Io's sodium directional feature: Evidence for ionospheric escape

We present evidence that Io's sodium directional feature is produced by the neutralization of pickup ions over the anti-Jovian hemisphere of Io. The l ocalized production region requires ion pickup to occur at or below Io's ex obase and is consistent with an ionospheric escape mechanism. Numerical mod eling reproduces the feature in at least four distinct data sets spanning 1 5 years. The directional feature is sensitive to local magnetic fields and seems to indicate little or no perturbation to the local Jovian magnetic fi eld beyond that associated with the Io/torus interaction. The sensitivity o f the directional feature to electric and magnetic fields near Io makes it a convenient ground-based observable phenomenon for quantifying the complex interaction of Jupiter's plasma torus with Io's atmosphere.