37th DPS Meeting, 4-9 September 2005
Session 60 Planetary Magnetospheres
Poster, Thursday, September 8, 2005, 6:00-7:15pm, Music Foyer

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[60.16] The Effect of Pick-up Ion Interactions on the Structure of Titan's Upper Atmosphere

M.C. Wong (JPL), R.E. Johnson (UVA)

Recent Cassini observations have indicated noticeable variability in both temperature and density in Titan's upper atmosphere at different flyby conditions (latitude, longitude, local time, etc) during past encounters. This is supported by a collection of independent measurements from the Ion and Neutral Mass Spectrometer (INMS), Huygens Atmospheric Structure Instrument (HASI), the Ultraviolet Imaging Spectrograph (UVIS), and the Attitude & Articulation Control Subsystem (AACS) onboard the spacecraft. The variability is not unexpected since this is a complex system in which general atmospheric circulation, atmospheric waves, solar illumination, and interactions with the magnetosphere/solar wind can all contribute. In this paper we will focus on the effect of ion bombardment on the structure of Titan's upper atmosphere, using a Direct Simulation Monte Carlo (DSMC) model that tracks incident ions and their subsequent collisions in the atmosphere at regions near the exobase. Similar methods have been applied to study the neutral loss rate of nitrogen from Titan's atmosphere [Michael et al 2005, Shematovich et al. 2003]. It was found that the secondary pickup ions were mainly responsible for the ion-neutral interactions that initiated cascades of interactions between the neutrals. In addition, the ions exiting from the ion-neutral collisions can be reenergized by the local electric fields, which can, in principal, lead to considerable heating near the exobase. Here we concentrate on the effect of these interactions on the atmospheric density and temperature profiles in an effort to understand one of the likely contributors to the observed variability in Titan's upper atmosphere. This is not only of interest scientifically, but is also of interest to Cassini navigation for the purpose of designing lowest safe flyby altitude for future encounters of Titan. Model studies will be performed for different location/geometry in the atmosphere and the results will be presented at the meeting.

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Bulletin of the American Astronomical Society, 37 #3
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