Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005dps....37.6007h&link_type=abstract
American Astronomical Society, DPS meeting #37, #60.07; Bulletin of the American Astronomical Society, Vol. 37, p.757
Astronomy and Astrophysics
Astronomy
Scientific paper
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.
Berthelier Jean-Jacques
Bolton Scott
Coates Andrew J.
Crary F. F.
Hartle Richard E.
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