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 
), 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"
( 
1000 R 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 across) show, for the first
time, narrow pick-up ion neutralization features extending out to at least
15 R . 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.