Spectroscopic observations of Io Neutral Clouds: constraints on Na-bearing dust grains ejected from Io

We report the results of model simulations performed to explain the nature of a recently detected sodium emission feature in Io Neutral Clouds via high-resolution (λ/Δλ ~ 115,000) spectroscopic observations from the Telescopio Nazionale Galileo (TNG) Italian telescope. The emission feature is blueshifted compared to the main emission (the banana-shaped Neutral Cloud) by a few tens of km/s, and it is most prominent when Io is a few tens of degrees from opposition (before eclipse behind Jupiter's shadow). The feature changes morphology with time, indicative of a geometry effect. We constrained its direction, velocity, and brightness (i.e. column density) with a model of sodium atom trajectories under the influence of Io and Jupiter gravity and solar radiation pressure. The model that best explains this emission feature has the atoms injected into the exosphere from the leading/sub-Jovian hemisphere of Io, with a range of velocities from 10 to 70 km/s. These trajectories are consistent with those of negatively charged dust grains (size ~10 nm) which follow the co-rotational electric field of Jupiter's magnetosphere. Both modeling and observational constraints provide an order-of-magnitude estimate of the sodium production rate (via transport of dust) of ~10²⁶ s^-1. Our findings are consistent with previous in situ dust detection (by Ulysses, Galileo, and Cassini) showing that Io is the source of dust in the Jovian system, and provide another method to monitor the amount of material that Io is supplying to its plasma torus.