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Io's sodium directional features are created by the escape of Io's ionosphere. The molecular ion stream, B-cloud, and extended sodium nebula are probably also produced by ionospheric escape.
We use a Monte Carlo model to simulate the ejection of sodium atoms from Io. The model simulates sodium pick-up ion neutralization over Io's anti-Jupiter side. Pedersen currents transport ionospheric ions away from Jupiter, so no directional features point towards Jupiter. The escaping sodium ions are neutralized by charge exchange with sodium atoms near or above Io's exobase.
Our model matches images and spectra from several data sets. Below are comparisons of our model with several images from Pilcher et al. 1984 (Astrophysical Journal, 287, pp. 427-444), really the best published images of directional features to date.
Our model also successfully reproduces images taken by Nick Schneider and John Trauger, and data from the following published papers: Goldberg et al. 1984 (Science, 226, pp. 512-516), Cremonese et al. 1992 (Astronomy and Astrophysics, 256, pp. 286-298).