dc.description.abstract | From HST observations, Voyager flyby measurements and the Cassini in-situ measurements, we have learned that the Saturnian system is immersed in a vast neutral gas cloud of oxygen molecules, water molecules and their photodissociative products like OH, O and H. Most of the gas molecules originate from the plumes in the south pole of Enceladus plus some small contribution from other inner icy satellites. In addition, the ring system is an important source of oxygen atoms and molecules which can be injected into the distant Saturnian magnetosphere via scattering processes. Titan’s exosphere is another major source contributing neutral gas like H2 and H, and probably also CH4 and N2. These neutral materials will be fed into the thermal plasma disk in the inner Saturnian magnetosphere. In this work, the model calculations have been performed to simulate the structures and compositions of the neutral gas clouds of different origins making use of an updated photochemical and plasma chemistry model based on the latest plasma measurements from Cassini CAPS instrument.
The present modeling efforts have first led to the picture that an exospheric population of neutral oxygen molecules can be maintained in the vicinity of the main rings by means of photolytic decomposition of ice and other surface reactions. The momentum exchange effect via charge exchange collisions has been taken into consideration in the computation. The ring atmosphere, therefore, serves as a source of O2+ ions throughout Saturn’s magnetosphere. By the same token, our results also show that the magnetopheric O2+ ions should be nearly depleted at Saturn’s equinox if O2 is produced mainly by photolysis of the ring material.
Secondly, we have examined the mass budget of the ring oxygen atmosphere of Saturn taking into account of the possibility of an “exogenic” source i.e. Enceladus’ neutral gas cloud. The maximum O2 source rate from recycling of Enceladus-originated plasma and neutrals might be comparable to the maximum value from photolytic decomposition of the icy ring particles. In this case, the neutral O2 source rate in the Saturnian magnetosphere would be independent of the solar insolation angle. It is also shown that the O2 source from other inner icy satellites is smaller comparable to the scattered O2 component of ring-origin.
The third part of our work is about Titan’s exospheric interaction with the corotating magnetospheric plasma. From the Cassini observations, we know that the magnetic field configuration and plasma flow field are highly variable. We have employed the numerical results of the three dimensional MHD simulation of Kopp and Ip (2001) to study possible spatial and temporal variations in the pickup ion influx. The computation of the ion influx and energy deposit into Titan’s exobase for the H2+, CH4+ and N2+ pickup ions separately are shown. The model results of four different Titan’s orbital locations are also presented.
Finally, we consider the distribution of hydrogen atoms escaping from Titan due to the long-term perturbation effects of the solar radiation pressure and planetary oblateness as Saturn orbits Sun.
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