diffusion equations for all the major neutral species above the ammonia cloud tops: H(,2), He, H, CH(,4), CH(,3), C(,2)H(,2), C(,2)H(,4), and C(,2)H(,6). It also contained a complete ionosphere model including the diffusing ion H('+) and the photochemical ions C(,2)H(,5)('+), CH(,5)('+), CH(,4)('+), CH(,3)('+), CH(,2)('+), CH('+), He('+), HeH('+), H(,2)('+), and H(,3)('+). Solutions to the neutral, ion, and electron conduction equations were also obtained. Photoelectron and electron precipitation processes were included by solving a two stream electron transport code. All components of the model were coupled together to provide a self-consistent solution to the structure, composition, and temperature of the upper atmosphere and ionosphere. The dearth of information concerning the upper atmosphere of Saturn makes it necessary to examine the solutions in terms of a number of free parameters. Two of the most important unknown parameters are the eddy diffusion coefficient and the neutral temperature structure. The eddy diffusion coefficient can be derived from satellite or rocket measurements of the planetary Lyman-(alpha) albedo, since the column abundance of atomic hydrogen above the homosphere is strongly affected by the amount of mechanical mixing in the lower atmosphere. The column abundance of atomic hydrogen above the methane absorbing layer as a function of the eddy diffusion coefficient has been determined and used in conjunction with a curve of growth of the planetary Lyman-(alpha) resonance emission to determine an approximate value of the eddy diffusion coefficient in the upper atmosphere of Saturn. A value of (TURN)10('7)cm('2)s('-1) at the methane homopause was indicated at the time of the Voyager encounter. The neutral temperature profile is another important parameter that effects the composition and structure of the Saturn ionosphere. High exospheric temperatures of (TURN)1000 K are indicated on both Saturn and Jupiter, but the source of heating is still uncertain. This dissertation examined the possibility of electron precipitation and joule heating in the thermosphere of Saturn. Various energy requirements were examined. A differential velocity of 400 ms('-1) between the ionosphere and neutral atmosphere produced a temperature profile with a 1000(DEGREES)K exospheric temperature. Many combinations of electron energies and fluxes resulted in the observed exospheric temperature of 1000 K. One such combination was a 7.5 keV monoenergetic electron beam with a total energy flux of 0.75 ergs cm(' -2)s('-1). The eddy diffusion coefficient and neutral temperature profile play an important role in determining the atmospheric composition and vertical structure and thereby strongly affect the ionospheric structure. The effects of the neutral temperature and eddy diffusion coefficient on the ionosphere were examined and possible changes due to plasma transport discussed. Values of the peak electron density varied between 10('3) and 3 x 10('5)cm('-3) depending on the combination of temperature structure and eddy diffusion coefficient which was used. The first measurements of the Saturn ionosphere were made by Pioneer 11 on September 1, 1979. They indicated an unexpectedly low peak ionospheric density of 2 x 10('4)cm(' -3) and surprisingly high plasmaspheric densities near the ring plane. Full analysis of the data is proceeding, however, current best agreement with the measurements occurred with a high eddy diffusion coefficient of 5 x 10('8)cm('2)s(' -1) coupled with a temperature profile that was isothermal at 122 K up to 900 km and increased linearly to 1000 K at 1300 km. This combination of parameters resulted in a relatively low peak electron density of 4 x 10('4)cm(' -3) at 1750 km. Possible explanations of these findings are discussed and a guideline for future studies using the recent Voyager/Saturn findings is suggested.