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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[60.07] Preliminary Methane Exosphere Densities at Titan Derived from CH4+ Pickup Ions Observed by CAPS

R. E. Hartle, E. C. Sittler (NASA Goddard Space Flight Center), F. M. Neubauer (University of Koln), R. E. Johnson (University of Virginia), F. Crary, D. J. McComas, D. T. Young (Southwest Research Institute), A. J. Coates (Mullard Space Flight Center), D. G. Simpson (NASA Goddard Space Flight Center), S. Bolton (Jet Propulsion Laborartory), D. Reisenfeld (University of Montana), K. Szego (KFKI Research Institute for Particle and Nuclear Physics), J. J. Berthelier (Centre d'Etude des Environnements Terrestre et Planetaires), A. Rymer (Mullard Space Flight Center), J. Vilppola (University of Oulu), J. T. Steinberg (Los Alamos National Laboratory)

It has been known for some time that measurement of pickup ions, born from neutral exospheres imbedded in moving plasmas, can be used to determine the composition and structure of the parent neutral exospheres [1]. Pickup ions were observed in Saturn’s rotating magnetosphere near Titan by the Cassini Plasma Spectrometer (CAPS) instrument during the Cassini orbiter’s TA encounter on October 26 and the TB encounter on Dec. 13, 2004. Two of the principal pickup ions observed in and near Titan's ion mass loading region by CAPS are CH4+ and N2+. Accounting for the spatial variation of the neutral CH4 exosphere, a model phase space density of CH4+ [2], derived from the Vlasov equation, is used in an algorithm to obtain CH4+ density and velocity moments from CAPS measurements. Because the gyroradius of CH4+ is much greater than the scale height of the source gas, CH4, the ion fluxes are beamlike with velocities distributed over a narrow range. The observed pickup ion velocities are found to be in ring distributions, with the light ion H+ occupying all of its allowed velocities and CH4+ only a small portion of its ring velocities. Further downstream from the source, CH4+ occupies a more energetic portion of its ring distribution. We determine the exosphere densities of CH4 at Titan. For a given CH4+ velocity interval observed by CAPS, the corresponding trajectories are to be mapped back to their source points, where the velocities essentially vanish. A density is obtained by equating the observed CH4+ flux in a given velocity interval to the corresponding ion column production rate at the source.

1. Hartle, R. E., K. W. Ogilvie and C. S. Wu, Planet Space Sci., 21, 2181, 1973.

2. Hartle, R. E. and E. C. Sittler, EOS Trans. AGU, 85(17), P33D-04, 2004.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.