Modeling the production and the imaging of energetic neutral atoms from Titan's exosphere

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Abstract
Energetic neutral atoms (ENAs) are formed when singly charged magnetospheric ions undergo charge exchange with exospheric neutral atoms. The energy of the incident ions is almost entirely transferred to the charge exchange produced ENAs, which then propagate along nearly rectilinear ballistic trajectories. Thus the ENAs can be used like “photons” in order to form an image of the energetic ion distribution. The Cassini spacecraft will be equipped with INCA, a magnetospheric imaging energetic neutral atom camera [Mitchell et al., 1993]. In this context, we undertook the study of the energetic neutral atom production resulting from the interaction of Titan's exosphere with Saturn's magnetosphere. This necessitated the development of a model for the altitude density profile and composition of both the inner and the outer exosphere of Titan. We used the Chamberlain model for the inner exosphere (altitude < 6000 km) that included the five major species: H, H2, N, N2, and CH4. In the lower part of the exosphere, where our model overlaps the high altitude end of existing lower atmosphere models, the results are compatible. The density and composition profiles obtained from our model were then used to calculate the ENA production, considering a proton spectrum measured by Voyager in the Saturnian magnetosphere as the parent ion population. It is shown how the exospheric species contribute in a different way, as a function of the altitude, to the ENA flux spectra. In the outer exosphere (altitude > 6000 km), molecular hydrogen is the dominant species, and we utilized a Chamberlain model to estimate the ENA intensities several Titan radii from the moon. The Titan exosphere is nearly as bright in ENA as the inner magnetosphere of Saturn, making it possible to image the exosphere out to at least 5 Titan radii, and even to resolve it when the spacecraft is several Saturn radii distant from the moon. Our results also show that the magnetospheric imaging technique via energetic neutral atom detection is a powerful diagnostic tool for the interaction of Saturn's magnetosphere with Titan and for the remote sensing of the composition profile of Titan's exosphere.