Pickup ion Phase Space Distributions at Titan in a Three Dimensional Exosphere

Details Pickup ion Phase Space Distributions at Titan in a Three Dimensional Exosphere Pickup ion Phase Space Distributions at Titan in a Three Dimensional Exosphere

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p23b1369h&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P23B-1369

Physics

1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 2459 Planetary Ionospheres (5435, 5729, 6026), 3265 Stochastic Processes (3235, 4468, 4475, 7857), 6033 Magnetospheres (2756), 6281 Titan

Scientific paper

The composition and structure of neutral exospheres imbedded in moving plasmas can be determined by measurements of the velocity distributions of their pickup ion progeny. In turn, the velocity distributions are dependent on the spatial structure of the neutral source gases. Since Titan's neutral exosphere extends into the Saturn's magnetosphere (or solar wind) and well above its ionopause, it serves as a good place to analyze such characteristics. They are analyzed using pickup ion measurements made by the Cassini Plasma Spectrometer (CAPS) at Titan [e.g., Hartle et al., 2006] and an ion kinetic model. An early version of the model [Hartle and Sittler, 2007] is an expression describing the phase space density of pickup ions, which is derived from the Vlasov equation with an ion source that explicitly accounts for the velocity and spatial variation of the exosphere source gases. The current version used here includes exosphere source gases in three dimensions and is applicable where the uniform flow approximation is valid. A fundamental parameter of the phase space densities is the ratio of the gyroradius to the neutral scale height, α = rg/H. Titan's exosphere structure yields pickup ions whose phase space distributions are beam-like when α ≫ 1 and fluid-like when α ≪ 1. Downstream from the source peak, the light pickup ions, with α ≪ 1, are easily observed because their phase space densities are almost uniform over the orbit phases. In contrast, the phase space distributions of the heavier ions, with α ≫ 1, peak over narrow velocity and spatial ranges. This beam-like nature makes it considerably more difficult to observe heavy ions because their downstream positions and viewing directions are narrowly constrained. Examples of these extremes will be discussed. The results will also be compared with the distributions obtained from a new 3D hybrid simulation [Lipatov, Sittler and Hartle, 2007], which is applicable over a larger region, from the ionosphere to many Titan radii. Hartle et al., Planet. Space Sci., 54, 1211, 2006. Hartle and Sittler, J. Geophys. Res., 112, A07104, doi:10.1029/2006JA012157, 2007. Lipatov, Sittler and Hartle, Eos Trans. AGU, this meeting Abstract, Fall 2007.

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