Neutral Exosphere Densities and Structures at Titan Inferred from Pickup Ions Observed by CAPS

Details Neutral Exosphere Densities and Structures at Titan Inferred from Pickup Ions Observed by CAPS Neutral Exosphere Densities and Structures at Titan Inferred from Pickup Ions Observed by CAPS

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p43b..07h&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P43B-07

Other

0328 Exosphere, 2400 Ionosphere (6929), 2455 Particle Precipitation, 2732 Magnetosphere Interactions With Satellites And Rings, 6281 Titan

Scientific paper

Measurements of pickup ions, born from neutral exospheres imbedded in moving plasmas, can be used to determine the composition and structure of the parent neutral exosphere constituents [1]. Pickup ions have been observed in Saturn's rotating magnetosphere near Titan by the Cassini Plasma Spectrometer (CAPS) instrument during the Cassini orbiter's recent flybys of the moon. Pickup ions observed by CAPS include H+, H2+, N+/CH2+, CH4+, and N2+. These ions slow down Saturn's magnetospheric plasma beyond Titan's ionosphere through mass loading. Because of its relatively high mass and high concentration, CH4+ is the dominant mass loading ion. The other ions make negligible contributions to the mass loading process except for N2+ just above the ionopause, where its concentration becomes important. The phase space densities of pickup ions are sensitive functions of the spatial variations of the parent exosphere gasses of the pickup ions [2]. Accounting for such variations, model phase space densities [2], derived from the Vlasov equation, are used in an algorithm to obtain ion density and velocity moments from CAPS measurements. The model implicitly maps an ions trajectory from its observation point to its source point. The analysis shows that 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. Applying the algorithm, exosphere densities are inferred. Using CAPS time-of-flight data and empirical cracking patterns, we show that the 14 amu ion is more likely N+. We compare ratios of the inferred N and CH4 exosphere densities with existing exosphere models. 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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