在第二個研究主題中，我們將分析從卡西尼號過去十年間從泰坦飛掠中所獲得的大量數據。本研究試圖通過使用太陽天頂角依賴性的簡單近似來生成不同離子物種的平均三維離子密度分佈。這種方式可以對泰坦電離層的全球特性進行全面的可視化，可作為將來執行泰坦任務的工程模型。此外，我們將討論太陽週期以及泰坦大氣層與土星磁層之間相互作用引起的泰坦電離層結構的不對稱性。 ;The work in this thesis use a multi-instruments data from the Cassini spacecraft to study the atmosphere and ionosphere of Saturn’s largest moon Titan. The Ion and Neutral Mass Spectrometer (INMS) provided the ion and neutral number density distributions, the Ion Beam Spectrometer (IBS) of the Cassini Plasma Spectrometer (CAPS) experiment provided positive ion density profiles, and the Radio and Plasma Wave Science (RWPS) instrument provided information on the 3D distributions of electron density and electron temperature. In addition, the Cassini magnetometer (MAG) probed the magnetic region when these targeted flybys happened. Two subjects are included in this thesis.
The first subject is a statistical study of variations in Titan′s thermospheric and exospheric temperature. The upper atmosphere of Titan is highly variable as characterized by the variations of the thermospheric and exospheric temperatures from in-situ measurements by Cassini at different Titan encounters. In the context of a bi-Maxwellian approximation, we examined the atmospheric density profiles obtained by the INMS experiment on Cassini and formed a global change of Titan’s atmospheric temperature at relatively short timescales. Some cases showed a cold exosphere overlying a hotter upper atmosphere during the transition. In this study, we review a series of measurements of N2 and CH4 density profiles. A comparison with the temperature variations of N2 and CH4, and statistical correlations between episodic events and plasma environments are included.
In the second part, we analyze the acquired extensive data set from the decadal coverage of the Cassini measurements during the Titan encounters. A statistical study of the spatial variations of Titan’s ionospheric composition has been performed. An attempt is made in this study to generate average 3D ion density distributions for different ion species by using a simple approximation of the solar zenith angle dependence. This empirical approach allows a comprehensive visualization of the global properties of Titan′s ionosphere that could be useful as engineering models for future missions to Titan. Moreover, we will discuss the asymmetry in the Titan’s ionospheric structure caused by of interaction between Titan’s atmosphere with the Saturnian magnetosphere.
