Imaging and Modeling Io's Energetic Sodium Atoms at Three Spatial Scales

M. Mendillo, J.K. Wilson, J. Baumgardner (Boston University/CSP), N.M. Schneider (Univ. of Colorado/LASP)

Ground-based observations of Io's extended sodium clouds provide a convenient means of studying the complex interaction of Jupiter's plasma torus with Io's atmosphere, and the accompanying atmospheric escape from Io. Interpretations of Io's energetic sodium atom (i.e., ``fast sodium") sources have depended significantly on the spatial scale of related observations. Charge exchange of thermalized torus sodium ions, collisional exospheric ejection, and atomic and molecular pickup ion neutralization have all been proposed as sources of the energetic neutral atoms (ENA's). Images taken at small spatial scales, out to less than 8 Jovian Radii (R tex2html_wrap_inline11 ), show the narrow features indicative of pickup ion neutralization. However, under ideal conditions, these mechanisms would produce distinct features in the large scale sodium ``magneto-nebula" ( tex2html_wrap_inline13 1000 R tex2html_wrap_inline11 across) which have not been seen. Rather, the magneto-nebula is best explained by the more diffuse sources, which do not always produce distinct features at small spatial scales.

Our new observations at a medium scale (70 R tex2html_wrap_inline11 across) show, for the first time, narrow pick-up ion neutralization features extending out to at least 15 R tex2html_wrap_inline11 . In addition, the medium field of view captures the diffuse interior of the magneto-nebula. Our combined analysis of all three spatial scales allows for a less ambiguous determination of the relative strengths of diffuse and concentrated ENA sources. The data set is also sensitive to the pitch-angle distribution of pickup ions before they are neutralized - which is an important characterization of the Io/torus interaction.